Phased Array Leseprobe

Moles Sjerve
Phased Array
IIW Handbook

Bibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie;
detailed bibliographic data are available in the Internet at http://dnb.d-nb.de.
English Edition
Volume 4
ISBN 978-3-87155-608-1
All rights, also for translation, are reserved. The reproduction of this volume or of parts of it only
with approval of the DVS Media GmbH, Düsseldorf.
DVS Media GmbH, Düsseldorf 2012
Printing: Griebsch & Rochol Druck GmbH & Co. KG, Hamm

Foreword
IIW Commission V – NDT and Quality Assurance – has had, from its beginning, strong activities
in compiling reviews, handbooks and guidelines on the application of newly developed NDTtechniques
and on the interpretation of recently developed standards. Whereas Sub-Commission
VA (Radiography-based weld inspection) has primarily concentrated on standardization, Sub-
Commission VC (Ultrasound-based weld inspection) has been working on guidelines, handbooks
and recommendations. Sub-commission VE (Electric, magnetic and thermal techniques for weld
inspection) has been working on standardization as well as on handbooks.
When Hermann Wüstenberg, Germany, as chairman of Sub-Commission VC retired in 2002 two
handbook activities were in preparation. The first one was the compilation of a handbook for
automated ultrasonic inspection and the second was a handbook for ultrasonic inspection of
austenitic and dissimilar welds.
However, the inspection of welds using ultrasonic phased arrays has become popular in industrial
sectors like nuclear, petrochemical and the aerospace industries, and therefore this special
ultrasonic examination technique has been qualified for specific applications in these industrial
sectors, even if no standard existed. The topic ‘phased arrays’ has been always a part of the stateof-the-art
discussions in Sub-commission VC.
At the IIW Annual Assembly 2003 in Bucharest, Romania, Eric Sjerve, Canada, was elected as the
new chairman of Sub-Commission VC and he took over the responsibility of finishing the ongoing
handbook actions. In addition he planned a new working party for an IIW handbook on
‘inspection using ultrasonic phased arrays’. In 2004 at the Annual Assembly in Osaka, Japan,
Commission V approved the plan to work on this handbook; in the same year at the World
Conference on NDT (WCNDT) at Montréal, Canada, the phased array working party was
established, chaired by Michael Moles, also from Canada.
A first draft of the handbook was presented for in-depth discussion in Commission V at the 2006
IIW Annual Assembly in Québec. Collaborators in that work came from the USA, Canada, France,
UK and Germany. In 2007 draft editing was performed and in 2008 at the Annual Assembly in
Graz, Austria, the completed draft version was presented, approved by Commission V and
transmitted to an internal reviewing board before a final drafting.
It was in Graz too that according to the IIW constitution and three periods of chairmanship, Gerd
Dobmann retired as chairman of Commission V and Philippe Benoist, France, was elected. Under
his new chairmanship, the reviewing was finished and the handbook compilation in 2009/2010
was finally scientifically edited by Professor Bruce Drinkwater and others, as an independent
reviewers. In 2010 the document was submitted to the IIW secretariat, which in August 2010
issued a call for tender to all welding societies in the world for printing the booklet. The German
Welding Society (DVS) obtained the copyright for publishing in March 2012.
Gerd Dobmann, past (1999-2008) chairman, and
Philippe Benoist, current chairman Commission V

Acknowledgements
Collecting, writing and editing a book as substantial as the “IIW Phased Array Handbook” is a
major effort. Many people have contributed sections, written portions and edited the Handbook.
The IIW Commission V working group to write this Handbook consisted of:
Dr. Michael Moles
Dr. Eric Sjerve
Significant contributors:
Dr. Mark Lozev
Mr. Rick Cahill
Mr. Peter Ciorau
Mr. Philippe Benoist
Chair, Handbook Working Group, Olympus NDT, Canada
Chair, IIW Commission VC, IRISNDT Corp, Canada
Edison Welding Institute, USA
GE Inspection Technologies, USA
Ontario Power Generation, Canada
CEA, France
Additional contributions were made by:
Mr. Glenn Gagner Westinghouse Amdata, USA
Dr. Anton Erhard BAM, Germany
Mr. Gottfried Schenk BAM, Germany
Mr. Ray Shepard Kakivik, USA
Mr. Robert Ginzel Eclipse Scientific, Canada
Mr. Walter Weber UTEX, Canada
Mr. Ed Ginzel Materials Research Institute, Canada
Mr. Wence Daks CadWire, Canada
Thanks must go to the Handbook external editors:
Dr. Gerd Dobmann IZfP, Germany
Dr. Tom Siewert NIST, USA
Dr. Alain Lhemery CEA, France
Dr. Uwe Ewert BAM, Germany
Dr. Bruce Drinkwater University of Bristol, United Kingdom

1 Introduction
1.1 Why an IIW Phased Array Handbook?
Industrial phased arrays have been in development for decades, following medical phased arrays
and other areas like radar, sonar and geophysics. In the past, a small market, difficult data
manipulation, crude software, and extensive training and experience requirements have hampered
the wide use of phased arrays for industrial applications. The first industrial commercial phased
array systems appeared in the early 1990’s, but were large, expensive, and required good software
skills. Most of these units were used in the nuclear industry on specific advanced applications.
Currently, the phased array industry is maturing rapidly and phased arrays are becoming pervasive
in inspection markets worldwide. This is true both for industrial field applications as well as for
more academic applications in universities and research institutes. From a technical perspective,
this is being driven by the flexibility that phased arrays bring to an inspection task, with their
ability to generate a wide range of ultrasonic beam characteristics with one phased array unit. This
has allowed phased arrays to displace other inspection techniques, and in many instances provide
inspection data that is superior.
From a practical perspective, the new phased array systems are relatively easy to use and are
portable, thus allowing easy access to the inspection location. This is in sharp contrast to older
phased array systems that are large, immobile and thus difficult to deploy. Lastly, from the
commercial perspective these systems have now come down in price to a level that they can be
used by organizations that span many different fields. This has also led to a corresponding
lowering of rates that has allowed their usage in many new commercial applications. It is these
factors acting together that have driven the large increase in phased array usage.
1.2 IIW
The IIW is an independent organization that brings together experts from a number of different
industries and countries to work together on projects that are relevant for welded components [1].
Within the IIW there are over fifty member countries that work within sixteen Commissions and a
number of other sub groups. Commission V of the IIW deals with NDT issues; within this
Commission, there are 4 sub-commissions that are focused on different aspects of NDT and report
to the IIW through Commission V.
Sub-commission VC is focused on ultrasonic inspection and in the past has been involved with
publishing Handbooks on a number of different ultrasonic inspection techniques. Examples of this
are the Handbook on the Ultrasonic Examination of Austenitic and Dissimilar Welds, Handbook
Automated Ultrasonic Testing Systems and this Handbook on Phased Arrays. The intent of this
Handbook is to provide a best practices document that can be used by practitioners of phased
arrays as a guideline when configuring and performing inspections. This same Sub-commission
also designed the IIW calibration block that is now ubiquitous in the field of ultrasonic inspection.
This IIW Phased Array Handbook was compiled by a group of experts, and thus represents a wide
consensus across different industries, with varying academic and industrial perspectives on the
state of the art in industrial phased array inspection.
1

1.3 Handbook layout
The Handbook is laid out in the following format:
Chapter 1 provides an introduction to the Handbook.
Chapter 2 covers Ultrasonic Phased Array Principles and Design; this chapter describes the basic
physics required for an understanding of phased arrays, how arrays are manufactured, and some
inherent limitations.
Chapter 3 covers Scanning Patterns and Ultrasonic Views; this chapter shows the basic scan
patterns used for data acquisition, plus some advanced scans primarily of academic interest. Many
different combinations of common data views are also described, with the addition of some less
common special views. In addition, this chapter includes a brief description of TOFD (Time of
Flight Diffraction), which is a complementary technique to pulse echo phased array inspection.
Chapter 4 covers Codes and Calibration; this chapter provides an update of phased array code
developments, plus indications of future trends with codes. Calibration of phased arrays is also
described, as it specifically relates to code inspections.
Chapter 5 covers Modeling and Imaging; this chapter describes four different levels of computer
modeling to optimize inspections, starting at a basic geometric model and going to a full beam
propagation model. The latest methods of data plotting showing two-dimensional and threedimensional
defect data superimposed on the component being inspected are also shown.
Chapter 6 covers Construction Weld Inspections; this chapter is a series of application specific
descriptions for pipelines, process piping, pressure vessels, butt welds, seamless pipe, T joints,
ERW pipe, superaustentics and friction stir welds. These are highly regulated inspections, and the
examples show how phased arrays fulfill the code requirements.
Chapter 7 covers In-Service Weld Inspections; this chapter is a series of application specific
descriptions for detecting in-service weld defects in heavy-walled reactors, core shrouds,
environmental cracking and transverse defects. These inspections are often less regulated than
construction weld inspection, so imaginative use of phased arrays is possible.
Chapter 8 covers Non-Weld Inspections; this chapter is a series of application specific descriptions
that demonstrate how phased arrays can be applied to non-weld applications, such as railway
wheels and axles, bridge pins, turbines, fasteners, and thickness measurements.
Lastly, the Handbook presents a Glossary of phased array terms. As phased arrays were developed
in different countries and industries, some functions have multiple terms (like S-scans), while
other terms have two very different meanings (e.g. linear scanning). All the common terms have
been collected.
References
[1-1] See the IIW Web site (www.iiwelding.org) or contact the member country welding society
for more details.
2

2 Ultrasonic phased array principles and design
2.1 Ultrasonic phased array instruments
Conventional ultrasonic instruments have a single pulser, driving a probe with a single transmit
element. By comparison, phased array instruments have multiple pulsers that drive multiple array
probe transmit elements. The number of elements that can be pulsed simultaneously is a way to
classify phased array instruments. The more pulser channels an instrument has, the more complex
and costly it is to build. Portable phased array instruments typically have 16 or 32 pulser channels
while fixed systems may have 128 channels or more. Instruments with higher channel counts
allow an inspector to configure larger virtual probes, which are more suitable for applications
requiring deeper focusing in the inspection material. A virtual probe is a group of individual array
elements that are pulsed to create the desired acoustic aperture.
Array elements may be pulsed individually or in virtual probe groups to match the ultrasonic
inspection requirements. There are several physical configurations for industrial phased array
probes; the more common ones are discussed in Section 2.4.1, Phased Array Probe Configurations.
Figure 2-1 shows a block diagram of an eight channel phased array instrument. Eight individual
pulsers activate eight array elements. The resulting acoustic wave travels through the test material
and is received by eight separate receiver elements. For simplicity, this example depicts a through
transmission set-up but the same concept applies to pulse echo.
Figure 2-1: Diagram of phased array instrument (through transmission)
2.2 How phased array probes work
Phased array probes have numerous individual acoustic elements. For industrial ultrasonic
inspection, the acoustic elements are typically manufactured from piezocomposite material.
Piezoelectric materials convert electrical energy to mechanical energy and vice versa. A short
duration high voltage pulse is applied to individual elements, which in turn, creates a mechanical
vibration or acoustic wave. When multiple elements are pulsed simultaneously their individual
acoustic waves interfere to generate an ultrasonic sound field that behaves in a manner similar to
3

that of a conventional ultrasonic probe with an aperture of the same dimensions.
Simultaneously pulsing virtual probe elements generates a longitudinal wave in the test material as
shown in Figure 2-2. A phased timing pulse sequence, referred to as delay laws or focal laws,
creates multiple wave fronts that interfere to form a distinct wave front within the test material.
When appropriate delay laws are applied to the virtual probe elements, an angle beam will be
generated in the test material as shown in Figures 2-3 and 2-5. The direction or angle the wave
front propagates in the material results from the constructive interference of the individual
waveforms. Phased array elements can operate in transmit or receive mode so the array can be
used for pulse echo or through transmission testing. Figure 2-2 is a simulation, and the turbulent
regions at the beginning are caused by near field interference.
Figure 2-2: Five-element virtual probe, pulsed
simultaneously, generates a longitudinal wave
front
4
Figure 2-3: Five-element virtual probe with
delay laws applied generates an angled wave
front

2.2.1 Delay laws
In what follows, formulas for beam steering are given for a one-dimensional linear array of
identical and regularly spaced rectangular elements. This is the simplest case mathematically, and
will be sufficient to illustrate basic phased array principles. There will be different formulae that
determine beam steering characteristics for more complex phased array probe geometries, such as
irregularly spaced linear elements, annular arrays and two-dimensional arrays, but these are
beyond the scope of this Handbook.
Delay laws determine the focal characteristics and steering behavior of the ultrasonic beam. Angle
beam steering is accomplished by applying a linear time delay function to the individual elements.
For unfocused beams, the angle of sound in the material is directly proportional to the applied time
delay; as the applied time delay increases, so does the angle. The formula governing this behavior
is shown in Equation 2-1, where θ = requested angle of incidence, n = 0, 1, 2, ... – number of
array elements with respect to the central array element, с = sound velocity in material, t 0 = a
constant value for avoiding negative delay values, and d = distance between the centre of two side
by side elements.
t
d
n sin
Equation 2-1
c
n t o
When using this equation, the central array element time delay is calculated by using n = 0 in the
equation, which in this case is t n = t 0 . The time delay of the element directly on one side is the
central element is then calculated by setting n = 1, and the element directly on the other side by
n = –1. By varying the value of n over all of the elements in the virtual aperture, the time delays of
all of the elements are derived.
Focusing is accomplished by applying a delay function; various functions such as parabolic
function can be used for this purpose. The focal depth is inversely proportional to the applied time
delay; therefore, as the maximum applied time delay decreases, the focal depth increases. The
formula governing this behavior is shown in Equation 2-2 for a focussed ultrasonic beam
propagating at = 0 into material, where F = requested focal depth, and the other variables are
defined above.
t
n
2
F nd 2
1
1
1/
to
Equation 2-2
c F
Equation 2-3 is the equation governing the creation a focused angle beam by combining angle
beam steering and focusing simultaneously, and reduces to the first two equations for simpler
cases. If Equation 2-3 is considered in the limit F (i.e. in the limit of an unfocussed ultrasonic
sound beam), then the result is Equation 2-1. If Equation 2-3 is considered in the limit of
(i.e. normal incident sound), then the result is Equation 2-2. Lastly, if Equation 2-3 is considered
in the limit of n = 0 only (i.e. only one element in the phased array probe), then t n = t 0 , or the time
delay is arbitrary, as is expected with conventional ultrasonic testing with single crystal probes.
Note that Equations 2-1, 2-2 and 2-3 apply only when the phased array probe is directly coupled to
the test material with normal incidence, and do not take into consideration a wedge attached to the
probe. Equation 2-3 and Figure 2-4 are the sum of equations 2.1 and 2.2.
2
F nd nd 2
t 1/
n 1 1 2 sin
t o
Equation 2-3
c F F
5

Figure 2-4: Equation 2-3 shown in graphical form
During angle beam weld inspection, a plastic wedge is normally attached to the phased array
probe. The wedge provides angular incidence and a refractive index so angle beams can be
generated in the test material. Snell’s law governs the angle of refraction in the test material.
Longitudinal angle beams and shear wave angle beams can be produced. When using a wedge, the
delay laws must be adjusted to compensate for the additional propagation delay in the wedge. This
can be handled automatically, in the instrument’s software, by scanning a known reference
reflector and applying an electronic compensation algorithm or by manually entering wedge
geometry information through an instrument’s user interface.
6
Figure 2-5: 16 element phased
array probe with delay laws
applied to produce an angled
beam
Normally, a wedge is selected that produces a nominal refracted angle, around which other angles
can be scanned. Delay laws are typically programmed to steer the sound beam above and below
the nominal wedge angle to scan a range of angles. This technique improves ultrasonic coverage
and increases the probability of detecting anomalies in the test material. It is normal to use several

wedges with a single phased array probe to produce different angular steering ranges. As with
conventional angle beam testing, an angled wedge produces an asymmetric sound beam along the
axis of refraction. Asymmetry causes variation in signal amplitude and beam width as a function
of angular position. As the refracted angle increases, signal amplitude decreases and greater beam
widths are observed. The same is true for phased array inspections.
2.2.2 Constructive and destructive interference
Phased array probes make use of constructive and destructive interference to form the sound field.
Typical industrial ultrasound applications utilize short pulse length ultrasonic waves compared to
the round-trip transit time of these waves in the material being tested. Hence, the waves can be
modeled as propagating waves, not standing waves. Therefore, interference of waves is a local
phenomenon associated with the physical superposition of waves emanating from various sources
(at least two). This superposition creates locally more or less constructive or destructive
interferences, mathematically expressed as algebraic additions of the amplitude of the various
wave contributions at the point considered. Both constructive and destructive interferences are
produced by the same set of sources depending on the position of the observation point relative to
these sources, the wavelength and the acoustic quantity considered.
When two or more ultrasonic waves of the same wavelength propagate through a material, the
waves interact. For example, consider two distinct waves propagating in a homogenous material
interacting at a location. If the two waves have the same amplitude and are in-phase, the waves
reinforce one another, resulting in a higher amplitude wave. The resulting wave is the sum of
vector amplitudes of the wave contributions and has a new characteristic amplitude. This behavior
is referred to as constructive interference. Note that these waves will propagate through each other
and continue through the material, as they would if they had not interacted. This is due to the local
nature of the interaction.
When two or more propagating ultrasonic waves interact at a location that is out-of-phase,
destructive interference occurs. The resulting wave is the sum of vector amplitudes of the wave
contributions, and results in a lower amplitude wave. In most cases, the waves will not be exactly
in-phase or exactly out-of-phase. In this case, sum of vector amplitudes of the wave contributions
must be calculated to determine the resulting amplitudes and propagation angle.
This description assumes that the waves are monochromatic; i.e. they have only one frequency. In
actuality, a finite duration electrical pulse excites the individual phased array elements to generate
the ultrasonic wave. This results in an ultrasonic pulse that exists for a finite time, and thus has a
finite bandwidth. Therefore, each ultrasonic pulse coming from each individual phased array
element has a band of frequencies associated with it. Furthermore, the variation in the performance
of the individual elements and the nonlinearity in the refraction at the wedge interface ensure that
each ultrasonic pulse generated by the individual elements will have slightly different properties.
Therefore, when these waves interact, they will never completely add constructively or subtract
destructively. There are references for wave physics available (see Bibliography).
2.2.3 Beam steering
Phased array ultrasonic sound beams can be steered by modifying the delay laws. Regardless of
which delay laws are applied, the virtual probe’s steering angle is limited by the divergence of an
individual array element, ignoring the limitations a wedge might impose on an angle beam test.
7

A phased array probe can only be reliably steered within the limits of the angle of divergence for a
single element. Therefore, it is possible to steer arrays with small and/or low frequency array
elements at higher angles because their sound field is more divergent. Most phased array probes
have either square or rectangular elements. Equation 2-4 describes the relationship of element size
and frequency on the beam divergence of square and rectangular elements. This formula has been
derived to give the half angle beam divergence in the Fraunhofer field for the first zero crossing
for infinitely-long elements (i.e. width D in one dimension and infinitely long in the other
dimension). Rectangular elements produce an asymmetrical lobe-shaped sound beam as shown in
Figure 2-6. The short dimension of the rectangular element has one divergence characteristic and
the long dimension has another. Both can be calculated using Equation 2-4. Although this formula
is derived for rectangular elements, it applies to circular elements within the same approximations
when the constant in front of the equation is changed from 1.0 to 1.2.
Beam divergence for square and rectangular elements:
Figure 2-6: Lobe-shaped sound beam of a rectangular
element (minus side lobes)
sin 0 = 1.0 C/FD Equation 2-4
C = Material Velocity
F = Frequency
D = Element dimension (Length or width)
0 = The half angle beam divergence
Based on the physical limitations of element beam divergence, there is a range of steering angles
for each phased array probe. This range of angles is finite (i.e. phased array probes do not have the
ability to be steered through all angles) and will depend on the manufacturing details of the probe,
material and other factors.
Phased array ultrasonic sound beams can only be focused at distances shorter than a virtual probe’s
near field length. Near field length is determined by the virtual probe aperture, frequency, and
bandwidth. Equation 2-5 and Figure 2-7 describe the relationship that element diameter,
frequency, and material velocity have on near field length for circular elements. The actual near
field length will be shorter than this equation states because it is based on very narrowband,
monotonic, continuous-wave theory. Industrial ultrasonic probes are pulsed and exhibit broadband
performance. Because most phased array virtual probes are either square or rectangular, Equation
2-5 can only be used to derive a close approximation of near field length. The actual near field
length will always be shorter than the calculated values.
Square virtual probe apertures behave in a way similar to circular elements. Rectangular virtual
probe apertures essentially have two near fields that combine. The short dimension of the rectangle
has one near field characteristic and the long dimension has another. The circular element equation
can be used for rough approximation. Calculate the near field of each aperture side, replacing
8

element diameter in Equation 2-5 with the dimension of each side. This yields one short near field
and long near field distance. The actual near field length of a rectangular virtual probe aperture is
somewhere between these two values.
Near field length of a circular element:
Figure 2-7: Diagram of sound field
zones
N = D 2 * F/4C Equation 2-5
N = Near-field length
D = Element Diameter
F = Frequency
C = Material Velocity
When considering the propagation of sound through a medium, some assumptions are made.
Assumptions are necessary to predict the direction and velocity of propagation as well as the
intensity of sound at any point in the medium and how it interacts with discontinuities. A short
discussion about these assumptions is presented here before the more general discussion on sound
beam characteristics.
Ray tracing is a standard method of modeling the way sound propagates through isotropic media,
which involves first order approximations to determine the sound field characteristics. These
assumptions are reasonable in the far field when considering isotropic materials. Within this
assumption, the sound is treated as a ray that is reflected or refracted by the interfaces it
encounters. The grain structure is considered isotropic and does not affect the sound beam. The
sound is also assumed to have no intensity variation across the pressure front. There is no
distinction between the near field and the far field within this assumption.
In reality, the propagation of sound is far more complex, and includes diffraction and nonlinear
effects. Sound beams are better approximated by Gaussian equations, which have far different
behavior in the near field (or Fresnel zone) than in the far field (or Fraunhofer zone). They also
have a nonlinear intensity profile in a direction perpendicular to the direction of beam travel.
Using beam mapping, one can determine the area of maximum amplitude, which is typically
within the centre of the beam. Amplitude decreases as the perimeter of the beam is approached,
but the function is not linear as assumed in many ultrasonic applications. At any particular cross
section within the beam, energy density is not constant. This is seen in Figure 2-8 where the
intensity of a sound beam is shown as colour. In the top series of figures, the cross section of the
9

eam is taken in a direction orthogonal to the direction or propagation at a series of different sound
paths. This initially shows near field behaviour, then a maximum intensity at the focal length of
the probe of roughly 23 mm (0.9 inch), and then regular far field behaviour beyond this point with
a reducing intensity. The two figures at the bottom show this same behaviour, but have been taken
in two cross sections along the direction of beam travel. It is clear from this figure that the sound
intensity profile is nonlinear.
Figure 2-8: Typical sound field plot
2.2.4 Sound field zones
Figure 2-8 shows a plot of a typical sound field that is emitted from an ultrasonic probe. With
conventional ultrasonic probes or single phased array elements, the near field is the location where
10

the sound field exhibits a complex (though predictable) behavior appearing as strong wave
amplitude variations from one observation point to another. Normally, industrial inspections are
not performed in this region due to the difficulty in interpreting the signals reflected from
discontinuities.
In the far field, the sound has become a standard diverging cone of sound that propagates into the
material being inspected. A continually diverging sound beam allows inspections to be performed
in the far field with predictable results. An example of this is when a distance amplitude curve is
constructed in the far field and is able to accurately reproduce reflected amplitudes from
calibration notches. When focusing is used, the near field is compressed, which results in a
predictable pressure distribution at the focus.
2.2.5 Beam width
The beam width is defined as the spatial extent, or beam cross section of the ultrasonic sound field
in a direction perpendicular to the direction of travel. In general, there will be two beam widths in
the two directions orthogonal to the direction of sound travel. These dimensions are determined
with respect to the two different spatial orientations of the rectangular phased array elements,
which will have separate beam divergence qualities. There are a number of definitions that can be
used to determine the edge of the beam, and one of the more common definitions is a –6 dB
intensity difference compared to the beam maximum. This is also referred to as the FWHM (full
width at half maximum). In materials that are not isotropic, this definition is not correct, as there
can be beam skewing that occurs due to differences in the group and phase velocities. In this case,
the direction of beam travel within the material will not be straight, but will be bent by the
anisotropic grain structure.
A phased array probe can only be focused within the near field length. When focusing, the beam
width is inversely proportional to the ratio of the near field length to focal length. A probe with a
long near field and short focal length will have a sound beam with a small cross-sectional area.
The closer the ratio gets to 1:1, the weaker the focusing behavior (larger sound beam) is for any
given probe. This indicates that it becomes increasingly difficult to obtain a tight focus as the focal
point approaches the end of the near field. Focusing is always a compromise between sensitivity,
beam dimensions, and depth of field.
2.2.6 Depth of field
Depth of field is the distance over which a probe maintains ultrasonic signal strength of a certain
level. In a phased array probe it is determined by the f-value of the virtual probe. F-value is the
ratio of focal length to acoustic aperture. Longer depths of field are observed as the f-value
approaches 1:1. In general, there is a trade off between the sound intensity at the focal point and
the depth of field. With all other parameters being equal, if a probe has a long depth of field then
there will be a lower intensity of sound at the focal point than for a short depth of field. When
focusing is used, the near field is compressed, and results in a predictable pressure distribution at
the focal point.
A probe having a long depth of field is generally more desirable for industrial inspection than a
probe with a short depth of field. This is because probes with a short depth of field lose sensitivity
very rapidly outside the focal point. This can be seen in Figure 2-8 where the sound intensity
11