Extension of near-field acoustical holography to measurements in a moving medium

Abstract

Near-field acoustical holography (NAH) has been widely used to study acoustic radiation from stationary sources. Recently, it has been applied to study tire noise on a vehicle in which the source and receiver are both moving in the medium. Since the source and the measurement plane are moving at the same speed, the frequency Doppler effect is absent. However, a wave-number Doppler effect exists. This leads to errors when reconstructing the acoustic field both toward and away from the source, even at low Mach number speeds. To investigate these errors, a point source is studied analytically using planar NAH with flow in one direction. The effect of the moving medium is noted by a shift in the radiation circle in the wave-number spectrum. From the nonlinear wave equation, a k-space Green’s function and a k-space filter are developed that includes the effects of the moving medium. The newly developed methods are then applied to tire noise using measured data. [Work supported by DOT.]