NC - Noise Criterion

For a given noise spectrum, the NC rating can be obtained by plotting its octave band levels on the set of NC curves

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Noise Criterion - NC - were established in U.S. for rating indoor noise, noise from air-conditioning equipment etc. In Europe it is common to use Noise Rating Curves - NR.

The method consists of a set of criteria curves extending from 63 to 8000 Hz, and a tangency rating procedure. The criteria curves define the limits of octave band spectra that must not be exceeded to meet occupant acceptance in certain spaces.

The NC rating can be obtained by plotting the octave band levels for a given noise spectrum - the NC curves. 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

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Recommended Noise Criterion - NC

The noise in different types of rooms should not exceed the Noise Criterion limits listed below:

Type of Room - Space Type Recommended NC Level
NC Curve		 Equivalent Sound Level
dBA
Residences
Apartment Houses 25-35 35-45
Assembly Halls 25-30 35-40
Churches 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 - Estimating NC - Noise Criterion

The Noise Criterion - NC - of a noise spectrum like

  • 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

can be estimated to NC = 57 dB as indicated in the diagram below.

nc - noise criteria - example

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Related Topics

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

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