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I have an ambisonic multitrack with several spatialized sound effects (I'm using 1st order ambisonics). However, I'd like to add some mono sources (e.g. narration) to my project. My initial thought was just to add the mono source to each channel with the same amplitude, but that doesn't seem reasonable when I see how the XYZW channels are decoded into stereo.

How should I add mono sources to an ambisonic project?

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    $\begingroup$ Even though I have had conversations with Michael Gerzon (back in the 1990s), I never learned exactly what Ambisonics does that's different from using HRTFs to locate sounds. Dunno how they do reverberation and localizing components of the reverb tail. Do you have the equations that maps XYZW to stereo LR? If you do, you can try to locate your voice (in the XYZW context) that maps to a specific place on the LR spectrum that's in the front. $\endgroup$ Jul 27, 2023 at 21:07

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The best approach to add a mono source to an ambisonic project really depends on how you want the mono source to behave spatially so you will have to elaborate more on that end.

If you want the mono source to appear as if it's coming from a specific direction, you can use an ambisonic encoder to convert the mono source to a B-format signal (W, X, Y, Z channels for 1st order ambisonics). The encoder will take parameters like the azimuth and elevation to control the direction of the sound source. Once encoded, you can then add the B-format signals of the mono source and the rest of the ambisonic mix together.

On the other hand, if you want the mono source (such as a narration) to be non-spatial, you can add it only to the W channel, which iirc is the omni-directional channel in the B-format. This will make the mono source appear as if it's positioned in the center of the sound field, no matter where the listener is facing.

Note that, ambisonics is a full-sphere surround sound technique, so every source you add will be perceived to be coming from a specific point in space (or uniformly from all directions if added to the W channel). Therefore, you need to make a creative decision about where your mono source should be in the soundscape.

Alternatively, if you want to create a binaural mix for headphone playback, you can use a binaural rendering engine to convert the ambisonic mix, including the mono sources, into a binaural format. Binaural rendering algorithms simulate the perception of 3D sound through headphones by applying head-related transfer functions (HRTFs).

Or you could simply pan the mono sources within the ambisonic soundfield using a virtual panning algorithm. This involves placing the mono source at a specific azimuth and elevation within the soundfield. The panning algorithm will then distribute the mono source's signal across the appropriate ambisonic channels to create the illusion of spatialization.

As mentioned previously, it all depends on the mono source and what you want to get out of it.

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