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 | ||
| Rooms | Noise Criteria NC | Equivalent dBA Level |
| Rooms for excellent listening conditions: concert halls, opera houses, broadcasting and recording studios, auditoriums etc. | NC-20 | less than 30 |
| small auditoriums, large meeting rooms, teleconference rooms, theaters, executive offices etc. | NC-20 to NC-30 | 30 - 38 |
| sleeping quarters, hospitals, residences, apartments, hotels etc. | NC-25 to NC-35 | 34 - 42 |
| offices, classrooms, libraries, small conference rooms etc. | NC-30 to NC-35 | 38 - 42 |
| for moderately good listening conditions: offices, restaurants, reception areas, retail stores etc. | NC-35 to NC-40 | 42 - 47 |
| for fair listening conditions: engineering rooms, lobbies, laboratory work spaces, general secretarial areas etc. | NC-40 to NC-45 | 47 - 52 |
| school and industrial shops, computer equipment rooms, kitchens etc. | NC-45 to NC-55 | 52 - 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 | ||
| Rooms | Noise Rating NR | |
| Concert halls, broadcasting and recording studios, churches | NR 25 | |
| Private dwellings, hospitals, theatres, cinemas, conference rooms | NR 30 | |
| Libraries, museums, court rooms, schools, hospitals operating theaters and wards, flats, hotels, executive offices | NR 35 | |
| Halls, corridors, cloakrooms, restaurants, night clubs, offices, shops | NR 40 | |
| Department stores, supermarkets, canteens, general offices | NR 45 | |
| Typing pools, offices with business machines | NR 50 | |
| Light engineering works | NR 60 | |
| Foundries, heavy engineering works | NR 70 | |
