Bass build-up

 An increase in molecular pressure variation, not molecular velocity, which occurs at low frequencies at room boundaries. The pressure nodes for all frequencies build up in the corners of rooms, particularly at the intersections of the walls and floor or walls and ceiling. These regions can roughly be thought of as resonant spaces, and energy can be trapped quite effectively by placing frictional absorbers at the desired quarter wavelength out into the room from the corner. See bass trapboundary effect.