Ambisonics Formats Explained: A-Format, B-Format, FuMa, AmbiX, FOA and HOA

Level: Beginner to Intermediate | Audience: Composers, students, producers, technicians, studio residents.

Use this page when you see terms such as A-Format, B-Format, FuMa, ambiX, ACN/SN3D, FOA, or HOA and need to know what they mean in practice.

Quick checklist ICST recommendation B-Format Archive

1. Why “format” is confusing in Ambisonics

Ambisonics uses the word format for several different things:

  • the raw output of a microphone
  • the internal spatial signal in a DAW
  • the channel order inside a file
  • the normalization convention used by the spherical harmonics
  • the Ambisonics order, which determines the channel count

That is why “Is this B-Format?” is often not specific enough. A better practical question is:

Is this an ambiX B-Format file, with ACN channel ordering and SN3D normalization, and which Ambisonics order does it use?

2. A-Format vs B-Format

A-Format is usually the raw signal from an Ambisonic microphone. In a first-order tetrahedral microphone, this means four capsule signals before spatial decoding or correction. A-Format is microphone-specific and normally should not be treated as a finished Ambisonics master.

B-Format is the Ambisonics signal representation used for production, exchange, decoding, and archiving. It describes a sound field around a listening point. First-order B-Format has four channels; higher-order B-Format has more.

Practical rule:

  • A-Format comes from the microphone and must be converted.
  • B-Format is the spatial scene you can edit, route, decode, archive, or render.

3. FuMa vs ambiX

FuMa is the older Ambisonics convention. It uses named first-order channels such as W, X, Y, Z and MaxN-style normalization. It is still encountered in older tools, historical material, and some first-order workflows.

ambiX is the modern exchange convention for Ambisonics. It uses:

  • ACN channel ordering
  • SN3D normalization
  • ordinary multichannel audio files, usually WAV or RF64

In current ICST workflows, ambiX is the default. If a tool asks for channel ordering and normalization, choose ACN/SN3D unless you have a clear reason to do otherwise.

4. ACN, SN3D and N3D

ACN means Ambisonic Channel Number. It defines the channel order. Instead of using historical names such as W, X, Y, Z, each spherical harmonic component gets a numbered channel.

SN3D and N3D are normalization conventions. They define the scaling of Ambisonics components. The spatial scene may be conceptually the same, but a mismatch between SN3D and N3D changes levels between channels and can make decoding wrong.

For most plugin and DAW work today:

  • ACN/SN3D = ambiX, recommended for exchange and production
  • ACN/N3D = common in some research, analysis, and mathematical contexts
  • FuMa = legacy convention, mainly first-order

5. FOA vs HOA

FOA means First-Order Ambisonics. It uses 4 channels and is useful for basic recording, simple spatial work, compatibility, and many entry-level microphone workflows.

HOA means Higher-Order Ambisonics. It starts at second order and increases the spatial resolution by adding more spherical harmonic components.

Higher order can give sharper localization, more stable rendering, and a larger useful listening area, but it also needs more channels and more careful routing.

6. Channel counts by order

For full 3D Ambisonics, the channel count is:

channels = (order + 1)^2
OrderNameChannels
1FOA4
2HOA-29
3HOA-316
4HOA-425
5HOA-536
6HOA-649
7HOA-764

In REAPER, this is why ICST sessions often use a 64-channel B-Format bus: it can carry HOA-7 without silently cutting off higher-order channels.

7. Scene-based vs channel-based audio

Ambisonics is scene-based. The channels are not speaker feeds. They are components of a spatial sound-field representation.

This is different from channel-based audio, where channel 1 might mean left speaker, channel 2 right speaker, channel 3 center speaker, and so on.

In Ambisonics, the decoder decides how the B-Format scene becomes loudspeaker signals or binaural headphone output. That is the reason the same B-Format master can be rendered for a dome, an octagon, a studio array, headphones, or stereo.

8. Practical ICST recommendation

ICST Recommendation

For new Ambisonics work, use ambiX: ACN channel ordering + SN3D normalization.

For higher-order work in REAPER, use a 64-channel B-Format bus for HOA-7 when needed. Decode only at the monitoring or rendering stage.

This keeps the session open: the same B-Format master can feed loudspeaker decoding, binaural monitoring, archive export, and later reinterpretation on another playback system.

9. Quick checklist: Which format should I use?

Use ambiX B-Format, ACN/SN3D when:

  • you are starting a new Ambisonics production
  • you are working with ICST, IEM, SPARTA, or other modern HOA tools
  • you are exporting a B-Format archive master
  • you need reliable exchange between DAWs, plugins, and rendering tools

Use A-Format only when:

  • you are handling raw Ambisonic microphone recordings before conversion
  • you still need the manufacturer-specific A-to-B conversion step

Use FuMa only when:

  • an older tool or archive specifically requires it
  • you are converting historical first-order material

Always document:

  • Ambisonics order
  • channel count
  • channel ordering
  • normalization
  • sample rate and bit depth
  • decoder or renderer used for monitoring
ambisonicsb-formatambixhoa