When a building services engineer asks "what NC are we targeting?" they are asking the single most important question about background noise in the building. NC — Noise Criteria — is the standard system used primarily in North America and Asia to rate and specify acceptable background noise levels in occupied spaces. It has been the lingua franca of building noise specifications since the 1950s, and understanding it is essential for anyone designing rooms where people need to hear, concentrate, or sleep.
TLDR
NC (Noise Criteria) is a single-number rating system for steady-state background noise in buildings, based on a family of spectral curves defined across octave bands from 63 Hz to 8 kHz. Each NC curve represents the maximum acceptable sound pressure level at each octave band for a given rating. A room's NC rating is determined by the highest NC curve that is reached or exceeded by any single octave band measurement. The system was developed by Leo Beranek in 1957 and is referenced in ASHRAE handbooks, ANSI standards, and building specifications worldwide. NC-25 is typical for concert halls and recording studios. NC-30 to NC-35 for classrooms and private offices. NC-40 to NC-45 for open-plan offices. The NC system is being supplemented (though not replaced) by the RC (Room Criteria) system, which adds spectral quality assessment.
Real-World Analogy
Think of NC curves like speed limits on a highway, but with different limits for each lane. Each lane represents a frequency band — bass rumble in the left lane, midrange hum in the middle, high-frequency hiss in the right lane. The NC rating is the highest speed limit reached by any lane. If the "bass lane" is going 45 (NC-45) but all other lanes are at 35 or below, the room is rated NC-45 because that one lane controls. Just as a highway is only as fast as its fastest lane, a room is only as quiet as its noisiest frequency band.
Technical Definition
The NC Curve Family
NC curves are a family of inverse-loudness-contour curves spanning octave bands from 63 Hz to 8 kHz. Each curve is identified by a number (NC-15, NC-20, NC-25, etc.) representing the sound pressure level at 1000 Hz on that curve. The curves slope downward from low to high frequencies, reflecting the human ear's greater sensitivity to mid and high-frequency noise.
Selected NC curve values (sound pressure level in dB re 20 µPa):
| NC | 63 Hz | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz | 8 kHz |
|---|---|---|---|---|---|---|---|---|
| 15 | 47 | 36 | 29 | 22 | 17 | 14 | 12 | 11 |
| 25 | 54 | 44 | 37 | 31 | 27 | 24 | 22 | 21 |
| 35 | 60 | 52 | 45 | 40 | 36 | 34 | 33 | 32 |
| 45 | 67 | 60 | 54 | 49 | 46 | 44 | 43 | 42 |
How to Determine the NC Rating
- Measure background noise in octave bands (63 Hz to 8 kHz) using a Class 1 or Class 2 sound level meter per IEC 61672-1.
- Plot the measured levels on the NC curve chart.
- Identify the highest NC curve that is touched or exceeded by any octave band.
- That curve number is the room's NC rating.
NC vs dBA
NC and dBA both describe noise level, but they serve different purposes:
- dBA is a single number (A-weighted) that approximates human loudness perception. It does not reveal spectral content — a 40 dBA room with a low-frequency rumble sounds very different from a 40 dBA room with high-frequency hiss.
- NC retains spectral information. An NC-35 room has specific maximum levels at each octave band. This is critical for identifying whether the noise problem is HVAC rumble (low-frequency), duct noise (mid-frequency), or diffuser hiss (high-frequency).
Why It Matters for Design
NC targets drive mechanical system design. An NC-25 classroom requires significantly quieter HVAC than an NC-40 open-plan office. This affects duct sizing (larger ducts = lower velocity = less noise), diffuser selection (low-noise types), equipment isolation (spring mounts, inertia bases), and silencer specification. HVAC noise control often accounts for 3 to 8% of a building's mechanical budget, and it is the NC target that determines that cost.
For acoustic consultants, NC is the bridge between room function and noise specification. Each room type has an established NC range based on decades of research and occupant satisfaction data:
- NC-15 to NC-20: Concert halls, recording studios, audiometric test rooms — near silence
- NC-25 to NC-30: Classrooms, courtrooms, libraries, hospital patient rooms — very quiet
- NC-30 to NC-35: Private offices, conference rooms, hotel rooms — quiet
- NC-35 to NC-40: Lobbies, corridors, retail shops — moderate
- NC-40 to NC-45: Open-plan offices, restaurants, workshops — noticeable but acceptable
How AcousPlan Uses This
AcousPlan's noise criteria module lets you input background noise levels in octave bands and instantly see the NC rating, with the controlling band highlighted. The platform compares the measured or estimated levels against the target NC for your selected room type and flags any exceedances. The STI calculation uses the same noise data to determine how background noise affects speech intelligibility. The AI co-pilot suggests mitigation measures when the NC target is exceeded.
Related Concepts
- What is an NR Rating? — European equivalent of NC
- What is an RC Rating? — Enhanced criteria with spectral quality
- What is a Background Noise Survey? — How NC is measured
- Noise Criteria (NR/NC/RC) Explained — Comprehensive comparison
- What is STI? — How noise affects speech intelligibility
Calculate Now
Input your octave-band noise levels into AcousPlan to determine the NC rating and see how it compares to target values for your room type.