Doors must have gaps between the door leaf and its frame, and between the bottom of the door and the floor. These gaps allow doors to be opened and closed easily, and accommodate normal building movement. However, they also allow the leakage of sound, as well as fire, smoke, cold draughts, dust, rain, wind, and light.
Sealing the gaps around a door is of prime importance when reducing the amount of sound entering or leaving a room. Wherever noice influences human activity, effective acoustic sealing is essential. The solution is to fit an appropriate sealing system which provides a complete and continuous barrier around the door when it is closed. Acoustic seals can provide excellent resistance to airborne sound, and significantly improve the acoustic attenuation of the door assembly (door leaf, frame, and seals). Seals designed for the gap between the door and frame, or a pair of doors, are termed perimeter seals. Seals designed for the gap between the bottom of the door and the floor are termed threshold seals.