NRC and STC Ratings Explained

NRC and STC Ratings Explained

By Sarah Okonkwo · October 17, 2026

What Are NRC and STC?

NRC and STC are the two primary acoustic ratings for building materials. Understanding them prevents costly mistakes in studio construction.

NRC: Noise Reduction Coefficient

Measures sound absorption, rated 0.0 (reflector) to 1.0 (absorber). Tested per ASTM C423 across 250-2000 Hz. Does not account for bass below 250 Hz.

STC: Sound Transmission Class

Measures how well a partition blocks sound. Standard wall: STC 33. Studio wall targets STC 55+. Does not predict low-frequency isolation well.

OITC for Low Frequencies

OITC extends down to 80 Hz, better predicting bass isolation. For studios, check both STC and OITC.

Practical Guide

NRC 0.90+: Ceiling clouds, bass trap faces
NRC 0.70-0.85: Wall panels at reflection points
STC 50+: Minimum for music room walls
STC 60+: Professional studio isolation