(A) study on the characteristics of parametric speakers in near field

Abstract

A parametric loudspeaker is an equipment for generating highly directional sound beam. It is essentially a by-product that comes with nonlinearity of ultrasound. Using its high directivity, the parametric loudspeaker can focus the sound in a small space. It is noteworthy, however, that this directional beam exists only in the far field. Therefore, we need to study its near field characteristics if we desire to use this in the near field. Theory of the parametric loudspeaker and far field approximation were introduced for selecting physical variables used in experiments. Prototype of the parametric loudspeaker is devised using ultrasonic transducers, and a measurement method using 90-degree inclined microphones is also selected. Directivity and attenuation characteristics were measured in near field with varying the variables. The findings are that increases of aperture size and difference frequency make higher directivity similarly to the linear piston source, but show more than twice the beam width in far field approximation. The attenuation characteristics also do not follow the inverse square law, which describes the decrease of sound pressure by 6 dB as the distance doubled and is dominant in far field. The results, conclusively, explain the ongoing formation of the sound beam by the virtual sound source and reveal the consecutive generation and decay of the audible sound.