Noise criteria

The Handbook of Facilities Planning (1990) explains:

The noise criteria (sic) (NC) is a single numerical index commonly used to define design goals for the maximum allowable noise in a given space. They primarily apply to the noise produced by a ventilation system, but they may be applied to other noise sources. The NC criteria consist of a family of curves that define the maximum allowable octave-band sound pressure level corresponding to a chosen NC design goal. Although alternate contours have been proposed (most notably the room criterion or RC curves), the NC criteria remain the most widely accepted.

... Acceptable Limits of Noise

Space Noise Criterion
(NC)
Lab office 35
Laboratory 45
Instrument room 40
Contr.-temp. room 45
Darkroom 35
Glasswash 45

Noise Criteria Levels for Rooms
Type of Space (and acoustical requirements)NC level
Conference rooms, churches, lecture halls, classroomsNC 30-35
Open-offices, schools, lobbies, public areasNC 35-40
Large public officesNC 40-45
Shops, garages, etc. (for just acceptable speech and telephone communication)NC 50-60
For work spaces where speech or telephone communication is not required,

but where there must be no risk of hearing damage*NC 55-70

* Most cleanrooms are at this NC level


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