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Ambisonic Microphones

ajocular
Honored Guest
I'd love to hear people weigh in on ambisonics. I'm looking to pick up an ambisonic mic pretty soon, but I'd like to know who else is using them for VR already and what you think about them vs. other gear.

My background with 3D audio is more on the omni-binaural side, but lately I've been feeling like I want to capture the sound field neutrally in addition to binaurally. Ambisonic mics make it easier to isolate elements from the sound field and play with them in post.

With binaural, you want to leave the track alone as much as possible, but maybe sometimes I want more verb or a flanger or whatever, and maybe I want it on one object but not the rest of the field.

Anybody have recommendations for the best ambisonic mics out there?

My primary concern is this: if ambisonic mixes aren't panned correctly, they can really gum up spatialization in post. Ambisonics are excellent at isolating unique elements of the sound field, but they're not as good at direction cues as binaural, so I think a lot of people will try to mix the two, and that's where you run into trouble. There are ways to avoid the pitfalls, and I hope we as a community can all help each other get up to speed on the strengths and weaknesses.

I'm worried that if ambisonics become super popular in VR, it will lead to blurry spatialization in lots of VR experiences (unless we heed the pitfalls). Ambisonics are a fascinating concept - you can position the sound anywhere within the volume of a sphere around your head. In VR though, we'll want to stay on the surface of that sphere. You have to crank your X,Y, and Z channels all the way to one side (whichever side you want to isolate).

That limits the versatility of the format because it prevents the mic from providing any of the distance cues it is famous for. It's the graphics equivalent of only being allowed to render at infinite. However, it's the only way to get a binaural plugin properly into the mix later in the chain. Those distance cues are virtually simulated by the hardware anyway, so you may as well virtually simulate them using any number of other methods later in the chain, and your results should be just as good.

Anyone have experience with this? It's hard to find people who are breaking ground here.
21 REPLIES 21

Anonymous
Not applicable
Hi AJ,

You already know my attitude about ambisonics vs. omni-binaural. First off, you'll want to join the sursound mailing list if you're not already a member. That's where the internet's foremost ambisonics experts chat.

My personal feeling is you are over-thinking the problem. As others have stated, object sound rendered in real-time will yield the best possible results in a game situation when you're concerned with localizing point sources. Ambisonics provides the best possible solution for ambient soundfield capture, and provides the benefit of allowing for customized HRTF processing at playback. In a game, I think an ambisonic environmental field recording would make a fantastic complement to object sound. For VR video of course the field recording is a 1-to-1 analog to the panoramic array, but for high-end productions we are using Dolby Atmos via their new Dolby VR technology.

At Jaunt we use CoreSound TetraMics. They are inexpensive and made with obsessive attention to detail, sourcing excellent components and undergoing rigorous capsule matching & calibration. Paired with a Tascam DR-680 the whole setup costs around $2k.

An Eigenmike will set you back at least $22k. BTW the Eigenmike is not an A-format soundfield microphone. True ambisonics arrays either natively capture spherical harmonics (such as the Nimbus/Halliday configuration), or consist of a regular polyhedron of which the tetrahedral variety (first order) is the only possible solution. MH Acoustics does however provide an Eigenmike-to-b-format (third-order) conversion, but band-limited at 8kHz due to spatial aliasing caused by the spacing between capsules. It is true that directional cues originate predominantly from lower frequencies, so a solution involving mixed-order ambisonics is probably the way to go when using the Eigenmike.

Due to our large volume of content we have developed a proprietary b-format processing pipeline for asset management, a-to-b conversion, and playback. There are off-the-shelf tools available for all of these things, however, including plugins from Blue Ripple Sound, VVAudio, and Harpex. These are best used in the Reaper DAW due to its flexible channel handling.

If you really want to understand ambisonics you'll be well-served reading Aaron Heller's papers.

ajocular
Honored Guest
Reading Heller's work now. I appreciate the link.

I'm wondering which part I'm overthinking, though. I think that if two different methods of audio localization (ambisonics and HRTFs) inherently prevent each other from working as intended under certain conditions when both are employed within the same signal, we should all be careful to watch out for those conditions. I think there's no such thing as overthinking it. If you do it wrong, you can't hear the cues as well. One reason I want to get my hands on an ambisonic mic is so that we can have some objective examples that demonstrate this. I think we should think about it as much as necessary to ensure the cues are as clear as possible.

Omni binaural is a whole other can of worms. Y'all will be seeing a new thread on that topic before too long.

ajocular
Honored Guest
Also, I had a long conversation with Marc from Dolby at GDC, and he explained their approach for Dolby VR in detail. Not sure if he intended any part of that conversation to stay between us, so I'll try to be as diplomatic as possible.

In general, I got the impression that the folks at Dolby won't be chiming in on this forum any time soon because their VR solution is intended as exclusively available to established cinema production companies. They clearly had no desire to get into any pissing contests with competitors over fidelity or performance, which means lowly developers like myself are not allowed to objectively measure their solution against any other options. Curious.

I told Marc I'll try to refrain from disparaging their contribution to the VR community until I have a chance to see how they measure up, but it seems to me that they wouldn't have anything to hide if their solution really was tops. Their brand is often seen as "the best there is" in audio, and I can't help feeling like that brand is becoming too easy to hide behind. Exclusive back-room deals only help to propagate the "he-said she-said" nonsense that is so prevalent in pro audio. Most of this stuff is objectively measurable, and the quality differences are easily perceptible to laypeople.

jlangford
Honored Guest
I'm very much interested in this also. Within the next year, I'll be doing sound design on an open cockpit vehicle simulator and am already planning on placing a tetramic in the location of the virtual driver's head as part of the microphone array when I go to record.

On the VR side, what is the technical feasibility of transcoding from b-format to binaural in real time to allow dynamic "panning" of the 3D sound via HMD rotational input? Are there any plans to support anything like this?

henkSPOOK
Explorer
"jlangford" wrote:
On the VR side, what is the technical feasibility of transcoding from b-format to binaural in real time to allow dynamic "panning" of the 3D sound via HMD rotational input? Are there any plans to support anything like this?


--> You can use 3Dception as a unity plugin for this. First turn your B-format recording into two equal length stereo tracks (W&X) and (Y&Z) which can be easily done with audacity for example, or any other DAW that supports multi channel audio files. Then you load them into the 3Dception AmbiArray Component. If you use a standard asset first person character controller and freeze its x y and z position (so it cannot move around but only look around) you have it set up in no time. Works like a charm. You can then also add sound design elements as mono sources. I have done some test and this works really really well. Drawback is of course that this is only suitable for when you have a character that does not move around, like with 360 film.

jlangford
Honored Guest
"henkSPOOK" wrote:
--> You can use 3Dception as a unity plugin for this. First turn your B-format recording into two equal length stereo tracks (W&X) and (Y&Z) which can be easily done with audacity for example, or any other DAW that supports multi channel audio files. Then you load them into the 3Dception AmbiArray Component. If you use a standard asset first person character controller and freeze its x y and z position (so it cannot move around but only look around) you have it set up in no time. Works like a charm. You can then also add sound design elements as mono sources. I have done some test and this works really really well. Drawback is of course that this is only suitable for when you have a character that does not move around, like with 360 film.

I was actually not expecting this to be a thing already! Great, I will do some test recordings and experiment. Thank you.

Regarding the spatial movement issue, you are right - traditionally I would not think of using this technique for nearfield sound, although I think in this particular scenario all will be ok; there will of course be scope in the simulation for the driver to lean around in the seat to some extent, but in the real world, this type of engine has so much aural volume/presence, such small positional changes *should* have a negligable effect in all but the most extreme cases. That is the theory anyway! Will see how it plays out in practice. If it doesn't work, hey - the b-format recording will be a great base for a fixed speaker surround mix 🙂

ajocular
Honored Guest
TwoBigEars has an ambi input? That's pretty cool. I didn't know that.

Though it's a pain, you can mix any component signal from your ambi tracks down to mono and then feed the output into any 3D audio plugin for binaural rendering (I covered some of the rigors and pitfalls of that above). This is what the Jaunt guys are doing, if I remember right. Not sure what binaural plugin they use, but my personal favorite is the AstoundSound plugin. TwoBigEars is cool too.

There is no restriction on motion when you do it this way, as long as you track the component motion from your ambi mix appropriately (which of course can be quite difficult if the source was moving quickly). Alternatively, you could just point a bunch of binaural instances of the field equidistantly in all directions. The more you do, the better it'll sound, but it'll render in 3D better if you're able to follow the exact motion of the source in the mixdown, prior to feeding it into the DSP. Also, you should not do that with more than 6 instances if you're running on a mobile processor.

I hope someone automates tracking sources and syncing them to video one day for ambisonic mixes, but that's a pie-in-the-sky dream on my wishlist. It would make our lives much easier in post. I've actually spoken with several of the 360 video gurus about this, but most of them have enough battles on their hands with visuals alone right now.

When I really want to capture all of the unique characteristics of a live physical environment, I often opt for my omni-binaural mic, especially in situations where I'm in control of the sound field (i.e. I can isolate each source). This is also the highest performance option by a factor of 10, so if you need LOTs of instances to render simultaneously, accept no substitute. If you're on Unity3D, you can use my VRSFX plugin to drop in omni-binaural output such that it will automatically track dynamically when you put it in motion digitally. I think my plugin is the only way you can do this with binaural hardware, so if you're using Unreal, I'm sorry. Still haven't had time to port it.

If for some reason I preferred to use a DSP plugin instead, I'd probably still wouldn't capture with an ambisonic mic. I'd just use a dynamic mic unless the isolated source couldn't be successfully tracked for some reason. The tetra is great for full-field, but that's really the only good use of it. At the end of the day, it's a condenser array that acts like a good chameleon of other patterns. You'd be using it as a dynamic wannabe. I'd just use a dynamic instead if you can. If the field is out of your control (i.e. film shoot with dialog and source SFX), go tetra if you need to do a lot of post isolation, filtering, etc. In studio, there are better options.

If you go dynamic, as with omni-binaural, you can avoid B-format. Your mono output is ready to go straight off the mic and into your binaural rendering plugin of choice.

JaeT
Honored Guest
"ajocular" wrote:
I'd love to hear people weigh in on ambisonics. I'm looking to pick up an ambisonic mic pretty soon, but I'd like to know who else is using them for VR already and what you think about them vs. other gear.

My background with 3D audio is more on the omni-binaural side, but lately I've been feeling like I want to capture the sound field neutrally in addition to binaurally. Ambisonic mics make it easier to isolate elements from the sound field and play with them in post.

With binaural, you want to leave the track alone as much as possible, but maybe sometimes I want more verb or a flanger or whatever, and maybe I want it on one object but not the rest of the field.

Anybody have recommendations for the best ambisonic mics out there?

My primary concern is this: if ambisonic mixes aren't panned correctly, they can really gum up spatialization in post. Ambisonics are excellent at isolating unique elements of the sound field, but they're not as good at direction cues as binaural, so I think a lot of people will try to mix the two, and that's where you run into trouble. There are ways to avoid the pitfalls, and I hope we as a community can all help each other get up to speed on the strengths and weaknesses.

I'm worried that if ambisonics become super popular in VR, it will lead to blurry spatialization in lots of VR experiences (unless we heed the pitfalls). Ambisonics are a fascinating concept - you can position the sound anywhere within the volume of a sphere around your head. In VR though, we'll want to stay on the surface of that sphere. You have to crank your X,Y, and Z channels all the way to one side (whichever side you want to isolate).

That limits the versatility of the format because it prevents the mic from providing any of the distance cues it is famous for. It's the graphics equivalent of only being allowed to render at infinite. However, it's the only way to get a binaural plugin properly into the mix later in the chain. Those distance cues are virtually simulated by the hardware anyway, so you may as well virtually simulate them using any number of other methods later in the chain, and your results should be just as good.

Anyone have experience with this? It's hard to find people who are breaking ground here.


@ajocular : I have just created an account with the hope of contacting you in regards to your posts on this topic via a private message but the forum isn't letting me do so... If you wouldn't mind, could you please message me or reply to this post with another form of contact, I would really appreciate a moment of your time.

Many thanks,

J

nosys70
Expert Protege
as far as the subject is gaming, i do not see the relation with recording, since most gaming sound is issued from prerecorded sound (mostly basic sounds like explosions, squeeks and laser guns shots)
unless you are creating a game that is heavily relying on real life simulation, i doubt you need to fiddle with 3D sound at this level.

BonzoDog
Honored Guest

ajocular said:

I'd love to hear people weigh in on ambisonics. I'm looking to pick up an ambisonic mic pretty soon, but I'd like to know who else is using them for VR already and what you think about them vs. other gear.


Anybody have recommendations for the best ambisonic mics out there?



I don't think there is a "best" ambisonic mic.

I have rented both  the Soundfield MK5 and the Core TetraMic from Audio Rents in Burbank CA.

The MK5 is a great studio mic, with large diameter condenser capsules, very flat, very low noise, but it's a large mic, and has all the problems that go along with that.

The Core TetraMic has small capsule electret capsules, so it has a higher noise floor, and it isn't as flat.  But it's great for field work.

So, if I am going for audiophile quality in a studio, I reach for the MK5.  If I am in the field and recording ambiances or effects, I grab the TetraMic.