This page concerns itself with techniques for reproducing three-dimensional sound images. The focus is on use of multiple speakers to reproduce complete `fields' of sound in which the listener can sit or move around. This is rather different to HRTF-based techniques which are either reliant on headphones or a motionless listener facing in the correct direction at the `sweet spot' of a listening area. Tools are introduced which also construct more conventional mono or stereo recordings.
Ambisonics is a technique developed initially by Michael Gerzon in the early 70s. It provides a way to encode (sometimes by recording) three dimensional soundfields. These encoded soundfields can then be reproduced over various different speaker arrangements. This is known as Ambisonic decoding.
Ambisonics, unlike some other `surround' techniques, is based on solid mathematics. It records an approximation to the complete soundfield, the Ambisonic `order' indicating what level of accuracy is in use. (For the mathematicians out there, each order corresponds to an order of spherical harmonic.) Zeroth order corresponds to Mono, first order to the prevailing form in use at present, B-Format. This uses a four channel encoding which is usually decoded over a square or cube of speakers. Note that the four channels that make up B-Format are not themselves a speaker feed, merely an efficient way to carry the soundfield information.
"Ambisonics" is a registered trademark of Nimbus Communications International.
Work is ongoing on second order Ambisonics as modern personal computers are now capable of performing the required decoding mathematics live and the number of speakers required is realistic for small venues. Hopefully the Furse-Malham Higher Order Format will become a new standard for second-order Ambisonics. For the Ambisonic experts out there, the proposed encoding is:
| Label | Angle/Elevation Representation | Cartesian Representation |
|---|---|---|
| W | 0.707107 |
0.707107 |
| X | cos(A)cos(E) |
x |
| Y | sin(A)cos(E) |
y |
| Z | sin(E) |
z |
| R | 1.5sin(E)sin(E)-0.5 |
1.5zz-0.5 |
| S | cos(A)sin(2E) |
2zx |
| T | sin(A)sin(2E) |
2yz |
| U | cos(2A)cos(E)cos(E) |
xx-yy |
| V | sin(2A)cos(E)cos(E) |
2xy |
Dave Malham also provides a further discussion of this encoding including an extension to handle mixed first and second order recordings.
Speaker decoding equations and other information for the Furse-Malham Set is available.
B-Format soundfields can be encoded using Ambisonic Microphones and it may not be long before second order recordings can be made in this way. For the moment, second order encodings are most easily generated in software or hardware, live or by batch process. VSpace can be used to generate first or second order recordings using soundfiles and a script language.
The encoded soundfield can be worked with in its own right. Soundfields can be mixed by simple linear combination and rotated by matrix operations. Other manipulations are possible.
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