National Floor Covering Association

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Two Categories of Sound


There are two sound transmission categories that products are tested under:

Impact Insulation Class (IIC)

This is a measurement obtained by standard / controlled laboratory testing methods which classify or rate a flooring assembly in regard to amount of impact sound transmitted through the assembly. Each rating is called an impact insulation class (IIC). The higher the IIC number the better the impact insulation.

Sound Transmission Class (STC)

This rating system measures the ability of building elements, such as floors and walls, to reduce the passage of audible sounds when tested in specific laboratory conditions. The higher the STC number the better sound barrier.


Impact sound
Below is a test video taken in a four story wood frame building.
Figure 1 shows a water bottle being dropped from 2′ onto a vinyl plank floor with an acoustical underlayment installed beneath. The acoustical underlay came with an IIC of 73 when tested according to ASTM 492, a test that is performed over a concrete subfloor, not a wood frame subfloor.
Figure 2 records the sound from the same test heard in the suite immediately below.