Noise Criteria rating per ASHRAE standards

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What is NC (Noise Criteria)?

NC (Noise Criteria) rates background noise levels from HVAC systems, traffic, and other steady-state sources. Lower NC = quieter space. It considers the frequency spectrum because low-frequency rumble is more intrusive than high-frequency hiss at the same volume.

Target values by space: Recording studios need NC-15 to NC-25. Private offices typically need NC-35 to NC-40. Open offices can tolerate NC-40 to NC-45. Exceeding these targets causes distraction, fatigue, and speech interference.

Octave Band SPL Measurements (dB) ?
Load Example Spectrum
NC Curve Visualization

About NC Curves

Noise Criteria (NC) curves were developed by Leo Beranek (1957) to evaluate acceptable background noise levels in buildings. The NC rating is determined by the highest NC curve touched or exceeded by the measured spectrum at any frequency.

NC curves are contoured to match human hearing sensitivity, allowing higher SPL at low frequencies where hearing is less sensitive.

Sources: ASHRAE Handbook - HVAC Applications, Engineering Toolbox, Beranek L.L. (1957) "Revised criteria for noise in buildings"
Target Room Type
Measured NC Rating ?
NC-35
Tangent method (highest curve exceeded)
Target: NC-35 On Target
Limiting Frequency ?
500 Hz
dB Over Target ?
0 dB
A-weighted Leq ?
42 dBA
Spectrum Shape ?
Neutral

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