Noise Criterion (NC) Levels: Definitions & Standards

NC Noise Criterion rating system for indoor noise from air-conditioning and similar equipment. Displays octave band sound pressure level criteria from 63 to 8000 Hz, and recommended NC levels for various room types and spaces.

Noise Criterion63 Hz125 Hz250 Hz500 Hz1000 Hz2000 Hz4000 Hz8000 Hz
NC-154736292217141211
NC-205140332622191716
NC-255444373127242221
NC-305748413531292827
NC-356052454036343332
NC-406456504541393837
NC-456760544946444342
NC-507164585451494847
NC-557467625856545352
NC-607771676361595857
NC-658075716866646362
NC-708379757271706968
Type of Room - Space TypeRecommended NC LevelEquivalent Sound Level dBA
Apartment Houses25-3535-45
Assembly Halls25-3035-40
Churches, Synagogues, Mosques30-3540-45
Courtrooms30-4040-50
Factories40-6550-75
Private Homes, rural and suburban20-3030-38
Private Homes, urban25-3034-42
Hotel Individual rooms or suites25-3535-45
Hotel Meeting or banquet rooms25-3535-45
Hotel Service and Support Areas40-4545-50
Hotel Halls, corridors, lobbies35-4050-55
Offices Conference rooms25-3035-40
Offices Private30-3540-45
Offices Open-plan areas35-4045-50
Offices Business machines/computers40-4550-55
Hospitals Private rooms25-3035-40
Hospitals Operating rooms25-3035-40
Hospitals Wards30-3540-45
Hospitals Laboratories35-4045-50
Hospitals Corridors30-3540-45
Hospitals Public areas35-4045-50
Schools Lecture and classrooms25-3035-40
Schools Open-plan classrooms35-4045-50
Movie motion picture theaters30-3540-45
Libraries35-4040-50
Legitimate theaters20-2530-65
Private Residences25-3535-45
Restaurants40-4550-55
TV Broadcast studios15-2525-35
Recording Studios15-2025-30
Concert and recital halls15-2025-30
Sport Coliseums45-5555-65
Sound broadcasting15-2025-30

About Noise Criterion (NC)

Noise Criterion (NC) levels were established in the U.S. to rate indoor noise from air-conditioning equipment and similar sources. In Europe, the alternative Noise Rating Curve (NR) is commonly used.

How NC Works

The NC method is based on measurements of sound pressure levels across octave bands ranging from 63 to 8000 Hz. A set of NC criteria curves defines the limits that must not be exceeded to meet occupant acceptance in specific spaces.

The NC rating is determined by:

  1. Plotting measured sound pressure at each octave band
  2. Finding the lowest NC curve not exceeded by the measured spectrum
  3. Assigning that NC curve number as the rating

Example

For measured sound pressure levels:

  • 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 resulting Noise Criterion rating would be approximately NC-57 dB.

NCB Curves

Balanced Noise Criteria (NCB) curves are a newer standard used to specify acceptable background noise levels in occupied spaces, including noise from air-conditioning systems and other ambient noise sources. They are intended to replace the traditional NC curves.

Noise Criterion (NC) Levels: Definitions & Standards | MEPKit Engineering Tools | MEPKit