An ultrasonic digital lock-in probing technique for monitoring fast changes within an elastic body

A probing technique using ultrasonic waves and a digital lock-in detection scheme is developed to monitor changes within an elastic body. The scheme integrates products of the signal and reference rather than low pass filtering them. Two applications are illustrated: detecting acoustic emissions and monitoring slow dynamic nonlinear elasticity. The probe is capable of resolving the earliest times (<10-3 s) of the slow dynamic recovery and does not suffer distortion from the pump wave.