Cost-Effective Acoustic Cameras Use Coherence Scanning Holography

Cost-Effective Acoustic Cameras Use Coherence Scanning Holography

CSH or Coherence Scanning Holography is a patented method which is developed by Seven Bel. In Seven Bel acoustic cameras which are named as “Sound Scanner“, sound imaging is done based on audio data captured with moving microphones. This novel method is fundamentally different than the method of beamforming which is used in conventional acoustic cameras.

What is Coherence Scanning Holography?

The Coherence Scanning Holography requires a microphone to catch the incident sound field along a ring-shaped trajectory. Since the microphone moves relatively towards a sound source, the audio data which are captured over a complete revolution have a Doppler shift of the frequencies. In fact, there is a stationary microphone at the center of the scanning area which is free from Doppler shift.

Also, there is a magnetic rotary encoder which measures the angular position with respect to the constrained axis of rotation. This encoder is co-axially aligned with the rotation axis of the linear array.


Computation of Acoustic Image

After capturing data, we need to compute the acoustic image from the acquired data, and in order to do so, a reconstruction plane is chosen at a distance where the distribution of sound sources is expected. The so-called reconstruction plane is parallel to the measurement plane.

Firstly, the audio signal which comes from the moving microphone is travelled back in time to a specific point in the reconstruction plane. In other words, the audio signal is manipulated so it seems to appear to have been emitted from that specific point caught by the moving microphone. Please note that, at this specific point, the Doppler shift is completely compensated for a point where a sound source is actually located. And, an additional Doppler shift is induced in the backpropagated audio signal for any other points away from the point source.

acoustic camera p132 reconstruction_plane_measurement_plane

Now, we get a weak coherence value for points where there is no or little sound emission happening, and a strong coherence value for points where a sound source is actually located using the coherence as a measure of similarity between the two signals coming from the moving and reference microphones.

In fact, this measure makes it possible for us to produce a heatmap that shows the distribution of sound sources over the entire reconstruction plane. Finally, as the last step, to give the user a clear indication of the spatial position of the sound sources, the heatmap is finally overlaid onto an optical image of the measurement scene.

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