Exploiting fluid nonlinearity for sound control and sensing with parametric speakers

Parametric ultrasonic loudspeakers can modulate ultrasonic sound waves to produce sound beams. A high-intensity sound field generates sum and difference frequencies. Specifically, a narrow sound beam is produced by modulating a carrier wave at ultrasonic frequencies with a 'message' signal containing low frequencies (typically in the range of 100 to several 1000 Hz). Due to large amplitude pressure variations, fluid nonlinearity manifests as a narrow sound beam encompassing itself. As a result, physical demodulation occurs, leading to the emergence of low-frequency sound beyond the traditional 'Rayleigh' zone. Notably, this approach actively amplifies weak low-frequency signals, surpassing the capabilities of ordinary passive microphones. To validate these concepts, numerical simulations will be followed by an analysis of experimental evidence. This technology has potential applications in directional audio projection, underwater communication, and directional underwater sensing.