The microphone array can be focused on any point in front of it by
varying the calculated amplitude and phase correction terms. In this
way, an image of the sound pressure distribution on a single image
plane is obtained without mechanically moving the array.
Theseparation of the sound sources in terms of position and frequency
depends on the selected microphone geometry. Any sound situation can be
reproduced as a colored two-dimensional image of the absolute sound
power distribution. For the better visualization of the sound situation
the localization result is superimposed on a photograph of the object
under examination.
Typical applications for AcoustiCam are:
- Localization and separation of sound sources for reducing noise emission
of vehicles, machines, white goods and electric tools
- Sourceanalysis of wind tunnel models, complex vibrating structures,
screeching or rattling structures as a basis for the acoustic design.
Two MSX16 units controlled by a notebook or a standard PC are used as
hardware for the data acquisition. In cases where it is necessary to
improve the position and frequency resolution, the hardware may be
extended to 64 measurement channels. Compared to other concepts, our
solution has the following advantages:
- The hardware for the data acquisition may also be used for other measurement tasks.
- The system allows data to be recorded continuously on HDD from 32 channels over 8 hours.
- The entire measuring hardware may be independently powered by buffer batteries.
- The system works with various array geometries depending on the requirements.
- The solution features a high accuracy for a relatively low price.
- Orthogonal beamforming
-
This new algorithm of AcoustiCam is based on the decomposition of the
localization result into uncorrelated, i.e. independent sound sources.
These are based on different source mechanisms which may result in
strongly differing source sound levels. The orthogonal beam forming
leads to separate images of the individual sound sources which enables
the separate localization of not only the high-level main sound
sources, but also of the lower-level masked sound sources without
having to enclose parts or perform several measurements. This method
increases the signal-to-noise ratio to > 25 dB, which is a clear
improvement in comparison to the 10 ... 15 dB achievable by methods
currently in common use.
- Source analysis
-
The AcoustiCam measuring system is able to analyze single sound sources by
determining the source-typical sound pressure spectra related to
specific points. Even under acoustically unfavorable conditions (e.g.
in a wind tunnel) the existing sound sources are localized reliably. No
special acoustic rooms are necessary.
- Boundary layer array
-
The use of the so called boundary layer microphone array suppresses the
sound propagating from behind the microphone array. For this reason, no
special acoustic rooms are necessary and various geometries may be used
for those measurements. Special inexpensive microphones are integrated
in the array.
- Ring array
-
The considerably lighter ring array (in comparison with the boundary layer array)
consists of 32 microphones on a metal ring which can be mounted on a
tripod. Standard measuring microphones with ICP supply and BNC
connectors are used, which are easy to mount and which may also be used
for other measuring tasks. Easy positioning is possible by using a
wheeled tripod with a swivel head.
- Aperture angle
-
As in an optical system, increasing the angle between object direction and
the camera alignment causes distortions of the object’s mapping. In the
acoustic far field these distortions can be neglected up to a maximum
aperture angle of ±30°. The larger the dimensions of the test object,
the greater the distance between the microphone array and the object
has to be.
The AcoustiCam measuring system is based on the near-
field beamforming algorithm. This algorithm allows acoustic
examinations of test objects at small distances because the suggested
maximum aperture angle increases with decreasing distance to the test
object. The minimum distance which must be maintained is approximately
25 cm. Smaller objects can be placed very close to the microphone
array, whereas larger objects require a greater distance in order to
obtain an optimal image. For the application of AcoustiCam there are
virtually no constraints regarding the dimensions of the object under
examination.
- Spatial resolution
-
Various resolutions can be obtained depending on the microphone array applied.
Apart from the geometric arrangement of the microphones, the frequency
and the distance to the sound source also determine the spatial
resolution of the system. The spatial resolution of the array increases
with increasing frequency, i.e. with decreasing wavelength.
Technical data (for Ring 32)
| Object size | 1 m x 1 m |
| Object distance | 1 ... 5 m |
| Frequency range | 300 Hz ... 8 kHz |
| Frequency resolution | octaves, 1/3 octaves |
| Calculation time/image | approx. 30 s |
| Preview image | approx. 1 s |
| Spatial resolution | 28 cm @ 1 kHz, 14 cm @ 2 kHz, 7 cm @ 4 kHz, 3 cm @ 8 kHz |
| Signal-to-noise ratio | 12 dB @ 1 kHz, 12 dB @ 2 kHz, 12 dB @ 4 kHz, 12 dB @ 8 kHz |
| Microphone ring array | with 32 x 1/4" microphones on tripod |
| Front-end | 2 x MSX16 with ICP inputs; CardBus interface, D-SUB cable |
| Notebook | P4 2 GHz, 80 GB HD, 1 GB RAM, WindowsXP |
| Microphone calibration | using centric point sound source or 1/4" calibrator |
- Results:
-
- Sound pressure distribution as colored map superimposed over photograph
- Triggered time data on all channels
- Transfer functions between all channels
- Listening-in to the measurement at one point of the image area
|
