Audio occlusion
Muffling a sound whose path to the listener is blocked by geometry — usually a low-pass filter plus a volume cut, triggered by a ray between source and ear.
When a source slips behind a wall, the ear expects it dull, not at full volume. The engine fires a ray from listener to source; if it hits geometry, it layers on a low-pass filter and drops the gain, so high frequencies die first, exactly as they do in air. The field distinguishes occlusion (both the direct sound and its reflections are blocked) from obstruction (the direct path is blocked but the echo still arrives). Middleware like FMOD, Wwise and Steam Audio ship ready-made models; full acoustic wave simulation is too costly in real time, so nearly every game approximates it with a handful of rays.