February 7, 2023 at 7:37 am #4751
As a manufacturer of new technology microphones, I’ve got the challenging task of educating about new (and potentially better) ways of doing things that audio engineers have done for years and years.
It’s very challenging, because people have spent big chunks of their lifetimes learning how to use the older technologies to get good results. They’ve invested lots of time and lots of money to get to where they are. They’re rightly reluctant to learn about new technologies and processes that might – in the end – be simpler and much more flexible, and that may yield better sonic results.
First-order ambisonic microphones date back to the 1970s. While that technology is still useful, it’s very limited. They are the first of what could be considered to be the larger class of “spatial microphones.”
Higher-order ambisonic microphones are new technology. What they can do and how they do it can change the world of recording. They are true spatial microphones.
They don’t work in the way that traditional mono microphones work. They are fundamentally different.
If folks here are interested, I’d like this forum topic to address how they work and what they can do.
- This topic was modified 1 month, 3 weeks ago by Len Moskowitz.
February 7, 2023 at 4:42 pm #4770Bob OlhssonModerator
I’d love to hear more. The best “immersive” experience I’ve ever had was with ambisonic recordings played on one pair of omni-directional speakers.
February 9, 2023 at 10:45 pm #4777
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.
February 12, 2023 at 8:34 pm #4855
So what’s fundamentally different about spatial microphones?
They capture and preserve everything going on acoustically at the point they are positioned. They record the entire soundfield.
What you do with that very rich spatial recording later, in post-production, is up to you.
You can decode it into a simple mono recording, with a single virtual microphone pointed in any direction you please, and with any directivity you want. It can have any polar pattern ranging along the first-order continuum from omnidirectional, to sub-cardioid, to cardioid, to super-cardioid, to hyper-cardioid, to figure-8. Or higher-order polar patterns that traditional mono mics can’t offer, including tighter polar patterns and smaller back and sidelobes than traditional mono shotgun microphones.
Or you can decode the spatial recording to any number of coincident virtual microphones, each pointed in any direction, and having any polar pattern you want. You can select two crossed figure-8s for Blumlein and rotate the array to point in any direction. Or two cardioids for XY. Or three cardioids for LCR, and adjust the spacing angle. Or 5 cardioids or sub-cardioids for 5.1 playback. Or 7 of whatever polar patterns you want for 7.1. Or 11 for Atmos 7.1.4, including the four positioned above. Or three rings of 6 or 8 or 12 microphones for fully immersive playback. Or two omni microphones modified by personal HRTFs and driven by a headtracker for VR360 headset binaural playback. Or as a tight-pattern boom mic that automatically follows a sound source moving in space.
All of those options (and more) are available from a single spatial recording.
All of those options are available in post-production.
From a single high-resolution/high accuracy spatial recording.
And the same range of options are available for spaced microphone arrays. So an ORTF-3D array that required 8 traditional mono microphones and compromises in the microphone spacing and angles, takes only four spatial microphones. The four spatial microphones provide exactly correct 110-degree angles and 17 cm spacings for all eight ORTF arrays, and each of the microphones acts as four virtual microphones.
And it’s all selectable in post-production, offering flexibility unavailable with traditional mono microphones.
February 12, 2023 at 9:28 pm #4856
How can you tell a good spatial microphone from a not-so-good one?
The simplest way is to look at the decoded virtual microphone polar pattern graphs. If you decode a microphone’s spatial recordings to, for example, a virtual mono cardioid, how good are its polar patterns? How deep is its rear null? 20 dB? 30 dB? 40 dB? How consistent are the patterns over frequency and angle? At what frequency does the polar pattern start to break down? How quickly and how badly does it break down?
Look at some other polar patterns, including higher-order polar patterns. See if the performance holds up or not.
Some spatial microphones are as good or better than the world’s finest mono microphones. Some are not.
February 13, 2023 at 1:57 pm #4870JasonHillerParticipant
This all sounds very interesting to me, even though I record mostly rock n’ roll, it seems like it could be fun, even in just plain old stereo. Might be a bit tough to use my Ampex 8 track, but that’s ok I guess. 🙂
February 13, 2023 at 4:31 pm #4872
If you can hold the gain variation across the eight channels of pre-amps and Ampex to +/- 0.1 dB, it could be used.
Best of luck!
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