Measurement of Airborne Ultrasound Using Laser Doppler Vibrometry

The speed of light in air is dependent on the air’s instantaneous density. Since air density is modulated by sound, sound in the air can be observed and measured using optical methods. One such optical method is laser Doppler vibrometry (LDV). In this method, the time derivative of a laser beam’s optical path is measured. The optical path length is the product of the physical path length traversed by a laser beam and the refractive index of the medium. Most commonly, such instruments measure the time rate-of-change of the physical path (i.e., the mechanical velocity of a surface), and index changes of the medium are small in comparison. In contrast, by placing a rigid reflector beneath a sound beam in air, it is possible to measure the time rate-of-change of refractive index and to therefore measure dynamic changes in air density, or sound. Because modern LDVs enable rapid and convenient scanning, entire sound fields and sound beams can be observed. In prior demonstrations by other teams, this method has been used to visualize sound fields in the audible frequency range and ultrasound range underwater. In this work, we present the first measurements of high-intensity airborne ultrasound beams in the frequency range spanning 60–500 kHz. We present a sound field generated by a piezoelectric MEMS transducer at 228 kHz. We also observe interesting phenomena predicted from nonlinear acoustics including accumulated distortion, wave steepening, and weak shock formation as a sound beam propagates. LDV measurements are compared against more conventional measurement methods using pressure transducers.