HOA Convolution Testing

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Pan Athen
SoundFellas Crew
Posts: 78
Joined: 04 Dec 2021 20:51
Location: Athens / Greece

I'm starting this thread as a way to gather collective knowledge for everyone that's using ambisonics in production. Music, games, radio dramas, immersive podcasts, NGA production, all those benefit from the use of ambisonics on secondary layers.

Ambisonics is a great format for reverberation as even in 1st order can give a very good impression of the acoustic space and also provides full audience envelopment because the format is by nature spherical.

Algorithmic reverbs are abundant and also work in various ways, but with ambisonics, we have a good opportunity with convolution processors, especially for free as the convolution can work channel-to-channel, so it's easier to build those plugins.

There are many convolvers out there and in an effort to test them I create a test impulse response file.

You can download it from here: https://1drv.ms/u/s!AoFZ1MP3ewRggqY6Dmw ... A?e=sLiUqt

Here is a video rendering of the impulse's behavior inside the ambisonic sphere.

Some information and tips are:

- This file comes in 3rd order ambisonic format with a resolution of 96 kHz and 32-bit.
- It features an inward impulse emission from the weighted centers of the surfaces of a rhombicuboctahedron with the listener positioned in the center of the ambisonic sphere.
- The series of signals start from the front center of the horizontal plane at middle height and rotate clockwise, then moves to the upper height, then to the lower height, and in the end, one signal is emitted from the top surface and one from the bottom.
- There is also a short screencast included that shows the signal emission so that you know what behavior to expect from your convolution.

Common uses:

- Use this file to check the directivity behavior of ambisonic convolution algorithms.
- Proposed chain: Mono source --> Ambisonic encoder with positioning --> Send/insert to convolver --> Convolver with this file loaded --> Master ambisonic bus --> Decode to binaural or studio monitor channels.


- You can modify the file's resolution. Have in mind that this file contains impulse signals, so depending on the conversion algorithm, the impulse signals might change.
- Try playing a short full-spectrum sound encoded with an ambisonic positioner plugin and then stop the playback and only listen to the convolver output. Does that move the same way the IR was supposed to work? What if you change the wet/dry settings of the convolver? What happens when you change the convolver's width settings?
- This might be a 3rd order ambisonic file but you can easily try lower orders by deleting/muting the higher-order channels of the file or by using different plugin routing permission where applicable (REAPER advanced plugin routing settings).
- Also works well for testing envelopment with various binaural processors and HRTF files as it moves around and behind the listener. Don't use the impulse response to test the envelopment as the impulses are not good for humans to localize, better use that to convolve a short good sound for localization.
- Can also be used to test production pipelines for channel-based formats derived from scene-based formats (ambisonics) of compatible orders.

Happy sound explorations!

This file is licensed under the Creative Commons Attribution/Non-Commercial/No-Derivatives
© 2021 by Pan Athen and SoundFellas Immersive Audio Labs is licensed under Attribution-NonCommercial-NoDerivatives 4.0 International
Read more here: https://creativecommons.org/licenses/by-nc-nd/4.0
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