STC

STC stands for sound transmission class. This is the most common rating used in North America for determining airborne sound transmission loss between 125 Hz and 4,000 Hz. This range covers the majority of common noises we hear including speech, television, music, dogs barking, and other similar annoyances. A higher STC rating often shows improved performance. However, the rating is essentially an average over the 16 frequency points tested. The STC rating for a basic wood structure floor is STC 40-43.
 

IIC
IIC stands for impact insulation class. Acoustic labs conduct the IIC test using a tapping machine with steel faced hammers. These hammers strike a test floor material generating sounds between 125 Hz to 4,000 Hz. The impact creates vibrations that travel through the floor into the receiving side (the room below). The engineer plots the results of each tap on a graph, compares the results to the reference assembly, and determines the IIC rating from comparing these two tests. A higher number shows better performance. The IIC rating does not account for any squeaking or rattling caused by loose wood frame construction. Nor does it account for low frequency footfall noise or structural deflection. The IIC ratings for basic wood structure with no resilient underlayment is around 40 IIC to 45 IIC. Without the aid of sound control products, the IIC rating of basic wood structures will be noticeably higher than concrete structures as they are inherently more resilient.

PERFORMANCE
The STC rating can be improved by up to 25 points and the IIC rating can be improved by up to 15 points with the use of an approved Sound Attenuation Mat. The performance (increased ratings) of the sound mat depend on the in situ conditions and the quality of the installation. Manufacturers recommend that sound mat be installed as a part of a completed assembly which includes a perimeter isolation strip and taping of all joints. Building codes and HOA policy often require an STC rating above 55 and an IIC rating of above 50. Caution: There are many products on the market that make unsubstantiated claims. Use only a product with the applicable ULC ratings and available test data to back up any performance claims.