The practice is intended for application when the background noise is essentially a steady state sound, without strong time-varying components.
The background noise may be produced by air-handling systems, by adding noise to the monitoring system, or by a combination of these. An advantage to adding noise electronically is the ability to switch off the noise, for reproduction of the fullest possible dynamic range of the program material, exceeding that of most theaters, so long as the air-handling system note of the dubbing stage is quieter than most theaters.
More SMPTE standard pdf
SMPTE EG 27-2004
Supplemental Information for ANSI/SMPTE 170M and Background on the Development of NTSC Color Standards
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