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

Use of the dB in Sound Measurements
Sound intensity is defined as the sound power per unit area perpendicular to
the wave. Units are typically in watts/m2 or watts/cm2. For sound intensity,
the dB equation becomes:
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. Therefore, the equation used to quantify a difference in sound
intensity based on a measured difference in sound pressure becomes: