Measurement of the acoustic field outside a vehicle

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.

 

Fig. 1: Top and Side Microphone Array.

Planar Beamforming

 

Fig.2 Examples of time –averaged results achieved respectively with Top and Side Array

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 are plotted in fig.2 e fig.3.
 

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Fig. 3: Time – resolved Noise Map achieved with the side array.

 

3D Beamforming

 

Fig. 4: “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. In fig. 5 is plotted an example of noise map achieved with the 3D BF.
 

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Fig 5 Noise map example achieved with the 3D Beamforming (1.25 kHz – 1/3 Octave Band).

The 3D BF algorithm and the optical scanning system have been designed and developed by the Pininfarina Aerodynamic and Aeroacoustic Research Center. For more details see reference [1].

ARRAY SPECIFICATIONS

  • 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

REFERENCES

  • [1] “Improvements of the Beamforming Technique in Pininfarina Full Scale Wind Tunnel by using a 3D Scanning System ", M. Maffei, A. Bianco, SAE Congress Paper n° 2008-01-0405. Detroit, April 2007
  • [2] "Upgrade of the Pininfarina Wind Tunnel – The New “13-Fan” Drive System", A. Cogotti, SAE Congress Paper n° 2006-01-0569. Detroit, March 2006
  • [3] "Update on the Pininfarina 'Turbulence Generation System' and its effects on the car Aerodynamics and Aeroacoustics", A. Cogotti, SAE Congress Paper n° 2004-01-0807. Detroit, March 2004
  • [4] "Genaration of Controlled Level of Turbulence in the Pininfarina Wind Tunnel for the Measurement of Unsteady Aerodynamics and Aeroacoustiocs", A. Cogotti, SAE Congress Paper n° 2003-01-0430. Detroit, March 2003
  • [5] "Ottimizzazione Aeroacustica dei Veicoli in Galleria del Vento", A. Cogotti, III Convegno Nazionale PFT2 - Taormina, Novembre 1997
  • [6] "Aeroacoustics Development at Pininfarina", A.Cogotti, SAE Congress Paper n° 970402. Detroit, February 1997
  • [7] "Aeroacoustic Testing Improvements at Pininfarina", A. Cogotti, SAE Congress Paper n° 940417. Detroit, February 1994