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

In spread spectrum ultrasonics (see spread spectrum page), encoded
sound is generated by an arbitrary waveform generator continuously
transmitting coded sound into the part or structure being tested. Instead of
receiving echoes, spread spectrum ultrasonics generates an acoustic
correlation signature having a one-to-one correspondence with the acoustic
state of the part or structure (in its environment) at the instant of
measurement. In its simplest embodiment, the acoustic correlation signature
is generated by cross correlating an encoding sequence (with suitable cross
and auto correlation properties) transmitted into a part (structure) with
received signals returning from the part (structure).