NC - the Noise Criterion

Noise Criterion - NC - level is a standard uesd to describe the relative loudness of a space within a range of frequencies.

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Noise Criterion - NC - were established in the U.S. to rate indoor noise like noise from air-conditioning equipment and similar. In Europe it is common to use the alternative Noise Rating Curve - NR.

The method is based on a measurement of sound pressure levels and a set of sound pressure criteria curves ranging frequencies from 63 to 8000 Hz - and a tangent rating procedure. The criteria curves defines the limits of the octave band spectra that must not be exceeded to meet the occupants acceptance in the actual spaces.

The NC rating can be determined by plotting the measured sound pressure at each octave band. The noise spectrum is specified as having a NC rating same as the lowest NC curve which is not exceeded by the spectrum.

NC - noise criteria diagram

Noise Criterion	Octave Band Center Frequency (Hz)
	63	125	250	500	1000	2000	4000	8000
	Sound Pressure Levels (dB)
NC-15	47	36	29	22	17	14	12	11
NC-20	51	40	33	26	22	19	17	16
NC-25	54	44	37	31	27	24	22	21
NC-30	57	48	41	35	31	29	28	27
NC-35	60	52	45	40	36	34	33	32
NC-40	64	56	50	45	41	39	38	37
NC-45	67	60	54	49	46	44	43	42
NC-50	71	64	58	54	51	49	48	47
NC-55	74	67	62	58	56	54	53	52
NC-60	77	71	67	63	61	59	58	57
NC-65	80	75	71	68	66	64	63	62
NC-70	83	79	75	72	71	70	69	68

Related Mobile Apps from The Engineering ToolBox

NC Noise Criterion Calculator App

- free apps for offline use on mobile devices.

Recommended Noise Criterion - NC

NC rated noise should not exceed the limits listed below:

Type of Room - Space Type	Recommended NC Level NC Curve	Equivalent Sound Level dB_A
Residences
Apartment Houses	25-35	35-45
Assembly Halls	25-30	35-40
Churches, Synagogues, Mosques	30-35	40-45
Courtrooms	30-40	40-50
Factories	40-65	50-75
Private Homes, rural and suburban	20-30	30-38
Private Homes, urban	25-30	34-42
Hotels/Motels
- Individual rooms or suites	25-35	35-45
- Meeting or banquet rooms	25-35	35-45
- Service and Support Areas	40-45	45-50
- Halls, corridors, lobbies	35-40	50-55
Offices
- Conference rooms	25-30	35-40
- Private	30-35	40-45
- Open-plan areas	35-40	45-50
- Business machines/computers	40-45	50-55
Hospitals and Clinics
- Private rooms	25-30	35-40
- Operating rooms	25-30	35-40
- Wards	30-35	40-45
- Laboratories	35-40	45-50
- Corridors	30-35	40-45
- Public areas	35-40	45-50
Schools
- Lecture and classrooms	25-30	35-40
- Open-plan classrooms	35-40	45-50
Movie motion picture theaters	30-35	40-45
Libraries	35-40	40-50
Legitimate theaters	20-25	30-65
Private Residences	25-35	35-45
Restaurants	40-45	50-55
TV Broadcast studies	15-25	25-35
Recording Studios	15-20	25-30
Concert and recital halls	15-20	25-30
Sport Coliseums	45-55	55-65
Sound broadcasting	15-20	25-30

Example - Estimate Rated Noise by using the Noise Criterion

Sound pressure levels are measured for different frequencies as:

62.5 Hz : 40 dB
125 Hz : 50 dB
250 Hz : 55 dB
500 Hz : 60 dB
1000 Hz : 50 dB
2000 Hz : 55 dB
4000 Hz : 45 dB
8000 Hz : 45 dB

The Noise Criterion can be estimated to

NC = 57 dB

as indicated in the spreadsheet diagram below.

NC - Noise Criterion Example

NC - Noise Criterion - Interactive Spreadsheet

You can save and modify your own copy (in Google Docs or as a Excel spreadsheet) of the example below.

NCB Curves

Balanced Noise Criteria (NCB) curves are used to specify acceptable background noise levels in occupied spaces and include noise from air-conditioning systems and any other ambient noise. They are intended to replace the NC curves.