This chapter deals with transmission of sound between two spaces through a dividing partition such as a wall, floor or ceiling. Much of the chapter is concerned with airborne sound insulation, although impact and structureborne sound transmission are also briefly discussed. In connection with airborne sound insulation it is useful to think of the incident sound wave generated by a loudspeaker on the source side of a partition as exciting the partition into vibration. The vibrating partition then, in effect, acts as a large loudspeaker radiating sound into this space.