Ambisonics is the technique of sound transmission (capture and reproduction) in full sphere surround.
In other words, it is meant to be a technique for reproducing a faithful three-dimensional sound image. It supports the horizontal plane (as in traditional surround techniques) but also the vertical planes (above and below).
It was invented in the 70s and as you can see in this brochure, it was pretty promising.
From the brochure NRDC Ambisonics of 1979:
"In nature, sounds come from all around our ears. Reproduced sounds come from only a few loudspeakers. Directional distortion results whenever our ears can hear the difference. As other distortions in the audio chain have been progressively lessened, so directional distortion has become more noticeable.
"The earliest widely used attempt to mitigate directional distortion is stereo, which however gives a directional illusion only over a frontal sound stage. The Ambisonic technology is the culmination of over two decades of systematic research into how directional distortion can be reduced as much as possible using any given number of audio channels and loudspeakers.
"Just as the accurate reproduction of performed music is the crucial test of audio fidelity, so the ability to reproduce correctly the directionality of natural sounds is the crucial test of a surround sound system. Unless it can do this, there will not be the correct disposition of indirect sound which provides the acoustic ambience of the performance and gives the position-dependent labelling of direct sounds by their wall reflections, which is an important aspect of the appreciation of music. "If a system can cope with this difficult task, it should go without saying that it can easily deal with the relatively simple problems of synthetic source material. A system of surround sound which is able to reproduce the directionality of indirect reverberant sounds, as well as of direct sources, is termed 'Ambisonic'"
--NRDC Ambisonics brochure, 1979.
Ambisonics Sound Capture:
The capture of sound through the ambisonics technique can look like something difficult. It involves at least 3 or 4 microphones or capsules.
In the other hand it exists tetrahedral microphone , the most wide-spread kind of gear in this field. It can result in a more accurate sound field. And it is also more easy and convenient than other types of techniques or microphones. It consists of a coincidental Four-Capsule microphone. Many manufacturer proposes their own solution. The most known are from SoundField and Core Sound (Tetramic), you can also find budget solution like the Brahma. And more recently Sennheiser got its own version with the Ambeo microphone.
Furthermore, it exists higher-order microphone (for ex: the eigenmike) but they do not get the same equalization result and in the end it can get really difficult to manage them as it involves a lot of channels.
Reproduction of the Sound:
Ambisonics differs from any other multi-channel surround formats since it doesn't convey a speaker-dependent signal.
The data that is transmitted through Ambisonics is usually done by the so-called B-Format. This format needs a decoder process in order to get the reproduction of the right sound. In addition, to be able to fully experience the power of full-sphere surround, a reproduction hardware system needs to be put in place. Very few spaces or rooms contain the system that enables such a reproduction. It sometimes implies hundreds of speakers (for example ESPRO at IRCAM has 350 speakers).
But now with the advent of Virtual Reality, head tracking systems combined with headphone speakers are able to reproduce full-sphere surround experience with no need for big devices. With their headsets, VR has a big advantage on room-based/speaker-based system. Since the technology of VR is currently a big trend with many applications and developers, more and more people become interested in this technique.
Additionally of course, it is possible to decode the Ambisonics format and adapt them to more 'classic' reproduction system, for example via virtual microphones:
Using special decoding algorithm, it is possible to virtually reproduced any setting of microphone with diverse possibility of polar patterns. Thus making it possible for the sound to be reproduced on most of the speaker-based devices : mono, stereo, surround,... But in return it obviously loses some information that ambisonics still includes. For example the free plugin Surround Zone from SoundField is pretty convenient (however SoundField has been bought by Rode recently)
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