File Format for B-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.

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:

• Custom version of WAVE-EX for B-Format defined at http://dream.cs.bath.ac.uk/researchdev/wave-ex/bformat.html.
• Generic WAVE-EX is defined at http://www.ambisonia.com/Members/mleese/file-formats/multichaud.pdf. (A more readable explanation is at http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/WAVE.html.)
• 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.
• Channels use the Furse-Malham set of weighting factors defined at http://www.york.ac.uk/inst/mustech/3d_audio/higher_order_ambisonics.pdf. (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 at http://members.tripod.com/martin_leese/Ambisonic/Harmonic.html.
• The dwChannelMask field should be set to zero for all B-Format configurations.
• The number of channels defines the order of the soundfield:

  Number of channels	Malham notation	Soundfield type	Horizontal order	Height order	Channels
  3	h	horizontal	1	0	WXY
  4	f	full-sphere	1	1	WXYZ
  5	hh	horizontal	2	0	WXYUV
  6	fh	mixed-order	2	1	WXYZUV
  9	ff	full-sphere	2	2	WXYZRSTUV
  7	hhh	horizontal	3	0	WXYUVPQ
  8	fhh	mixed-order	3	1	WXYZUVPQ
  11	ffh	mixed-order	3	2	WXYZRSTUVPQ
  16	fff	full-sphere	3	3	WXYZRSTUVKLMNOPQ

Many people already offer plug-ins and other Ambisonic software. Such people are encouraged to support the above format (in addition to other formats).

Martin Leese
http://members.tripod.com/martin_leese/