Noise Criterion NC

NC curves are used to evaluate sound levels in rooms and to specify the continuous background noise levels. The NC rating can be obtained for a given noise spectrum by its octave band levels as a NC curve. Each NC curve is defined by its sound pressure level for eight octave-band center frequencies.
The NC rating means the lowest NC curve that is not exceeded by any octave-band sound pressure level. It is mostly critical to achieve low noise levels within the frequency bands of 63Hz and 125Hz (These are the bands where hearing-impaired persons have most of their residual hearing).
The term Noise Criterion was established in the United States for rating indoor noise and noise from air-conditioning equipment etc.

Recommended Noise Criterion - NC
RoomsNoise Criteria NCEquivalent dBA Level
Rooms for excellent listening conditions: concert halls, opera houses, broadcasting and recording studios, auditoriums etc.NC-20less than 30
small auditoriums, large meeting rooms, teleconference rooms, theaters, executive offices etc. NC-20 to NC-3030 - 38
sleeping quarters, hospitals, residences, apartments, hotels etc.

NC-25 to NC-3534 - 42
offices, classrooms, libraries, small conference rooms etc.

NC-30 to NC-3538 - 42
for moderately good listening conditions: offices, restaurants, reception areas, retail stores etc.NC-35 to NC-4042 - 47
for fair listening conditions: engineering rooms, lobbies, laboratory work spaces, general secretarial areas etc.NC-40 to NC-4547 - 52
school and industrial shops, computer equipment rooms, kitchens etc.

NC-45 to NC-5552 - 61

In Europe the Noise Rating Curves NR are more commonly used for the ratings of maximum indoor room noises. This standard was developed by the International Organization for Standardization (ISO).

Recommended Noise Ratings - NR
RoomsNoise Rating NR
Concert halls, broadcasting and recording studios, churchesNR 25
Private dwellings, hospitals, theatres, cinemas, conference roomsNR 30
Libraries, museums, court rooms, schools, hospitals operating theaters and wards, flats, hotels, executive officesNR 35
Halls, corridors, cloakrooms, restaurants, night clubs, offices, shopsNR 40
Department stores, supermarkets, canteens, general officesNR 45
Typing pools, offices with business machinesNR 50
Light engineering worksNR 60
Foundries, heavy engineering worksNR 70

Maximum Noise Level for Studio Facilities, German Standards