Microphone array method for the characterization of rotating sound sources in axial fans

Abstract

Methods based on microphone array measurements provide a powerful tool for determining the location and magnitude of acoustic sources. For stationary sources, sophisticated algorithms working in the frequency domain can be applied. By using circularly arranged arrays and interpolating between microphone signals it is possible to treat rotating sources, as are present in fans, as being non-moving. Measurements conducted with a four-bladed fan and analyzed with the "virtual rotating array" method show that it is not only possible to identify the main noise contributors, but also to determine a full spectrum for any rotating component of interest.

Comparison of the Rotating Source Identifier and the Virtual Rotating Array Method

“…Comparing the figures, one can see that both methods have localised the source successfully, and the strength estimates are very similar, as well, meaning that both methods are applicable. The point spread functions are also very similar, reported also in [16]. This observation was found true for all the investigated cases.…”

“…It can be seen that for increasing M values, ∆L b tends towards zero, as expected, however, for decreasing M, the magnitude of ∆L b increases, and at a very low M of 12, an error above 2 dB is possible. The degradation of the beamform maps is shown in [16], but no numerical results are presented there. It should be noted that the location of the source was successfully determined even with M = 16, while at M = 12, only a negligible position error was obtained.…”

“…Another method, termed Virtual Rotating Array method (VRAM) was proposed in [6]. Earlier, a similar algorithm was applied in [10] for a ducted fan.…”

Implementation of a Virtual Microphone Array to Obtain High Resolution Acoustic Images

“…The problem with these solutions is that the capture time is too high, because it is necessary to repeat the capture for each of the selected positions. One solution to this problem can be the implementation of 2D virtual arrays using only one sensor that moves by means of a 2D positioning system [24,25], or using a linear (1D) array that moves by means of a 1D positioning system [26,27,28]. These systems used to be built with analogic sensors and high-cost analogic acquisition systems.…”

In-duct beamforming and mode detection using a circular microphone array for the characterisation of broadband aeroengine fan noise.

“…It is possible to employ virtual rotating microphones as explained by Lowis in (2) The latter technique was not employed in the present study. For convenience, we opted for using the interpolation technique as described by Herold in (18). A brief introduction of the method is presented as follows, but intuitively it is equivalent to stroboscopically sampling the signals.…”