Resonant Ultrasound Spectroscopy Detection Using a Non-Contact Ultrasound Microphone

We observed vibrational eigenmodes for a variety of millimeter-scale objects, including glass and sapphire lenses, by placing them on a piezoelectric ‘shaker’ driven by a broadband noise or frequency sweep signal, and using an optomechanical microphone to pick up their vibrational signatures emitted into the surrounding air. High-quality vibrational modes were detected over the ~0–8 MHz range for a typical object–microphone spacing of 1–10 mm. The observed eigenfrequencies are shown to be in excellent agreement with numerical predictions. Non-contact detection of resonant vibrational eigenmodes in the MHz ultrasound range could find application in the quality control of numerous industrial parts, such as ball bearings and lenses.