Ambisonics Is About Much More Than Reproducing Soundfields
When people think about using an ambisonic microphone, they usually think of the one classical application for ambisonic microphones that's been used since the 1970s. That's using one ambisonic microphone to record everything going on acoustically at its position. Then we decode the recording to a speaker array (or headphones) for playback, reproducing the soundfield.
In recent years, "ambisonic recording" has broadened to include three more applications.
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First, let's state a simple definition of ambisonic recording:
- Ambisonic recording is the process of capturing everything going on acoustically at a point-in-space in a way that preserves the spatial information. The spatial information is stored in a specific format.
The format is called "B-format." B-format allows us to determine where sounds are coming from.
B-format can provide different amounts of spatial resolution. The more resolution the description has in capturing the spatial information, the higher its "order." For example, second-order ambisonic recording has more information about where sounds are coming from than first-order - it has greater spatial resolution.
First-order has greater resolution than zero-order. ("Zero-order" is an omnidirectional description, with no spatial information). Zero-order B-format uses one channel to store its spatial information. First-order B-format uses four channels. Second-order B-format uses nine channels. Higher orders use even more channels
Now back to our original topic: the three other applications of ambisonic recordings.
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The second application is to use a single ambisonic microphone as a single "virtual microphone."
What's a "virtual microphone"?
Since an ambisonic recording ideally captures everything going on acoustically at a point-in-space, we can use that information to model - in a computer - how any imaginary microphone would record that acoustic event. That imaginary microphone could have any conceivable pickup pattern. That imaginary modelled microphone is called a "virtual microphone."
The commonly used mono microphones we're all familiar with have a limited range of pickup patterns: omnidirectional, sub-cardioid, cardioid, super-cardioid, hyper-cardioid and figure-eight. Those are called "first-order directivity patterns."
A first-order ambisonic microphone has the spatial resolution to model any or all of the first-order directivity patterns. So a first-order ambisonic microphone can function as a virtual cardioid microphone. Or a virtual super-cardioid microphone. Or a virtual figure-eight microphone. All at the same time, from a single recording.
A second- (or higher-) order ambisonic microphone can model those common mono microphone pickup patterns even better than the best mono microphone. That's because they have much finer spatial resolution.
If you wanted even more spatial resolution than common mono microphones, and if your ambisonic microphone could capture that level of spatial detail, you could create virtual microphones with much more spatial resolution than the mono microphones we're so familiar with. These "second-order" (or "higher-order") ambisonic microphones could have tighter directivity and almost no backlobes. They would pick up sounds pretty much only in the directions we want, and almost nothing in the directions we don't want. They could be asymmetrical if we wanted. And they could potentially reject sounds coming from any desired angle.
So higher-order ambisonics allow us to synthesize types of microphones that we've never had before. It's very difficult to build them physically, so you'll never be able to buy a well-behaved second-order mono microphone. But it's pretty much trivial to make one (or as many as you want, each simultaneously pointed in different directions) - truly excellent ones - using higher-order ambisonics.
Using an ambisonic microphone as a virtual microphone has another very interesting application. Since an ambisonic microphone can be used as a single virtual mono microphone, we can use a bunch of them in spaced arrays. (Examples of common spaced arrays are ORTF, ORTF-3D, Decca Tree, ESMA, Hamasaki square and cube, Fukada tree.) Since a well-designed-and-calibrated higher-order ambisonic microphone can have more stable directivity patterns and more extended frequency responses than traditional mono microphones, the spaced arrays perform better. And with higher-order ambisonic microphones placed around a space, you can interpolate between their output for 6-degrees of freedom walkarounds in Virtual Reality.
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The third application of "ambisonic recording" is an elaboration of the second one: we can use a single ambisonic microphone as a coincident array of virtual microphones. It's not necessarily intended to recreate a complete soundfield.
Just as mono microphones can be used in coincident arrays for stereo recording (e.g., Blumlein, X-Y), so can ambisonic virtual microphones. But where each of the mono microphones in a coincident array takes up real space and so can't create a truly coincident array (because the microphones are displaced in space from the array's theoretical center), ambisonic virtual microphones are truly coincident. For an ambisonic virtual microphone, the displacement of its capsules from the central point-in-space is mathematically compensated, so we end up with a truly coincident array. This results in much better performance than an array made with mono microphones.
So, for example, the world's finest Blumlein array uses a single higher-order ambisonic microphone (e.g., a Core Sound OctoMic). Its two orthogonal figure-eight patterns stay consistent across the frequency band, and the nulls stay deep and true.
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And finally the fourth application is not about live recording at all, but rather the creation of spatial recordings in the studio using audio editing software, to be played back over speaker arrays or headphones. The original audio sources for the recording might be live recordings made with ambisonic microphones. Or they could use live recordings made with mono microphones and inserted into an ambisonic editing framework. Or they could be sounds created artificially - not live recordings.
What makes this fourth application "ambisonic" is that all of the sound sources end up in B-format, and can be decoded for spatial playback using the ambisonic process. The result when played back over speakers is potentially an accurate soundfield in space. And when played back over headphones, we hear that soundfield presented binaurally.
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Summarizing, there are currently four applications of ambisonic recording:
1. Recording with a single ambisonic microphone and playing back over speakers or headphones to re-establish the recorded soundfield
2. Using an ambisonic microphone as a virtual microphone - alone or in spaced arrays
3. Using an ambisonic microphone as a coincident array of virtual microphones, often for stereo or binaural playback
4. Creating spatial recordings in the studio using B-format
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Disclaimer: I own Core Sound, a manufacturer of ambisonic and binaural microphones.