MOF wrote:I went to an ambisonic demonstration many years ago and I seem to remember that the placement of speakers was proscribed. The two front speakers were at (L-R) 315º and 45º and the rear speakers were at 135º and 225º they were all above your head pointing down at you, I seem to remember. I can't find anything on the web to confirm this.
The speaker feeds are decoded from the B-format (or higher order) ambisonic format specifically for any given number and location of speakers. Its just a load of matrix calculations and some filtering -- very easy in the digital domain.
The arrangement of speakers is normally expected to be symmetrical left/right and front/back, but the four channel decoders I've used have had facilities to cope with square or rectangular arrays, and the latter with the long axis fore-aft or left-right. The Meridian B-format/UHJ decoder I use now can decode to 4, 5, 6, or 7 speaker layouts, with a lot of flexibility in actual speaker positions. I have it set up to decode to a fairly conventional ITU 5 channel layout.
Four speakers is the minimum for horizontal soundfield's, but can suffer from unstable imaging at positions between the speakers, especially atbthe sides. The more speakers available to the decoder, the more accurate and stable the spatial imagining will be... and there are decoders that can derive height channels as well, of course...