Old 05-24-2021, 03:23 PM   #1
Jensus
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Default Ambisonic IR

Hi, I am trying to figure out how to create the best and easiest ambisonic impulse responses for software like ambi verb and wwise convolution.
I would like to
1. record IR for a specific place in the sphere wehere I will pan my monophonic sound source.
2. record IR to be used more freely across the whole sphere.
Should I for no. 1 place the speaker with the white noise/sine sweep at the position I want to place the virtual sound source and record it there?
For no. 2 record several IR around the location and combine them? Or fx recording it from a 2 m distance and use that for the entire sphere?
Does anyone have experience with this?
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Old 05-25-2021, 09:55 AM   #2
Kewl
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If a "true stereo" IR reverb is four IRs (2*2 matrix), a "true 1st order" IR reverb would be 16 IRs (4*4 matrix). It's been on my "to experiment list" for the past ten years...

https://www.avosound.com/en/tutorial...ono-and-stereo
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Old 05-25-2021, 11:02 AM   #3
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Quote:
Originally Posted by Kewl View Post
If a "true stereo" IR reverb is four IRs (2*2 matrix), a "true 1st order" IR reverb would be 16 IRs (4*4 matrix). It's been on my "to experiment list" for the past ten years...

https://www.avosound.com/en/tutorial...ono-and-stereo
Would it not simply be 16 independent IR processes for first order and not 16 different channels in the IR itself? Or wouldn't it actually be 12 since the W channel doesn't need to be convolved independently for each axis but rather as a matrix of the x/y/z signals? A stereo impulse is still only 2 channels, the true stereo part is to convolve the left channel and right channel independently through that stereo impulse. This is then mixed from the the now 4 channel result back down to stereo. I was just thinking, when I take an impulse with my first order mic I don't need to do it 4 times from 4 locations to get a true ambisonic IR. Perhaps that's what you meant all along though? I'm just trying to get it straight in my mind.

Anyways, for number 1 you do want to place the speaker at the location you want the emitter at and the mic in the spot you want the listener to be located. In my understanding, if you move that emitter around the listener it will be like rotating the entire room as you can't virtually move that speaker after the fact.

However...Zylia just did a very interesting demo of synthesizing multiple ambisonic recording positions in a room. I assume the same idea could be used to synthesize multiple ambisonic IRs, especially since they used wwise and unity to do it. That is my answer to question 2.

It is labor and processor intensive so make sure it's going to pay off in the end.
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Old 05-27-2021, 03:03 AM   #4
Jensus
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My main goal is to do a 360 video and audio recording and then ad sound sources with a reverb that is coherent with the space.
Knowing that I can make an ambisonic IR of the speaker placed at the right position of the panned mono source, must be my primary focus after getting into the complexity of this problem.
Ambi Verb and Wwise Convolution lets you import a 4 channel ambisonic IR recording. Regarding panning a sound source in the entire sphere it seems to be the course of action to use one IR and trust that the localization information from the direct sound source will overshadow the mismatch with the spatial reverb.
AudioEase is offering a set of ambisonic reverbs in their 360 pan suite, but I just thought it would be great to enhance the realism with the correct reverb information.
I have also heard of examples where you can re-render the IR using a decoder plugin and then do a soundfield rotation and process the next mono sound source at the new position. And that some are blurring the localization of the direct sound if it clashes with the reverb.
Is it possible at all to create a 16 channel IR file and use it with a 360 panner in any software today?

Last edited by Jensus; 05-27-2021 at 06:25 AM.
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Old 05-27-2021, 05:16 AM   #5
jm duchenne
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How to create the 16 channels IR file I don't know, but to process it you can use X-MCFX Volver or the MConvolutionEZ from MeldaProduction, both free :
http://www.angelofarina.it/X-MCFX.htm
https://www.meldaproduction.com/MFreeFXBundle
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Old 05-27-2021, 08:02 AM   #6
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Quote:
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Is it possible at all to create a 16 channel IR file and use it with a 360 panner in any software today?
A bit of work, but, yes.

For the IR capture and processing, I would use Logic Pro's Impulse Response Utility with the "Quadraphonic" preset for an A-Format mic or "Quadraphonic B-Format encoded" preset for a B-Format microphone. Both have 16 IRs.

For the convolution with the IRs, I would probably use X-Volver Essential. Signal flow:

For A-Format microphone IRs:
B-Format -> Decode to position of emitters -> A-Format IRs convolution -> B-Format encoding

B-Format microphone IRs:
B-Format -> Decode to position of emitters -> B-Format IRs convolution
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Old 05-27-2021, 06:43 PM   #7
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The problem you are going to continually run into in what you describe is that the sound source position of the IR determines the localization of the IR. The relationship between the impulse (speaker, acetylene balloon, clapper) and the ambisonic microphone is fixed when you record that IR. You can rotate it but you can't really move the speaker around after the fact so to speak. You could potentially use the information learned from the IR to synthesize the properties of the room and create a reverb model (using something like CATT) that would allow you to freely position sources in that room. It's an interesting question, is there an IR reverb that models in that fashion. Some of the shoebox options (like the one in IEM) offer something like this? Dear VR Pro allows for free position within certain hard-coded (not user created) spaces.

Easiest would be to get a good ambisonic IR of the space and then tweak that to simulate your different positions, or just make several IRs with the source in the likely places you'll want to put your instruments.

As an aside, I'm loving the melda MconvolutionEZ for its simplicity and non-crashiness (for me anyways) in comparison to xvolvler.

I use gratisvolver to create my impulses as I am not on a Mac.
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Old 05-28-2021, 02:19 AM   #8
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Yes, it will take some trial and error to find a sufficient solution. Perhaps some situations can be handled with simple tools and others requires more precision. Great ideas on where to dive in.
I will primarily record outside for the soundscapes I am working with so it is also important it is relatively easy to handle in that situation.

Just an idea.
- I have seen people using the possibility of summing the IR's. This I guess would make the reverb less directional and cover the IR's of a larger area.
https://www.openair.hosted.york.ac.uk/?page_id=483

- In the Ambi Verb tutorial it says you should record 1 IR 2m from the emitter. Ex. Speaker on stage and microphone 2m from it in the audience. This could be a bit low resulotion imo.
The sweep file is only 7 sec, but otherwise I am thinking of sending a white noise signal from a recorder directly into the speaker and then pressing stop, to get a broadband IR signal.
https://www.noisemakers.fr/faq/#1524...-004522eb-722b

- If I record 4 IR's (n,s,e,w) in the horizontal plane around the ambisonic microphone at a 2m distance and sum them. The convolution would then be a panned mono signal with an ambisonic IR.

Would that give the reverb a rough directivity during a convolution in accordance to this logic https://www.avosound.com/en/tutorial...no-and-stereo?
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Old 05-28-2021, 03:07 AM   #9
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Quote:
Originally Posted by Jensus View Post
If I record 4 IR's (n,s,e,w) in the horizontal plane around the ambisonic microphone at a 2m distance and sum them. The convolution would then be a panned mono signal with an ambisonic IR.
If it's horizontal only, you can capture from three positions.
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Old 05-28-2021, 05:40 AM   #10
Jensus
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Ok, equally distributed from the front?
I will be looking into your description on Logic Pro's Impulse Response Utility Kewl.

FYI documentation on the Wwise convolution can be seen here:
https://www.audiokinetic.com/library...reverb_plug_in

and

https://www.audiokinetic.com/learn/videos/H50NRzZnd5k/
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Old 05-28-2021, 10:37 AM   #11
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Quote:
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Ok, equally distributed from the front?
I will be looking into your description on Logic Pro's Impulse Response Utility Kewl.
120° from one another. It could be 0, 120, 240 or 60, 180, 300.
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Old 05-29-2021, 10:29 AM   #12
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some notes i just read in the sparta matrix convolver source code:


Example 1, spatial reverberation: if you have a B-Format/Ambisonic room impulse response (RIR), you may convolve it with a monophonic input signal and the output will exhibit (much of) the spatial characteristics of the measured room. Simply load this Ambisonic RIR into the plug-in and set the number of input channels to 1. You may then decode the resulting Ambisonic output to your loudspeaker array (e.g. using sparta_ambiDEC) or to headphones (e.g. using sparta_ambiBIN). However, please note that the limitations of lower-order Ambisonics for signals (namely, colouration and poor spatial accuracy) will also be present with lower-order Ambisonic RIRs; at least, when applied in this manner. Consider referring to Example 3, for a more spatially accurate method of reproducing the spatial characteristics of rooms captured as Ambisonic RIRs.

Example 3, more advanced spatial reverberation: if you have a monophonic recording and you wish to reproduce it as if it were in your favourite concert hall, first measure a B-Format/Ambisonic room impulse response (RIR) of the hall, and then convert this Ambisonic RIR to your loudspeaker set-up using HOSIRR. Then load the resulting rendered loudspeaker array RIR into the plug-in and set the number of input channels to 1. Note it is recommended to use HOSIRR (which is a parametric renderer), to convert your B-Format/Ambisonic IRs into arbitrary loudspeaker array IRs as the resulting convolved output will generally be more spatially accurate when compared to linear (non-parametric) Ambisonic decoding; as described by Example 1"
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Old 08-17-2021, 03:48 AM   #13
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Quote:
Originally Posted by Jensus View Post
The sweep file is only 7 sec, but otherwise I am thinking of sending a white noise signal from a recorder directly into the speaker and then pressing stop, to get a broadband IR signal.
Hi! I'm loving this thread.
I have not tried this yet, but Wave Arts offers a completely free and cross-platform application for making IT files using white noise to capture a broadband response. I think its manual said the number of channels it can handle are only limited by your the channel count on your interface. They also co-created the excellent freeware true stereo Convology XT convolution reverb plug-in, which I do use.
Pick your platform from the popup. MIs Tool is the name of the IR capture app...
https://wavearts.com/downloads/

In addition, you'll find the manual for Apple's Impulse Response tool contains numerous graphical, tried-and-true suggestions for multi-speaker, multi-microphone, and single mic and speaker capture methods. I bet something in there can be adapted to your needs.

https://tinyurl.com/AppleIR

I hope this is if use. Stay well!

Cheers,
Glenn in Rochester, NY, USA
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