In a stereo mix, if sounds are simply moving, this alone can be enough to stimulate our brains to stay engaged. Ambisonics, first developed in the 1970s, has had a recent resurgence due to the possibilites it offers in immersive audio for Virtual Reality. Ambisonics allow us to create a 360 degree sound field that can be decoded into a number of different formats
Most of the time when recording, we typically use two mics, forming whats known as an X-Y configuration. When played back, one mic goes to the left channel, while the other goes to the right creating whats known as stereo playback.
Yet we can also record in stereo using other methods such as Mid-side recording. In this technique a cardioid microphone (top) is recording the middle sound while the other mic (Bi directional) records both sides in a figure of 8 pattern. Since one mic is recording the centre space and the other is recording the sides, it would not be aurally viable to just assign them to the left and right speaker as we would with stereo recordings. As a result, the signals need to be added together and decoded. This is also the case for Ambisonic recordings.
With the mid side technique, two microphones create one dimension of audio – left and right. For ambisonic recording all we need now in addition to left and right is another bi directional microphone for front and back and another for up and down, giving us a 3D recording.
Most ambisonic recorders, such as the H3-VR, don’t use bi directional microphones. Instead they have 4 cardioid microphones set up in a tetrahedral mic array. This creates 4 audio tracks that we combine together to create a 3d sonic image .
The raw recording from one of these tetrahedral microphones comes in the form of a file known as the A-Format. These files are 4 channel audio files that contain the input of each mic in the tetrahedral array.
With the ambisonic tetrahedral array, the mics are separated into mid, side, up/down and front/back. More specifically they are normally labelled W for the centre omni-directional channel, X for the bi-directional front and back, Y for the left/ right and Z for the up/down.
Moreover, another file format known as B-format ambisonics, are essentially A-format files that have gone through one stage of conversion. There are two types of B-format files, namely AmbiX and FuMa. They order the channels in differeing ways and have different relative amplitudes so it is important to inspect the software or programme you intend to use the ambisonic files with, and understand what format is appropriate. One can convert AmbiX, FuMa and various other file formats between each other using softwares such as the Zoom Ambisonics Player or the downloadable Ambeo Orbit conversion tool.
Recording with first the tetrahedral mic array is known as first order Ambisonics. When more than four mics and audio channels are used, the resulting recording is known as higher order ambisonics. The additional tracks help create a fuller 360 degree sphere of sound, filling in the diagonal dead spots between the microphones.
Furthermore, ambisonic files must be decoded for playback. The great thing about ambisonic files is that they’re not channel dependent. This means that an ambisonic file can be decoded into any number of speaker configurations ( stereo or quad for e.g.)
Ambisonics offers us an intuitive way to of recording and hearing the sounds around us in a very realistic fashion. It use in immersive and virtual environments may hold answers to some of the future and contemporary audio challenges.
After going on a sound-walk with the H3-VR, I was able to familiarise myself with many elements of ambisonic recordings, such as the mics test tones and the various formats in which I could record in. I also experimented by recording certain sounds using different mic positions, specifically front facing, upside and end fire, in order to get a feel of how the recordings respective stereo images would differ when played back.