Phased Array Leseprobe

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
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