Sound
Impact Insulation Class
IIC
A single-number rating of how well a floor-ceiling assembly limits impact noise, measured with a standardized tapping machine. The test source is far lighter than footfall or a dropped weight.
In more detail
IIC rates how well a floor-ceiling assembly limits impact noise transmitted to the room below. A standardized tapping machine under ASTM E492 strikes the floor at a fixed rate, sound levels are measured below, and the result is fitted to a reference contour to produce one number.
The tapping machine is the key to reading the rating. Its hammers are light and its impacts are sharp and high in frequency. That makes the test repeatable and comparable, and it makes IIC a better predictor of hard-heeled walking and small dropped objects than of what people actually complain about — a child jumping, a dumbbell set down, a chair leg dragged across engineered wood. Those are heavier, lower-frequency events, which is why low-frequency impact ratings have been developed alongside IIC.
Finish flooring is part of the assembly rather than an accessory to it. The same structure can vary by a wide margin between carpet over pad and glued hard flooring, and field results — AIIC, under ASTM E1007 — are measured with whatever is actually installed.
Why it matters
Impact noise is the most common acoustic complaint in multi-story homes and the hardest to fix afterward, because the correction usually lives under the finish floor. Resilient underlayment, a floating assembly or a decoupled ceiling are all decisions made before flooring goes down. Once it is down, the remaining options are removing the floor or changing what people do on it.
The common misunderstanding
That a good rating means footfall will not be audible below. The tapping machine's impacts are far lighter and higher in frequency than a heel strike or a dropped weight, so an assembly can rate well and still transmit the low-frequency thud that generates the complaint. Gyms and children's rooms are where the gap shows up most.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.