Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

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.