New Microphone Technologies – Spatial Microphones
Since microphones were invented in the late 1800’s, in one specific way they haven’t changed: traditional mono microphones can’t capture and preserve the spatial information associated with the sounds that they record. They discard spatial information at the time of recording. It can’t be retrieved. It is lost forever.
Traditional mono microphones may record sounds in all directions more or less equally (what we call an omnidirectional microphone), or preferentially in one direction or another (what we call a directional microphone). What’s lost is all information about where the sounds were located in relation to the recording space, and in relation to other sounds in the space.
Back in the 1970s, a new kind of microphone was developed that preserved spatials. Since it was new technology it was just a starting point, not well developed. It had poor spatial resolution and accuracy. But it worked.
It was called Ambisonics, and was developed by Michael Gerzon and Peter Craven. It built on earlier work done by Alan Blumlein for stereo recording and playback.
Ambisonics preserved spatial information using an audio storage format called B-format. Once the spatial information is captured in B-format, we can play it back, reproducing the spatial audio. Or we can process it in many different ways, to achieve many different goals.
Preserving Spatial Precision and Accuracy
A spatial microphone should ideally capture everything going on acoustically at the point in space where it is located. Instead of discarding the spatial information, it should preserve it. Once it is preserved, the spatial information can be used in many ways.
The first generation of ambisonic microphones used only four traditional mono microphone capsules. Each capsule was mounted on one face of a tetrahedron. This configuration was later called “first-order ambisonics.” It had limited spatial resolution and precision.
Core Sound’s TetraMic is an example of a first-order ambisonic microphone. It improves on the original first-order microphone by being a very well calibrated A-format microphone. (A-format is the raw audio from the four capsules.) A-format is transformed (encoded) in software to accurate B-format using precisely measured calibration information. Each TetraMic is individually calibrated.
Other current examples of first-order ambisonic microphones from well-known manufacturers are the Sennheiser Ambeo and Rode NT-SF1. Zoom makes a recorder with a built-in first-order array. From very small manufacturers there are the microphones from Brahma, Saramonic, Mini-DSP and a few others.
Over roughly the last ten years, higher-order versions of ambisonic microphones have been developed. They have much finer spatial resolution. And their software-based calibration and reproduction techniques increased spatial accuracy tremendously.
Higher-order microphones don’t look the same as the first generation first-order microphones. Instead of a simple four-face tetrahedron, they use more capsules and complex geometries. Some use capsules mounted on relatively large spheres.
Core Sound’s OctoMic is an example of a higher-order ambisonic microphone. Like TetraMic, each OctoMic is individually calibrated.
Other higher-order ambisonic microphones from larger manufacturers include those from mh acoustics and Zylia. Smaller manufactures include Harpex, Brahma and Voyage Audio.
It’s very important to note that the audio outputs of the individual capsules used in ambisonic microphones are never, ever used alone. They are only (and always) used to derive the B-format recording.
Since a spatial microphone captures and preserves all of the acoustic information in a digital audio format, a computer can then be used to model what a traditional mono microphone would have heard at that point in space. The computer processes the spatial recording to reproduce what a traditional mono microphone – having any directivity pattern and pointed in any specific direction – would have recorded.
The result is a recording as would have been made by a “virtual” microphone. “Virtual” means that no physical mono microphone was used to make the recording – it was derived in a computer from a precise and accurate spatial recording.
The virtual microphone process can be repeated to create as many virtual microphones as you desire from the original spatial recording. Each of the virtual microphones can have different (or the same) directivities, and can be pointed in any direction you please.
So a single spatial microphone can function as any coincident array of mono microphones you can imagine. Since their virtual polar patterns can be better than traditional mono microphones, and they are truly coincident in both the horizonal and vertical planes, the resulting coincident arrays perform better than coincident arrays made with traditional mono microphones. They can be among the best coincident arrays in the world.
A virtual microphone can replace any traditional mono microphone. So you can use higher-order ambisonic microphones as virtual microphones in spaced microphone arrays like ORTF, ESMA Decca Tree and others.
Ambisonics is not the only microphone/playback system that preserves spatial information. Angelo Farina developed an alternative system called Spatial PCM Sampling (SPS). It has strengths and weaknesses that are different, and perhaps complementary, to Ambisonics. There are ways to transform Ambisonics to SPS, and vice versa.