NC Calculator Guide: ANSI/ASA S12.2 Noise Criterion Rating

An NC calculator rates room noise against Noise Criterion (NC) curves: enter octave-band sound pressure levels $L_p(f_i)$, find the lowest NC curve that is nowhere exceeded (ANSI/ASA S12.2 tangent method), then compare to an ASHRAE space maximum for Pass/Fail.

Open the live Noise Criterion Rating Calculator — SpecRoutes include Control Room NC-35, Office NC-30, Equipment Room NC-50, and Workshop NC-55.

Quick Summary (TL;DR)

• Takeaway: NC = lowest clear NC-15…65 curve under the measured octave-band spectrum; Pass when NC ≤ ASHRAE space max.
• Governing relation: $L_p(f_i) \le L_{\mathrm{NC}}(f_i)$ for all bands $f_i \in \{63,\ldots,8000\}\text{ Hz}$.
• Code / basis: ANSI/ASA S12.2 · ASHRAE HVAC Applications space maxima.

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How an NC calculator works

1. Select space type (sets ASHRAE max NC).
2. Enter $L_p$ at 63, 125, 250, 500, 1k, 2k, 4k, 8k Hz.
3. Read hero NC-XX and Pass/Fail.
4. Use the spectrum chart to see the tangent (governing) band.

Worked path — Control Room NC-35

SpecRoute control-room-nc35 loads a Control Room spectrum on the NC-35 tangent and rates NC-35 Pass (est. dBA ≈ 40–43). The calculator default Control Room duty also returns NC-35 Pass.

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Interactive tool CTA

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Frequently Asked Questions (FAQ)

Q1. What is an NC calculator?

A tool that computes a Noise Criterion (NC) rating from octave-band SPL using the ANSI/ASA S12.2 tangent method, then checks an ASHRAE space limit. FEK: Noise Criterion Rating Calculator.

Q2. Is estimated dBA exact?

No. Screening uses dBA ≈ NC + 5…8 dB. Use a calibrated A-weighted measurement for compliance records.

Q3. When should I use RC or NCB instead?

For low-frequency rumble or tonal complaints, use RC or NCB — NC alone can miss those modes.