Noise Criteria

Standard spectrum curves by which a given measured background noise may be described by a single NC number.

The noise criterion (NC) system was introduced by Beranek in 1957.

Some early problems with the levels set for NC curves lead to the introduction in 1971 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.

Noise Criteria
LocationNoise Criteria
Shops50 - 55
Clerical office45 - 50
Bank40 - 45
Restaurant / cinema35 - 40
Library / living room30 - 35
Church / courtroom / conference room / bedroom25 - 30
Theatre / classroom20 - 25
Concert hall / recording studio15 - 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 = Noise Criteria
PNC = Preferred Noise Criteria
LA = 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.

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Subjects: Noise & Vibration