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

5.7: Spread Spectrum Ultrasonics
Spread spectrum ultrasonics makes use of the correlation of continuous
signals rather than pulse-echo or pitch-catch techniques.
Spread spectrum ultrasonics is a patented new broad band spread-spectrum
ultrasonic nondestructive evaluation method. In conventional ultrasonics, a
pulse or tone burst is transmitted, then received echoes or throughtransmission
signals are received and analyzed.
In spread spectrum ultrasonics, encoded sound is continuously transmitted
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 the 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).