IIC (ASTM E492, E989) rates how well a floor-ceiling assembly reduces footsteps, dropped objects and moved furniture below. Like STC, higher is better. Building codes commonly require IIC 50 (and STC 50) between dwellings; many condominiums require 55 or more.
| Floor | Approx. IIC |
|---|---|
| Tile on concrete slab, no underlayment | 25–30 |
| Hardwood on wood joists | 30–40 |
| Hardwood with acoustic underlayment | 50–55 |
| Carpet and pad | 65–75 |
- Resilient underlayment under hard floors: the biggest gain.
- Floating floors over isolation mats.
- Resilient ceiling below.
Panels on the ceiling below do not change IIC.