File Format for B-Format
The ".amb" Format
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.
An 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:
- Custom version of WAVE-EX for B-Format defined at
- Generic WAVE-EX is defined at
(A more readable description is at
- 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. (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.
- Channels use the Furse-Malham set of weighting factors defined at
(The F-M set includes the attenuation of W described above; it
should not be applied twice.)
- No shelf filters. These are a decoding option.
- The sequence of the channels is WXYZRSTUVKLMNOPQ, with unused channels
omitted. Pictures are available.
- The FormatChunk.dwChannelMask field should be set to zero for all B-Format
- The number of channels, given in FormatChunk.nChannels, defines
the order of the soundfield as follows:
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