3D Measurements of a Vehicle Interior

Interior Noise Measurements
Acoustic Camera: Sphere 48 in Car

To localize sound sources inside the car cabin while the car is in motion has always been a challenging task, either because the equipment does not fit inside the car, or because it needs external supply.

In this case study, the client had noticed an unknown noise when the car's speed was up to 160 km/h. In order to identify the noise source, the Acoustic Camera measured the noise interior of the car while the vehicle was driven up to 160 km/h. The data was fitted with a 3D model of the interior of the car cabin, and the results show that the unknown noise was produced by a defective windscreen wiper. 

Method

Application area

Acoustic analysis of vehicle interiors during operation.

Measurement task

Localization of a hissing sound with the vehicle in motion.

Measurement object

Three-door Peugeot 2006.

Measurement set up

The measurements have been carried out with the microphone array Sphere48, the 144 channels data recorder and a mobile power supply that offers up to 4 hours of autonomy.
The microphone array has been arranged between the two front seats in order to directly reach the maximum cabin surface from the microphones.

Results

Two measurements have been conducted and the results have been compared: one with the car being driven at 130 km/h and one with the car being driven at 175 km/h. Both measurements have been filtered using an A-Weighting filter.

On the one hand, the results show that the spectrogram of the measurement taken at 130 km/h does not have any special frequency marked. The same situation is shown by the 3D acoustic Photo, where it is possible to see a constant distribution of the noise scene inside the cabin.

On the other hand, the results obtained from the measurements taken at 175 km/h shown a peak of sound pressure level in the frequency range between 2200 Hz and 3000 Hz. This phenomena has a SPL between 52 dBA and 60 dBA and it is not visible in the spectrogram of the first measurement.
The acoustic photo calculated using the band frequency between 2200 Hz and 3000 Hz, shows a clear red spot where the windscreen wipers are.

Accepting the windscreen wipers as a possible cause of the noise, it was decided to remove them and repeat the measurement at 175 km/h.

The results of the new measurement show that in the frequency range between 2200 Hz to 3000 Hz, the peak seen in the measurement with the wipers has vanished. Besides, the Total Sound Preasure Level with windscreen wipers at 175 km/h was 77,4 dBA and at the same velocity but without wipers is now 76 dBA. 

Conclusion

The Acoustic Camera allows the user 3D- analyses of interiors. The mobile-battery supply allows to carry out acoustic measurements in reduced spaces in motion. Both noise types, stationary and impulse noise, can be analyzed and located accurately even in low frequency ranges and in loud environments.


The case under study has shown that the Acoustic Camera is fast, accurate and efficient, bringing light to the problem and that it correctly identifies the windscreen wipers as the noise source. Thus, the client will have to take actions on this part of the car to improve comfort inside the car.

System

Array Sphere48

  • 48 microphones
  • 35 cm diameter
  • Carbon fiber structure
  • Dynamic of the microphones: 35 dB -130 dB
  • Recommended mapping frequencies: 1 kHz - 20 kHz
  • Typical measurement distance: 0.4 - 2 m

Data Recorder

  • 192 kHz Sampling frequency
  • 48 to 144 channels per 10 inch rack (24 channels per card)
  • Ethernet Interface > high transfer rate > 20 MByte/s, network-compatible
  • Digital card with 12 extra channels for recordings of RPM, rotation angle, reversal point, etc.
  • Integrated PC with Windows XP (embedded)

Software

  • NoiseImage

Power Supply

  • Mobile power supply/battery pack