Prediction of acoustic radiation based on particle velocity measurements

Abstract

It has been shown [Wu and Hu, J. Acoust. Soc. Am. 103, 1763–1774 (1998); 104, 3251–3258 (1998)] that the radiated acoustic pressure can be determined directly once the particle velocity distribution over an imaginary surface enclosing the object under consideration is obtained. This alternative formulation is advantageous over the classical Helmholtz integral theory, which requires the surface acoustic quantities to be completely specified before the field acoustic pressure can be calculated. The difficulty with this approach is the measurement of the fluctuating part of particle velocity in the fluid medium. This paper describes an attempt to measure particle velocities using a laser anemometer. To facilitate measurements, fine particles are sprayed in the air by a fog generator. These particles oscillate in an insonified field at the excitation frequency. Both amplitudes of particle velocities in the normal and tangential directions and phases are measured. These data are used to predict the field acoustic pressures, which are validated by measurements taken at the same locations. Since the field acoustic pressures are calculated directly, the nonuniqueness difficulties inherent in the Helmholtz integral formulation are no longer existent and the efficiency of numerical computations is significantly enhanced. [Work supported by NSF.]