Imaging of directional distributed noise sources

This study relates to the imaging of noise sources that are distributed and strongly directional, such as in turbulent jets. The goal is to generate noise source maps that are self-consistent, i.e., their integration over the extent of the noise source region gives the far-field pressure autospectrum for a particular emission direction. This is possible by including a directivity factor in the formulation of the source cross-spectral density. The resulting source distribution is based on the complex coherence, rather than the cross-spectrum, of the measured acoustic field. For jet noise, which is not only directional but whose nature changes with emission angle, it is necessary to conduct the measurements with a narrow-aperture array. The resulting images must be devoid to the extent possible of spatial distortions caused by the array response. Two methods for obtaining clean images are addressed: deconvolution of the beamformer output and direct spectral estimation. The two techniques produced similar noise source maps for a Mach 0.9 cold jet.