Impact Insulation Class (or IIC) is an integer-number rating of how well a building floor attenuates impact sounds, such as footsteps. A larger number means more attenuation. The scale, like the decibel scale for sound, is logarithmic.
The IIC number is derived from sound attenuation values tested at sixteen standard frequencies from 100 to 3150 Hz. Unfortunately, "real world" footstep noise is also generated at frequencies below 100 Hz, so the IIC value may not accurately describe the complete noise attenuation profile of a floor.
IIC In a multi-level home or business, when a floor covering in one of the upper rooms is impacted, by dropping an object or moving furniture for example, the impact creates a vibration that travels through the floor, subfloor, and through the ceiling to the room below. These vibrations result in unwanted and annoying sounds in those rooms. This is called impact sound transmission. Floor coverings with a high IIC rating help to reduce impact sound transmissions to lower levels, thus reducing or eliminating those bothersome noises. The lowest IIC rated floors/ceiling assemblies come in at around 25 and the highest rated systems can come in at 85 and up.