Electro-acoustic study of airborne standing-wave acoustic levitators based on arrays of ultrasonic transducers

Airborne acoustic levitation has seen great advances in recent years largely due to the development of devices utilizing arrays of compact, low-power, piezoelectric transducers. In particular, standing wave acoustic levitators are acoustic cavities, consisting of either a single piezoelectric boundary opposing a reflector or two opposing piezoelectric boundaries operating simultaneously. The multiple intra-cavity reflections can be significant enough to generate voltages and currents at the electrical port due to the direct piezoelectric effect, providing valuable information about the optimal operating conditions of the cavity. In this work, we present a theoretical model that employs two planar piezoelectric boundaries to analyze the electrical response (voltage and current) at the cavity's electrical port and to identify the resonance conditions as a function of two key parameters: the cavity length and the phase configuration of the boundaries. We implemented a numerical approach to estimate the voltage and the current in cavities with pairs of planar and spherical boundaries. Additionally, we conduct a comparative study between numerical simulations and experimental results using cavities with spherical cap boundaries to validate our approach. The results demonstrate that source-reflector (S-R) and source-source (S-S) cavities can be set to resonance by tuning the cavity length from directly monitoring in real time the electric response of both S-R and S-S cavities, with voltage (current) exhibiting direct (inverse) correlation with the acoustic pressure amplitudes.