The IIC was developed by ATSM International (formerly the American Society for Testing and Materials) using ATSM Method E989, incorporating a five-hammer tapping machine that's specified in ATSM Method E492. The five hammers on the tapping machine are placed in a line, and during testing, each hammer is lifted and then dropped in sequence, causing a total of 20 impacts per second hitting the floor. A sound level meter is placed in the room below to measure the difference between the initial sound, the frequency and force of which is known, and the resulting sound below. An industry standard is used to determine the sound reduction, or transmission loss, in dB (decibels).
Impact Sound Transmission
Unwanted sound vibrations transmitted through floor/ceiling assemblies are annoying, and assemblies that can reduce or eliminate these noise levels are highly desirable. These impact sound transmissions are attenuated, or lessened, by a variety of different flooring materials and floor coverings. Consider the difference in noise transmission to the room below if you have a bare hardwood floor or a floor covered in a thick carpet on top of a quality pad. The difference is significant.
Each of these floors and floor covering types have an IIC rating, typically falling somewhere between 25 and 85+. Higher numbers signify better noise reduction. IIC 50 would be considered the least amount of acceptable impact sound transmission between occupied floors and would be unsatisfactory to many occupying the lower floor. IIC 60 is considered medium noise reduction, and IIC 65 would likely be an acceptable noise reduction level for most lower-level occupants.
In testing a sample floor/ceiling assembly in the laboratory using a five-hammer tapping machine, sounds are generated at 16 standard frequencies between 100 Hz and 3150 Hz. Results from each tap get plotted on a graph, each point depending on the amount of lost impact sound from each tap. The resulting graph is then compared to a standard reference graph to determine the IIC rating.
This testing, done in a laboratory on a single sample section of a floor/ceiling assembly, is the least accurate of two methods for determining a specific IIC rating. A more accurate and realistic rating is obtained through a testing method occurring in an actual building after installation of the floors. When testing is conducted under these real-world conditions, a total IIC value is obtained:
- Floor covering
- Floor joists
- Lower room ceiling
- Sealants and adhesives used for installation
- Sound-deadening materials such as insulation and resilient channels
This method of testing is known as the field impact insulation class (FIIC).
Cushioning the Blow
Quite simply, one of the easiest and most effective ways of reducing impact sound transmission is by “cushioning the blow.” Adding a thick carpet with a high-quality carpet pad is extremely effective in lessening impact sound transmission and improving a floor's IIC rating.
Floors made from resilient materials such as rubber, cork or vinyl can also provide a slight improvement in sound reduction. Floating floors, such as a wood finished floor sitting on a resilient underlayment can also provide a higher IIC rating value. On the other hand, a concrete floor covered directly with a non-resilient material such as wood, tile or stone has a low IIC due to the fact that there is no “give” in this type of flooring system.
Additionally, since the current IIC rating system only tests sounds within a range of 125 Hz to 3150 Hz, which is approximately the sound range of the human voice, noises existing below 125 Hz, such as those that may be heard when someone is walking above on a floor with a lightweight joist system, may be audible. Noises made by people walking on a floor with a loose joist construction are also not accurately accounted for by standard IIC testing.