Wave field synthesis
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'Wave field synthesis'
(or WFS) is a spatial sound reproduction principle. It is physically based, not reliant upon psychoacoustic effects like the hitherto known sound rendition procedures.
Berkhout invented the procedure in 1988 at the University of Delft, based on the Huygens principle. The mathematical base is the Kirchhoff- Helmholtz- Integral which describes, that if sound pressure and velocity regarding the limits of a volume are known, in any space point inside this volume pressure and velocities determinate.
This idea is convincingly: The wave front of a natural acoustic source according Huygens principle is build up from several elementary waves. A computer synthesis moves each of the single loudspeakers, arranged in rows around the listener, just in that moment, to which the wave front of the virtual source would go through its space point. Thus, according to the Huygens principle, the primary wave front is physically restored.
Their starting point portrays a virtual acoustic source, which does hardly differ from a material acoustic source here. It does not move any longer if we move ourselves in the listening area, like the phantom acoustic source between conventional loudspeakers is doing. Just as a material acoustic source, we always locate it at their virtual starting point. This starting point can exist before or behind the loudspeaker row. Convex or also concave wave fronts may be produce. Because the loudspeaker lines arranged around the listener, a virtual acoustic source may be represented at any point within the infinite level of the listener. By means of the delay and level information stored in the impulse response of the recording room, or according the model based mirror source approach, also it is possible to restore the acoustic behaviour of the recording room.
At the German “Fraunhofer Institut” the transmission standard MPEG-4 was developed for such separate transmission of content (the audio signal) and form (the acoustic data). However, each dry recorded conventional audio signal may reproduced by means of the procedure in a stored acoustic environment. Each virtual acoustic source needs its own audio channel. The result of such a reproduction widely approximates a material presentation. By today the most significant difference is the reduction of the horizontal plane of the listener. Because of the otherwise almost perfect rendition with the test setups it seems to become more audible than using conventional loudspeaker arrangements. A fundamental problem is, furthermore, the acoustic of the rendition area has to be diminished because the speaker rows generate the acoustic of the recording area including early strong reflections and reverberation. In consequence the rendition room reflexions are disturbing signals.
Another, “sound field transformation” approach, purposely includes those reflections into the synthesis. It seems to be comprehensive, but hardly realizable till this day. However, the prospects for the wave field synthesis are breathtaking. In future it seems to become possible to restore the entire sound field of an opera house physically into the living room.
Primary problem till now is the high effort. On the one hand must be aligned the single speakers very closely; otherwise, spatial aliasing effects arise audibly. On the other hand the speaker column or field expansion determinate low frequency limit and depiction range. However, the close speaker arrangement cannot been given up, what causes their high number whereat each one needs its own signal. That justifies the reduction on the lines till this day, although principle itself is not fundamentally reduced onto the plane .
Furthermore especially produced audio material is hardly available at the moment. Nevertheless the reproduction of conventional material already may be clearly improved. By means of wave field synthesis it is capable to generate “virtual panning spots”. Such virtual sources headed on the related channel content can be placed far beyond the listening room walls. Thereby the influence of listener moves inside the rendition area drops because its influence regarding level and angle diminish. To solve the sweet spot problem in this manner is a main facility of the wave field synthesis. The procedure shall to undergo radical changes in the audio technology up to the next years.
[edit] Demo
DEMO Wave field synthesis ( Animation )
[edit] References
Berkhout, A.J.: A Holographic Approach to Acoustic Control, J.Audio Eng.Soc., vol. 36, Dezember 1988, pp. 977-995
Berkhout, A.J.; De Vries, D.; Vogel, P.: Acoustic Control by Wave Field Synthesis, J.Acoust.Soc.Am., vol. 93, Mai 1993, pp. 2764-2778
