Noise Rating Curves: NR vs NC Explained

By Jake Morrison · October 12, 2026

What Are Noise Rating Curves?

NRC (Noise Rating Curves) and NC (Noise Criteria) are standardized methods for rating background noise in indoor spaces. They convert a full octave-band spectrum into a single number.

NC vs NR: Key Differences

NC curves (American standard, ANSI S12.2) cover 63 Hz to 8 kHz in octave bands. NR curves (ISO standard) cover 31.5 Hz to 8 kHz and include 1/3 octave bands for more precision.

How to Measure

Use a Class 1 sound level meter with octave or 1/3 octave filters. Measure at the listener position. Record the spectrum, then overlay it on the NC or NR curve chart. The rating is the highest curve that the spectrum touches.

Typical Targets

Recording studios: NC-15 to NC-20. Concert halls: NC-15 to NC-20. Offices: NC-30 to NC-35. Classrooms: NC-25 to NC-30. Residential bedrooms: NC-25 to NC-30.

Common Noise Sources and Their NC Ratings

A quiet HVAC system produces NC-25 to NC-30. Computer fans produce NC-45 to NC-55. Traffic noise through single-pane windows is NC-40 to NC-50.

Improving Your NC Rating

Replace HVAC diffusers with low-velocity types, add duct silencers, isolate mechanical equipment on spring mounts, seal gaps around doors and windows, and add mass to partitions.