Acoustic Ratings Explained IIC • STC • ΔIIC • Why “assembly” matters

Q: What is IIC in flooring?

IIC (Impact Insulation Class) is a lab rating that describes how well a floor/ceiling assembly reduces impact noise, like footsteps, under a specific standardized test setup.

Q: What is STC in flooring?

STC (Sound Transmission Class) is a lab rating that describes how well a wall or floor/ceiling assembly reduces airborne sound, like voices or TV, under a standardized test setup.

Q: What does ΔIIC mean?

ΔIIC (Delta IIC) is a lab-reported improvement number showing how much impact sound rating changed when a specific underlayment or layer was added to a defined test assembly.

Acoustic ratings like IIC and STC often appear on flooring spec sheets, especially for products marketed for multi-family or commercial settings. This page explains what those ratings mean and why the test setup matters. Reference-only: no product recommendations.

Quick Answer

IIC relates to impact noise (footsteps), STC relates to airborne noise (voices/TV), and ΔIIC is a reported change in impact rating when a layer like underlayment is added. These numbers are only comparable when the test method and full assembly are the same.

Ratings Table Explainers FAQ

Common Acoustic Ratings on Flooring Spec Sheets

Ratings depend on the full assembly and test method.

Rating	What it describes	Noise type	Important caveat
IIC	How a floor/ceiling assembly reduces impact noise under a standardized lab test.	Impact (footsteps, objects dropping)	Only comparable when assembly + test method match.
STC	How a wall or floor/ceiling assembly reduces airborne sound under a standardized lab test.	Airborne (voices, TV, music)	Not a direct measure of impact noise.
ΔIIC	Change in impact rating when a specific layer (often underlayment) is added to a defined assembly.	Impact (difference/improvement value)	It’s an “improvement number,” not a final system rating by itself.

Reminder: A flooring product alone usually can’t be assigned one universal IIC/STC number — the assembly matters.

Quick explainers

Impact vs Airborne Sound

Impact sound is created by contact with the floor (footsteps, dropped items). Airborne sound travels through air first (voices, TV, music). Because these behave differently, lab standards use different tests and ratings to describe them.

What “Assembly” Means on a Spec Sheet

Acoustic ratings typically apply to a floor/ceiling assembly — not just the finished flooring. “Assembly” can include the subfloor, underlayment, ceiling construction below, and sometimes a specific test slab or joist system depending on the method.

That’s why spec sheets may list an IIC/STC number alongside a description of the tested setup. If the setup changes, the measured rating can change.

Why ΔIIC Is Often Misunderstood

ΔIIC is commonly used to describe a change in impact performance when a layer is added. It does not automatically tell you the final IIC rating of your real building assembly. It’s most useful when it clearly states the baseline and the exact tested assembly.

How To Read Acoustic Numbers Without Over-Interpreting Them

Look for the test method and the assembly description near the rating.
Check whether the number is a final rating (IIC/STC) or an improvement number (ΔIIC).
Compare ratings only when the assembly and method match.
Remember: lab ratings describe results under specific conditions, not guarantees for every building.

FAQ

Short answers. Reference only.

What is IIC in flooring? ⌄

IIC (Impact Insulation Class) is a lab rating describing how well a floor/ceiling assembly reduces impact noise under a standardized test setup.

What is STC in flooring? ⌄

STC (Sound Transmission Class) is a lab rating describing how well an assembly reduces airborne sound under a standardized test setup.

What does ΔIIC mean? ⌄

ΔIIC is a reported change in impact rating when a specific layer (often underlayment) is added to a defined test assembly. It’s an improvement number, not a universal final rating.

Can you compare acoustic ratings between products? ⌄

Only if the test method and full assembly details match. Acoustic ratings depend heavily on the entire floor/ceiling assembly, not just the flooring product by itself.