Noise Criteria

Standard spectrum curves by which a given measured background noise may be described by a single NC number. Some early problems with the levels set for NC curves lead to the introduction of Preferred Noise Criteria (PNC).

Each NC curve is defined by its sound pressure level at eight octave-band centre frequencies: 63, 125, 250, 500 1000, 2000, 4000 and 8000 Hz. The lower the NC rating, the lower the background noise level.

Location	Noise Criteria
Shops	50 - 55
Clerical office	45 - 50
Bank	40 - 45
Restaurant / cinema	35 - 40
Library / living room	30 - 35
Church / courtroom / conference room / bedroom	25 - 30
Theatre / classroom	20 - 25
Concert hall / recording studio	15 - 20

Note: All levels are for unoccupied rooms.

NC and PNC are based on the A-weighting, therefore, they may be estimated from the measured Sound Pressure Level in dB(A):

where

NC = Preferred Noise Criteria

PNC = Preferred Noise Criteria

L_A = A-weighted Sound Pressure Level [dB(A)]

The NC 40 curve has been used for years as the basis of design for electronic masking sound systems. The NC 40 curve is typically "rolled down" below 100 Hz, and above 5 kHz.

See also: Room Modes, Sound Transmission Class.