How to determine NC rating?

Measure octave bands 63-8 kHz. Plot against NC curves. Rating = lowest curve not exceeded by any band. If 250 Hz touches NC-35 but rest below NC-30, rating is NC-35.

NC for conference room?

ASHRAE: NC-25-30. NC-25 good for video conferencing. NC-30 acceptable for small meetings.

NC: max levels without spectral balance. RC: evaluates shape (neutral/rumbly/hissy). RC provides better diagnostic and complaint correlation.

Can met NC still cause complaints?

Yes. NC misses tonal components, temporal variation, infrasound. Prominent tones annoy despite meeting NC. RC with tonal analysis provides better assessment.

How to determine NC rating?

Measure octave bands 63-8 kHz. Plot against NC curves. Rating = lowest curve not exceeded by any band. If 250 Hz touches NC-35 but rest below NC-30, rating is NC-35.

NC for conference room?

ASHRAE: NC-25-30. NC-25 good for video conferencing. NC-30 acceptable for small meetings.

NC: max levels without spectral balance. RC: evaluates shape (neutral/rumbly/hissy). RC provides better diagnostic and complaint correlation.

Can met NC still cause complaints?

Yes. NC misses tonal components, temporal variation, infrasound. Prominent tones annoy despite meeting NC. RC with tonal analysis provides better assessment.

Noise Criteria (NC/RC) — HVAC Assessment

NC curves per ASHRAE Chapter 48 define maximum background noise in octave bands 63 Hz-8 kHz. The NC rating equals the lowest curve not exceeded. NC is the primary method for specifying and verifying HVAC noise in commercial buildings.

ASHRAE Handbook Ch. 48ANSI/ASA S12.2-2019

Structure

Octave bands 63-8 kHz, sloping down. NC-35: 62 dB at 63 Hz, 36 dB at 4 kHz. Rating = highest tangent band.

Targets

Concert halls NC-15-20. Churches NC-25-30. Offices NC-30-35. Open plan NC-35-40. Retail NC-40-45.

RC Method

Evaluates balance: average 500/1k/2k Hz, check low/high deviation. RC-N (neutral), RC-R (rumbly), RC-H (hissy). Better diagnostic than NC.

Troubleshooting

Low-freq excess: duct rumble, fan. Mid: airflow at diffusers. High: turbulence at grilles.

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Measure octave bands and NC rating

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Frequently Asked Questions

→SPL Measurement Guide — IEC 61672 Accuracy Standards →A-Weighting Explained — Why dBA Matters for Noise Assessment →Ideal RT60 by Venue Type →Sound Absorption — NRC, Sabine & Material Selection

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