Application For Cabine Noise
Sound and Vibration
Applications for Cabin Noise
One of the most promising applications is for interior noise diagnostics inside automobiles, aircraft, etc. The 31 element rigid sphere was placed at the operator location inside an automobile driven on a dynamometer at 3000 rpm and a reference accelerometer was placed on the engine block. Partial reference holography was used to extract the hologram related to the fundamental of the engine tonal, at 105 Hz, and the measured pressure hologram correlated very highly to the engine block accelerometer.
The volumetric acoustic intensity was imaged from this hologram, as described above on a cubic grid with lattice spacing 0.06 m. Figure above shows two views of the resulting intensity field with an outline of the quarter section of the automobile including driver-side window and windshield framing the display (looking from the back seat on the left, and into the driver door on the right).
The mean sound pressure level over the sphere microphones was about 89 dBA at 105 Hz. In the figure, it can be seen that the intensity originates from the rear, coming from the left side of the back seat. The hemisphere plots in the figure indicate the radial intensity on an imaginary spherical surface of radius 0.3 m, red entering the sphere and blue leaving the sphere in units W/cm2. The seat-side hemisphere is shown on the left and the windshield side hemisphere is shown on the right oriented aligned with the photo on the extreme right as well as the top left vector display.
On the left hemisphere, one can see the large influx of power (red area) from the left of the back seat of the cabin. The right hemisphere also indicates an inward power flow at a lower level coming from the windshield orange area. It also shows a fairly large outflow of power (deep blue) in an area near the left under-dash and floor area.
It is very difficult to create comprehensive yet intelligible displays of the vector fields. Rotating the graphics display is very helpful and ultimately a stereographic display would be ideal. One final note on the resolution is obtained with the vector display.
At 105 Hz, the acoustic wavelength is over 3.2 m, and the vector display covers a cube 0.6 m on a side which corresponds to about 1/5 of a wavelength. Clearly the displayed field presents a spatial resolution much better than half-wavelength. The Tikhonov filter values Fn (a) are displayed at the bottom of the plot.
As a final demonstration of the potential of the theory, we open up the Tikhonov filter beyond its “correct” settings to increase the spherical harmonic content (adding most of the n=3 component) for the extracted hologram and repeat the intensity calculation for 105 Hz. The results in Fig. 13 show that increasing the harmonic contributions provides a more concentrated intensity yielding a more precise indication of the direction of the interior noise. The source now appears to be near the back door as opposed to the rear seat as conjectured above.
Extract from Original work: “Vector intensity reconstructions in a volume surrounding a rigid spherical microphone array”
Earl G. Williams – Acoustics Division, Naval Research Laboratory, Washington DC 20375
Kazuhiro Takashima – Nittobo Acoustic Engineering Co., Ltd., Tokyo, Japan
J. Acoust. Soc. Am. 127 (2), February 2010
This work was supported by the U.S. Office of Naval Research.
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