Indoor sound is potentially a greater source of noise hindrance than outdoor sound as the insulation in buildings is improved. Mechanical ventilation units make sound, which may be experienced as noise hindrance, as they displace the air through the air distribution system. They emit sound by vibration and through the casing and the air distribution system. Sound is also generated as air travels through the air distribution system, including the extract and supply valves in respectively the wet and habitable rooms. Therefore, special attention must be paid to the location and mounting of the ventilation unit, the sound attenuation of partition walls, ceilings and doors, and the design and installation of the air distribution system. Ubbink has introduced a range of sound attenuators to minimise the sound emitted by ventilation units through the air distribution systems.