Measurement of an aeroacoustic dipole using a linear microphone array

Abstract

It is shown that the standard beamformer technique is inadequate for both the source location and the measurement of a simple dipole and that this is due to the assumption of monopole propagation in the calculation of the phase weights used to steer the focus of the array. A numerical simulation is used to illustrate the problem and to develop a correction to the signal processing algorithm to account for the dipole propagation characteristic. This is then applied to array measurements for an aeroacoustic dipole produced by a cylinder in a cross flow. The resulting source map and the beamformed spectrum are shown to give a true representation of the source energy and frequency content. A secondary effect of this correction is that the array becomes insensitive to other source types so that in addition to acting as a spatial filter, the array can perform as a source filter. This work also demonstrates how an array measurement can be misinterpreted if applied without consideration of the source mechanism.

Measurement and simulation of surface roughness noise using phased microphone arrays

“…The technique of Jordan et al [16], however, is not applicable in the case of a distribution of dipoles because their directivities and hence the required corrections vary over the source region, unlike the case of a single dipole. We do not have access to the source code for array analysis software to modify the algorithm for a distribution of axial dipoles.…”

“…Jordan et al [16] has shown that the standard beamforming technique is inadequate for both the source location and the measurement of a simple dipole, and that this is due to the assumption of monopole propagation in the calculation of the phase weights used to steer the focus of the array. They developed a correction to the beamforming algorithm to account for the dipole propagation characteristics, and applied it to array measurements for an aeroacoustic dipole produced by a cylinder in a cross flow.…”

“…, ω n , and is used as the input source strength for the frequency band [ω 1 , ω n ] in SIMSRC. By equating the total source powers in the prediction (16) and simulation (17), the predicted equivalent source strength for the two coherent monopoles of DPL 1 is given by…”

“…The technique developed by Jordan et al [16] to account for the propagation characteristics of a single dipole is not applicable because the roughness dipoles are distributed over such a large region that the directivity varies considerably. The array analysis source code is inaccessible and so it was not possible to modify the algorithm to base the beamforming on dipole sources.…”

Beamforming with microphone arrays for directional sources

“…We found an example of beamforming on directional sources in two papers where the sources are aeroacoustic dipoles [Jordan 2002] [Liu 2008]. The authors were interested in finding the locations of the dipoles.…”