Audio occlusion simulates how physical barriers in the game world affect sound. In real life, a gunshot behind a concrete wall sounds completely different from one in the open -- it's muffled, the high frequencies are absorbed, and it might reach you through an indirect path like a doorway or vent. Game audio systems replicate this by casting rays or checking line-of-sight between the listener and the sound source. If something is in the way, the system applies low-pass filters, volume reduction, and delay based on the material's properties. Concrete blocks more than wood, which blocks more than glass. Open doors let more sound through than closed ones.