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|