Section 3: Equipment & Transducers

Section 3: Equipment & Transducers

Typical sound velocities

Wavelength in mm for Steel

Content: Section 3: Equipment & Transducers 

3.1: Piezoelectric Transducers 

3.2: Characteristics of Piezoelectric Transducers 

3.3: Radiated Fields of Ultrasonic Transducers 

3.4: Transducer Beam Spread 

3.5: Transducer Types 

3.6: Transducer Testing I 

3.7: Transducer Modeling 

3.8: Couplants 

3.9: Electromagnetic Acoustic Transducers (EMATs) 

Continues Next Page

3.10: Pulser-Receivers 

3.11: Tone Burst Generators In Research 

3.12: Arbitrary Function Generators 

3.13: Electrical Impedance Matching and Termination 

3.14: Transducer Quality Factor “Q” 

3.15: Data Presentation 

3.16: Testing Techniques 

3.17: UT Equipment Circuitry 

3.18: Further Reading on Sub-Section 3 

3.19: Questions & Answers

3.1: Piezoelectric Transducers 

The Definitions: 

• Nominal frequency (F) - nominal operating frequency of the transducer 

(usually stamped on housing) 

• Peak frequency (PF) - the highest frequency response measured from 

the frequency spectrum 

• Bandwidth center frequency (BCF) - the average of the lowest and 

highest points at a -6 dB level of the frequency spectrum 

• Bandwidth (BW) - the difference between the highest and lowest 

frequencies at the -6 dB level of the frequency spectrum; also % of BCF or 

of PF 

• Pulse width (PW) - the time duration of the time domain envelope that is 

20 dB above the rising and decaying cycles of a transducer response

• Sensitivity is the ability of the search unit to detect reflections or echoes 

from small defects or flaws. 

• The acoustic impedance of a transducer is the product of its density and 

the velocity of sound within it. 

• Resolution is the resolving power includes the ability to separate 

reflections from two closely spaced flaws or reflectors. 

• Front surface pulse (at crystal face), Initial pulse, or “Main Bang” - the 

first indication on the screen, represents the emission of ultrasonic energy 

from the crystal face. 

• Front surface pulse (at interface) - ?

Pulse width (PW) - the time duration of the time domain envelope that is 20 

dB above the rising and decaying cycles of a transducer response

Bandwidth (BW) - the difference between the highest and lowest frequencies 

at the -6 dB level of the frequency spectrum; also % of BCF or of PF

Piezoelectric Properties 

The conversion of electrical pulses to mechanical vibrations and the 

conversion of returned mechanical vibrations back into electrical energy is the 

basis for ultrasonic testing. The active element is the heart of the transducer 

as it converts the electrical energy to acoustic energy, and vice versa. The 

active element is basically a piece of polarized material (i.e. some parts of the 

molecule are positively charged, while other parts of the molecule are 

negatively charged) with electrodes attached to two of its opposite faces. 

When an electric field is applied across the material, the polarized molecules 

will align themselves with the electric field, resulting in induced dipoles within 

the molecular or crystal structure of the material. 

The effectiveness of the search unit for a particular application depends on 

Q factor, bandwidth, frequency, sensitivity, acoustic impedance, and resolving 

power.

This alignment of molecules will cause the material to change dimensions. 

This phenomenon is known as electrostriction. In addition, a permanentlypolarized 

material such as quartz (SiO2) or barium titanate (BaTiO3) will 

produce an electric field when the material changes dimensions as a result of 

an imposed mechanical force. This phenomenon is known as the 

piezoelectric effect. Additional information on why certain materials produce 

this effect can be found in the linked presentation material, which was 

produced by the Valpey Fisher Corporation. 

Keyword: 

SiO2- Quartz 

BaTiO3- Barium Titanate 

Electric field is applied causing dimensional change: electrostriction 

Electric field is generated by dimensional change: piezoelectric effect

Fig. 5.10: Basic design of a single 

transducer Ultrasound head 

Piezoelectric materials have two nice 

properties: 

1. Piezoelectric materials change their 

shape upon the application of an 

electric field as the orientation of the 

dipoles changes. 

2. Conversely, if a mechanical forces 

is applied to the crystal a the 

electric field is changed producing a 

small voltage signal. 

The piezoelectric crystals thus function 

as the transmitter as well as the 

receiver!

Transducer Effectiveness 

The effectiveness of the search unit for a particular application depends on 

Q factor, bandwidth, frequency, sensitivity, acoustic impedance, and resolving 

power.

Piezoelectric crystals 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/PiezoelectricEffect.ppt 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/PiezoelectricElements.ppt

Piezoelectric crystals 

http://www.ndt-kits.com/blog/wp-content/uploads/2013/05/What-is-piezoelectric-transducer.gif 

http://www.ndt-kits.com/blog/?cat=7

Piezoelectric crystals




The active element of most acoustic transducers used today is a 

piezoelectric ceramic, which can be cut in various ways to produce different 

wave modes. A large piezoelectric ceramic element can be seen in the image 

of a sectioned low frequency transducer. Preceding the advent of 

piezoelectric ceramics in the early 1950's, piezoelectric crystals made from 

quartz crystals and magnetostrictive materials were primarily used. The active 

element is still sometimes referred to as the crystal by old timers in the NDT 

field. When piezoelectric ceramics were introduced, they soon became the 

dominant material for transducers due to their good piezoelectric properties 

and their ease of manufacture into a variety of shapes and sizes. They also 

operate at low voltage and are usable up to about 300°C. The first 

piezoceramic in general use was (1) barium titanate, and that was followed 

during the 1960's by (2) lead Zirconate Titanate compositions, which are now 

the most commonly employed ceramic for making transducers. New materials 

such as piezo-polymers and composites are also being used in some 

applications. 

Keywords: 

(1) Barium Titanate 

(2) Lead Zirconate Titanate

The thickness of the active element is determined by the desired frequency of 

the transducer. A thin wafer element vibrates with a wavelength that is twice 

its thickness. Therefore, piezoelectric crystals are cut to a thickness that is ½ 

the desired radiated wavelength. The higher the frequency of the transducer, 

the thinner the active element. The primary reason that high frequency 

contact transducers are not produced is because the element is very thin and 

too fragile.

The fundamental frequency of the transducer is determined by its thickness: 

From the equation, it can be seen that for high frequency transducer, the 

thickness is very thin , thus fragile; making its only suitable for immersion 

techniques only.

At Interface: Reflection & Transmittance 

1,87 

Incoming wave 

1,0 

0,87 

Transmitted wave 

Reflected wave 

Perspex 

Steel


Incoming wave 

1,0 

Transmitted wave 

0,13 

Reflected wave 

Perspex 

-0,87 

Steel

At Interface: Reflection & Transmittance


At first glance a sound pressure exceeding 

100 % seems paradoxical and one suspects 

a contradiction of the energy law. However, 

according to Eq. (1.4) the intensity, i.e. the 

energy per unit time and unit area, is not 

calculated from the sound pressure 

(squared) only but also from the acoustic 

impedance of the material in which the wave 

travels. However, since this impedance in 

steel is very much greater than in water, the 

calculation shows that the intensity of the 

transmitted wave is very much smaller there 

than in water in spite of the higher sound 

pressure.

Piezoelectric crystals may be X or Y cut depending on which orientation 

they are sliced. The crystals used in UT testing are X cut, due to the mode of 

vibration they produced (longitudinal wave). This means that the crystal is 

sliced with it main axis perpendicular with the X axis.


Q153 A quartz crystal cut so that its major faces are parallel to the X, Y axes 

and perpendicular to the X axis is called: 

a) a Y-cut crystal/ longitudinal wave 

b) a Y-cut crystal/ shear wave 

c) a X-cut crystal/ longitudinal wave 

d) a X-cut crystal/ shear wave 

e) a XY-cut crystal/ longitudinal wave 

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html




3.1.1: Type of Piezoelectric Crystal 

■ Quartz is a Silicon Oxide (SiO 3 ) 

■ Lithium Sulphate LiSO 4 Decomposed 130°C 

■ Barium Titanate (BaTiO 3 ) Curies point 120°C 

■ Lead Metaniobate (PBNbO 6 ) 

■ Lead Zirconate Titanate (PBZrO 3 . PbTiO 3 )* Curies point 350°C 

*Pb[Zr x Ti 1-x ]O 3 (0≤x≤1).

■ Quartz is a Silicon Oxide (SiO 3 ) crystal found naturally and X cut across 

the crustal give compression wave, a Y cut produces shear wave. 

Advantages: 

1. Resistance to wear 

2. insoluble in water 

3. resistance to ageing 

4. easy to cut to give the required frequency 

Disadvantage 

1. It is inefficient, needs a lot of energy to 

produce small amount of ultrasound 

2. Quart crystals are susceptible to 

damages (nor robust) 

3. High voltage to produce low frequency 

sound

Quartz

SiO3-Silicon Quartz

■ 

Lithium Sulphate LiSO 4 , grows from Lithium Sulphate solution by 

evaporation. 

Advantages: 

1. Lithium Sulphate is the most efficient receiver of ultrasound 

2. It has low electric impedance 

3. Operate well at low voltage 

4. it does not age 

5. it has very good resolution 

6. crystals are easily damp and give a short pulse length 

Disadvantage 

1. It dissolves in water 

2. It breaks easily 

3. It decomposed at temperature above 130°C (what is Curie temperature?) 

All of which make it unsuitable for industrial used, except for medical 

ultrasonic where the temperature restriction is not a concern.

Lithium Sulphate LiSO 4 硫酸锂

Followings are Piezoelectric crystals- Polarized crystals made by heating up 

powders to high temperatures, pressing them into shape and allow them to 

cool in a very strong electric fields. 

Heat applied 

Pressed Powders 

Fused polarized PZT 

Heat applied

■ Barium Titanate (BaTiO 3 ) are polarized crystals made by baking Barium 

Titanate at 1250C and cooling in a 2KV/mm electric field. 

Advantages 

It is efficient ultrasound generator 

It requires low voltage 

It has good sensitivity 

Disadvantages 

Its curies point is about only 120°C, above which it loss it functionality 

It deteriorated over time

BaTiO 3

BaTiO 3

■ Lead Metaniobate (PBNbO 6 ) crystals are made the similar way as 

Barium Titanate 

Advantages 

It has high internal damping 

It gives narrow pulse of ultrasound, which gives good resolution 

Disadvantage 

It has much less sensitivity than Lead Zirconate Titanate PZT

Fig. 3: Comparison between PZT (left) and 1-3 piezocomposite transducer 

(right) on a prospect wedge

Fig. 4: Comparison between lead Metaniobate (left) and 1-3 piezocomposite 

transducer (right) for a WSY70-4 probe 

http://www.ndt.net/article/splitt/splitt_e.htm

■ Lead Zirconate Titanate (PBZrO 3 . PbTiO 3 )* is the best all round crystal 

for industrial use. 

Advantages 

■ It has high Curies point 350°C 

■ It has good resolution 

■ It does not dissolved in water 

■ It is tough 

■ It does not dissolve in water 

■ It is easily damp. 

Other Transducer> Polyvinylchloride probe for high frequency 15MHz, giving 

high resolution and very high sensitivity. 

*Pb[Zr x Ti 1-x ]O 3 (0≤x≤1).

■ Lead Zirconate Titanate PZT Curies point 350°C 

350°C

350°C is also goof for:



Curie Temperature: In physics and materials science, the Curie temperature 

(Tc), or Curie point, is the temperature where a material's permanent 

magnetism changes to induced magnetism. The force of magnetism is 

determined by magnetic moments. The Curie temperature is the critical point 

where a material's intrinsic magnetic moments change direction. Magnetic 

moments are permanent dipole moments within the atom which originate from 

electrons' angular momentum and spin. Materials have different structures of 

intrinsic magnetic moments that depend on temperature. At a material's Curie 

Temperature those intrinsic magnetic moments change direction. 

Permanent magnetism is caused by the alignment of magnetic moments and 

induced magnetism is created when disordered magnetic moments are forced 

to align in an applied magnetic field. For example, the ordered magnetic 

moments (ferromagnetic, figure 1) change and become disordered 

(paramagnetic, figure 2) at the Curie Temperature. Higher temperatures make 

magnets weaker as spontaneous magnetism only occurs below the Curie 

Temperature. Magnetic susceptibility only occurs above the Curie 

Temperature and can be calculated from the Curie-Weiss Law which is 

derived from Curie's Law.

Lead zirconium Titanate is an intermetallic inorganic compound with the 

chemical formula Pb[Zr x Ti 1-x ]O 3 (0≤x≤1). Also called PZT, it is a ceramic 

perovskite material that shows a marked piezoelectric effect, which finds 

practical applications in the area of electroceramics. It is a white solid that is 

insoluble in all solvents.

Lead zirconium Titanate PZT 

http://en.wikipedia.org/wiki/Lead_zirconate_titanate

http://www.ndt.net/article/platte2/platte2.htm

Properties of Piezoelectric Materials

Ceramic Transducer

Q67: Which of the following transducer materials is the most efficient receiver 

of ultrasonic energy? 

(a) Lead metaniobate 

(b) Quartz 

(c) Lithium sulphate 

(d) Barium titanate 

Q69: An advantage of using lithium sulphate in search units it that: 

(a) It is one of the most efficient generators of ultrasonic energy 

(b) It is one of the most efficient receivers of ultrasonic energy 

(c) It is insoluble 

(d) It can withstand temperatures as high as 700ºC

Q68: Which of the following transducer materials is the most efficient 

transmitter of ultrasonic energy? 

(a) Lead metaniobate 

(b) Quartz 

(c) Lithium sulphate 

(d) Barium titanate 

Q17: Which of the following is the least efficient receiver of ultrasonic Energy? 

(a) Quartz 

(b) Lithium sulphate 

(c) Lead metaniobate 

(d) Barium titanate

Q21: An advantage of using a ceramic transducer in search units is that: 

(a) It is one of the most efficient generators of ultrasonic energy 

(b) It is one of the most efficient receivers of ultrasonic energy 

(c) It has a very low mechanical impedance 

(d) It can withstand temperatures as high as 700 o C

Q73: Which of the following is the most durable piezoelectric material? 

A. Barium titanate 

B. Quartz 

C. Dipotassoium tartrate 

D. Rochelle salt 

Q12: The 1 MHz transducer that should normally have the best time or 

distance resolution is a: 

A. Quartz transducer with air backing 

B. Quartz transducer with phenolic backing 

C. Barium titanate transducer with phenolic backing 

D. Lithium Sulphate transducer with epoxy backing

3.2: Characteristics of Piezoelectric Transducers 

The transducer is a very important part of the ultrasonic instrumentation 

system. As discussed on the previous page, the transducer incorporates a 

piezoelectric element, which converts electrical signals into mechanical 

vibrations (transmit mode) and mechanical vibrations into electrical signals 

(receive mode). Many factors, including material, mechanical and electrical 

construction, and the external mechanical and electrical load conditions, 

influence the behavior of a transducer. Mechanical construction includes 

parameters such as the radiation surface area, mechanical damping, housing, 

connector type and other variables of physical construction. As of this writing, 

transducer manufacturers are hard pressed when constructing two 

transducers that have identical performance characteristics.

Transducer

Transducer PZT & Matching Layer Thicknesses

3.2.1 Transducer Cut-Out 

A cut away of a typical contact transducer is shown above. It was previously 

learned that the piezoelectric element is cut to ½ the desired wavelength. To 

get as much energy out of the transducer as possible, an impedance 

matching is placed between the active element and the face of the transducer. 

Optimal impedance matching is achieved by sizing the matching layer so that 

its thickness is ¼ of the desired wavelength. This keeps waves that were 

reflected within the matching layer in phase when they exit the layer (as 

illustrated in the image to the top). (HOW?) 

For contact transducers, the matching layer is made from a material that has 

an acoustical impedance “Z” between the active element and steel. 

Immersion transducers have a matching layer with an acoustical impedance 

“Z” between the active element and water. 

Contact transducers also incorporate a wear plate to protect the matching 

layer and active element from scratching.

Contact Transducer Types: 

socket 

crystal 

Damping 

Delay / protecting face 

Electrical matching 

Cable 

Straight beam probe 

TR-probe 

Angle beam probe

Transducer

Transducer: Straight Beam

Transducer: Angle Beam

Transducer Cut-Out

3.2.2 The Active Element (Crystal) 

The active element, which is piezo or ferroelectric material, converts 

electrical energy such as an excitation pulse from a flaw detector into 

ultrasonic energy. The most commonly used materials are polarized 

ceramics which can be cut in a variety of manners to produce different wave 

modes. New materials such as piezo polymers and composites are also 

being employed for applications where they provide benefit to transducer 

and system performance.

3.2.3 Design of Matching Layer 

The matching layer consists of a layer of material with acoustic impedance 

that of intermediate between the top & bottom mediums. The thickness its 

thickness is ¼ of the desired wavelength , determined from the center 

operating frequency of the transducer and the speed of sound of the matching 

layer.

Matching Layer: Immersion & Delay Transducers 

Backing 

As wear plate 

λ /2 

λ /4 

Active Element 

Matching Layer

3.2.4 Backing (Damping) 

The backing is usually a highly attenuative, high density material that is used 

to control the vibration of the transducer by absorbing the energy radiating 

from the back face of the active element. When the acoustic impedance 

of the backing matches the acoustic impedance of the active element, 

the result will be a heavily damped transducer that displays good range 

resolution but may be lower in signal amplitude. If there is a mismatch in 

acoustic impedance between the element and the backing, more sound 

energy will be reflected forward into the test material. The end result is a 

transducer that is lower in resolution due to a longer waveform duration, but 

may be higher in signal amplitude or greater in sensitivity.

Note on Backing: 

The backing material supporting the crystal has a great influence on the 

damping characteristics of a transducer. 

Using a backing material with an impedance similar to that of the active 

element will produce the most effective damping. Such a transducer will have 

a wider bandwidth resulting in higher sensitivity. 

As the mismatch in impedance between the active element and the backing 

material increases, material penetration increases but transducer sensitivity is 

reduced. 

Keywords: 

Backing impedance mismatch small: Higher sensitivity 

Backing impedance mismatch high: Higher penetration.

3.2.5 Wear Plate 

The basic purpose of the transducer wear plate is to protect the transducer 

element from the testing environment. In the case of contact transducers, the 

wear plate must be a durable and corrosion resistant material in order to 

withstand the wear caused by use on materials such as steel.

Matching Layer (Wear Plate) 

For immersion, angle beam, and delay line transducers the wear plate has 

the additional purpose of serving as an acoustic transformer between the 

high acoustic impedance of the active element and the water, the wedge 

or the delay line all of which are of lower acoustic impedance. 

This is accomplished by selecting a 

matching layer that is ¼ λ 

wavelength thick and of the desired 

acoustic impedance (the active 

element is nominally ½ λ wavelength). 

The choice of the wear surface 

thickness is based upon the idea of 

superposition that allows waves 

generated by the active element to be 

in phase with the wave reverberating 

in the matching layer as shown in 

Figure (4).

When signals are in phase, their amplitudes are additive, thus a greater 

amplitude wave enters the test piece. Figure (12) shows the active element 

and the wear plate, and when they are in phase. If a transducer is not tightly 

controlled or designed with care and the proper materials, and the sound 

waves are not in phase, it causes a disruption in the wave front.

Transducers

Transducers 

http://www.ndt-kits.com/Angle-Beam-Ultrasonic-Transducer-UT0013-s-381-428.html

3.2.6 Transducer Efficiency, Bandwidth and Frequency 

3.2.6.1 Resolution 

Some transducers are specially fabricated to be more efficient transmitters 

and others to be more efficient receivers. A transducer that performs well in 

one application will not always produce the desired results in a different 

application. For example, sensitivity to small defects is proportional to the 

product of the efficiency of the transducer as a transmitter and a receiver. 

Resolution, the ability to locate defects near the surface or in close proximity 

in the material, requires a highly damped transducer.

Resolution: BS4331 Pt 3. the 

recommended resolution should 

be able to distinguished two 

discrete echoes less than two 

wavelength apart. By discrete 

echoes mean they are split by 

more than 6dB. 

(Vertical spatial resolution) 

50% Amplitude or 

6dB line.

2 λ 

50% Amplitude or 

6dB line. 

2 λ

In the early days of ultrasonic testing we used the 100, 91 and 85mm steps, at the radius end of 

the V1 block to test resolving power. However, today this is regarded as too crude a test and BS 

4331 Part 3 (now obsolete) recommended that we should be able to recognise two discrete 

echoes less than two wavelengths apart. By discrete echoes they mean split by more than 6dB, 

or to more than half the total height of the signals.

3.2.6.2 Transducer Damping 

It is also important to understand the concept of bandwidth, or range of 

frequencies, associated with a transducer. The frequency noted on a 

transducer is the central or center frequency and depends primarily on the 

backing material. 

Highly damped transducers will respond to frequencies above and below the 

central frequency. The broad frequency range provides a transducer with high 

resolving power. Less damped transducers will exhibit a narrower frequency 

range and poorer resolving power, but greater penetration. 

The central frequency will also define the capabilities of a transducer. Lower 

frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a 

material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced 

penetration but greater sensitivity to small discontinuities. High frequency 

transducers, when used with the proper instrumentation, can improve flaw 

resolution and thickness measurement capabilities dramatically. Broadband 

transducers with frequencies up to 150 MHz are commercially available.

Transducer Damping (illustration with X-axis frequency domain) 

Less damped transducers will 

exhibit a narrower frequency range 

and poorer resolving power, but 

greater penetration. 

Highly damped transducers will 

respond to frequencies above and 

below the central frequency. The 

broad frequency range provides a 

transducer with high resolving 

power.

Transducer (Backing) Damping: 

• Highly damped transducers will respond to frequencies above and below 

the central frequency. The broad frequency range provides a transducer 

with high resolving power. 

• Less damped transducers will exhibit a narrower frequency range and 

poorer resolving power, but greater penetration.

Transducer Damping 

Narrow 

bandwidth 

X-axis time domain 

Wide 

bandwidth 

X-axis time domain

Transducer Damping

Transducer Damping

Transducer Damping at -20dB

Transducer Damping at -14dB

Transducer Damping

Transducer Damping- Pulse Length

Wave form Duration at -10dB

Transducer Damping- Low Damping (X-axis time domain)

Transducer Damping- High Damping (X-axis time domain)

48. A more highly damped transducer crystal results in: 

(a) Better resolution 

(b) Better sensitivity (mistake) 

(c) Lower sensitivity 

(d) Poorer resolution

3.2.6.3 Bandwidth: 

It is also important to understand the concept of bandwidth, or range of 

frequencies, associated with a ultrasonic transducer. The frequency noted on 

a transducer is the central or center frequency and depends primarily on the 

backing material. 

Highly damped ultrasonic transducers will respond to frequencies above and 

below the central frequency. The broad frequency range provides a 

transducer with high resolving power. 

Less damped transducers will exhibit a narrower frequency range and poorer 

resolving power, but greater penetration. 

The central frequency will also define the capabilities of a transducer. Lower 

frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in 

material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced 



resolution and thickness measurement capabilities dramatically. Broadband 

transducers with frequencies up to 150 MHz are commercially available.

Bandwidth: 

• The unit for bandwidth is MHz 

• The unit for pulse length is mm or time

The central frequency will also define the capabilities of a transducer. 

1. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and 

penetration in a material, 

2. while high frequency crystals (15.0MHz-25.0MHz) provide reduced 



resolution and thickness measurement capabilities dramatically.





The relation between MHz bandwidth and waveform duration is shown 

in Figure below. The scatter is wider at -40 dB because the 1% trailing end of 

the waveform contains very little energy and so has very little effect on the 

analysis of bandwidth. Because of the scatter it is most appropriate to specify 

waveforms in the time domain (microseconds) and spectra in the frequency 

domain.

Transducers are constructed to withstand some abuse, but they should be 

handled carefully. Misuse, such as dropping, can cause cracking of the wear 

plate, element, or the backing material. Damage to a transducer is often 

noted on the A-scan presentation as an enlargement of the initial pulse.

The approximate relations shown in Figure (6) above, can be used to assist in 

transducer selection. For example, if a -14 dB waveform duration of one 

microsecond is needed, what frequency transducer should be selected? 

From the graph, a bandwidth of approximately 1 to 1.2 MHz corresponds 

to approximately 1 microsecond -14 dB waveform duration. Assuming a 

nominal 50% fractional bandwidth transducer, this calculates to a nominal 

center frequency of 2 to 2.4 MHz. Therefore, a transducer of 2.25 MHz or 

3.5 MHz may be applicable. 

http://olympus-ims.com/data/File/panametrics/UT-technotes.en.pdf

Instrumentation Filtered Band Width: 

1. Broad band instrument means a wide array of frequencies could be 

processed by the instrument. The frequencies shown will be a close 

representation of the actual electrical signal measured by the receiver 

transducer. The S/N may not be very good, the shape of the amplitude 

tend to be the actual representation. 

2. Narrow band instrument, suppressed a portion of frequencies above and 

below the center frequency. With the high frequencies noise suppressed, 

gain could be increase, leading to improved sensitivity. However the shape 

and relative amplitude of pulse frequency components often altered

Instrumentation Band Width:

Q8: Receiver noise must often be filtered out of a test system. Receiver 

amplifier noise increases proportionally to: 

A. the square root of the amplifier bandwidth 

B. the inverse square of the amplifier bandwidth 

C. attenuation 

D. temperature

Q164: The resolving power of a transducer is directly proportional to its: 

A. Diameter 

B. Bandwidth 

C. Pulse repetition rate 

D. None of the above 

Bandwidth is the frequency range of the pulse, it is not the pulse length

Q48: The approximate bandwidth of the transducer with the frequency 

response shown in figure 1 (-3dB) is: 

A. 4 MHz (standard answer) 

B. 8 MHz 

C. 10 MHz 

D. 12 MHz 

≈6.5MHz

3.3: Radiated Fields of Ultrasonic Transducers 

The sound that emanates from a piezoelectric transducer does not originate 

from a point, but instead originates from most of the surface of the 

piezoelectric element. Round transducers are often referred to as piston 

source transducers because the sound field resembles a cylindrical mass in 

front of the transducer. The sound field from a typical piezoelectric transducer 

is shown below. The intensity of the sound is indicated by color, with lighter 

colors indicating higher intensity. 

Ɵ

Since the ultrasound originates from a number of points along the transducer 

face, the ultrasound intensity along the beam is affected by constructive and 

destructive wave interference as discussed in a previous page on wave 

interference. These are sometimes also referred to as diffraction effects. This 

wave interference leads to extensive fluctuations in the sound intensity near 

the source and is known as the near field. Because of acoustic variations 

within a near field, it can be extremely difficult to accurately evaluate flaws in 

materials when they are positioned within this area.

The pressure waves combine to form a relatively uniform front at the end of 

the near field. The area beyond the near field where the ultrasonic beam is 

more uniform is called the far field. In the far field, the beam spreads out in a 

pattern originating from the center of the transducer. The transition between 

the near field and the far field occurs at a distance, N, and is sometimes 

referred to as the "natural focus" of a flat (or unfocused) transducer. The 

near/far field distance, N, is significant because amplitude variations that 

characterize the near field change to a smoothly declining amplitude at this 

point. The area just beyond the near field is where the sound wave is well 

behaved and at its maximum strength. Therefore, optimal detection results 

will be obtained when flaws occur in this area.

Near Field

Angular characteristics for large distances from the oscillator. 

a: Values of the sound pressure in a linear plot; 

b: the same plotted in dB

Angular characteristics: Lines of equal sound pressure, plotted in dB. Also 

the distance from the radiator is plotted in a logarithmic measure

Angular characteristics: Spatial distribution of the sound pressure plotted in 

linear values on a half plane through the radiator

Angular characteristics: Sound-pressure mountain measured in a plane 

parallel to the oscillator

Angular characteristics: Sound pressure on the axis of a piston oscillator

For a piston source transducer of radius (a), frequency (f), and velocity (V) in 

a liquid or solid medium, the applet below allows the calculation of the 

near/far field transition point. In the Java applet below, the radius (a) and the 

near field/far field distance can be in metric or English units (e.g. mm or inch), 

the frequency (f) is in MHz and the sound velocity (V) is in metric or English 

length units per second (e.g. mm/sec or inch/sec). Just make sure the length 

units used are consistent in the calculation.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/applet_3_3/applet_3_3.htm

Spherical or cylindrical focusing changes the structure of a transducer field by 

"pulling" the N point nearer the transducer. It is also important to note that the 

driving excitation normally used in NDT applications are either spike or 

rectangular pulsars, not a single frequency. This can significantly alter the 

performance of a transducer. Nonetheless, the supporting analysis is widely 

used because it represents a reasonable approximation and a good starting 

point.

Beam Spreads 

http://www.eclipsescientific.com/Software/ESBeamToolAScan/index.html

Probe Dimension & Spread angle 

探子小 , 近场杂波短 , 声扩张度较大 .

Probe Dimension & Spread angle 

探子大 , 近场杂波长 , 声扩张度较小 .

Probe dimension & Z f , , Ɵ 


Probe dimension & Z f, , Ɵ 


3.4: Transducer Beam Spread 

As discussed on the previous page, round transducers are often referred to 

as piston source transducers because the sound field resembles a cylindrical 

mass in front of the transducer. However, the energy in the beam does not 

remain in a cylinder, but instead spreads out as it propagates through the 

material. The phenomenon is usually referred to as beam spread but is 

sometimes also referred to as beam divergence or ultrasonic diffraction. It 

should be noted that there is actually a difference between beam spread and 

beam divergence. Beam spread is a measure of the whole angle from side to 

side of the main lobe of the sound beam in the far field. Beam divergence is a 

measure of the angle from one side of the sound beam to the central axis of 

the beam in the far field. Therefore, beam spread is twice the beam 

divergence. 

Far field, or Fraunhofer zone

Although beam spread must be considered when performing an ultrasonic 

inspection, it is important to note that in the far field, or Fraunhofer zone, the 

maximum sound pressure is always found along the acoustic axis (centerline) 

of the transducer. Therefore, the strongest reflections are likely to come from 

the area directly in front of the transducer. 

Beam spread occurs because the vibrating particle of the material (through 

which the wave is traveling) do not always transfer all of their energy in the 

direction of wave propagation. Recall that waves propagate through the 

transfer of energy from one particle to another in the medium. If the particles 

are not directly aligned in the direction of wave propagation, some of the 

energy will get transferred off at an angle. (Picture what happens when one 

ball hits another ball slightly off center). In the near field, constructive and 

destructive wave interference fill the sound field with fluctuation. At the start of 

the far field, however, the beam strength is always greatest at the center of 

the beam and diminishes as it spreads outward.

As shown in the applet below, beam spread is largely determined by the 

frequency and diameter of the transducer. Beam spread is greater when 

using a low frequency transducer than when using a high frequency 

transducer. As the diameter of the transducer increases, the beam spread will 

be reduced. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/toplinks-rev2.swf

Near/ Far Fields 

Near field, constructive and 

destructive wave interference fill the 

sound field with fluctuation 

- reverberence 

Far field, however, the 

beam strength is always 

greatest at the center of the 

beam and diminishes as it 

spreads outward.

Beam angle is an important consideration in transducer selection for a couple 

of reasons. First, beam spread lowers the amplitude of reflections since 

sound fields are less concentrated and, thereby weaker. Second, beam 

spread may result in more difficulty in interpreting signals due to reflections 

from the lateral sides of the test object or other features outside of the 

inspection area. Characterization of the sound field generated by a transducer 

is a prerequisite to understanding observed signals. 

Numerous codes exist that can be used to standardize the method used for 

the characterization of beam spread. American Society for Testing and 

Materials ASTM E-1065, addresses methods for ascertaining beam shapes in 

Section A6, Measurement of Sound Field Parameters. However, these 

measurements are limited to immersion probes. In fact, the methods 

described in E-1065 are primarily concerned with the measurement of beam 

characteristics in water, and as such are limited to measurements of the 

compression mode only. Techniques described in E-1065 include pulse-echo 

using a ball target and hydrophone receiver, which allows the sound field of 

the probe to be assessed for the entire volume in front of the probe.

For a flat piston source transducer, an approximation of the beam spread may 

be calculated as a function of the transducer diameter (D), frequency (F), and 

the sound velocity (V) in the liquid or solid medium. The applet below allows 

the beam divergence angle (1/2 the beam spread angle) to be calculated. 

This angle represents a measure from the center of the acoustic axis to the 

point where the sound pressure has decreased by one half (-6 dB) to the side 

of the acoustic axis in the far field. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/applet_3_4/applet_3_4.htm

3.5: Transducer Types 

Ultrasonic transducers are manufactured for a variety of applications and can 

be custom fabricated when necessary. Careful attention must be paid to 

selecting the proper transducer for the application. A previous section on 

Acoustic Wavelength and Defect Detection gave a brief overview of factors 

that affect defect detectability. From this material, we know that it is important 

to choose transducers that have the desired; 

■ 

■ 

■ 

frequency, (thickness of piezoelectric material) 

bandwidth, (Back damping) 

Focusing (curvature probe) 

to optimize inspection capability. Most often the transducer is chosen either to 

enhance the sensitivity or resolution of the system. Transducers are classified 

into groups according to the application.

3.5.1 Contact transducers 

are used for direct contact inspections, and are generally hand manipulated. 

They have elements protected in a rugged casing to withstand sliding contact 

with a variety of materials. These transducers have an ergonomic design so 

that they are easy to grip and move along a surface. They often have 

replaceable wear plates to lengthen their useful life. Coupling materials of 

water, grease, oils, or commercial materials are used to remove the air gap 

between the transducer and the component being inspected.

Contact Transducers

Contact probe

Contact Transducer 

http://www.olympus-ims.com/en/ultrasonic-transducers/dualelement/ 

http://static2.olympus-ims.com/data/Flash/dual.swf?rev=6C5C

Practice Makes Perfect 

43. Which of the following is a disadvantage of contact testing? 

(a) Ability to maintain uniform coupling on rough surface 

(b) Ease of field use 

(c) Greater penetrating power than immersion testing 

(d) Less penetrating power than immersion testing

3.5.2 Immersion transducers 

In immersion testing, the transducer do not contact the component. These 

transducers are designed to operate in a liquid environment and all 

connections are watertight. Immersion transducers usually have an 

impedance matching layer that helps to get more sound energy into the water 

and, in turn, into the component being inspected. Immersion transducers can 

be purchased with a (1) planer, (2) cylindrically focused or (3) spherically 

focused lens. A focused transducer can improve the sensitivity and axial 

resolution by concentrating the sound energy to a smaller area. Immersion 

transducers are typically used inside a water tank or as part of a squirter or 

bubbler system in scanning applications.

Unfocused & Focused

Focusing Ration in water/steel (F=4) 

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/beam-characteristics/

Focused Transducer (Olympus) 

Z B 

F Z 

Z E 

D 

= Beginning of the Focal Zone 

= Focal Zone 

= End of the Focal Zone 

= Element Diameter

Focal Length Equation: 

The focal length F is determined by following equation; 

Where: 

F = Focal Length in water 

R = Curvature of the focusing lens 

n = Ration of L-velocity of epoxy to L-velocity of water 

F

Focal Length Variations 

Focal Length Variations due to Acoustic Velocity and Geometry of the Test 

Part. The measured focal length of a transducer is dependent on the material 

in which it is being measured. This is due to the fact that different materials 

have different sound velocities. When specifying a transducer’s focal length it 

is typically specified for water. Since most materials have a higher velocity 

than water, the focal length is effectively shortened. This effect is caused by 

refraction (according to Snell’s Law) and is illustrated in Figure (18).

Focal Length Variations

This change in the focal length can be predicted by Equation (13). 

For example, given a particular focal length and material path, this equation 

can be used to determine the appropriate water path to compensate for the 

focusing effect in the test material. 

Eqn. 13 

WP = F – MP.(C tm /C w ) 

WP 

MP 

F 

C tm 

C w 

= Water Path 

= Material Depth 

= Focal Length in Water 

= Sound Velocity in the Test Material 

= Sound Velocity in the water 

In addition, the curvature of surface of the test piece can affect focusing. 

Depending on whether the entry surface is concave or convex, the sound 

beam may converge more rapidly than it would in a flat sample or it may 

spread and actually defocus.

Cylindrical & Spherical Focused

Cylindrical & Spherical Focused

Q79: What type of search unit allows the greatest resolving power with 

standard ultrasonic testing equipment? 

a) Delay tip 

b) Focused 

c) Highly damped 

d) High Q 

Q165: Acoustic lens elements with which of the following permit focusing the 

sound energy to enter cylindrical surface normally or along a line of focus. 

a) Cylindrical curvature 

b) Spherical lens curvatures 

c) Convex shapes 

d) Concave shapes

Q18: Which of the following is an advantage of a focused transducer? 

(a) Extended useful range 

(b) Reduced sensitivity in localised area 

(c) Improved signal to noise ratio over an extended range 

(d) Higher resolution over a limited range 

Q67: A divergent sound beam is produced by: 

(a) Concave mirror 

(b) Convex mirror 

(c) Convex lens 

(d) None of the above

Q78: Which of the following is not an advantage of a focused transducer? 

(a) High sensitivity to small flaws 

(b) Deep penetration 

(c) High resolving power 

(d) Not much affected by surface roughness 

Q79: What type of search unit allows the greatest resolving power with 

standard ultrasonic testing equipment? 

(a) Delay tip 

(b) Focused 

(c) Highly damped 

(d) High Q

3.5.3 Dual element transducers 

contain two independently operated elements in a single housing. One of the 

elements transmits and the other receives the ultrasonic signal. Active 

elements can be chosen for their sending and receiving capabilities to provide 

a transducer with a cleaner signal, and transducers for special applications, 

such as the inspection of course grained material. Dual element transducers 

are especially well suited for making measurements in applications where 

reflectors are very near the transducer since this design eliminates the ring 

down effect that single-element transducers experience (when single-element 

transducers are operating in pulse echo mode, the element cannot start 

receiving reflected signals until the element has stopped ringing from its 

transmit function). Dual element transducers are very useful when making 

thickness measurements of thin materials and when inspecting for near 

surface defects. The two elements are angled towards each other to create a 

crossed-beam sound path in the test material. 

Keywords: For near surface effects 

■ Fresnel zone (near zone) 

■ Ring down effect

For a single crystal probe the length of the initial pulse is the dead zone and 

any signal from a reflector at a shorter distance than this will be concealed 

in the initial pulse. We deliberately delay the initial pulse beyond the left of 

the time base, by mounting the transducers of a twin (or double) crystal 

probe onto plastic wedges. This and the focusing of the crystals reduces the 

dead zone considerably and it is only where the transmission and receptive 

beams do not overlap that we cannot assess flaws. 

A twin or double crystal probe is designed to minimise the problem of dead 

zone. A twin crystal probe has two crystals mounted on Perspex shoes 

angled inwards slightly to focus at a set distance in the test material. Were 

the crystals not angled, the pulse would be reflected straight back into the 

transmitting crystal.

The Perspex shoes hold the crystals away from the test surface so that the 

initial pulse does not appear on the CRT screen. The dead zone is greatly 

reduced to the region adjoining the test surface, where the transmission and 

reception beams do not overlap. 

More on Dead Zone BS EN 12668-Part1 Section: 3.5 

Dead time after transmitter pulse 

time interval following the start of the transmitter pulse during which the amplifier is 

unable to respond to incoming signals, when using the pulse echo method, because of 

saturation by the transmitter pulse

There are other advantages 

1. Double crystal probes can be focused 

2. Can measure thin plate 

3. Can detect near surface flaws 

4. Has good near surface resolution 

Disadvantages 

1. Good contact is difficult with curved surfaces 

2. Difficult to size small defects accurately as the width of a double crystal 

3. probe is usually greater than that of a single crystal probe 

4. The amplitude of a signal decreases the further a reflector is situated 

5. from the focal distance - a response curve can be made out. 

Therefore single and twin crystal probes are complementary.

Other Reading (Olympus): Dual element transducers utilize separate 

transmitting and receiving elements, mounted on delay lines that are usually 

cut at an angle (see diagram on page 8). This configuration improves near 

surface resolution by eliminating main bang recovery problems. In addition, the 

crossed beam design provides a pseudo focus that makes duals more 

sensitive to echoes from irregular reflectors such as corrosion and pitting. 

One consequence of the dual element design is a sharply defined distance/ 

amplitude curve. In general, a decrease in the roof angle or an increase in 

the transducer element size will result in a longer pseudo-focal distance and 

an increase in useful range, as shown in Figure (13).

Advantages: 

Improves near surface resolution (sensitivity?) 

Provide a pseudo focus (improve sensitivity in the Far Zone?) 

Less affected by surface roughness due to the pseudo focus effect 

Disadvantage(?) 

The pseudo focus by tilting the active elements (roof angle?) reduces the 

useful range of transducer?

Figure (13).

Duo Elements Transducer 

Transmitting 

Crystal 

Acoustic 

Barrier 

Receiving 

Crystal 

Roof Angle 

Casing 

Cross Beam 

Sound path

Duo Elements Transducer

3.5.4 Delay line transducers 

provide versatility with a variety of replaceable options. Removable delay line, 

surface conforming membrane, and protective wear cap options can make a 

single transducer effective for a wide range of applications. As the name 

implies, the primary function of a delay line transducer is to introduce a time 

delay between the generation of the sound wave and the arrival of any 

reflected waves. This allows the transducer to complete its "sending" function 

before it starts its "listening" function so that near surface resolution is 

improved. They are designed for use in applications such as high precision 

thickness gauging of thin materials and delamination checks in composite 

materials. They are also useful in high-temperature measurement applications 

since the delay line provides some insulation to the piezoelectric element from 

the heat.

Delay Lined Transducer: 

Advantages: 

1. Heavily damped transducer combined with the use of a delay line provides 

excellent near surface resolution 

2. Higher transducer frequency improves resolution 

3. Improves the ability to measure thin materials or find small flaws while 

using the direct contact method 

4. Contouring available to fit curved parts 

Applications: 

1. Precision thickness gauging 

2. Straight beam flaw detection 

3. Inspection of parts with limited contact areas 

4. Replaceable Delay Line Transducers 

5. Each transducer comes with a standard delay line and retaining ring 

6. High temperature and dry couple delay lines are available 

7. Requires couplant between transducer and delay line tip

Other Reading (Olympus): Delay Line Transducers 

Delay line transducers are single element longitudinal wave transducers 

used in conjunction with a replaceable delay line. One of the reasons for 

choosing a delay line transducer is that near surface resolution can be 

improved. 

The delay allows the element to stop vibrating before a return signal from the 

reflector can be received. When using a delay line transducer, there will be 

multiple echoes from end of the delay line and it is important to take these 

into account. Another use of delay line transducers is in applications in 

which the test material is at an elevated temperature. The high 

temperature delay 

line options listed in this catalog (page 16, 17, 19) are not intended for 

continuous contact, they are meant for intermittent contact only. 

Advantages: 

■ Improve near surface resolution 

■ High temperature contact testing

Delay Lined Transducer

Delay lined Transducer

TR-Probe / Dual Crystal Probe- Transmitting Receiving Probe 

http://www.weldr.net/simple/skill/html/content_10802.htm

Probe Delay with TR-Probe

Cross Talk at High Gain

Probe Delay

Probe Delay

Delay Line UT 1 Lab 8 

www.youtube.com/embed/lelVZ9OGli8

3.5.5 Angle beam transducers 

Angle beam transducer and wedges are typically used to introduce a 

refracted shear wave into the test material. Transducers can be purchased in 

a variety of (1) fixed angles or in (2) adjustable versions where the user 

determines the angles of incidence and refraction. 

In the fixed angle versions, the angle of refraction that is marked on the 

transducer is only accurate for a particular material, which is usually steel. 

The angled sound path allows the sound beam to be reflected from the 

backwall to improve detectability of flaws in and around welded areas. They 

are also used to generate surface waves for use in detecting defects on the 

surface of a component.

Angle Beam Transducers- Angle beam transducers are typically used to 

locate and/or size flaws which are oriented non-parallel to the test surface.



















Angle Beam Transducers 

ϴ 1L 

ϴ 2L 

ϴ 2S

Angle Beam Transducers 

ϴ 1L 

ϴ 2L 

ϴ 2S

Angle Beam Transducers- Mode Conversion 

Figure (15) below shows the relationship between the incident angle and the 

relative amplitudes of the refracted or mode converted longitudinal, shear, 

and surface waves that can be produced from a plastic wedge into steel.

Angle Beam Transducers- Common Terms 

ϴ = Refracted angle T= Thickness LEG1=LEG2= T/Cos ϴ 

V PATH= 2x LEG= 2T/Cos ϴ 

SKIP= 2.T Tan ϴ 

ϴ

Angle Beam Transducers- Common Terms 

ϴ = Refracted angle T= Thickness Surface Distance= S.Sin ϴ 

Depth= S.Cos ϴ 

ϴ

Angle Beam Transducers- Longitudinal / Shear Wave Inspection 

Many AWS inspections are performed using refracted shear waves. 

However, grainy materials such as austenitic stainless steel may require 

refracted longitudinal waves or other angle beam techniques for successful 

inspections.

Angle Beam Transducer 

http://www.olympus-ims.com/en/ultrasonic-transducers/dualelement/ 

http://static4.olympus-ims.com/data/Flash/wedge_weld.swf?rev=EF60

3.5.6 Normal incidence shear wave transducers 

Normal Incidence Shear Wave transducers incorporate a shear wave crystal 

in a contact transducer case. These transducers are unique because they 

allow the introduction of shear waves directly into a test piece without the use 

of an angle beam wedge. Rather than using the principles of refraction, 

as with the angle beam transducers, to produce shear waves in a material, 

the crystal itself produces the shear wave (Y-cut). Careful design has enabled 

manufacturing of transducers with minimal longitudinal wave contamination. 

The ratio of the longitudinal to shear wave components is generally below - 

30dB. 

Because shear waves do not propagate in liquids, it is necessary to use a 

very viscous couplant when making measurements with these. When using 

this type of transducer in a through transmission mode application, it is 

important that direction of polarity of each of the transducers is in line with 

the other. If the polarities are 90° off, the receiver may not receive the signal 

from the transmitter.

Application of Normal incidence shear wave transducers 

Typically these transducers are used to make shear velocity measurements 

of materials. This measurement, along with a longitudinal velocity 

measurement can be used in the calculation of Poisson’s Ratio, Young’s 

Modulus, and Shear Modulus. These formulas are listed below for reference. 

Keys: 

S 

V L 

V T 

r 

E 

G 

= Poisson’s Ratio 

= Longitudinal Velocity 

= Shear Velocity 

= Material Density 

= Young’s Modulus 

= Shear Modulus

Normal incidence shear wave transducers 

http://static3.olympus-ims.com/data/Flash/shear_wave.swf?rev=3970

Normal incidence shear wave transducers 

Advantages: 

1. Generate shear waves which propagate perpendicular to the test surface 

2. For ease of alignment, the direction of the polarization of shear wave is 

nominally in line with the right angle connector 

3. The ratio of the longitudinal to shear wave components is generally below 

-30 dB 

Applications: 

1. Shear wave velocity measurements 

2. Calculation of Young's Modulus of elasticity and shear modulus (see 

Technical Notes, page 46) 

3. Characterization of material grain structure 

http://www.olympus-ims.com/en/ultrasonic-transducers/shear-wave/

3.5.7 Paint brush transducers 

Paint brush transducers are used to scan wide areas. These long and narrow 

transducers are made up of an array of small crystals that are carefully 

matched to minimize variations in performance and maintain uniform 

sensitivity over the entire area of the transducer. Paint brush transducers 

make it possible to scan a larger area more rapidly for discontinuities. Smaller 

and more sensitive transducers are often then required to further define the 

details of a discontinuity.

Q: To evaluate and accurately locate discontinuities after scanning a part with 

paintbrush transducer, it is generally necessary to uae a: 

A. Transducer with a smaller crystal 

B. Scrubber 

C. Grid map 

D. Crystal collimator

3.5.8 Wheel Transducer 

Wheel Transducer Probe Features: 

The main driving advantage of this dry coupled solid contact wheel probe is 

that it works to overcome problems with couplant contamination (application 

& removal) as well as eliminating the practicalities of immersion systems. 

The "tyre" or delay material is constructed of hydrophilic polymers which have 

acoustic properties that lend themselves ideally to the implementation of 

ultrasonics. Applications include thickness measurement, composite 

inspection, delamination detection and general flaw detection.

Q: A special scanning device with the transducer mounted in a tire-like 

container filled with couplant is commonly called: 

A. A rotating scanner 

B. An axial scanner 

C. A wheel transducer 

D. A circular scanner 

Q: A wheel transducer scanning method is consider as: 

A. Contact method 

B. Immersion method 

C. Wheel method 

D. Not allowed

UT Technician At works- Salute!

3.6: Transducer Testing 

Some transducer manufacturers have lead in the development of transducer 

characterization techniques and have participated in developing the AIUM 

Standard Methods for Testing Single-Element Pulse-Echo Ultrasonic 

Transducers as well as ASTM-E 1065 Standard Guide for Evaluating 

Characteristics of Ultrasonic Search Units. 

Additionally, some manufacturers perform characterizations according to 

AWS, ESI, and many other industrial and military standards. Often, 

equipment in test labs is maintained in compliance with MIL-C-45662A 

Calibration System Requirements. As part of the documentation process, an 

extensive database containing records of the waveform and spectrum of each 

transducer is maintained and can be accessed for comparative or statistical 

studies of transducer characteristics.

Manufacturers often provide time and frequency domain plots for each 

transducer. The signals below were generated by a spiked pulser. The 

waveform image on the left shows the test response signal in the time domain 

(amplitude versus time). The spectrum image on the right shows the same 

signal in the frequency domain (amplitude versus frequency). The signal path 

is usually a reflection from the back wall (fused silica) with the reflection in the 

far field of the transducer.

TRANSDUCER EXCITATION 

As a general rule, all of our ultrasonic transducers are designed for negative 

spike excitation. The maximum spike excitation voltages should be limited to 

approximately 50 volts per mil of piezoelectric transducer thickness. Low 

frequency elements are thick, and high frequency elements are thin. 

A negative-going 600 volt fast rise time, short duration, spike excitation can 

be used across the terminals on transducers 5.0 MHz and lower in frequency. 

For 10 MHz transducers, the voltage used across the terminals should be 

halved to about 300 volts as measured across the terminals. 

Although negative spike excitation is recommended, continuous wave or tone 

burst excitations may be used. However there are limitations to consider 

when using these types of excitation. First, the average power dissipation to 

the transducer should not exceed 125 mW to avoid overheating the 

transducer and depoling the crystal. 

http://www.olympus-ims.com/en/5072pr/

Excitation: Spiked Pulser (negative spike excitation) 

0V 

10% 

Pulse Width @50% 

90% 

ΔT 

Time 

http://www.olympus-ims.com/en/5072pr/

Square Wave Spiked Pulser: (negative spike excitation) 

Square wave has controlled rise and fall times with directly adjustable voltage 

and pulse width. Precautions on the average power dissipation to the 

transducer should not exceed 125 mW to avoid overheating the transducer 

and depoling the crystal. 

0V 

Adjustable Voltage 

Adjustable Pulse width 

Time →

Pulse energy: Broad band versus Narrow band. 

Energy (dB) 

0 5 10 15 20 25 30 

Narrow band 

Broad band 

0.1 1.0 5.0 10 20 

Frequency MHz

UT Flaw Detector – Olympus EPOCH 600

Other tests may include the following: 

Electrical Impedance Plots provide important information about the design 

and construction of a transducer and can allow users to obtain electrically 

similar transducers from multiple sources. 

Beam Alignment Measurements provide data on the degree of alignment 

between the sound beam axis and the transducer housing. This information is 

particularly useful in applications that require a high degree of certainty 

regarding beam positioning with respect to a mechanical reference surface. 

Beam Profiles provide valuable information about transducer sound field 

characteristics. Transverse beam profiles are created by scanning the 

transducer across a target (usually either a steel ball or rod) at a given 

distance from the transducer face and are used to determine focal spot size 

and beam symmetry. Axial beam profiles are created by recording the pulseecho 

amplitude of the sound field as a function of distance from the 

transducer face and provide data on depth of field and focal length.

Effects of Probe Frequencies: 

1. Higher frequencies give better resolution 

2. Higher frequencies give better sensitivity 

3. Lower frequencies give better penetration 

4. Lower frequencies less attenuation 

5. Lower frequencies probe wider beam spread with more coverage to detect 

reflectors and reflectors with unfavorable orientation. 

6. Higher frequencies the beams are more focused and the sensitivity and 

resolution are better.

Effects of Probe Sizes: 

1. The larger the probe produce more energy thus more penetration 

2. Small probe small near zone 

3. The larger the probe the poorer the contacts on a curve substrate. 

Single or Double Crustal Probe Selection: 

1. Single crystal probe should be used for material thickness 15mm and 

above, according to the probe the near zone 

2. Single crystal probe should be used for thickness above 30mm 

3. Double crystal should be used for thin material

As noted in the ASTM E1065 Standard Guide for Evaluating Characteristics 

of Ultrasonic Transducers, the acoustic and electrical characteristics which 

can be described from the data, are obtained from specific procedures that 

are listed below: 

Frequency Response--The frequency response may be obtained from one 

of two procedures: shock excitation and sinusoidal burst. 

Sinusoidal excitation.

Shock excitation

Relative Pulse-Echo Sensitivity--The relative pulse-echo sensitivity may be 

obtained from the frequency response data by using a sinusoidal burst 

procedure. The value is obtained from the relationship of the amplitude of the 

voltage applied to the transducer and the amplitude of the pulse-echo signal 

received from a specified target. 

Time Response--The time response provides a means for describing the 

radio frequency (RF) response of the waveform. A shock excitation, pulseecho 

procedure is used to obtain the response. The time or waveform 

responses are recorded from specific targets that are chosen for the type of 

transducer under evaluation, for example, immersion, contact straight beam, 

or contact angle beam.

Frequency Response--The frequency response of the above transducer has 

a peak at 5 MHz and operates over a broad range of frequencies. Its 

bandwidth (4.1 to 6.15 MHz) is measured at the -6 dB points, or 70% of the 

peak frequency. The useable bandwidth of broadband transducers, especially 

in frequency analysis measurements, is often quoted at the -20 dB points. 

Transducer sensitivity and bandwidth (more of one means less of the other) 

are chosen based on inspection needs. 

Complex Electrical Impedance--The complex electrical impedance may be 

obtained with commercial impedance measuring instrumentation, and these 

measurements may provide the magnitude and phase of the impedance of 

the search unit over the operating frequency range of the unit. These 

measurements are generally made under laboratory conditions with minimum 

cable lengths or external accessories and in accordance with specifications 

given by the instrument manufacturer. The value of the magnitude of the 

complex electrical impedance may also be obtained using values recorded 

from the sinusoidal burst.

Sound Field Measurements--The objective of these measurements is to 

establish parameters such as the on-axis and transverse sound beam profiles 

for immersion, and flat and curved transducers. These measurements are 

often achieved by scanning the sound field with a hydrophone transducer to 

map the sound field in three dimensional space. An alternative approach to 

sound field measurements is a measure of the transducer's radiating surface 

motion using laser interferometry.

3.7: Transducer Modeling 

In high-technology manufacturing, part design and simulation of part 

inspection is done in the virtual world of the computer. Transducer modeling 

is necessary to make accurate predictions of how a part or component might 

be inspected, prior to the actual building of that part. Computer modeling is 

also used to design ultrasonic transducers. 

As noted in the previous section, an ultrasonic transducer may be 

characterized by detailed measurements of its electrical and sound radiation 

properties. Such measurements can completely determine the response of 

any one individual transducer.

There is ongoing research to develop general models that relate electrical 

inputs (voltage, current) to mechanical outputs (force, velocity) and vice-versa. 

These models can be very robust in giving accurate prediction of transducer 

response, but suffer from a lack of accurate modeling of physical variables 

inherent in transducer manufacturing. These electrical-mechanical response 

models must take into account the physical and electrical components in the 

figure below.

The Thompson-Gray Measurement Model, which makes very accurate 

predictions of ultrasonic scattering measurements made through liquid-solid 

interfaces, does not attempt to model transducer electrical-mechanical 

response. The Thompson-Gray Measurement Model approach makes use of 

reference data taken with the same transducer(s) to deconvolve electrophysical 

characteristics specific to individual transducers. See Section 5.4 

Thompson-Gray Measurement Model. 

The long term goal in ultrasonic modeling is to incorporate accurate models of 

the transducers themselves as well as accurate models of pulser-receivers, 

cables, and other components that completely describe any given inspection 

setup and allow the accurate prediction of inspection signals.

3.8: Couplants 

A couplant is a material (usually liquid) that facilitates the transmission of 

ultrasonic energy from the transducer into the test specimen. Couplant is 

generally necessary because the acoustic impedance mismatch between air 

and solids (i.e. such as the test specimen) is large. Therefore, nearly all of the 

energy is reflected and very little is transmitted into the test material. The 

couplant displaces the air and makes it possible to get more sound energy 

into the test specimen so that a usable ultrasonic signal can be obtained. In 

contact ultrasonic testing a thin film of oil, glycerin or water is generally used 

between the transducer and the test surface.

Couplant

Immersion Method - Water as a couplant 

When scanning over the part or making precise measurements, an immersion 

technique is often used. In immersion ultrasonic testing both the transducer 

and the part are immersed in the couplant, which is typically water. This 

method of coupling makes it easier to maintain consistent coupling while 

moving and manipulating the transducer and/or the part.

Squirter Column (bubbler)- Water as a couplant

Squirter Column (bubbler)- Water as a couplant 

https://www.youtube.com/user/UltrasonicSciences

Couplant

Couplant

3.9: Electromagnetic Acoustic Transducers (EMATs) 

As discussed on the previous page, one of the essential features of ultrasonic 

measurements is mechanical coupling between the transducer and the solid 

whose properties or structure are to be studied. This coupling is generally 

achieved in one of two ways. In immersion measurements, energy is coupled 

between the transducer and sample by placing both objects in a tank filled 

with a fluid, generally water. In contact measurements, the transducer is 

pressed directly against the sample, and coupling is achieved by the 

presence of a thin fluid layer inserted between the two. When shear waves 

are to be transmitted, the fluid is generally selected to have a significant 

viscosity.

Electromagnetic-acoustic transducers (EMAT) acts through totally different 

physical principles and do not need couplant. When a wire is placed near the 

surface of an electrically conducting object and is driven by a current at the 

desired ultrasonic frequency, eddy currents will be induced in a near surface 

region of the object. If a static magnetic field is also present, these eddy 

currents will experience Lorentz forces of the form 

F = I x B 

F the Lorentz force is the body force per unit volume, I is the induced 

dynamic current density, and B is the static magnetic induction. 

The most important application of EMATs has been in nondestructive 

evaluation (NDE) applications such as (1) flaw detection or (2) material 

property characterization. Couplant free transduction allows operation without 

contact at elevated temperatures and in remote locations. The coil and 

magnet structure can also be designed to excite complex wave patterns and 

polarizations that would be difficult to realize with fluid coupled piezoelectric 

probes. In the inference of material properties from precise velocity or 

attenuation measurements, using EMATs can eliminate errors associated 

with couplant variation, particularly in contact measurements.

F is the body force per unit volume, I is the induced dynamic current 

density, and B is the static magnetic induction.

EMAT

A number of practical EMAT configurations are shown below. In each, the 

biasing magnet structure, the coil, and the forces on the surface of the solid 

are shown in an exploded view. The first three configurations will excite 

beams propagating normal to the surface of the half-space and produce 

beams with radial, longitudinal, and transverse polarizations, respectively. 

The final two use spatially varying stresses to excite beams propagating at 

oblique angles or along the surface of a component. Although a great number 

of variations on these configurations have been conceived and used in 

practice, consideration of these three geometries should suffice to introduce 

the fundamentals. 

http://www.mie.utoronto.ca/labs/undel/index.php?menu_path=menu_pages/projects_menu.html&content_path=content_pages/fac2_2.html&main_menu=projects&side_menu=page1&sub_side_menu=s2

Electromagnetic acoustic transducer 

http://en.wikipedia.org/wiki/Electromagnetic_acoustic_transducer 

Electromagnetic Acoustic Transducer (EMAT) is a transducer for non-contact 

sound generation and reception using electromagnetic mechanisms. EMAT is 

an ultrasonic nondestructive testing (NDT) method which does not require 

contact or couplant, because the sound is directly generated within the 

material adjacent to the transducer. Due to this couplant-free feature, EMAT 

is particularly useful for automated inspection, and hot, cold, clean, or dry 

environments. EMAT is an ideal transducer to generate Shear Horizontal (SH) 

bulk wave mode, Surface Wave, Lamb waves and all sorts of other guidedwave 

modes in metallic and/or ferromagnetic materials. As an emerging 

ultrasonic testing (UT) technique, EMAT can be used for thickness 

measurement, flaw detection, and material property characterization. After 

decades of research and development, EMAT has found its applications in 

many industries such as primary metal manufacturing and processing, 

automotive, railroad, pipeline, boiler and pressure vessel industries.

Comparison between EMAT and Piezoelectric Transducers 

As an Ultrasonic Testing (UT) method, EMAT has all the advantages of UT 

compared to other NDT methods. Just like piezoelectric UT probes, EMAT 

probes can be used in pulse echo, pitch-catch, and through-transmission 

configurations. EMAT probes can also be assembled into phased array 

probes, delivering focusing and beam steering capabilities. 

Advantages 

Compared to piezoelectric transducers, EMAT probes have the following 

advantages: 

1. No couplant is needed. Based on the transduction mechanism of EMAT, 

couplant is not required. This makes EMAT ideal for inspections at 

temperatures below the freezing point and above the evaporation point of 

liquid couplants. It also makes it convenient for situations where couplant 

handling would be impractical. 

2. EMAT is a non-contact method. Although proximity is preferred, a physical 

contact between the transducer and the specimen under test is not required.

3. Dry Inspection. Since no couplant is needed, the EMAT inspection can be 

performed in a dry environment. 

4. Less sensitive to surface condition. With contact-based piezoelectric 

transducers, the test surface has to be machined smoothly to ensure 

coupling. Using EMAT, the requirements to surface smoothness are less 

stringent; the only requirement is to remove loose scale and the like. 

5. Easier for sensor deployment. Using piezoelectric transducer, the wave 

propagation angle in the test part is affected by Snell’s law. As a result, a 

small variation in sensor deployment may cause a significant change in 

the refracted angle. 

6. Easier to generate SH-type waves. Using piezoelectric transducers, SH 

wave is difficult to couple to the test part. EMAT provide a convenient 

means of generating SH bulk wave and SH guided waves.

Challenges and Disadvantages 

The disadvantages of EMAT compared to piezoelectric UT can be 

summarized as follows: 

1. Low transduction efficiency. EMAT transducers typically produce raw 

signal of lower power than piezoelectric transducers. As a result, more 

sophisticated signal processing techniques are needed to isolate signal 

from noise. 

2. Limited to metallic or magnetic products. NDT of plastic and ceramic 

material is not suitable or at least not convenient using EMAT. 

3. Size constraints. Although there are EMAT transducers as small as a 

penny, commonly used transducers are large in size. Low-profile EMAT 

problems are still under research and development. Due to the size 

constraints, EMAT phased array is also difficult to be made from very 

small elements. 

4. Caution must be taken when handling magnets around steel products.

Applications of EMATs 

EMAT has been used in a broad range of applications and has potential to be 

used in many other applications. A brief and incomplete list is as follows. 

1. Thickness measurement for various applications 

2. Flaw detection in steel products 

3. Plate lamination defect inspection 

4. Bonded structure lamination detection 

5. Laser weld inspection for automotive components 

6. Various weld inspection for coil join, tubes and pipes. 

7. Pipeline in-service inspection. 

8. Railroad and wheel inspection 

9. Austenitic weld inspection for power industry 

10. Material characterization

http://mdienergy.com/emat.html

Cross-sectional view of a spiral coil EMAT exciting radially polarized shear 

waves propagating normal to the surface.

EMAT Transducer 

http://www-ndc.me.es.osakau.ac.jp/pmwiki_e/pmwiki.php?n=Research.EMATs

Cross-sectional view of a tangential field EMAT for exciting polarized 

longitudinal waves propagating normal to the surface.

Cross-sectional view of a normal field EMAT for exciting plane polarized 

shear waves propagating normal to the surface.

EMATS 

The bulk-shear-wave EMAT 

consists of a pair of permanent 

magnets and a spiral-elongated 

coil. Driving currents in the coil 

generate the electromagnet 

forces (Lorentz force and 

magnetostriction force) parallel 

to the surface to generate the 

shear waves propagating 

normal to the surface

Cross-sectional view of a meander coil EMAT for exciting obliquely 

propagating L or SV waves, Rayleigh waves, or guided modes (such as Lamb 

waves) in plates.

Cross-sectional view of a periodic permanent magnet EMAT for exciting 

grazing or obliquely propagating horizontally polarized (SH) waves or guided 

SH modes in plates.

Practical EMAT designs are relatively narrowband and require strong 

magnetic fields and large currents to produce ultrasound that is often weaker 

than that produced by piezoelectric transducers. Rare-earth materials such as 

Samarium-Cobalt and Neodymium-Iron-Boron are often used to produce 

sufficiently strong magnetic fields, which may also be generated by pulsed 

electromagnets. 

The EMAT offers many advantages based on its couplant-free operation. 

These advantages include the abilities to operate in remote environments at 

elevated speeds and temperatures, to excite polarizations not easily excited 

by fluid coupled piezoelectrics, and to produce highly consistent 

measurements. 

These advantages are tempered by low efficiencies, and careful electronic 

design is essential to applications.

3.10: Pulser-Receivers 

Ultrasonic pulser-receivers are well suited to general purpose ultrasonic 

testing. Along with appropriate transducers and an oscilloscope, they can be 

used for flaw detection and thickness gauging in a wide variety of metals, 

plastics, ceramics, and composites. Ultrasonic pulser-receivers provide a 

unique, low-cost ultrasonic measurement capability

The pulser section of the instrument generates short, large amplitude electric 

pulses of controlled energy, which are converted into short ultrasonic 

pulses when applied to an ultrasonic transducer. Most pulser sections 

have very low impedance outputs to better drive transducers. Control 

functions associated with the pulser circuit include: 

1. Pulse length or damping (The amount of time the pulse is applied to the 

transducer.) 

2. Pulse energy (The voltage applied to the transducer. Typical pulser circuits 

will apply from 100 volts to 800 volts to a transducer.) 

100 volts to 800 volts (1KV~2KV could be used)

Transducer Cut-out

Pulse characteristics 

Pulse energy 

N= Pulse Rate 

Pulse length

Pulse Length: BS4331 Pt2. 

N= Pulse Rate 

Pulse length 

Pulse energy

Pulse Length: BS EN 12668- Part 1 Instrumentation 

3.22 

pulse duration 

time interval during which the modulus of the amplitude of a pulse is 10 % or 

more of its peak amplitude.

Pulse Length: A long pulse length may be 15 wavelength λ, a short pulse 

length may be only 2 λ and a normal pulse length usually about 5 λ. 

The longer the pulse length the more energy, thus more penetrating, however 

the resolution and sensitivity deteriorated.

Pulse Length

Pulse Length

Pulse Length

Pulse Length

Pulse Length and Wave form

Pulse-Length and Wave form Quality Factor 

Two different pulses with the same frequency, but different duration (pulse 

length), i.e. Number of oscillations. The shortest pulse has a wider dispersion 

of frequencies, i.e. a greater bandwidth.

Wave form Quality Factor

Pulse Length- x axis time domain 

Quality factor- x axis frequency domain 

Frequency 

Q Factor = f o /(f 1 -f 2 )

Pulse-Echo mode of operation, narrow band excitation (tone burst). 

Conventional air-coupled transducer with passive matching layers 

Two types of excitation: Sinusoidal/Shock. 

http://www.mdpi.com/1424-8220/13/5/5996/htm

Pulse-echo mode of operation, wideband excitation (spike). 1. (Red) Aircoupled 

transducer with active matching layer. 2. (Blue) Conventional aircoupled 

transducer with passive matching layers. 

λ /4 impedance 

matching layers

Modulus of the electrical impedance of the piezocomposite disk vs frequency. 

Circles: experimental measurements, solid red line: theoretical calculation. 

Z= pV

Sensitivity in pulse-echo mode of operation wideband excitation (spike). 1. 

(Red) Air-coupled transducer with active matching layer. 2. (Blue) 

Conventional air-coupled transducer with passive matching layers

Transducers

Damping: 

Shock wave transducer and low damped transducer : Shock wave 

transducers should always be used for wall thickness measurement. For 

smaller wall thicknesses this is as important for the pulse separation as is the 

frequency itself. For large wall thickness the shock wave is required also for a 

perfect start and stop trigger of the time measurement. Low damped 

transducers are not recommended. 

http://www.ndt.net/article/rohrext/us_pk/us_pk_e.htm

In the receiver section the voltage signals produced by the transducer, which 

represent the received ultrasonic pulses, are amplified. The amplified radio 

frequency (RF) signal is available as an output for display or capture for 

signal processing. Control functions associated with the receiver circuit 

include: 

1. Signal rectification (The RF signal can be viewed as positive half wave, 

negative half wave or full wave.) 

2. Filtering to shape and smooth return signals 

3. Gain, or signal amplification 

4. Reject control

The pulser-receiver is also used in material characterization work involving 

sound velocity or attenuation measurements, which can be correlated to 

material properties such as elastic modulus. In conjunction with a stepless 

gate and a spectrum analyzer, pulser-receivers are also used to study 

frequency dependent material properties or to characterize the performance 

of ultrasonic transducers.

Pulse/Beam Characteristics 

High frequency, short duration pulse exhibit better depth resolution but allow 

less penetration. A short time duration pulse only a few cycle is known as 

broad band pulse, because its frequency domain equivalent is large. Such 

pulse exhibit good depth resolution. 

http://www.olympus-ims.com/en/ndt-tutorials/thickness_gage/transducers/beam_characteristics/

Transducers of the kind most commonly used for ultrasonic gauging will have 

these fundamental functional properties, which in turn affect the properties of 

the sound beam that they will generate in a given material: 

Type - The transducer will be identified according to its design and function 

as a contact, delay line, or immersion type. Physical characteristics of the test 

material such as surface roughness, temperature, and accessibility, as well 

as its sound transmission properties and the range of thickness to be 

measured, will all influence the selection of transducer type. 

Diameter - The diameter of the active transducer element, which is normally 

housed in a somewhat larger case. Smaller diameter transducers are often 

most easily coupled to the test material, while larger diameters may couple 

more efficiently into rough surfaces due to an averaging effect. Larger 

diameters are also required for design reasons as transducer frequency 

decreases.

Frequency - The number of wave cycles completed in one second, normally 

expressed in Kilohertz (KHz) or Megahertz (MHz). Most ultrasonic gauging is 

done in the frequency range from 500 KHz to 20 MHz, so most transducers 

fall within that range, although commercial transducers are available from 

below 50 KHz to greater than 200 MHz. Penetration increases with lower 

frequency, while resolution and focal sharpness increase with higher 

frequency. 

Waveform duration - The number of wave cycles generated by the 

transducer each time it is pulsed. A narrow bandwidth transducer has more 

cycles than a broader bandwidth transducer. Element diameter, backing 

material, electrical tuning and transducer excitation method all impact 

waveform duration. A short wave duration (broadband response) is desirable 

in most thickness gauging applications.

Bandwidth - Typical transducers for thickness gauging do not generate 

sound waves at a single pure frequency, but rather over a range of 

frequencies centered at the nominal frequency designation. Bandwidth is the 

portion of the frequency response that falls within specified amplitude limits. 

Broad bandwidth is usually desirable in thickness gauging applications 

involving contact, delay line, and immersion transducers.

Sensitivity - The relationship between the amplitude of the excitation pulse 

and that of the echo received from a designated target. This is a function of 

the energy output of the transducer. 

Beam profile - As a working approximation, the beam from a typical 

unfocused disk transducer is often thought of as a column of energy 

originating from the active element area that travels as a straight column for a 

while and then expands in diameter and eventually dissipates, like the beam 

from a spotlight.

In fact, the actual beam profile is complex, with pressure gradients in both the 

transverse and axial directions. In the beam profile illustration below, red 

represents areas of highest energy, while green and blue represent lower 

energy. 

The exact shape of the beam in a given case is determined by transducer 

frequency, transducer diameter, and material sound velocity. The area of 

maximum energy a short distance beyond the face of the transducer marks 

the transition between beam components known as the near field and the far 

field, each of which is characterized by specific types of pressure gradients. 

Near field length is an important factor in ultrasonic flaw detection, since it 

affects the amplitude of echoes from small flaws like cracks, but it is usually 

not a significant factor in thickness gauging applications.

Focusing - Immersion transducers can be focused with acoustic lenses to 

create an hourglass-shaped beam that narrows to a small focal zone and 

then expands. Certain types of delay line transducers can be focused as well. 

Beam focusing is very useful when measuring small diameter tubing or other 

test pieces with sharp radiuses, since it concentrates sound energy in a small 

area and improves echo response.

Attenuation - As it travels through a medium, the organized wave front 

generated by an ultrasonic transducer will begin to break down due to 

imperfect transmission of energy through the microstructure of any material. 

Organized mechanical vibrations (sound waves) turn into random mechanical 

vibrations (heat) until the wave front is no longer detectable. This process is 

known as sound attenuation. Attenuation varies with material, and increases 

proportionally to frequency. As a general rule, hard materials like metals are 

less attenuating than softer materials like plastics. Attenuation ultimately limits 

the maximum material thickness that can be measured with a given gage 

setup and transducer, since it determines the point at which an echo will be 

too small to detect. 

http://www.olympus-ims.com/en/ndt-tutorials/thickness_gage/transducers/beam_characteristics/

Q15: A significant limitation of a lower frequency, single element transducer is: 

a) Scatter of sound beam due to microstructure of test object 

b) Increased grain noise or ‘hash’ 

c) (Less beam spread 

d) Impaired ability to display discontinuities just below the entry surface 

How & Why ? 

Reasoning: Pulse/Beam Characteristics 

High frequency, short duration pulse exhibit better depth resolution but allow 

less penetration. 

Lower frequency, longer duration pulse.

3.11: Tone Burst Generators In Research 

Tone burst generators are often used in high power ultrasonic applications. 

They take low-voltage signals and convert them into high-power pulse trains 

for the most power-demanding applications. Their purpose is to transmit 

bursts of acoustic energy into a test piece, receive the resulting signals, and 

then manipulate and analyze the received signals in various ways. High 

power radio frequency (RF) burst capability allows researchers to work with 

difficult, highly attenuative materials or inefficient transducers such as EMATs. 

A computer interface makes it possible for systems to make high speed 

complex measurements, such as those involving multiple frequencies.

Tone burst generators

Tone burst generators 

http://www.seekic.com/circuit_diagram/Signal_Processing/SINGLE_TONE_BURST_GENERATOR.html

3.12: Arbitrary Function Generators 

Arbitrary waveform generators permit the user to design and generate 

virtually any waveform in addition to the standard function generator signals 

(i.e. sine wave, square wave, etc.). Waveforms are generated digitally from a 

computer's memory, and most instruments allow the downloading of digital 

waveform files from computers. 

Ultrasonic generation pulses must be varied to accommodate different types 

of ultrasonic transducers. General-purpose highly damped contact 

transducers are usually excited by a wideband, spike-like pulse provided by 

many common pulser/receiver units. The lightly damped transducers used in 

high power generation, for example, require a narrowband tone-burst 

excitation from a separate generator unit. Sometimes the same transducer 

will be excited differently, such as in the study of the dispersion of a material's 

ultrasonic attenuation or to characterize ultrasonic transducers.

Section of biphase modulated spread spectrum ultrasonic waveform 

http://www.mpi-ultrasonics.com/content/mmm-signal-processing-examples

In spread spectrum ultrasonics (see spread spectrum page), encoded 

sound is generated by an arbitrary waveform generator continuously 

transmitting coded sound into the part or structure being tested. Instead of 

receiving echoes, spread spectrum ultrasonics generates an acoustic 

correlation signature having a one-to-one correspondence with the acoustic 

state of the part or structure (in its environment) at the instant of 

measurement. In its simplest embodiment, the acoustic correlation signature 

is generated by cross correlating an encoding sequence (with suitable cross 

and auto correlation properties) transmitted into a part (structure) with 

received signals returning from the part (structure).

3.13: Electrical Impedance Matching and Termination 

When computer systems were first introduced decades ago, they were large, 

slow-working devices that were incompatible with each other. Today, national 

and international networking standards have established electronic control 

protocols that enable different systems to "talk" to each other. The Electronics 

Industries Associations (EIA) and the Institute of Electrical and Electronics 

Engineers (IEEE) developed standards that established common terminology 

and interface requirements, such as EIA RS-232 and IEEE 802.3. If a system 

designer builds equipment to comply with these standards, the equipment will 

interface with other systems. But what about analog signals that are used in 

ultrasonics?

Data Signals: Input versus Output 

Consider the signal going to and from ultrasonic transducers. When you 

transmit data through a cable, the requirement usually simplifies into 

comparing what goes in one end with what comes out the other. High 

frequency pulses degrade or deteriorate when they are passed through 

any cable. Both the height of the pulse (magnitude) and the shape of the 

pulse (wave form) change dramatically, and the amount of change 

depends on the data rate, transmission distance and the cable's electrical 

characteristics. Sometimes a marginal electrical cable may perform 

adequately if used in only short lengths, but the same cable with the same 

data in long lengths will fail. This is why system designers and industry 

standards specify precise cable criteria. 

1. Recommendation: Observe manufacturer's recommended practices for 

cable impedance, cable length, impedance matching, and any 

requirements for termination in characteristic impedance. 

2. Recommendation: If possible, use the same cables and cable dressing for 

all inspections.

Cable Electrical Characteristics 

The most important characteristics in an electronic cable are impedance, 

attenuation, shielding, and capacitance. In this page, we can only review 

these characteristics very generally, however, we will discuss capacitance in 

more detail. 

Impedance (Ohms) represents the total resistance that the cable presents to 

the electrical current passing through it. At low frequencies the impedance is 

largely a function of the conductor size, but at high frequencies conductor size, 

insulation material, and insulation thickness all affect the cable's impedance. 

Matching impedance is very important. If the system is designed to be 100 

Ohms, then the cable should match that impedance, otherwise errorproducing 

reflections are created. 

Attenuation is measured in decibels per unit length (dB/m), and provides an 

indication of the signal loss as it travels through the cable. Attenuation is very 

dependent on signal frequency. A cable that works very well with low 

frequency data may do very poorly at higher data rates. Cables with lower 

attenuation are better.

Shielding is normally specified as a cable construction detail. For example, 

the cable may be unshielded, contain shielded pairs, have an overall 

aluminum/mylar tape and drain wire, or have a double shield. Cable shields 

usually have two functions: to act as a barrier to keep external signals from 

getting in and internal signals from getting out, and to be a part of the 

electrical circuit. Shielding effectiveness is very complex to measure and 

depends on the data frequency within the cable and the precise shield design. 

A shield may be very effective in one frequency range, but a different 

frequency may require a completely different design. System designers often 

test complete cable assemblies or connected systems for shielding 

effectiveness.

Capacitance in a cable is usually measured as picofarads per foot (pf/m). It 

indicates how much charge the cable can store within itself. If a voltage signal 

is being transmitted by a twisted pair, the insulation of the individual wires 

becomes charged by the voltage within the circuit. Since it takes a certain 

amount of time for the cable to reach its charged level, this slows down and 

interferes with the signal being transmitted. Digital data pulses are a string of 

voltage variations that are represented by square waves. A cable with a high 

capacitance slows down these signals so that they come out of the cable 

looking more like "saw-teeth," rather than square waves. The lower the 

capacitance of the cable, the better it performs with high speed data.

3.14 Transducer Quality Factor “Q” 

The quality factor “Q” of tuned circuit, search units or individual transducer 

element is a performance measurement of their frequency selectivity. It is thru 

ration of search unit fundamental (resonance ) frequency f o to the band width 

(f 2 -f 1 ) at 3dB down point at both sides.

Quality Factor “Q”

Quality Factor “Q” 

High quality Q-factor has a narrow frequency range (bandwidth) (i.e. little 

damping) and a correspond long spatial pulse length, where as a Low quality 

Q-factor transducer has a wide frequency range (bandwidth) and a shorter 

spatial pulse length. 

As discussed previously highly damped transducer, gives a wider frequency 

range provide better spatial resolution. Thus a Low quality Q-factor does not 

mean poor choice of transducer. 

Continuous-wave ultrasound testing usually employed High qiality Q-factor 

transducer. 

http://www.slideshare.net/vsrbhupal/echo-meet-final?related=2&utm_campaign=related&utm_medium=1&utm_source=6

3.15: Data Presentation 

Ultrasonic data can be collected and displayed in a number of different 

formats. The three most common formats are know in the NDT world as: 

A-scan, 

B-scan 

C-scan presentations 

D-scan presentations. 

Shadow Methods (modified A-Scan ?) 

Each presentation mode provides a different way of looking at and evaluating 

the region of material being inspected. Modern computerized ultrasonic 

scanning systems can display data in all three presentation forms 

simultaneously.

Data Presentation: A, B and C-scan recording and principle of scanning

Data Presentation:

3.15.1 A-Scan Presentation 

The A-scan presentation displays the amount of 

received ultrasonic energy as a function of time. 

The relative amount of received energy is 

plotted along the vertical axis and the elapsed 

time (which may be related to the sound energy 

travel time within the material) is displayed 

along the horizontal axis. Most instruments with 

an A-scan display allow the signal to be 

displayed in its: 

natural radio frequency form (RF), 

as a fully rectified RF signal, or 

as either the positive or negative half of the RF 

signal. 

In the A-scan presentation, relative discontinuity size can be estimated by 

comparing the signal amplitude obtained from an unknown reflector to that 

from a known reflector. Reflector depth can be determined by the position of 

the signal on the horizontal sweep.

In the A-scan presentation, relative discontinuity size can be estimated by 

comparing the signal amplitude obtained from an unknown reflector to that 

from a known reflector. Reflector depth can be determined by the position of 

the signal on the horizontal sweep. 

Reflector depth 

Relative discontinuity size

A-Scan

A-Scan 

http://static3.olympus-ims.com/data/Flash/HTML5/a_scan/A-scan.html?rev=F2E2

In the illustration of the A-scan presentation to the right, the initial pulse 

generated by the transducer is represented by the signal IP, which is near 

time zero, the transducer is scanned along the surface of the part, four other 

signals are likely to appear at different times on the screen. When the 

transducer is in its far left position, only the IP signal and signal A, the sound 

energy reflecting from surface A, will be seen on the trace. As the transducer 

is scanned to the right, a signal from the backwall BW will appear later in time, 

showing that the sound has traveled farther to reach this surface. When the 

transducer is over flaw B, signal B will appear at a point on the time scale that 

is approximately halfway between the IP signal and the BW signal. Since the 

IP signal corresponds to the front surface of the material, this indicates that 

flaw B is about halfway between the front and back surfaces of the sample. 

When the transducer is moved over flaw C, signal C will appear earlier in time 

since the sound travel path is shorter and signal B will disappear since sound 

will no longer be reflecting from it.

3.15.2 B-Scan 

http://static2.olympus-ims.com/data/Flash/HTML5/B_Scan/B-scan.html?rev=5E4D

B-Scan

B-Scan 


B-Scan Presentation 

The B-scan presentations is a profile (cross-sectional) view of the test 

specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is 

displayed along the vertical axis and the linear position of the transducer is 

displayed along the horizontal axis. From the B-scan, the depth of the 

reflector and its approximate linear dimensions in the scan direction can be 

determined. The B-scan is typically produced by establishing a trigger gate on 

the A-scan. Whenever the signal intensity is great enough to trigger the gate, 

a point is produced on the B-scan. The gate is triggered by the sound 

reflecting from the backwall of the specimen and by smaller reflectors within 

the material. In the B-scan image above, line A is produced as the transducer 

is scanned over the reduced thickness portion of the specimen. When the 

transducer moves to the right of this section, the backwall line BW is 

produced. When the transducer is over flaws B and C, lines that are similar to 

the length of the flaws and at similar depths within the material are drawn on 

the B-scan. It should be noted that a limitation to this display technique is that 

reflectors may be masked by larger reflectors near the surface.

It should be noted that a limitation to this display technique is that reflectors 

may be masked by larger reflectors near the surface. 

Masked by “C” above

B-Scan

Q: In a B-scan display, the length of a screen indication from a discontinuity is 

related to: 

A. A discontinuity’s thickness as measured parallel to the ultrasonic beam 

B. The discontinuity’s length in the direction of the transducer travel 

C. Both A and B 

D. None of the above

3.15.3 C-Scan Presentation 

The C-scan presentation provides a plan-type view of the location and size of 

test specimen features. The plane of the image is parallel to the scan pattern 

of the transducer. C-scan presentations are produced with an automated data 

acquisition system, such as a computer controlled immersion scanning 

system. Typically, a data collection gate is established on the A-scan and the 

amplitude or the time-of-flight of the signal is recorded at regular intervals as 

the transducer is scanned over the test piece. The relative signal amplitude or 

the time-of-flight is displayed as a shade of gray or a color for each of the 

positions where data was recorded. The C-scan presentation provides an 

image of the features that reflect and scatter the sound within and on the 

surfaces of the test piece.

C-Scan 

The (1) relative signal 

amplitude or (2) the timeof-flight 

is displayed as a 

shade of gray or a color 

for each of the positions 

where data was recorded. 

http://www.ndt.net/article/pohl/pohl_e.htm

C-Scan

C-Scan / A-Scan

High resolution scans can produce very detailed images. Below are two 

ultrasonic C-scan images of a US quarter. Both images were produced using 

a pulse-echo technique with the transducer scanned over the head side in an 

immersion scanning system. For the C-scan image on the left, the gate was 

setup to capture the amplitude of the sound reflecting from the front surface of 

the quarter. Light areas in the image indicate areas that reflected a greater 

amount of energy back to the transducer. In the C-scan image on the right, 

the gate was moved to record the intensity of the sound reflecting from the 

back surface of the coin. The details on the back surface are clearly visible 

but front surface features are also still visible since the sound energy is 

affected by these features as it travels through the front surface of the coin.

C-Scan

C-Scan Recording

C-Scan Recording

3.15.4 The D scan- The D scan gives a side view of the defect seen from a 

viewpoint normal to 

the B scan. It is usually automated, and shows the length, depth and 

through thickness of a defect. The D scan should not be confused with the 

delta technique.

The D scan- The D scan gives a side view of the defect seen from a 

viewpoint normal to 

the B scan. It is usually automated, and shows the length, depth and 

through thickness of a defect. The D scan should not be confused with the 

delta technique.

AUT Displays

3.15.5 The Through Transmission Shadow Method 

This method is also called the intensity-measurement or through-transmission 

method and is explained in Fig. 12.1. The shadow of an in-homogeneity, 

which is illuminated by an ultrasonic wave, reduces under certain conditions 

the intensity of the wave received by a second probe. The name throughtransmission 

method arises obviously from the fact that two probes are often 

positioned face to face on opposite sides of the specimen but that may not 

always be the case. Figure 12.2 shows an alternative arrangement of the 

shadow method where the beam is reflected before being influenced by the 

defect, and equally is could also be reflected afterwards. 

The transmission method, which may include either reflection or through 

transmission, involves only the measurement of signal attenuation. This 

method is also used in flaw detection. 


In the pulse-echo method, it is necessary that an internal flaw reflect at least 

part of the sound energy onto a receiving transducer. However, echoes from 

flaws are not essential to their detection. Merely the fact that the amplitude of 

the back reflection from a test piece is lower than that from an identical 

workpiece known to be free of flaws implies that the test piece contains one 

or more flaws. The technique of detecting the presence of flaws by sound 

attenuation is used in transmission methods as well as in the pulse-echo 

method. The main disadvantage of attenuation methods is that flaw depth 

cannot be measured.

Fig. 12.1 Principle of the shadow method

Fig. 12.2 Shadow method with reflection 

Fig. 12.3 Shadow method with guidance of the sound

3.15.6 Other Presentations

3.16 Testing Techniques 

3.16.1 Pulse Echo Method 

1. The advantages of pulse echo method is that the deflector could be locate 

and assess accurately from one side of specimen. 

2. The disadvantage ids that the sound path has to travel twice the distance, 

thus more attenuations. 

3. The presentation is an A-Scan Dispaly

3.16.2 Through Transmission Techniques 

Two probes are used, positioned on opposite sides. The present of reflector is 

indicated by reduction or loss of receiving signal amplitude. 

1. The advantages is that the sound has to travel a single path, thus material 

with higher attenuation could be checked, thicker material could be 

checked and higher frequency with improved sensitivity and resolution 

could be realized. 

2. The disadvantages is that there is no indication of depth, access to both 

sides of specimen is required and 

change in coupling condition may 

be mistaken as defect. More 

elaborate set-up 

3. The presentation is a Shadow Method

Through Transmission Techniques

The Through Transmission Shadow Method 


3.16.3 The Tandem Techniques 

The tandem method employed 2 probe on the same side , with each other 

spaced at a predetermined length. One transmitting signal the other set to 

received signal if reflected from a defect,\. The distance between the probe 

depends on the probe angle, material thickness and the depth of expected 

defects. The techniques are used to find for defects at predetermined depth 

such as in the root of double V weld. The presentation could be a A-Scan 

display.

The Tandem Techniques 

Phased array: Complete coverage 

with two probes 

Conventional UT: Complete 

coverage with > 24 probes 

Illustration showing the inspection of one 

zone. Phased array technology allows the 

simultaneous inspection of all zones with 

the same probe. Phased array offers 

complete coverage of the weld with one 

probe on either side of the weld. 

Illustration showing the inspection of 

one zone. With conventional UT 

technology several probes are needed 

to cover all zones. 

http://www.olympus-ims.com/en/pipewizard/

3.16.4 Immersion Methods 

In immersion method, compressional probe is mounted on a bridge immersed 

in water. The probe could be normal to the test piece as compressional probe 

or the bridge could be tilted to generate shear wave of various shear angle. 

Probe frequency of 25MHz is not uncommon for immersion method unlike the 

contact methods where the thin crustal may be too fragile to handle. The 

display could be a A, B, C Scans or through transmission shadow display.

During the set-up of immersion methods, the water path between the probe 

and the material surface is delay off the screen, so that the Zero starting point 

at the screen represent the front surface of the test material. 

It is important to note that the longitudinal velocity in steel is 4 times of that of 

water, so the testing of steel the water gap should be greater than one quarter 

( ¼ ) of steel thickness 

Gap water > ¼ Steel Thickness, < 

(e.g. for 100mm steel the water gap shall be >25mm)

¼ T 

T

3.17 UT Equipment Circuitry & Controls 

As with computers, the technology concerning ultrasonic equipment and 

systems is becoming somewhat transitory. Ultrasonic systems are either 

battery operated portable units, multi-component laboratory ultrasonic 

systems, or something in between. Whether they are based on modern digital 

technology or the fast disappearing analog original, systems (often defined as 

instrument plus transducer and cable) basically comprise the following 

components circuitry and controls.

3.17.1 Instrument Circuitry: 

Although the electronic equipment used for ultrasonic inspection can vary 

greatly in detail among equipment manufacturers, all general-purpose units 

consist of a power supply, a pulser circuit, a search unit, a receiver-amplifier 

circuit, an oscilloscope, and an electronic clock. Many systems also include 

electronic equipment for signal conditioning, gating, automatic interpretation, 

and integration with a mechanical or electronic scanning system. Moreover, 

advances in microprocessor technology have extended the data acquisition 

and signal-processing capabilities of ultrasonic inspection systems.

Instrument Circuitry: Time base 

The function of the time base, also called "sweep generator" in analog-display 

instruments, is to establish a display of sound travel time on the horizontal 

scale of the display. The horizontal scale can then be used for distance 

readout. The range (coarse range, test range) control adjusts the scale for the 

range of distance to be displayed.

Instrument Circuitry: Screen picture of a specimen with back echo R and a 

group of defect indications F, with normal sweep at I-m range (a) and with 

scale expansion to 250 mm (b)

Instrument Circuitry: Power Supply. 

Circuits that supply current for all functions of the instrument constitute the 

power supply, which is usually energized by conventional 115-V or 230-V 

alternating current. There are, however, many types and sizes of portable 

instruments for which the power is supplied by batteries contained in the unit. 

Instrument Circuitry: Pulser Circuit. 

When electronically triggered, the pulser circuit generates a burst of 

alternating voltage. The principal frequency of this burst, its duration, the 

profile of the envelope of the burst, and the burst repetition rate may be either 

fixed or adjustable, depending on the flexibility of the unit.

Instrument Circuitry: Receiver-amplifier circuits 

electronically amplify return signals from the receiving transducer and often 

demodulate or otherwise modify the signals into a form suitable for display. 

The output from the receiver-amplifier circuit is a signal directly related to the 

intensity of the ultrasonic wave impinging on the receiving transducer. This 

output is fed into an oscilloscope or other display device. 

Instrument Circuitry: Oscilloscope. 

Data received are usually displayed on an oscilloscope in either video mode 

or radio frequency mode. In video mode display, only peak intensities are 

visible on the trace; in the RF mode, it is possible to observe the waveform of 

signal voltages. Some instruments have a selector switch so that the operator 

can choose the display mode, but others are designed for single-mode 

operation only

Instrument Circuitry: Signal-conditioning and gating 

circuits are included in many commercial ultrasonic instruments. One 

common example of a signal-conditioning feature is a circuit that 

electronically compensates for the signal-amplitude loss caused by 

attenuation of the ultrasonic pulse in the test piece. Electronic gates, which 

monitor returning signals for pulses of selected amplitudes that occur within 

selected time-delay ranges, provide automatic interpretation. The set point of 

a gate corresponds to a flaw of a certain size that is located within a 

prescribed depth range. Gates are often used to trigger alarms or to operate 

automatic systems that sort test pieces or identify rejectable pieces.

Instrument Circuitry: Image- and Data-Processing Equipment. 

As a result of the development of microprocessors and modern 

electronics, many ultrasonic inspection systems possess substantially 

improved capabilities in terms of signal processing and data acquisition. This 

development allows better flaw detection and evaluation (especially in 

composites) by improving the acquisition of transient ultrasonic waveforms 

and by enhancing the display and analysis of ultrasonic data. The 

development of microprocessor technology has also been useful in portable 

C-scan systems with hand-held transducers (see the section "Scanning 

Equipment" in this article).

Instrument Circuitry: Clock 

The clock circuit initiates a chain of events that results in one complete cycle 

of a UT examination. The clock sends a trigger signal, at a regular interval, to 

both the (1) time base and to the (2) pulser. As the name “clock”' implies, this 

trigger signal is repeated at a given frequency, called the pulse repetition rate 

(PRR). On some instruments pulse repetition rate is adjustable by the 

examiner; other instruments do it automatically. The electronic clock, or timer, 

serves as a source of logic pulses, reference voltage, and reference 

waveform. The clock coordinates operation of the entire electronic system.

Instrument Circuitry: Pulse Repetition Rate PRR 

The pulse repetition rate establishes the number of times per second that a 

complete test cycle will occur. In instruments with adjustable pulse repetition 

rate, adjustment is made by a pulse repetition rate control, sometimes labeled 

REP RATE.

Instrument Circuitry: Pulser-Receiver 

The pulser emits the electrical signal that activates the transducer. This 

signal, known as the initial pulse, is quite brief, usually lasting only several 

nanoseconds (10 -9 , billionths of a second). The output of the initial pulse is in 

the order of hundreds of volts; the brief duration provides a fast rise time to 

the full voltage. The pulser is connected via output connectors on the 

instrument front panel to the transducer cable. The pulser is also connected, 

internally, through the receiver circuit, to the display, thus making available 

(depending upon the delay setting) a displayed initial pulse signal. This signal 

is, of course, present whether or not a transducer is connected to the 

instrument. When a transducer is connected, it is in the signal path between 

the pulser and the receiver and its output is displayed.

3.17.2 Instrument Control: 

Even though the nomenclature used by different instrument manufacturers 

may vary, certain controls are required for the basic functions of any 

ultrasonic instrument. These functions include power supply, clock, pulser, 

receiver-amplifier, and display. In most cases, the entire electronic assembly, 

including the controls, is contained in one instrument.

Instrument Control: REJECT Control 

It is intended for preventing the display of undesired low amplitude signals, 

called grass or hash, caused by metal noise such as echoes from material 

grain boundaries or inherent fine porosity. There are two types of REJECT 

controls installed on UT instruments: nonlinear REJECT and the more 

recently linear REJECT controls. Linear REJECT controls offer the advantage 

in that they do not affect vertical linearity of the display.

Instrument Control: DELAY and RANGE Controls 

The controls are used to adjust the instruments time base for proper display 

of distances. The delay control shifts the horizontal signals to the left and right 

without altering the spacing between them. The RANGE control expands or 

contracts the spacing between horizontal signals, corresponding to the Range 

of the sound travel to be displayed.

Instrument Control: GAIN Control 

The sound amplitudes of individual reflectors returning to the transducer 

determine the relative heights of the corresponding vertical signals on the 

CRT. By adjusting the Gain Control, vertical display sensitivity and therefore 

determines the actual amplitude at which signals are displayed. 

Gain controls for the receiver-amplifier circuit usually consist of fine- and 

coarse-sensitivity selectors or one control marked "sensitivity." For a clean 

video display, with low-level electronic noise eliminated, a reject control can 

be provided.

Instrument Control: Display Control 

The display (oscilloscope) controls are usually screwdriver-adjusted, with the 

exception of the scale illumination and power on/off. After initial setup and 

calibration, the screwdriver-adjusted controls seldom require additional 

adjustment. The controls and their functions for the display unit usually 

consist of the following: 

• Controls for vertical position of the display on the oscilloscope screen. 

• Controls for horizontal position of display on the oscilloscope screen. 

• Controls for brightness of display. 

• Control for adjusting focus of trace on the oscilloscope screen. 

• Controls to correct for distortion or astigmatism that may be introduced as 

the electron beam sweeps across the oscilloscope screen. 

An optical system with astigmatism is one where rays that propagate in two 

perpendicular planes have different foci. If an optical system with astigmatism 

is used to form an image of a cross, the vertical and horizontal lines will be in 

sharp focus at two different distances.

• A control that varies the level of illumination for a measuring grid usually 

incorporated in the transparent faceplate covering the oscilloscope screen. 

• Timing controls, which usually consist of sweep-delay and sweep-rate 

controls, to provide coarse and fine adjustments to suit the material and 

thickness of the test piece. The sweep-delay control is also used to 

position the sound entry point on the left side of the display screen, with a 

back reflection or multiples of back reflections visible on the right side of 

the screen. 

• On/off switch.

Instrument Controls: 

• A marker circuit, which provides regularly spaced secondary indications 

(often in the form of a square wave) on or below the sweep line to serve 

the same purpose as scribe marks on a ruler. This circuit is activated or 

left out of the display by a marker switch for on/off selection. Usually there 

will also be a marker-calibration or marker-adjustment control to permit 

selection of marker-circuit frequency. The higher the frequency, the closer 

the spacing of square waves, and the more accurate the measurements. 

Marker circuits are controlled by timing signals triggered by the electronic 

clock. Most modern ultrasonic instruments do not have marker circuits 

• A Gain circuit to electronically compensate for a drop in the amplitude of 

signals reflected from flaws located deep in the test piece. This circuit may 

be known as distance-amplitude correction, sensitivity-time control, timecorrected 

gain, or time-varied gain 

• Damping controls that can be used to shorten the pulse duration and 

thus adjust the length of the wave packet emanating from the transducer. 

Resolution is improved by higher values of damping

• High-voltage or low-voltage driving current, which is selected for the 

transducer with a transducer voltage switch. 

• Gated alarm units, which enable the use of automatic alarms when flaws 

are detected. This is accomplished by setting up controllable time spans 

on the display that correspond to specific zones within the test piece. 

Signals appearing within the gates may automatically operate visual or 

audible alarms. These signals may also be passed on to display devices 

or strip-chart recorders or to external control devices. Gated alarm units 

usually have three controls: the gate-start or delay control, which adjusts 

the location of the leading edge of the gate on the oscilloscope trace; the 

gate-length control, which adjusts the length of the gate or the location of 

the gate trailing edge; and the alarm-level or sensitivity control, which 

establishes the minimum echo height necessary to activate an alarm 

circuit. A positive/negative logic switch determines whether the alarm is 

triggered above or below the threshold level.

Instrument Control: Gates 

Most UT equipment is equipped with “gates” that can be superimposed on the 

time base so that a rapid response from a particular reflector can be obtained 

when they reach a certain predetermined amplitude. This can be adapted as 

a “go/no-go” monitoring device for some examinations. Gates can be set for 

an alarm to be triggered at a pre-determined amplitude (positive) with an 

increasing signal or (negative) with a decreasing signal amplitude. Gates are 

essential for some types of recording systems where they also serve to 

provide information to the recording devices or storage systems.

3.17.3 Pulse-Echo Instrumentation (A-Scan) 

The UT system includes: the instrument, transducers, calibration standards, 

and the object being examined. These elements function together to form a 

chain of events during a typical UT that can be summarized as follows: 

1. The instrument’s pulser electrically activates the transducer, causing it to 

send sound pulses into the test object. 

2. The activation signal, called the initial pulse, is displayed as a vertical 

signal on the CRT. 

3. As sound travels through the test object, it reflects from boundaries as well 

as from discontinuities within the material. 

4. The instrument's time base initiates readout of time/distance information 

on the horizontal scale of the display. 

5. A reflection from the surface opposite the entry surface is called a back 

reflection. These reflections reach the transducer, which converts them 

into electrical signals that are displayed on the CRT.

Figure above Block diagram circuitries are: 

1. Transducer 

2. Pulser (clock) 

3. Receiver/amplifier 

4. Display (screen) 

To understand how a typical ultrasonic system operates, it is necessary to 

view one cycle of events, or one pulse. The sequence is as follows. 

1. The clock signals the pulser to provide a short, high-voltage pulse to the 

transducer while simultaneously supplying a voltage to the time-base trigger 

module. 

2. The time-base trigger starts the spot in the CRT on its journey across the 

screen.

3. The voltage pulse reaches the transducer and is converted into mechanical 

vibrations (see piezoelectricity ), which enter the test piece. These vibrations 

(energy) now travel along their sound path through the test piece. All this 

time, the spot is moving horizontally across the CRT. 

4. The energy in the test piece now reflects off the interface (back wall) back 

toward the transducer, where it is reconverted into a voltage. (The 

reconverted voltage is a fraction of its original value.) 

5. This voltage is now received and amplified by the receiver/amplifier

Pulse Repetition Rate 

- Gain 

- Frequency 

-Reject 

Sweep & Range

Typical block diagram of an analog A-scan setup, including video-mode 

display, for basic pulse-echo ultrasonic inspection

A basic instrument contains several circuits: 

• power supply, clock (also called synchronizer or timer), 

• time base (called sweep generator), 

• pulser (also called transmitter), 

• receiver (also called receiver-amplifier), and 

• display.

3.17.4 B Scan Block diagram: 

B-scan display is a plot of time versus distance, in which 

• one orthogonal axis on the display corresponds to elapsed time (depth), 

• while the other axis represents the position of the transducer along a line 

on the surface of the test piece relative to the position of the transducer at 

the start of the inspection. 

Echo intensity is not measured directly as it is in A-scan inspection, but is 

often indicated semi quantitatively by the relative brightness of echo 

indications on an oscilloscope screen. A B-scan display can be likened to an 

imaginary cross section through the test piece where both front and back 

surfaces are shown in profile. Indications from reflecting interfaces within the 

test piece are also shown in profile, and the position, orientation, and depth of 

such interfaces along the imaginary cutting plane are revealed.

Applications. 

The chief value of B-scan presentations is their ability to reveal the 

distribution of flaws in a part on a cross section of that part. Although B-scan 

techniques have been more widely used in medical applications than in 

industrial applications, B-scans can be used for the rapid screening of parts 

and for the selection of certain parts, or portions of certain parts, for more 

thorough inspection with A-scan techniques. Optimum results from B-scan 

techniques are generally obtained with small transducers and high 

frequencies.

Typical B-scan setup, including video-mode display, for basic pulse-echo 

ultrasonic inspection

• First, the display is generated on an oscilloscope screen that is composed 

of a long-persistence phosphor, that is, a phosphor that continues to 

fluoresce long after the means of excitation ceases to fall on the 

fluorescing area of the screen. This characteristic of the oscilloscope in a 

B-scan system allows the imaginary cross section to be viewed as a whole 

without having to resort to permanent imaging methods, such as 

photographs. (Photographic equipment, facsimile recorders, or x-y plotters 

can be used to record B-scan data, especially when a permanent record is 

desired for later reference.) 

• Second, the oscilloscope input for one axis of the display is provided by an 

electromechanical device that generates an electrical voltage or digital 

signals proportional to the position of the transducer relative to a reference 

point on the surface of the test piece. Most B-scans are generated by 

scanning the search unit in a straight line across the surface of the test 

piece at a uniform rate. One axis of the display, usually the horizontal axis, 

represents the distance traveled along this line.

• Third, echoes are indicated by bright spots on the screen rather than by 

deflections of the time trace. The position of a bright spot along the axis 

orthogonal to the search-unit position axis, usually measured top to bottom 

on the screen, indicates the depth of the echo within the test piece. Finally, 

to ensure that echoes are recorded as bright spots, the echo-intensity 

signal from the receiver-amplifier is connected to the trace-brightness 

control on the oscilloscope. In some systems, the brightness 

corresponding to different values of echo intensity may exhibit enough 

contrast to enable semi quantitative appraisal of echo intensity, which is 

related to flaw size and shape.

Signal Display. 

The oscilloscope screen in Fig. 11 above illustrates the type of video-mode 

display that is generated by B-Scan equipment. On this screen, the internal 

flaw in the test piece shown at left in Fig. 11 above is shown only as a profile 

view of its top reflecting surface. Portions of the test piece that are behind this 

large reflecting surface are in shadow. The flaw length in the direction of 

search-unit travel is recorded, but the width (in a direction mutually 

perpendicular to the sound beam and the direction of search-unit travel) is not 

recorded except as it affects echo intensity and therefore echo-image 

brightness. Because the sound beam is slightly conical rather than truly 

cylindrical, flaws near the back surface of the test piece appear longer than 

those near the front surface.

3.17.5 C-scan display 

C-scan display records echoes from the internal portions of test pieces as a 

function of the position of each reflecting interface within an area. Flaws are 

shown on a readout, superimposed on a plan view of the test piece, and both 

flaw size (flaw area) and position within the plan view are recorded. Flaw 

depth normally is not recorded, although it can be measured semi 

quantitatively by restricting the range of depths within the test piece that is 

covered in a given scan. With an increasing number of C-scan systems 

designed with on-board computers, other options in image processing and 

enhancement have become widely used in the presentation of flaw depth and 

the characterization of flaws. An example of a computer-processed C-scan 

image is shown in Fig. 11, in which a graphite-epoxy sample with impact 

damage was examined using time-of-flight data. The depth of damage is 

displayed with a color scale in the original photograph.

Typical C-scan setup, including display, for basic pulse-echo ultrasonic 

immersion inspection

System Setup. 

In a basic C-scan system, shown schematically in Fig. 12 above, the search 

unit is moved over the surface of the test piece in a search pattern. The 

search pattern may take many forms; for example, a series of closely spaced 

parallel lines, a fine raster pattern, or a spiral pattern (polar scan). Mechanical 

linkage connects the search unit to x-axis and y-axis position indicators, 

which in turn feed position data to the x-y plotter or facsimile device. Echo 

recording systems vary; some produce a shaded-line scan with echo intensity 

recorded as a variation in line shading, while others indicate flaws by an 

absence of shading so that each flaw shows up as a blank space on the 

display (Fig. 12) above.

Gating. (Depth Gate) 

An electronic depth gate is another essential element in C-scan systems. A 

depth gate is an electronic circuit that allows only those echo signals that are 

received within a limited range of delay times following the initial pulse or 

interface echo to be admitted to the receiver-amplifier circuit. Usually, the 

depth gate is set so that front reflections and back reflections are just barely 

excluded from the display. Thus, only echoes from within the test piece are 

recorded, except for echoes from thin layers adjacent to both surfaces of the 

test piece. Depth gates are adjustable. By setting a depth gate for a narrow 

range of delay times, echo signals from a thin slice of the test piece parallel to 

the scanned surface can be recorded, with signals from other portions being 

excluded from the display. 

Some C-scan systems, particularly automatic units, incorporate additional 

electronic gating circuits for marking, alarming, or charting. These gates can 

record or indicate information such as flaw depth or loss of back reflection, 

while the main display records an overall picture of flaw distribution.

Q79: In the pulse echo instrument, the synchronizer, clock, or timer circuit 

determine the: 

a) Pulse length 

b) Gain 

c) Pulse repetition rate 

d) Sweep range

Q1: In an ultrasonic test system where signal amplitudes are displayed, an 

advantage of a frequency independent attenuator over a continuously 

variable gain control is that: 

A. The pulse shape is less distorted 

B. The signal amplitude measured using the attenuator is independent 

of frequency 

C. The dynamic range of the system id decreased 

D. The effect of amplification threshold is avoided. 

Definition: Switch that controls the output power of the HV generator is 

the attenuator.

Q1: The rate generator in B-scan equipment will invariably be directly 

connected to the: 

A. The display intensity circuit 

B. The pulser circuit 

C. The RF amplifier circuit 

D. The horizontal sweep circuit

Q30: The time from the start of the ultrasonic pulse to the reverberations 

complete decay limit the maximum usable: 

A. Pulse time-flaw rate 

B. Pulse/receiver rate 


D. Modified pulse-time rate 

Hint: A/B/D could not be the correct answers as they were not even the standard terms used.

Q129: An A-scan display, which shows a signal both above and below the 

sweep line is called: 

A. A video display 

B. A RF display 

C. An audio display 

D. Frequency modulated display

Q166: In a basic pulse echo instrument, the sunchronizer, clock or timer 

circuit determines the: 

A. Pulse length 

B. Gain 


D. Sweep length

Q32: On many ultrasonic testing instruments, an operator conducting an 

immersion test can remove that portion of the screen presentation that 

represents water distance by adjusting a: 

A. Pulse length control. 

B. Reject control. 

C. Sweep delay control. 

D. Sweep length control.

121. In an ultrasonic instrument, the number of pulses produced by an 

instrument in a given period of time is known as the: 

A. Pulse length of the instrument 

B. Pulse recovery time 

C. Frequency 

D. Pulse repetition rate 

122. In a basic pulse echo ultrasonic instrument, the component that 

coordinates the action and timing of other components is called a: 

A. Display unit 

B. Receiver 

C. Marker circuit or range marker circuit 

D. Synchronizer, clock, or timer


produces the voltage that activates the transducer is called: 

A. An amplifier 

B. A receiver 

C. A pulser 

D. A synchronizer 

124. In basic pulse echo ultrasonic instrument, the component that produces 

the time base line is called a: 

A. Sweep circuit 

B. Receiver 

C. Pulser 

D. Synchronizer


produces visible signals on the CRT which are used to measure distance is 

called a: 

A. Sweep circuit 

B. Marker circuit 

C. Receiver circuit 

D. Synchronizer 

126. Most basic pulse echo ultrasonic instruments use: 

A. Automatic read-out equipment 

B. An A-scan presentation 

C. A B-scan presentation 

D. A C-scan presentation

3.18 Further Reading on Sub-Section 3 

3.18.1 What is reflection, refraction, diffraction, and interference? 

What exactly is reflection, refraction, diffraction, and interference? 

Reflection occurs when a wave hits something and then bounces it off it. 

Refraction is the bending of a wave caused by a change in its speed as it 

moves from one medium to another. 

Diffraction occurs when an object causes a wave to change direction and 

bend around it. Interference is when two or more waves overlap and combine 

to make a new wave of lesser or more amplitude. 

This picture shows how reflection of light works 

and the names of the beams in a reflection. 

http://light-and-soundproject.wikispaces.com/3.+What+is+reflection,+refraction,+diffraction,+ 

and+interference%3F

3.18.2 Reflection 

西塘

In this picture there is two different beams, and those beams create angles. 

The beams are referred to as the reflected beam and the incident beam. The 

dotted line is the line that is perpendicular to the mirror, and it splits the large 

angle into the two different angles. The first angle is the angle of reflection, 

and it is formed by the reflected beam and the perpendicular line. The other 

angle is the angle of incidence which is formed by the incident beam and the 

perpendicular line. These two angles are always the same measure, although 

it sometimes might be a larger or smaller angle.

How do reflection, refraction, and diffraction relate to light? 

Reflection happens when a light is turned on, and it is in an enclosed area. If 

someone is in a enclosed area, and a light is turned on they are going to be 

able to see it. Then the light will continue, hit a wall, and it would reflect back 

to the human eye.

This picture shows how water waves will diffract around an island. This 

picture also shows constructive and destructive interference. 

The diffraction happens in this picture when the water waves pass between 

the two rocks. When the waves get onto the other side of the two rocks the 

waves are shaped as an arc (a U shape). The constructive and destructive 

interference happens by the rock in the middle of the picture to the left. The 

waves that are passing between the two rocks meet up with the waves 

passing around the one rock to the left, and the waves combine. Some waves 

will cancel each other out, and some will add to each other and make a 

bigger amplitude.

3.18.3 Refraction happens 

when light is shown through 

another material, and it changes 

the way it is being shown. An 

example is when you fill a cup 

with water, and then you place 

a pencil in the water. When you 

look at the pencil from the side 

it looks as though the pencil is 

broken where the pencil enters 

the water. This is due to 

refraction, and the bending of 

the waves before it enters your 

eyes. This picture shows the 

broken pencil experiment.

3.18.4 Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction happen when light tries to go through an opening. If you are in a 

dark hallway, and a room has a light on, you will be able to see he light, but it 

will only light up a section of the hallway, and you won't be in the light until 

you are almost directly in front of the room.

This diagram shows an interference. In this diagram it happens to be 

constructive interference, but this is not the only type of interference.

3.18.5 Interference

Interference

Interference

3.19 Questions & Answers

Q11: When maximum sensitivity is required from a transducer: 

A. Straight beam transducer should be used 

B. Large diameter crystals are required 

C. The piezoelectric element should be driven at its fundamental 

frequency 

D. The bandwidth of the transducer should be as large as possible.

Q12: The 1 MHz transducer that should normally have the best time of 

distance resolution is a: 

A. Quartz crystal with air backing 

B. Quartz crystal with phenolic backing 

C. Barium titanate transducer with phenolic backing 

D. Lithium Sulphate transducer with epoxy backing 

Hint: 1 MHz as Lithium Sulphate is not easily cut to very thin thickness, best 

distance resolution due to the fact the Lithium Sulphate is the best receiver of 

ultrasound energy.

Q3: The ultrasonic instrument used for examination of welding shall be 

capable of generating frequencies: 

A. more than 5 MHz 

B. more than 10 MHz 

C. less than 1 MHz 

D. 1 MHz to 5 MHz 

Q4. Calibration of ultrasonic equipment shall be done 

A. at beginning of examination 

B. both at beginning and end of the examination 

C. both at beginning and also at every two hours interval 

D. at beginning end, every two hours interval and whenever a change 

operator

Q4. Calibration of ultrasonic equipment shall be done 

• at beginning of examination 

• both at beginning and end of the examination 

• both at beginning and also at every two hours interval 

• at beginning end, every two hours interval and whenever a change 

operator

Q15: Entry surface resolution is a characteristic of an ultrasonic testing 

system which defines its ability to: 

A. Detect discontinuities oriented in a direction parallel to the ultrasonic beam. 

B. Detect discontinuities located in the center of a forging containing a fine 

metallurgic structure. 

C. Detect minute surface scratches. 

D. Detect discontinuities located just beneath the entry surface in the 

part being tested.

Discussion Topic: Factors affecting the Entry Surface Resolution 

Q15: Entry surface resolution is a characteristic of an ultrasonic testing system which defines its ability to: 

A. Detect discontinuities oriented in a direction parallel to the ultrasonic beam. 

B. Detect discontinuities located in the center of a forging containing a fine metallurgic structure. 

C. Detect minute surface scratches. 

D. Detect discontinuities located just beneath the entry surface in the part being tested. 

List of factors:

Expert at Works-Salute!

Experts at Work-Salute!

Section 3: Equipment & Transducers

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