We are all familiar with the way channel based audio reproduction works. If we sit halfway between a pair of speakers and audio is played at the same level through both then we perceive the sound to be coming from halfway between the speakers, even though there is no speaker there. This is how stereo reproduction works over speakers. It’s also how surround formats like 5.1 and 7.1 work.
It’s not without its problems though, one of them is that if we change the position of the speakers we change the position of these channel based phantom sources. This is the motivation for the development of Object based workflows like Dolby Atmos which render channel independent audio objects to appear at the correct locations regardless of the number of replay channels deployed in any particular room. Objects are great!
However Objects don’t help with another weakness of channel-based reproduction, and once rendered for playback an object becomes channel based like everything else which is played out of the speakers in an immersive install. This issue is the sweet spot.
If you move more than a small distance from exactly halfway between the speakers you are listening to you’ll very quickly lose the contribution of the furthest speaker and the illusion of a phantom centre will collapse. This is physics. The inverse square law applies to point sources and there is no getting around that, is there?
Wave Field Synthesis
Wave Field Synthesis is fundamentally different from familiar channel based reproduction as it uses loudspeakers in a different way and rather than trying to reproduce sounds by creating a stereophonic illusion in the small sweet spot in the centre of the room, it reproduces the entire sound field in the room in which the performance was made. Because of this it doesn’t suffer from the sweet spot issue common to other systems.
Wave Field Synthesis relies on Huyghens' Principle which states that the propagation of a wave through a medium can be formulated by adding the contributions of all of the secondary sources positioned along a wave front. What this means is that, taking a simple example, the wavefront produced by a rock thrown into a still pond could be accurately synthesised by dropping an infinite number of smaller rocks (secondary sources) along any position defined by the passage of the primary wave front.
By using a very large number of loudspeakers to act as secondary sources, the wavefront of any number of primary sources from outside the loudspeaker array could be reproduced in a way which wouldn’t suffer from any sweet spot issues, you can walk around the room freely without disrupting the effect and the position of the sources remains stable in space. Amazing stuff.
Unfortunately there is a downside. Wave Field Synthesis isn’t practical for any commercial application due to the sheer number of speakers required. While interesting from a research perspective, Wave Field Synthesis is simply too complex and too expensive to be commercially viable.
Reducing The Cost And Complexity Of Wave Field Synthesis
Is there a way to achieve anything similar to the performance of Wave Field Synthesis without the prohibitive complexity and expense? The Fraunhofer Institute for Digital Media Technology believe there is and their research has led to the development of SpatialSound Wave Technology which promises to create a 3D sound experience within the entire room, so no sweet spot, with 30 loudspeakers, and the potential of using fewer. To create a Wave Field Synthesis system in the same room would need about 90 speakers so this is a huge saving in terms of the required equipment.
A SpatialSound Wave system is built using a windows computer with a suitable MADI or Dante interface which feeds an array of 30 Neumann KH 310 monitors. I love these monitors, see my review and you’ll read just how impressed I was by them. Apparently the development team liked the KH 120 but found that in this application they needed greater level and bass extension for times when a sound was placed in a single speaker only. Sounds are Objects in the same way as we find in Dolby Atmos so panning information isn’t system specific.
The array consists of roughly half the speakers located at ear level around the room with the rest installed in the ceiling and four KH805 subwoofers in the corners of the room. The results are that the sound reproduction of this system is remarkable: The localisation of virtual sound sources is sharp even far outside the central listening position. The listener can move through the room almost like in an actual acoustic environment. Moving toward a sound source, it appears louder; the levels change in a natural way.
The secret is in the processing Daniel Beer, Head of Electroacoustics explains:
“The intelligence is in the algorithm that controls the loudspeakers. Originally, SpatialSound Wave was based on wave field synthesis which controls the loudspeakers so the individual signals are superimposed in such a way that the sound field is reproduced. In recent years, however, we included a lot of psychoacoustics. The algorithm was altered in such a way that reproduction of the sound field is not physically correct anymore, but the loudspeakers may now be spaced further apart.”
He goes on to add that the system is also very suitable for modifying room acoustics using an active acoustics approach:
“For theaters in particular it is very interesting that the same computer that allows positioning of audio objects in a 3D space can also be used for room simulation. The reverberation time of the room can be increased. It is a regenerative room simulation system, so there are microphones above, which record everything that happens in the room. Their signals are then used for artificial reverb so the room acoustics can be modified according to the music genre. Straight theatre, for instance, requires a different reverberation time than a symphony concert. The big advantage is that the actual acoustics are picked up and modified, so it is not an alien room that is grafted upon the existing one. You can move through the room without the sound appearing artificial. There are solutions for such applications by competitors, but our system has the advantage to be able to do both, room simulation and 3D audio playback.”
We investigated this fascinating area of audio technology some time ago in this piece on active variable acoustics in a venue in Denmark.
SpatialSound Wave Technology sounds amazing and though I don’t know if I’m likely to hear it I’d be fascinated to experience this immersive audio system. I’ll settle for plain old stereo for now but at least its on a pair of KH 310s.
Follow the link to find out more about SpatialSound Wave Technology