Noise
Noise is sound which is undesirable. It is any kind of sound and depends only on which sounds are wanted and which sounds are not. Noise interferes with the sound you wish to hear. The degree of noise depends on volume and frequency.
See also broad band noise.

Noise rating
To define demands for the maximum sound pressure level for background noise like noise from HVAC components, different type of noise rating curves are defi ned:
NR curves – value of the curve is the SPL value at 1000 Hz
NC curves – value of the curve is the SPL value at 2000 Hz
RC curves (room criteria) – value of the curve is the SPL at 1000 Hz
A given background noise has to fulfil the requirements in all frequencies of the chosen noise rating curve.

Noise Reduction Coefficient

The Noise Reduction Coeffi cient NRC is the average sound absorption coefficient measured at four frequencies: 250, 500, 1,000 and 2,000 Hz expressed to the nearest integral multiple of 0.05.