Decibel Shield
FAQ › Glossary
What is a noise criterion (NC) rating?
A noise criterion rating grades the steady background noise in a room, mostly from ventilation, against a family of octave-band curves from 63 Hz to 8 kHz. The room's NC is the lowest curve its spectrum stays under. Designers specify NC 15 to 20 for concert halls, 25 to 35 for bedrooms and offices, and 40 to 45 for restaurants. The A-weighted level usually runs 5 to 10 dB above the NC number.
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How the rating is read
The noise in the room is measured in eight octave bands. Each NC curve allows more level at low frequencies than at high, because rumble is tolerated better than hiss. The rating is the number of the lowest curve that the measured spectrum does not cross in any band, so one loud band, often 63 or 125 Hz from a fan, sets the whole rating. The curves and the procedure are defined in ANSI/ASA S12.2, Criteria for Evaluating Room Noise.
| Space | NC range |
|---|---|
| Recording studios, concert halls | 15–20 |
| Bedrooms, hospital patient rooms | 25–30 |
| Classrooms, private offices, conference rooms | 25–35 |
| Open-plan offices, libraries | 35–40 |
| Restaurants, retail, lobbies | 40–45 |
Sources: ANSI/ASA S12.2-2019, Criteria for Evaluating Room Noise; ASHRAE Handbook, HVAC Applications, chapter on sound and vibration control (design guidelines table). Ranges are the commonly published design targets; a project specification governs.
NC, RC and dBA
NC dates from 1957 and remains the most quoted rating. ASHRAE also uses RC (room criterion) and RNC, which grade rumble and hiss separately. A single dBA number cannot be converted to NC exactly, because NC depends on the spectrum, but for typical ventilation noise the dBA value sits about 5 to 10 dB above the NC rating. A classroom at NC 30 therefore reads around 35 to 40 dBA, which is the level our classroom noise meter shows as green with the default thresholds.
What NC level is a quiet room?
NC 25 to 30 is what designers aim for in bedrooms and small offices, roughly 30 to 40 dBA. NC 35 is noticeable as a steady hum; NC 40 and above interferes with quiet conversation.
What is the difference between NC and dBA?
dBA is one A-weighted number from any noise; NC is a rating against curves that need the noise measured in octave bands. NC is stricter about rumble and hiss, which is why an HVAC system can meet a dBA target and still fail its NC specification.
Can a phone decibel meter measure NC?
Not directly. An NC rating needs octave-band levels, which a phone meter does not report. A phone can give you the overall dBA, and the 5 to 10 dB rule of thumb gives a rough idea of the NC range.
What NC does a classroom need?
ANSI S12.60, the classroom acoustics standard, sets an unoccupied background limit of 35 dBA for core learning spaces, which corresponds to roughly NC 25 to 30. Many older classrooms measure NC 40 or more with the ventilation running.