A first-order Ambisonic B-format signal carries full-sphere spatial information in four channels: W, X, Y, Z

B-format is the encoding format at the heart of first-order Ambisonics. It stores spatial audio in four channels: W (omnidirectional pressure), X (front-back bidirectional), Y (left-right bidirectional), and Z (up-down bidirectional). These four components together capture a complete description of the soundfield at a point in space, in the same way that Mid-Side recording extends to three dimensions. Any B-format signal can be decoded for playback on any speaker arrangement — from headphones to multichannel arrays — by applying an appropriate matrix or convolution. The key insight is that spatial information travels with the signal, not with the speaker layout, so a single B-format file works for stereo, quad, surround, or binaural reproduction. A common misconception is that B-format is tied to a specific speaker count; it is not: the speaker topology only matters at decoding time.

Examples

In SuperCollider with the ATK: encode a mono source to B-format with FoaEncode.ar(mono_sig, FoaEncoderMatrix.newDirection(0, 0)), then decode to stereo with FoaDecode.ar(b_format_sig, FoaDecoderMatrix.newStereo(55.degrad, 'cardioid')). The signal expands from 1 to 4 channels after encoding, then back to 2 after decoding.

Assessment

Given a 4-channel B-format file, describe what each channel represents; then explain why you can play it back on headphones, a stereo system, or a quad array without re-encoding.