Audio demodulation of radiation pressure from modulated ultrasound

Abstract

Parametric arrays produce audible sound from the audio frequency modulation of high-intensity ultrasound as a result of nonlinear propagation. This paper presents observations of the audible sound resulting from modulated radiation pressure. Audio signals due to the parametric array effect depend strongly on the square of the modulation frequency. It is often shown that the low-frequency region shows a flat frequency response, although it has not been clarified whether this is related to the system noise floor, modulated radiation pressure, or microphone nonlinearity. In this investigation, microphone nonlinearity was found to be very strong at short distances unless acoustic filtering was used. A novel low-pass acoustic filter (passes less than 10 kHz) was constructed using four layers of polyethylene spaced at 4 mm. Using this scheme, the ultrasonic carrier was attenuated and microphone nonlinearity was reduced by six orders of magnitude. Modulated radiation pressure was successfully observed at distances of 1 to 3 m. The observed audio signal showed a flat frequency response and agreed with theoretical predictions for modulated radiation pressure. The reproduced audio signal from radiation pressure appears in a lower frequency region of parametric array audio systems, and the effect should be taken into the system design.