How to Verify NC/NR Compliance in a Finished Room

TL;DR

NC and NR curves evaluate HVAC noise quality by octave band rather than a single dBA number. A room can meet a 35 dBA target while failing NC-30 due to excessive energy in a single octave band. This guide shows how to measure octave band levels and determine the NC/NR rating using SonaVyx's built-in curve comparison.

NC vs NR: Which Standard?

NC (Noise Criteria) is used primarily in North America (ASHRAE). NR (Noise Rating) is the European equivalent (ISO 1996). Both use a family of curves across octave bands from 63 Hz to 8 kHz. The NC/NR rating is determined by the highest octave band tangent point — the band that touches the highest curve defines the rating.

Step-by-Step Procedure

Prepare the room. HVAC must be running at its design condition. Close doors and windows. Remove occupants and temporary noise sources. The room should represent normal unoccupied operating conditions.
Open SonaVyx SPL Meter at /tools/spl-meter. Select Z-weighting (linear, no weighting filter) for octave band measurement — this gives you the true octave band levels without A-weighting correction.
Enable octave band view. Switch to 1/1 octave display. SonaVyx shows 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 8 kHz bands — exactly the 8 bands used by NC and NR curves.
Position microphone. Measure at seated ear height (1.2 m) at the design listening position. For open-plan offices, measure at a representative workstation. For auditoriums, measure at the center of the seating area.
Measure for at least 5 minutes. Use Leq (equivalent continuous level) for each octave band. SonaVyx calculates running Leq per band. Five minutes captures at least one full HVAC cycle.
Read the NC/NR rating. SonaVyx automatically overlays NC curves and reports the NC rating. The rating equals the highest curve touched or exceeded by any octave band. For example, if all bands are below NC-30 except 250 Hz which touches NC-35, the room rates NC-35.
Compare against specification. Common targets: recording studios NC-15 to NC-20, concert halls NC-20, private offices NC-30 to NC-35, open offices NC-35 to NC-40, classrooms NC-25 to NC-30 (ANSI S12.60).
Identify failing bands. If the room exceeds the target NC, identify which octave band(s) are the offenders. Low-frequency excess (63-250 Hz) usually indicates duct rumble or fan noise. Mid-frequency excess (500-2k Hz) suggests air rush through diffusers. High-frequency excess (4-8 kHz) points to bearing whine or electrical noise.

Common Mistakes

Using A-weighted octave bands. NC/NR curves expect unweighted (Z or linear) octave band levels. A-weighting suppresses the low frequencies that often determine the NC rating.
Measuring with HVAC off. NC measures HVAC noise. With HVAC off, you are measuring ambient only — the NC rating will be artificially low and not representative.
Confusing NC with dBA. NC-35 is not the same as 35 dBA. A room at NC-35 typically measures around 40-42 dBA overall due to the frequency-weighted summation.
Not checking all VAV box settings. Variable air volume boxes change noise output based on demand. Measure at maximum airflow for the worst case.

Tool Bridge

Open the SonaVyx SPL Meter with octave band view enabled. The automatic NC rating appears above the bar chart. Export the octave band data as CSV for your compliance report.