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

However, the power or intensity of sound is generally not measured directly.
Since sound consists of pressure waves, one of the easiest ways to quantify
sound is to measure variations in pressure (i.e. the amplitude of the pressure
wave). When making ultrasound measurements, a transducer is used, which
is basically a small microphone. Transducers like most other microphones
produced a voltage that is approximately proportionally to the sound pressure
(P). The power carried by a traveling wave is proportional to the square of the
amplitude.
I α P 2 , I α V 2 where I=intensity, P=amplitude, V=voltage
Therefore, the equation used to quantify a difference in sound intensity based
on a measured difference in sound pressure becomes:
(The factor of 2 is added to the equation because the logarithm of the square of a
quantity is equal to 2 times the logarithm of the quantity.)