Ambisonic Surround Sound FAQ, version 2.8: Section 4

4. What is Ambisonic Surround Sound?

Ambisonic Surround Sound is a set of techniques, developed in the 1970s, for the recording, studio processing and reproduction of the complete sound field experienced during the original performance. Ambisonic technology does this by decomposing the directionality of the sound field into spherical harmonic components, termed W, X, Y and Z. The Ambisonic approach is to use all speakers to cooperatively recreate these directional components. That is to say, speakers to the rear of the listener help localise sounds in front of the listener, and vice versa.

Ambisonic technology is based on a meta-theory (a theory of theories) of sound localisation developed by the late Michael A Gerzon when he was with the Mathematical Institute, University of Oxford (see the Gerzon 1992a reference). Ambisonic decoder design aims to satisfy simultaneously and consistently as many as possible of the mechanisms used by the ear/brain to localise sounds. The Gerzon theory takes account of non-central as well as central listening positions.

In an Ambisonic decoder the spherical harmonic direction signals, W, X, Y and Z, are passed through a set of shelf filters which have different gains at low and high frequencies designed to match different ways the ear/brain localises sounds. (The different localisation mechanisms operate below and above about 700 Hz.) The speaker feeds are then derived by passing the outputs from the shelf filters through a simple amplitude matrix. An important aspect of Ambisonic decoder technology is that it is only at this final stage of processing that the number and layout of speakers is considered.

The listening area for Ambisonic Surround Sound is comparable with that for conventional stereo, but larger.