A sound reflection effect due to room modes ( standing waves) which accumulates at walls. sound wave reflections appear to make the localized sound level increase as all of the room modes terminate at the boundary (wall). Essentially as the wavefront approaches the wall, the amounts of molecular motion become smaller and smaller while the pressure differences become greater and greater as the wall resists the motion of the air molecules, the wall becoming a pressure node. The rigidity of the wall surface determines how much the pressure rises, i.e., how much of the pressure is reflected versus how much is absorbed. This occurs on a mode-by-mode basis at each resonant frequency. At very low frequencies, nothing large is rigid. However, at higher frequencies, the boundary effect is more pronounced, e.g., frequencies above 100 Hz in a room with typical walls. A related effect is often observed at a control room window, where the window itself will resonate at one or more resonant frequencies so that the window passes the resonant frequencies through to the (recording) space on the other side, somewhat reducing the boundary effect within the control room, but not providing sound isolation from the adjacent space(s). This last effect is worse for lower frequencies as higher frequencies tend to be absorbed by the glass in the window. Also called the pressure zone effect. See absorption coefficient, bass build-up, bass trap.