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 It has since been superseded by the Noise Rating method which functions on similar principles.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.

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.

Centre Frequency (Hz)

	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).

Preferred Noise Criterion curves. From: Beranek, Blazier and Figwer, "Preferred Noise Criterion (PNC) curves and their application to rooms", Journal of the Acoustical Society of America, vol. 50, p.1226, 1971, used by permission of the authors.