Airborne ultrasound pulse amplification based on acoustic resonance switching.

Airborne ultrasound radiation pressure, a nonlinear effect that appears as a static force in mid-air in the presence of strong ultrasound, has recently been applied in novel scientific and industrial fields. However, the output power of an ultrasound transducer remains low mainly due to the significant mismatch in acoustic impedance between a solid diaphragm and air. To circumvent this fundamental challenge, we propose to emit amplified airborne ultrasound pulses by instantaneously releasing stored acoustic energy into free-space. Specifically, we implement an acoustic cavity with a mechanically rotating shutter covering its open top. Once the acoustic cavity is fully charged, the stored energy is released by opening the shutter. By developing a choke structure that reduces leakage of the stored energy, we generate ultrasound pulses with 2.5 times higher peak power than the input continuous waves at 40 kHz. This preliminary result has a great potential to generate high-power ultrasound pulses using a conventional air-coupled transducer by separating the storage and radiation process, thus circumventing the fundamental limitation brought by impedance mismatch.