ISO 9568:1993

Specifies measurement methods and maximum ratings for indoor background sound pressure levels. Applies to noise emitted by heating, ventilating and air-conditioning systems, intrusive noise from the projector(s) and noise emitted by any other mechanical or electrical equipment, but not to intrusive noise from outside, such as traffic noise. It is intended for application when the background noise is essentially a steady-state sound, without strong time-varying components.