The paperwork: formats and conventions

Sooner or later — usually the day a collaborator sends "the ambisonic stems" — you will play a B-format file and hear something almost right: levels off, the image smeared, front and left subtly traded. Nothing is broken. You have met the field's history, encoded as channel order.

This chapter is short, practical, and worth its weight in un-smeared mixes.

Two dialects

Ambisonics predates its own standardization by four decades, so there are two families of convention in the wild:

FuMa (Furse–Malham), the historical dialect. First-order channels in the order W, X, Y, Z, and W recorded 3 dB down (a −3 dB factor, 1/√2, inherited from analog-era headroom practice, with further per-channel factors at higher orders). Four decades of recordings, .amb files, classic tools (and the SoundField microphone lineage) speak FuMa. Defined cleanly only up to order 3.

AmbiX (2011), the modern standard and the only thing new systems should emit. Channels in ACN order — W, then Y, Z, X (the n(n+1)+m indexing from Chapter 6) — at SN3D scaling, no W attenuation. Everything in AmbiTap, every current game engine, YouTube, and essentially all software written this decade speaks AmbiX.

Same soundfield, two filing systems:

What mismatch sounds like

Feed FuMa into a system expecting AmbiX and two things happen at once. The W attenuation reads as a level/balance error (the omni content 3 dB shy, so the scene sounds oddly hollow and over-directional). The X↔Y swap reads as a geometry error: front-back content lands on the left-right axis and vice versa — the image doesn't rotate so much as fold. It is exactly weird enough to waste an afternoon, because it still sounds "spatial."

The fix is one object:

[ambitap.format~ 3]     attribute: direction — fuma_to_ambix / ambix_to_fuma

Multichannel in, multichannel out, orders 0–3 (a FuMa limitation, not an AmbiTap one), exact published conversion factors (they're pinned by exact-value tests in the library, against the AmbiX specification's own tables). Put it at the border the moment anything historical crosses into your patch, and forget it's there.

The rest of the paperwork

Channel order and scaling are the big two; three smaller lines complete the customs form.

Angles and axes. AmbiTap: radians; azimuth 0 = front, positive = counterclockwise from above (+90° = left); elevation positive = up; rotations yaw-then-pitch-then-roll (Chapter 4's figure). Other tools may speak degrees, measure azimuth clockwise, or compose Euler angles in another order. None of this corrupts a file — it corrupts control data, which is why a head-tracker driver is where you'll meet it.

"B-format" is ambiguous on its own. Historically it implied FuMa; today people say it for any ambisonic bus. When a collaborator offers B-format, the professional reply is three questions: what order, what channel order (FuMa or ACN), what normalization (SN3D or N3D)? — that last one because some research tools emit N3D, a cousin scaling in which higher-order channels run hotter than SN3D by fixed per-order factors (√(2n+1); the fix is per-channel gains, and knowing the name is most of the battle).

Files. There is no dedicated ambisonic file format in common use — scenes travel as ordinary multichannel WAV (or WavPack/FLAC), and the convention travels as metadata at best, folklore at worst. .amb is a WAVE variant that reliably means FuMa. A 4- or 16-channel .wav means whatever its maker meant; a README beats a filename. When you export: AmbiX, state "AmbiX (ACN/SN3D), order N" in the delivery notes, and you've done your part for civilization.

For the curious. The AmbiX specification is Nachbar, Zotter, Deleflie & Sontacchi, AMBIX — A Suggested Ambisonics Format (Ambisonics Symposium 2011) — short and readable. The FuMa↔AmbiX factor tables it publishes are the ones format~ implements; the library's docs/COMPARISON.md records the exact-value tests, and its ACN⇄FuMa index map is eight integers you can read yourself in dsp/format_converter.h.

Checkpoint — and the end of the theory

Part II in three sentences: an ambisonic scene is a set of coincident microphone patterns (order 1: one omni, three figure-8s). More orders mean finer patterns, sharper images, quadratically more channels, and order 3 is the sensible default. Modern channels are filed ACN/SN3D — "AmbiX" — and one object repairs history when it knocks.

That is all the theory this book makes you carry. From here on, every chapter is a job: placing sources, adding distance, building rooms, decoding to arrays, and the rest of the craft. Patch cords from here to the end.