Material characterization via contact-free detection of surface waves using an optical microphone

The direct measurement of the frequency-dependent acoustic wave-velocity is a powerful method to assess the mechanical properties of structures. Changes of the wave propagation may indicate impending structural failure. State-of-the-art for dispersion measurements is the use of piezoelectric transducers as ultrasound sources, and laser Doppler vibrometers for spatially resolved imaging of the ultrasound propagation. The former devices require mechanical contact to the sample, the latter impose restrictions on its surface properties. Here, we present a non-contact system to determine the acoustic dispersion in materials featuring greatly varying compositions and surface properties. It combines laser excitation of ultrasound with the airborne detection of leaky or transmitted waves using an optical microphone, which is sensitive to acoustic frequencies from 10 Hz to 2 MHz. We show results from measurements on steel and aluminium plates, as well as the dispersion induced by surface treatment of sandstone samples, in comparison with previous reference measurements.