Understanding Ambisonics

Ambisonics is a surround sound technique that was developed in the 1970’s which enables full 3D soundfield reproduction, extending from Blumleim’s Mid-Side technique.

In this technique, four channels of information are used to convey all the information needed to produce the full soundfield over any number of loudspeakers. This differs from conventional stereo or surround sound, which work on a one-channel-per-speaker basis. The four channel signal, also known as B-format, relates to directionality and can be manipulated through a decoder, specific to your speaker setup, to recreate the desired soundfield.

In its simplest form, first order Ambisonics (FOA) include three figure-eight microphone patterns relating to left-right, up-down and foward-back pairs, plus an omnidirectional component. These four components can be summed to make virtual microphones with a any polar pattern, aimed at any direction, meaning the microphone pattern can essentially be chosen in post-production.

The spatial resolution in FOA is quite low, however we can add more components to the signal with a decoder that splits the signal up further using spherical harmonics, which leads to what we call higher order Ambisonics (HOA).

HOA enables more spatial information to be conveyed and requires more speakers to accurately represent the signal. Below, you can see the spherical harmonics from 0th to 5th order (each line represents the order 0-5). For example, 5th order ambisonics splits the signal into 36 components, therefore 36 loudspeakers are required to accurately reproduce the signal.

Spherical_Harmonics_deg5
(image: https://commons.wikimedia.org/wiki/File:Spherical_Harmonics_deg5.png)

 

One of the positives to this method is the source file can be played back over any loudspeaker array, and only a new decoder is needed to up- (or down) scale the audio, without losing any information. Furthermore, this technique can be transferred to the binaural setting by replacing the speakers during playback with HRTFs over headphones, meaning a virtual speaker array is created.

One of the main downsides to Ambisonics for loudspeaker playback is the need for sophisticated speaker arrays to fully take advantage of the technique. Like other surround sound techniques, there is also a ‘sweet spot,’ in which the audio is played back correctly inside, but not outside. Increasing the order improves the size of this sweet spot, however it comes at the expense of requiring additional speakers. This again is negated by using binaural based Ambisonics, as the listener is always within the sweet spot.

 

 

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