About STC and IIC Ratings


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.

Because of this, a product can perform exceptionally well in one range, poorly in another, and still end up with a better STC rating than a competing product that may have performed better in a frequency range more relevant to your project requirements. This is a pretty common occurrence that should not be ignored when comparing products. Sort through the misleading ratings by gaining access to actual sound test data showing the STC contour line on the transmission loss graph. The performance in relation to the STC contour line will reveal the consistency of the product over the most common frequencies.

Another little known fact about sound is that unless it is passing through an obvious crack or leak, sound does not pass through a wall. Rather the sound energy sets the wall in motion and the wall itself becomes the transmitter much like a radio wave that energizes a radio speaker, which transmits the broadcast signal. The materials that comprise the wall and the manner in which they are installed can have a great bearing on the performance of the wall. Many innovative methods are used to construct walls and floors.

The STC rating for a basic insulated 2×4 wall with a single layer of drywall on each side is ST 35-38. This rating increases to STC 42-45 with steel studs. The STC rating for a basic wood structure floor/ceiling is STC 40-43. The STC rating for a six inch concrete sub-floor is 52-55.

A good level of isolation for walls and ceilings is STC 50 plus, and reaching STC 55 provides a high level of privacy and quiet in a room. A high level of isolation for walls is STC 60 plus with ceilings at STC 50 plus.

STC Ratings and What Can Be Heard

STC What can be heard
25 Normal speech can be understood quite easily and distinctly through wall
30 Loud speech can be understood fairly well, normal speech heard but not understood
35 Loud speech audible but not intelligible
40 Onset of “privacy”
42 Speech audible as a murmur
45 Normal speech not audible; 90% of statistical population not annoyed
50 Moderately loud sounds such as group conversations or television can be faintly heard; 95% of population not annoyed.
60+ Superior soundproofing; most sounds inaudible; 99% of population not annoyed


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 concrete sub-floor with no resilient underlayment is around 28 IIC to 35 IIC. 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.