With Ambisonic technology, the directionality of the sound field is
composed of spherical harmonic components. The zero-order
component is termed W and is omnidirectional. The first-order
components are figure-of-eight (lemniscate) responses which point
forward, left and up. These are termed X, Y and Z, respectively.
There are also second-order components, and higher. The collection
of components of a soundfield is termed B-Format.
To be played, B-Format needs to be decoded, by an Ambisonic
decoder, into a set of speaker feeds.
A official file format for downloadable B-Format files has been defined
to allow easy exchange of audio files. The file format is in the
public domain for anybody to use. It has been carved in stone and
cannot be changed. Here are the details:
File extension is ".amb" (same three letter extension
irrespective of the number of channels). This is to allow the
operating system to route B-Format files to an Ambisonic decoder.
Note that when creating files, software must use this
file extension. However, when reading files, software should peek
inside any WAVE-EX file, irrespective of its extension, to see if
it contains the B-Format GUID in the 'fmt ' chunk. (This is an example of the
robustness principle, "Be liberal in what you read, and
conservative in what you write".)
The W channel is attenuated by -3 dB (1/sqrt(2)) relative to the
unnormalized spherical harmonic component. This is the case for
all orders. That is to say, a source at +45° azimuth (zero
elevation) would produce equal signals in W, X, and Y.