sound laboratory so designed as to minimize sound reflections as well as external noise. External sound is excluded by physical isolation of the structure, by elaborate acoustical filters in the ventilating ducts, and by thick masonry walls. Interior surfaces are covered with absorptive material, such as glass fibre or mineral wool in blankets or in horizontal and vertical wedges (see illustration). Ceiling and floor are similarly padded; a thin steel mesh just above the floor provides a surface for walking. Sound reflection can be reduced to one part in 1,000 in such a room, thus simulating the acoustical conditions of unobstructed free space.