Impact sound insulation

Frequently asked questions

Why does a better floor have a lower number in one market and a higher one in the other?

Because the two scales measure different quantities that happen to be about the same thing. Ln,w is a SOUND PRESSURE LEVEL: the machine taps, a microphone in the room below records how loud it is, and the rating is that loudness — so quieter is better and the number falls as the floor improves. IIC is an INSULATION CLASS: it is derived from the same measurement but expressed as how much the assembly reduces the noise, so more insulation is better and the number rises. They are not convertible by a fixed offset either, because they weight the frequency bands differently and reference different curves, though a rough rule of thumb circulates that adding the two gives a roughly constant figure for typical floors. The practical rule is the safe one: never compare a bare number, always carry the letters, and treat any specification that mixes the two scales as unverified until each figure is matched to its own standard.

Why does a hard floor covering ruin an acoustic floor that tested well?

Because the test result belongs to the whole build-up including its finish, and impact sound is created at the SURFACE. Airborne sound has to get through the mass of the floor, so the covering barely matters; impact sound starts when a heel or a dropped mug strikes the top layer, and a soft covering absorbs much of that energy before it ever enters the structure. Carpet on underlay is, in impact terms, an enormously effective treatment — which is why a floor that performed well in a carpeted flat can fail outright when the occupant replaces it with engineered wood or tile. That is the reason leases in apartment buildings so often specify floor coverings, why a resilient layer under a hard floor is a structural part of the specification rather than a comfort item, and why an impact rating quoted without naming the finish that was on the floor during the test says almost nothing.

What is flanking, and why is the field number always worse?

Flanking is every path the sound takes that does not go straight through the element being rated — around it through the walls, along a continuous screed, down a rigid skirting, through a shared joist that is built into both parties' walls. A laboratory test deliberately removes those paths, so a laboratory rating describes the floor in isolation and nothing else. In a real building the paths exist, so the measured performance is always lower, which is why the field and apparent ratings carry their own letters: FIIC in the United States, ASTC in Canada, the primed L'nT,w in Britain. The practical consequence is the same one the airborne sound concept records: a floor built exactly to a laboratory specification can still fail a completion test, because the failure is at the junctions. Isolating the resilient layer at its perimeter — turning it up the wall so the screed never touches the structure — is the detail that most often decides it.