Sound Source Localization - Wind-turbine noise: comparing different SSL techniques

In this example, sound sources are localized on a wind-turbine transmission. Both far-field beamforming and near-field acoustic holography techniques are assessed for result accuracy. For near-field acoustic holography (NAH) measurements, a vertical line array of 23 microphones with 10cm spacing is used. To cover the whole width of the transmission, this array is moved to 15 different positions, each time by 10cm. To allow correct phase results, an additional reference microphone is maintained at a permanent position. For beamforming measurements, the LMS Test.Lab High Definition Acoustic Camera, a 36-channel circular array is used. This array measures the distance to the source, and can be used in the far field as well as in the near field.

Near-field acoustic holography + Beamforming + irregular near - field acoustic holography fig17A.jpg

Near-field acoustic holography + Beamforming + irregular near - field acoustic holography fig17A.jpg

Near-field acoustic holography + Beamforming + irregular near - field acoustic holography fig17B.jpg

Near-field acoustic holography + Beamforming + irregular near - field acoustic holography fig17C.jpg

Figure 17: Beamforming at 58cm (right), focalization at 26cm (middle), NAH at 66cm (right)

Figure 18: near field at 630-730Hz: focalization at 26cm (left), irregular NAH (left) and NAH (right)

Figure 17 shows beamforming, focalization and NAH results at frequencies of 830-860Hz. All 3 techniques help find the correct source location but NAH results are not as accurate as expected. For practical reasons, the array could not be placed at a distance of 10-20 cm but only at 66 cm. Figure 18 compares results of 3 near-field techniques: focalization, irregular-NAH and NAH. The NAH technique does not provide best results. The irregular-NAH technique provides more accurate source localization, as measurements are taken much closer. NAH measurements at 10 or 20 cm distance could have resulted in best results. However, measuring with NAH technique is here impossible: due to obstacles on the wind-turbine, some of the 15 positions can not be measured at close distance.

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