This International Standard specifies measurement methods and maximum ratings for indoor background sound pressure levels in theatres, review rooms and dubbing rooms. It applies to noise emitted by heating, ventilating and air-conditioning systems, intrusive noise from the projector(s) associated with the theatre and noise emitted by any other mechanical or electrical equipment in the theatre building. It is intended for application when the background noise is essentially a steady-state sound, without strong time-varying components. It does not apply to intrusive noise from other sources outside the theatre, such as aircraft, highway traffic, or adjacent theatres, or to noise resulting from the operation of the sound system in the theatre, or the vibration of the theatre, i.e. movement of the building below 20 Hz.