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

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

3 Aims of the project What is a virtual acoustic window?

4 System description - Hardware

5

6 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

7 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

8 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

9 Multichannel acoustic echo canceller

10 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: dB Whole system runs on one Pentium4 class PC

11 Listening Test Description Methods: 1. Adaptive beamformer (GSC) + WFS 2. WFE 3. Close-talk microphones + WFS 4. Direct playback with two loudspeakers

12 Listening Test Description Method: Direct playback with two loudspeakers

13 Listening Test Description Method: Close-talk microphones+WFS

14 Listening Test Description Method: Adaptive beamformer (GSC)+WFS

15 Listening Test Description Method: Wave field extrapolation (WFE)

16 Listening Test Description Aspects: 1. Spatial naturalness (quality of spatial image) 2. Coloration 3. Distance of the sound sources 4. Separation of the sound sources

17 Listening Test Results

18 Listening Test Results

19 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