Generation of broadband airborne ultrasound using an Harmonic Acoustic Pneumatic Source

This paper presents a new type of airborne transducer for generating broadband ultrasound with a high Sound Pressure Level (SPL). The concept is based on the Harmonic Acoustic Pneumatic Source (HAPS) that uses pressurized air in conjunction with a flow chopper made up of a rotating cage with slots connected to a specific exhaust. The fundamental frequency depends on the number of slots and the rotation speed of the cage. An analytical model of the HAPS coupled with a numerical model of the exhaust is used to predict the radiated acoustic pressure and to estimate the influence of dimensional parameters on pressure level generated by the source. Experiments are conducted with two cages: one with one slot in order to generate pulses periodically and one with $122$ slots to generate periodic sound. The level of sound pressure is measured as a function of distance ($0.004$ to $0.5$ m), the cage rotation (up to $11$ krpm) and directivity ($0$ to $90$°). For the fundamental frequency at $22$ kHz, the maximum SPL of $150$ dB ($632$ Pa rms) is /measured at $0.004$ m, and decreases to $122$ dB ($35$ Pa rms) at $0.5$ m. At $0.5$ m, the second and third harmonics can generate a SPL equal or greater than $115$ dB above $22$ kHz and up to $66$ kHz. Discrepancies between the experiments results and numerical model are observed in terms of SPL, directivity and in-axis pressure.