Sound from ultrasound : the parametric array as an audible sound source

Abstract

A parametric array exploits the nonlinearity of the propagation medium to emit or detect acoustic waves in a spatially versatile manner, permitting concise, narrow directivity patterns otherwise possible only with physically very large transducer geometries. This thesis explores the use of the parametric array as an audible sound source, permitting audible sound to be generated with very high directivity compared to traditional loudspeakers of comparable size. The thesis begins with a review of basic underlying mathematics and relevant approximate solutions of nonlinear acoustic systems. Then, these solutions are used to construct suitable methods of ultrasonic synthesis for low-distortion audio reproduction. Geometrical modelling methods for predicting the acoustic distribution are presented and evaluated, and practical applications are explored experimentally. Issues of risk associated with ultrasonic exposure are presented, and the feasibility of a phased-array system for beam control is explored.