STC vs NRC Ratings: How to Choose the Right Soundproofing Material

What is STC (Sound Transmission Class)?

STC measures how well a material blocks sound from passing through it. If you want to stop traffic noise from coming in, or stop your home theatre bass from escaping into the next room, you need materials with a high STC rating.

STC is achieved through Mass, Density, and Decoupling. Materials with high STC ratings include brick walls, thick laminated acoustic glass, Mass Loaded Vinyl (MLV), and heavy solid-core doors. The higher the STC number, the better the partition is at blocking airborne noise.

What is NRC (Noise Reduction Coefficient)?

NRC measures how well a material absorbs sound that bounces around inside a room. If your room sounds like an echo chamber, or if voices sound hollow and reverberant, you need materials with a high NRC rating.

NRC is measured on a scale from 0 to 1 (where 0 is total reflection, like a mirror, and 1 is 100% absorption). Materials with high NRC ratings include acoustic foam, Rockwool, fiberglass panels, and heavy carpets. These materials stop echo, but they have virtually zero mass, meaning they cannot block sound from leaving or entering the room.

The Crucial Difference (Why Foam Fails at Soundproofing)

Acoustic foam has an NRC of 0.85 (excellent at stopping echo). However, it has an STC of almost 0 (terrible at blocking noise). Pasting foam on your shared apartment wall will make your voice sound better in your room, but your neighbor will still hear every word you say.

Conversely, a sheet of Mass Loaded Vinyl has an STC of 27 (great at blocking noise), but an NRC of 0.05 (it reflects sound and causes echo). To build a professional studio or cinema, you must use high-STC materials inside the walls to block noise, and high-NRC materials on the surface to control the echo.

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