Two-Way%20Acoustic%20Window%20using%20Wave%20Field%20Synthesis

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Two-Way Acoustic Window using Wave Field
Synthesis
Timo Haapsaari
Laboratory of Acoustics and Audio Signal
Processing
April 10, 2007
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Outline

• Aims of the project
• System description
• Wave Field Synthesis
• Acoustic echo canceller
• Performance analysis
• Results of listening tests
• Conclusion

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Aims of the project

• What is a virtual acoustic window?

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System description - Hardware


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System description - Hardware
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Wave Field Synthesis

• Originates from Huygens principle (1690)
• If the effect of a source O is known on surface
  S, sound pressure in volume V can be synthesized
  with continuous distribution of monopole and
  dipole sources on surface S

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Wave Field Synthesis

• Simplification needed for realization
• Reducing surface S to a plane (z0) and further
  to a line is possible
• Using a line array of monopole loudspeakers
  provides almost correct solution

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Wave Field Synthesis

• Creation of virtual audio sources with line array
  of loudspeakers by delaying and attenuating
  signals accordingly
• Wave Field Extrapolation (WFE)
• Extrapolation from 4 microphones to 12
  loudspeakers
• Sources in front the array also possible
• Bringing someone through the opening next to you

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Multichannel acoustic echo canceller

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Performance analysis

• A real time demonstrator was built
• Good localization of multiple sound sources, even
  in highly reverberant room
• Stable echo canceller enables normal conversation
• Echo reduction of 5-7 dB achieved in simulations
  with wide band noise (16 coefficients)
• With 64 coefficients 11-14 dB
• Whole system runs on one Pentium4 class PC

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Listening Test Description

• Methods
• Adaptive beamformer (GSC) WFS
• WFE
• Close-talk microphones WFS
• Direct playback with two loudspeakers

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Listening Test Description

• Method
• Direct playback with two loudspeakers

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Listening Test Description

• Method
• Close-talk microphonesWFS

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Listening Test Description

• Method
• Adaptive beamformer (GSC)WFS

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Listening Test Description

• Method
• Wave field extrapolation (WFE)

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Listening Test Description

• Aspects
• Spatial naturalness (quality of spatial image)
• Coloration
• Distance of the sound sources
• Separation of the sound sources

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Listening Test Results
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Listening Test Results
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Conclusions

• New aspects to communications system
• Opening to an another space
• Real spatial sound
• Persistent communication
• Viable solution for home-to-home conversation
• With flush-mounted or panel speakers no visible
  equipment
• Room for improvement with echo canceller
• Computational load
• Longer filters, system specific equalization
• Other solutions, e. g. frequency shifter
• WFE offers clear potential for usability in
  acoustic opening