Timo HaapsaariLaboratory of Acoustics and Audio Signal Processing

April 10, 2007

Two-Way Acoustic Window using Wave Field Synthesis

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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 Ω 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 (z=0) 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:1. Adaptive beamformer

(GSC) + WFS2. WFE3. Close-talk

microphones + WFS4. 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 microphones+WFS

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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:1. Spatial naturalness (quality of

spatial image)2. Coloration3. Distance of the sound sources4. Separation of the sound sources

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Listening Test Results

WFE GSC Close-talk Direct0

102030

405060

708090

100

Spat

ial n

atur

alne

ss

Dialog - Spatial naturalness

WFE GSC Close-talk Direct0

102030

405060

708090

100

Colo

ratio

n

Dialog - Coloration

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Listening Test Results

WFE GSC Close-talk DirectListener

Array

SourcesWall

Dialog - Distance

WFE GSC Close-talk Direct0

102030

405060

708090

100

Sepa

ratio

n

Dialog - Separation

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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