NOISE CRITERION (NC)
A noise rating system introduced by Leo Beranek in 1957 to take into account the distribution of noise energy in various frequency bands. It has since been superseded by the NOISE RATING method which functions on similar principles. A comparison of acceptable levels for residential bedrooms would be NC 25 - 35 and NR 25. Both of these are in the range of 30 dBA. The original noise criterion curves applied to continuous sounds in indoor environments, and took speech communication into consideration.
In 1971, Preferred Noise Criterion curves (PNC) were published, in which allowable levels for low and high frequency noise were lowered. The new PNC curves apply to steady AMBIENT NOISE LEVELs in enclosed spaces, and are often used to judge the acceptability of ventilation and other background BROAD BAND NOISE.
See also: INFRASONIC, MASKING. Compare: SPEECH COMMUNICATION CRITERION, SPEECH INTERFERENCE LEVEL.
Representative recommended PNC levels and their dBA equivalents (see SOUND LEVEL, SOUND LEVEL METER) are:
PNC dBA Excellent listening conditions
less than 20
less than 30
Sleeping, residential, private office, library & classroom spaces
25 - 40 34 - 47 Large offices, stores, cafeterias and restaurants
35 - 45 42 - 52 Lobbies, laboratory, engineering and secretarial spaces
40 - 50 47 - 56 Maintenance, equipment, kitchen and laundry rooms
45 - 55 52 - 61 Shops, garages, power-plant control rooms, etc.
50 - 60 56 - 66 From the table and curves, it can be seen that the equivalent allowed noise levels rise with lower frequencies. This can in part be attributed to the ear's decreasing sensitivity to low frequency sound (see EQUAL LOUDNESS CONTOURS). However, these levels allow exposure to very low frequency and INFRASONIC vibration, which have been shown to be the cause of various physical and psychological disorders.
As a verification of their criteria, the authors report that measurements in auditoria and offices which the occupants find satisfactory fall in the appropriate PNC range. Being generally inaudible, infrasonic vibrations are not likely to provoke conscious awareness, despite their ability to cause involuntary physiological effects. Therefore, PNC-type criteria provide no protection for low frequency sound exposure, and further, tend to rely on the public's increasing toleration of background noise and what seems to be decreasing auditory skills. Compare: SOUND INTRUSION, SOUND POLLUTION.
31.5 63.0 125 250 500 1000 2000 4000 8000 PNC - 15 58 43 35 28 21 15 10 8 8 PNC - 20 59 46 39 32 26 20 15 13 13 PNC - 25 60 49 43 37 31 25 20 18 18 PNC - 30 61 52 46 41 35 30 25 23 23 PNC - 35 62 55 50 45 40 35 30 28 28 PNC - 40 64 59 54 50 45 40 35 33 33 PNC - 45 67 63 58 54 50 45 41 38 38 PNC - 50 70 66 62 58 54 50 46 43 43 PNC - 55 73 70 66 62 59 55 51 48 48 PNC - 60 76 73 69 66 63 59 56 53 53 PNC - 65 79 76 73 70 67 64 61 58 58 Octave-band SPL values associated with the recommended 1971 preferred noise criterion (PNC) curves (from Beranek, Blazier and Figwer, J.A.S.A., 1971, p. 1226, used by permission of the authors).
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