IIC and STC: Sound Ratings Explained

Floor-ceiling assemblies are rated for two kinds of sound. Both scales work the same way: a higher number is better.

STC — Sound Transmission Class

STC measures airborne sound — voices, TV, music — passing through an assembly. It tells you how well a floor-ceiling keeps conversation in one unit from being heard in the unit below.

IIC — Impact Insulation Class

IIC measures impact sound — footsteps, dropped objects, rolling chairs — transmitted through the structure. This is usually the harder number to hit, and it is where soft surface and acoustic underlayment make the biggest difference.

What to target

• Code minimum — Many jurisdictions require IIC and STC of at least 50 for separating assemblies
• Spec preference — Developers and brands often require 55 or higher to reduce complaints
• Delta IIC — Shows how many points a product adds over the bare structure — the cleanest way to compare underlayments

One caveat

Ratings are assembly-dependent. A number is only valid for the specific structure it was tested over — for example, a 6-inch concrete slab versus a wood-frame floor. Always match the tested assembly to your project.