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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

NC - the Noise Criterion
Noise CriterionOctave Band Center Frequency (Hz)
631252505001000200040008000
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

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Recommended Noise Criterion - NC

NC rated noise should not exceed the limits listed below:

Noise Criterion - NC -  Recommended Levels
Type of Room - Space TypeRecommended NC Level
NC Curve		Equivalent Sound Level
dBA
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 - Interactive Spreadsheet

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

NC - noise criterion exampleNCB 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.

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Related Topics

  • Acoustics

    Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.

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