Ambisonics Demystified

Ambisonics Demystified
October 1, 2019 Voyage Audio

Ambisonics enables audio production and playback without using the traditional channel based methodology, thus enabling reproduction on any number of speakers in varying configurations.

When we record the signal from a traditional microphone, we put this on a track and assign it to a playback channel. We can play this back on one speaker and call it mono, or send it to two speakers, pan it left, right or center and call it stereo. We can also expand this to a larger 5.1 or even 22.1  surround system. In all of these cases we are relying on a channel based system to record and playback audio.

Channel based systems require the same number and configuration of loudspeakers to be used in production and playback. The advantage of using ambisonics vs channel-based audio (i.e. mono, stereo, 5.1, etc) is that the signals are not related to speaker feeds. Because of this, ambisonics is a future-proof way of recording.

Ambisonics enables audio production and playback without using the traditional channel based methodology, thus enabling reproduction on any number of speakers in varying configurations.

Ambisonics was first developed in the 1960’s and ‘70’s by passionate researchers as a way to encode a 3D sound field. To do this, the sound-field is decomposed into a series of spherical harmonics. While a spherical harmonic series goes indefinitely, we are most interested in the first few “orders”, which will give us enough spatial resolution to encode a realistic sounding representation of a soundfield. The encoded audio on DAW tracks will represent these spherical harmonics, instead of speaker feeds.

In Figure 1, you can see that, as order increases, more spherical harmonics are needed. For example, a second order spherical harmonic representation of a sound field contains 9 components – all of the harmonics from the zeroth and first order, plus 5 more.

Ambisonics Demystified

Figure 1: Microphone Capsule Arrays for Spherical Harmonic Reproduction

At the zeroth order, you can see that a single, omni-directional microphone capsule is capable of recording this harmonic. But what about the first and second order? To create these harmonics, the sound field must be sampled by a microphone array and then encoded. For first order, the traditional way is to use crossed figure-eight microphone capsules and was first pioneered by Michael Gerzon and Peter Craven in the 1970’s. The 8 microphone capsules in the Spatial Mic array are encoded by the Spatial Mic Converter plugin to create the spatial harmonics needed to represent a soundfield as second order. Any ambisonic soundfield representation greater than one is referred to as Higher Order Ambisonics (HOA).

When working with ambisonics, sometimes the raw capsule signals, pre-encode, are collectively called A-Format, while encoded signals are called B-Format. Ambisonics encoders, decoders and processors vary in the expected sequencing of the spatial harmonics and their encoded level (sometimes called weighting). The two most common are AmbiX and FuMa. Make sure the expected format for a given ambisonic processor matches the audio being sent to it.

Spatial Mic Converter encodes the raw capsule signals from Spatial mic to second order (9 channels) B-Format. Once in B-Format,  audio can be sent to numerous sound crafting plugins. Here are some that work with Higher Order Ambisonics:

Ambisonics may be decoded to almost any format desired (including channel-based) and stored in formats like MPEG-H. The most common decoding method for ambisonics is called binaural, which is played back on stereo headphones. When decoding to binaural a Head Related Transfer Function (HRTF) is used to approximate how human anatomy affects spatial perception. For example, how does the frequency response of a single sound source change as you rotate your head in front of it? An HRTF aims to take this into account.

Ambisonics moves us beyond what has been possible with channel based formats, enabling applications in Augmented and Virtual Reality, music production, broadcasting  and immersive environments. While ambisonics can be used in nearly any type of audio production, adoption is becoming more widespread. Here are some useful resources:

If you have any suggestions, corrections or interesting links you think would be useful to include in this article, please reach out!

 

Click here to learn about recording 2nd order Ambisonics with Spatial Mic!