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Wave field synthesis
Wave field synthesis (WFS) is a spatial audio rendering technique, characterized by creation of virtual acoustic environments. It produces artificial wavefronts synthesized by a large number of individually driven loudspeakers from elementary waves. Such wavefronts seem to originate from a virtual starting point, the virtual sound source. Contrary to traditional phantom sound sources, the localization of WFS-established virtual sound sources does not depend on the listener's position. Like a genuine sound source, the virtual source remains at a fixed starting point.
WFS is based on the Huygens–Fresnel principle, which states that any wavefront can be regarded as a superposition of spherical elementary waves. Therefore, any wavefront can be synthesized from such elementary waves. In practice, a computer controls a large array of individual loudspeakers and actuates each one exactly by the time and level at which the desired virtual wavefront would pass through its point. In that way, from a mono signal source, a genuine wave front of a sound source may be restored.
The basic procedure was developed in 1988 by Professor A.J. Berkhout at the Delft University of Technology. Its mathematical basis is the Kirchhoff–Helmholtz integral. It states that the sound pressure is completely determined within a volume free of sources if the sound pressure and velocity are determined on all points of its surface.
Therefore, any sound field can be reconstructed if sound pressure and acoustic velocity are restored at all points of the surface of its volume. This approach is the underlying principle of holophony.
For reproduction, the entire surface of the volume would have to be covered with closely spaced loudspeakers, each individually driven with its own signal. Moreover, the listening area would have to be anechoic, in order to avoid sound reflections that would violate the source-free volume assumption. In practice, this is hardly feasible. Because our acoustic perception is most exact in the horizontal plane, practical approaches generally reduce the array to a horizontal loudspeaker line, circle or rectangle around the listener. So origin of the synthesized wavefront restrict at any point on the horizontal plane of the loudspeakers. Real 3D audio is not possible with such loudspeaker rows. For sources behind the loudspeakers, the array will produce convex wavefronts. Sources in front of the speakers can be rendered using concave wavefronts that focus at the virtual source within the playback area and diverge again as convex wavefronts. Hence, the reproduction inside the volume is incomplete - it breaks down if the listener is situated between the speakers and the virtual source.
If the restriction to the horizontal plane is overcome, it becomes possible to establish a virtual copy of a genuine sound field indistinguishable from the real sound field. Changes of the listener position in the rendition area produce the same impression as an appropriate change of location in the recording room. Two-dimensional arrays can establish parallel wavefronts, which are direct at the loudspeakers not louder as in some meter distance. The horizontal arrays can only produce cylinder waves, which lose 3 dB level at any doubling of distance. But even with horizontal arrays, listeners are no longer relegated to a sweet spot area within the room.
The Moving Picture Expert Group standardized the object-oriented transmission standard MPEG-4, which allows a separate transmission of content (dry recorded audio signal) and form (the impulse response or the acoustic model). Each virtual acoustic source needs its own (mono) audio channel. The spatial sound field in the recording room consists of the direct wave of the acoustic source and a spatially distributed pattern of mirror acoustic sources caused by the reflections by the room surfaces. Reducing that spatial mirror source distribution onto a few transmitting channels causes a significant loss of spatial information. This spatial distribution can be synthesized much more accurately by the rendition side.
Compared to conventional channel-oriented rendition procedures, WFS provides a clear advantage: Virtual acoustic sources guided by the signal content of the associated channels can be positioned far beyond the conventional material rendition area. This reduces the influence of the listener position because the relative changes in angles and levels are clearly smaller compared to conventional loudspeakers located within the rendition area. This extends the sweet spot considerably; it can now cover nearly the entire rendition area. WFS thus is not only compatible with, but potentially improves the reproduction for conventional channel-oriented methods.
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Wave field synthesis AI simulator
(@Wave field synthesis_simulator)
Wave field synthesis
Wave field synthesis (WFS) is a spatial audio rendering technique, characterized by creation of virtual acoustic environments. It produces artificial wavefronts synthesized by a large number of individually driven loudspeakers from elementary waves. Such wavefronts seem to originate from a virtual starting point, the virtual sound source. Contrary to traditional phantom sound sources, the localization of WFS-established virtual sound sources does not depend on the listener's position. Like a genuine sound source, the virtual source remains at a fixed starting point.
WFS is based on the Huygens–Fresnel principle, which states that any wavefront can be regarded as a superposition of spherical elementary waves. Therefore, any wavefront can be synthesized from such elementary waves. In practice, a computer controls a large array of individual loudspeakers and actuates each one exactly by the time and level at which the desired virtual wavefront would pass through its point. In that way, from a mono signal source, a genuine wave front of a sound source may be restored.
The basic procedure was developed in 1988 by Professor A.J. Berkhout at the Delft University of Technology. Its mathematical basis is the Kirchhoff–Helmholtz integral. It states that the sound pressure is completely determined within a volume free of sources if the sound pressure and velocity are determined on all points of its surface.
Therefore, any sound field can be reconstructed if sound pressure and acoustic velocity are restored at all points of the surface of its volume. This approach is the underlying principle of holophony.
For reproduction, the entire surface of the volume would have to be covered with closely spaced loudspeakers, each individually driven with its own signal. Moreover, the listening area would have to be anechoic, in order to avoid sound reflections that would violate the source-free volume assumption. In practice, this is hardly feasible. Because our acoustic perception is most exact in the horizontal plane, practical approaches generally reduce the array to a horizontal loudspeaker line, circle or rectangle around the listener. So origin of the synthesized wavefront restrict at any point on the horizontal plane of the loudspeakers. Real 3D audio is not possible with such loudspeaker rows. For sources behind the loudspeakers, the array will produce convex wavefronts. Sources in front of the speakers can be rendered using concave wavefronts that focus at the virtual source within the playback area and diverge again as convex wavefronts. Hence, the reproduction inside the volume is incomplete - it breaks down if the listener is situated between the speakers and the virtual source.
If the restriction to the horizontal plane is overcome, it becomes possible to establish a virtual copy of a genuine sound field indistinguishable from the real sound field. Changes of the listener position in the rendition area produce the same impression as an appropriate change of location in the recording room. Two-dimensional arrays can establish parallel wavefronts, which are direct at the loudspeakers not louder as in some meter distance. The horizontal arrays can only produce cylinder waves, which lose 3 dB level at any doubling of distance. But even with horizontal arrays, listeners are no longer relegated to a sweet spot area within the room.
The Moving Picture Expert Group standardized the object-oriented transmission standard MPEG-4, which allows a separate transmission of content (dry recorded audio signal) and form (the impulse response or the acoustic model). Each virtual acoustic source needs its own (mono) audio channel. The spatial sound field in the recording room consists of the direct wave of the acoustic source and a spatially distributed pattern of mirror acoustic sources caused by the reflections by the room surfaces. Reducing that spatial mirror source distribution onto a few transmitting channels causes a significant loss of spatial information. This spatial distribution can be synthesized much more accurately by the rendition side.
Compared to conventional channel-oriented rendition procedures, WFS provides a clear advantage: Virtual acoustic sources guided by the signal content of the associated channels can be positioned far beyond the conventional material rendition area. This reduces the influence of the listener position because the relative changes in angles and levels are clearly smaller compared to conventional loudspeakers located within the rendition area. This extends the sweet spot considerably; it can now cover nearly the entire rendition area. WFS thus is not only compatible with, but potentially improves the reproduction for conventional channel-oriented methods.