Ultrasonic transducer characterization in air based on an indirect acoustic radiation pressure measurement

We present proof-of-concept measurement results from a simple indirect method that allows us to determine the acoustic radiation pressure acting on air coupled ultrasonic transducers in form of acoustic thrust. The simple and inexpensive method utilizes this acoustic thrust, acting on the ultrasonic transducer mounted on a clamped aluminum cantilever (850 × 20 × 2mm³). This approach is successful in air, because it exploits resonance amplification in return of a longer measurement time. By using a self-tuning circuit, the beam oscillates at its resonance frequency of approximately 2 Hz with a quality factor of 330. Modeling the system as spring-mass-dashpot system allows using the equations of forced damped oscillation to determine the acoustic thrust of various transducers based on only the measured beam displacement, obtained via e.g. strain gauges or a laser distance sensor. We used two commercially available ultrasonic transducers (MA40B8S and MA40S4S, both from MURATA, Japan) of different size and weight to test the setup. Excited with their specified maximum excitation voltages, acoustic thrust forces of up to 61 μN and 161 μN for the MA40S4S and the MA40B8S, respectively, are measured. Over a wide range these measurements are in good agreement with results from a digital high precision scale. Thus, our results show that the setup is able to measure small values of acoustic thrust in the μN-range. For future work, this approach can be used to compare different types of air-coupled ultrasonic transducers in terms of their efficiency, based on their generated acoustic thrust force.