Ambisonics as Sound Fields
Ambisonic sounds, either recorded or synthetic, are great for implementing ambient sounds. Although higher order ambisonics are typically used in the context of a game’s intermediate spatial representation, low orders are usually sufficient for ambient sounds.
When used as a sound field with 3D Spatialization, ambisonics, like other multichannel formats, collapses into a mono point source if its spread is left at 0%, which is the default. In order to be fully surrounded by the sound field, you must set its spread to 100% by adding an appropriate Attenuation ShareSet to the sound object.
As explained in Using Ambisonics for Dynamic Ambiences, with spread set to 100% and 3D Spatialization set to Position + Orientation, the ambisonic sound field will be rotated based on the relative orientation of the emitter and listener game objects, effectively giving the illusion that the sound field is tied to the world.
Ambisonics represents wavefronts coming towards the listener, so the sources that constitute the sound field are always away. It is thus difficult to use this representation to translate within the sound field. However, spread can help approximate the effect of going in and out of a sound field. Spread less than 100% has the effect of contracting the sound field towards the emitter game object, as explained in Effect of Spread. This is exactly how Wwise Spatial Audio Rooms handle room tones and reverberation (see Room Tones):
When the listener is inside the room, the spread is close to 100%, and the room tone and reverb bus are tied to the room’s orientation per rotation of the sound field.
When the listener is located outside the room, the room’s game object is placed on the closest portal, with spread less than 50% and dependent on the portal’s aperture. The sound field is thus contracted towards the location of the portal, all while being rotated based on the orientation of the room. As the listener walks away, the sound field is further contracted into a point source at the location of the portal.
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