What is a Noise Criterion (NC) rating?

An NC rating is the lowest standard NC curve (NC-15 to NC-65) that is nowhere exceeded by measured octave-band sound pressure levels, per ANSI/ASA S12.2 tangent method.

How does this calculator decide Pass or Fail?

Pass/Fail compares the rated NC to an ASHRAE HVAC Applications screening maximum for the selected space (e.g. Control Room NC-35, Office NC-30, Equipment Room NC-50, Workshop NC-55).

Is estimated dBA exact?

No. The tool reports a screening estimate dBA ≈ NC + 5…8 dB. Use a calibrated A-weighted measurement for compliance documentation.

When should I use RC or NCB instead of NC?

For low-frequency rumble or tonal noise isolation, perform RC (Room Criterion) or NCB (Balanced Noise Criterion) analysis — NC alone can miss those complaint modes.

Noise Criterion Rating Calculator

1. Core Formula & Variable Definitions

ANSI/ASA S12.2 Criteria for Evaluating Room Noise (Noise Criterion curves) · ASHRAE Handbook — HVAC Applications (recommended NC by space type)

L_p(f_i) ≤ L_NC(f_i) → NC

dBA ≈ NC + 5…8

Octave bands f_i ∈ {63, 125, 250, 500, 1000, 2000, 4000, 8000} Hz · NC = lowest clear NC-15…65 curve (ANSI/ASA S12.2 tangent) · ASHRAE space Pass/Fail

NC calculator S12.2 tangentControl Room NC-35Scope NC-15…65Space ASHRAE HVAC

Field NC calculator using the ANSI/ASA S12.2 octave-band tangent method. Rated NC is the lowest standard curve (NC-15 to NC-65) that is nowhere exceeded by measured L_p. Pass/Fail uses ASHRAE HVAC Applications recommended maxima by space type. SpecRoutes: control-room-nc35, office-nc30, equipment-room-nc50, workshop-nc55. Estimated A-weighted level is a coarse NC+5…8 rule — not a substitute for calibrated dBA measurement. For rumble or tonal complaints, use RC or NCB analysis.

L_p(f_i) (Octave-band SPL) — Sound pressure level in each standard octave band (dB).
L_NC(f_i) (NC curve limit) — Published NC-15…65 octave-band limit at frequency f_i.
NC (Noise Criterion rating) — Lowest NC curve that clears every band (tangent method).
dBA est. (A-weighted estimate) — Screening: dBA ≈ NC + 5 to 8 dB.

2. Screening Rules Matching This Calculator

Rates NC by the ANSI/ASA S12.2 octave-band tangent method and compares to an ASHRAE space maximum. Calibrated dBA and RC/NCB remain site measurements.

NC rating methodLowest clear NC-15…65 curve

Every octave band L_p(f_i) must sit at or below the chosen NC curve limit (tangent method).

Space Pass / FailNC ≤ ASHRAE max for space type

Control Room, office, equipment room, and workshop presets set the recommended maximum NC.

dBA estimate≈ NC + 5…8 dB

Coarse A-weighted screening only — not a substitute for a calibrated sound-level meter.

Out of scope hereRC · NCB · tonal / rumble metrics

Use RC or NCB analysis when low-frequency rumble or tonal complaints dominate.

Quick Reference Lookup Table

ANSI/ASA S12.2 NC curve octave-band limits (dB) — tangent screening table
NC	63 Hz	125 Hz	250 Hz	500 Hz	1 kHz	2 kHz	4 kHz	8 kHz
NC-15	47	36	29	22	17	14	12	11
NC-20	51	40	33	26	22	19	17	16
NC-25	54	44	37	31	27	24	22	20
NC-30	57	48	41	35	31	29	28	27
NC-35	60	52	45	40	36	34	33	32
NC-40	64	56	50	45	41	39	38	37
NC-45	67	60	54	49	46	44	43	42
NC-50	71	64	58	54	51	49	48	47
NC-55	74	67	62	58	56	54	53	52
NC-60	77	71	67	63	61	59	58	57
NC-65	80	75	71	68	66	64	63	62

Values from the ANSI/ASA S12.2 NC curve family used for FEK tangent screening. Verify against the current published standard for final HVAC design.

3. Applicability & Space Limits

NC screening applies to occupied HVAC spaces with measured octave-band spectra. Metallurgy is not applicable — limits are acoustic / space criteria.

Group / Regime	Range / Condition	Limit / Criterion	Engineering Notes
Quiet occupied rooms	N/A — acoustic criterion	Typical max NC-25…35	Control rooms, executive offices, and similar ASHRAE quiet spaces.
General offices / classrooms	N/A	Typical max NC-30…40	Speech intelligibility driven; verify project ASHRAE table.
Equipment / mechanical rooms	N/A	Typical max NC-45…55	Higher NC allowed; still rate the spectrum — do not skip NC for “loud rooms.”
Tonal / rumble complaints	N/A	Out of scope (use RC / NCB)	NC tangent can miss low-frequency or tonal issues that RC/NCB catch.

Code Applicability & Safety Boundaries

ANSI/ASA S12.2 tangent NC only — not a full room-acoustics model
ASHRAE maxima are recommended screening values; project specs may be stricter

4. Step-by-Step Worked Example

Field VerificationShow

Worked example — Control Room NC-35 duty

Rate a control-room octave-band spectrum that sits on the NC-35 tangent profile and check ASHRAE Pass/Fail.

Space:Control Room (max NC-35)Method:ANSI/ASA S12.2 tangentSpectrum (dB):55 / 48 / 41 / 36 / 31 / 27 / 24 / 22

1

Compare each band to NC curves

Spectrum clears NC-30 at most bands but exceeds NC-30 at 500 Hz (36 > 35)

Result:NC-30 not clear

2

Find lowest clear curve

All bands ≤ NC-35 limits → lowest clear = NC-35

Result:NC Rating = NC-35

3

Space compliance

NC-35 ≤ Control Room max NC-35

Result:Pass · est. dBA 40–43

Conclusion: Default Control Room duty returns NC-35 Pass. Exact NC-curve spectra on pSEO paths (office-nc30, equipment-room-nc50, workshop-nc55) rate exactly to those NC levels.

5. How to calculate Noise Criterion (NC) rating

1
Select space type
Choose Control Room, Executive Office, Equipment Room, or Workshop to set the ASHRAE max NC for Pass/Fail.
2
Enter octave-band SPL
Input L_p (dB) for 63 Hz through 8000 Hz from a sound-level meter or HVAC report.
3
Read NC hero and compliance
Hero shows NC-XX and Pass/Fail versus the selected space limit.
4
Review tangent band and chart
Use the spectrum vs NC curve chart and octave-band rows to see which band governs the rating.

6. Frequently Asked Questions & Technical References

Show

An NC calculator rates room noise by the ANSI/ASA S12.2 octave-band tangent method: find the lowest NC-15…65 curve that is nowhere exceeded by measured $L_p$, then compare to an ASHRAE space maximum. FEK SpecRoute /control-room-nc35 is the default Control Room path.