Acoustic Comfort: Beamforming 2D/3D
The noise measurements of the external acoustic fields are usually performed to detect the noise sources on the vehicle surface.
The 2 microphone arrays (see fig.1), placed respectively on the ceiling and on the side wall of the test section, localize the noise sources of the upper and lateral parts of the vehicle. The signals acquired by the microphones are processed with the “Beamforming” algorithm that is able to estimate the Sound Pressure Level (SPL) of a noise source localized in a specific area of the space.
The term Planar Beamfoming (PBF) refers to an investigation of the noise sources on a plane parallel to the array and close to the vehicle.
Some examples of time-averaged and time-resolved results of the PBF
“Beamforming” results of an experiment with two virtual sources (RS1 and RS2) with the same SPL level but located at different height. The maps A1 and A2 show the planar Beamforming results of investigations performed respectively in the plane of the first and the second virtual sources.
The result plotted in the maps A3 highlights the improved accuracy of the 3D Beamforming that allows for an investigation directly on the 3D vehicle’s geometry. 3D Beamforming (3D BF) is able to investigate the noise sources directly on the vehicle’s surface. In this way, the Sound Pressure Level (SPL) accuracy and the localization of the noise sources increase.
The survey of the 3D Geometry of the vehicle is preformed by an optical scanning system fully integrated in the Wind Tunnel.
The results plotted in fig. 4 highlight the better accuracy of the 3D BF compare with the PBF.
COHERENT BEAMFORMING, the correlation between external noise and its internal cab level
In a recent research we have do a correlation between external aeroacoustic noise caracterised with beamforming technique with the noise measured in the internal cab.
In the photo you can see the results of an experiment done positioning two source of external noise( with relative microphone to measure its level ) near to a generic car and an internal microphone.
In particular the source 2 has a higher level than source 3.
The noise source 3 is a pure tone at 3 Khz and the source 2 is a pink noise ( the choice is done to reproduce the actual situation )
The classic beamforming analisys is on the left and the coherent power approach on the right.
The analisys are conduct at two frequency ( 1600 Hz and 3000 Hz ).
The result show as in classic beamforming the higher source is always the source 2 as in the coherent power approach show as at 3 KHz the source 3 is more higher in the internal cab.
- Microphones Num. : 66 Top Array and 78 Side Array
- Array Diameter D: 3 m
- Freq. range f : 0.2 ÷10 (kHz)
- Array Resolution R : α·(z /D) ·λ
- α : Array Coefficient (~1.22)
- z : Distance between the measurement plane and the Array
- λ : Wave length