UT Testing-Section 2 Physics of Ultrasound

Pulse Length:
A sound pulse traveling through a
metal occupies a physical
volume. This volume changes
with depth, being smallest in the
focal zone. The pulse volume, a
product of a pulse length L and a
cross-sectional area A, can be
fairly easily measured by
combining ultrasonic A-scans and
C-scans, as will be seen shortly.
For many cases of practical interest, the inspection simulation models predict
that S/N (signal to noise ratio) is inversely proportional to the square root of the
pulse volume at the depth of the defect. This is known as the “pulse volume
rule-of-thumb” and has become a guiding principle for designing
inspections. Generally speaking, it applies when both the grain size and the
lateral size of the defect are smaller than the sound pulse diameter.
http://www.cnde.iastate.edu/ultrasonics/grain-noise