Some comments on acoustic sensing at a single point in space

Abstract

The directionality provided by an acoustic vector sensor can be viewed in the context of a Taylor series expansion of the acoustic pressure field. From this perspective, acoustic pressure and acoustic particle velocity are but two of an infinite number of acoustic field variables. The purpose of this presentation is to examine some of the properties of these higher order field variables, including their energetics and the behavior of their spatial cross spectra in homogeneous, isotropic noise. Analytical expressions for those spatial cross spectra involving the normal strain rate parallel to the direction of separation decrease rapidly with increasing separation, and then continue to oscillate with a sinc function behavior. In contrast, those involving the normal strain rate perpendicular to the direction of separation decrease more slowly so that significant positive coherence exists at half-wavelength spacing, but then decay to, and remain near, zero at larger spacings. Accurate estimates of the range to a near-field point source can be obtained from the in-phase and quadrature components of normal strain rate with respect to pressure. Do animals in the ocean exploit the properties of these higher-order field variables?The directionality provided by an acoustic vector sensor can be viewed in the context of a Taylor series expansion of the acoustic pressure field. From this perspective, acoustic pressure and acoustic particle velocity are but two of an infinite number of acoustic field variables. The purpose of this presentation is to examine some of the properties of these higher order field variables, including their energetics and the behavior of their spatial cross spectra in homogeneous, isotropic noise. Analytical expressions for those spatial cross spectra involving the normal strain rate parallel to the direction of separation decrease rapidly with increasing separation, and then continue to oscillate with a sinc function behavior. In contrast, those involving the normal strain rate perpendicular to the direction of separation decrease more slowly so that significant positive coherence exists at half-wavelength spacing, but then decay to, and remain near, zero at larger spacings. Accurate estimates of the range t...