The sound insulation provided by a door depends not only on the type of door but also on installation details (e.g. the gaskets around the perimeter). Generally, rubber or plastic compression seals, which are set around the door edges, provide the best result of insulation when closing the door. The degree of sound insulation of a door is usually specified by sound transmission class (STC). Table 18 gives typical STC values of some conventional types of doors. However, if doors are required to have STC values greater than 30, it is recommended that packaged soundproof door units are to be employed. They are usually supplied with them fixed in a frame together with suitable seals. A variety of soundproof doors is available in the market, offering average STC values up to 45 which is equivalent to a 112mm brick wall.