Ambient-noise horizontal directionality measurements with linear arrays

Abstract

A Fourier analysis method is developed for estimating the symmetric part of the horizontal ambient noise field about a single heading from noise measurements by a linear array. The antisymmetric part of the noise field about the array cannot be measured due to the cylindrical symmetry of the array. In this method, the ambient noise field is expanded in a Fourier series and the Fourier coefficients are determined from the measurements. The number of Fourier coefficients that can be determined is equal to the number of nonoverlapping beams since the beam pattern acts as a spatial low-pass filter. This Fourier analysis method is superior to the procedure of dividing the array beam data by the beamwidth, especially in highly anisotropic noise fields. The full beam pattern of the array is used in the Fourier analysis method while dividing by the beamwidth automatically assumes infinitely thin beams. Dividing the array beam data by the beamwidth is justified only in isotropic noise fields. The Fourier series method is extended to estimate the antisymmetric part of the noise field about an array heading from measurements made at two array headings. Knowing the symmetric part of the noise field about two array headings allows the estimation of the antisymmetric part of the noise field about either heading if the noise field is stationary. Subject Classification: [43]60.28, [43]60.30; [43]50.10.