Impact insulation class (also known as Impact Isolation Class, or IIC) is an integer-number rating of how well a building floor attenuates impact sounds, such as footsteps. A larger number means more attenuation. The scale, like the decibel scale for sound, is logarithmic. The IIC is derived from ASTM method E989, which in turn uses a tapping machine specified in ASTM method E492.1
The IIC number is derived from sound attenuation values tested at sixteen standard frequencies from 100 to 3150 Hz. "Real-world" footstep noise is also generated at frequencies below 100 Hz, so the IIC value may not accurately describe the complete noise attenuation profile of a floor.2
Field Measurements
Laboratory IIC ratings are measured in specialised test facilities under ASTM E492, which suppresses flanking transmission so that only the floor–ceiling assembly is rated. The corresponding rating for a completed building is the Field Impact Insulation Class (FIIC), determined from in situ impact-sound measurements in accordance with ASTM E1007.3 Because field measurements capture flanking transmission and other on-site conditions that are absent in the laboratory, the FIIC of an installed assembly is typically several points lower than its laboratory IIC.4
References
References
- National Research Council of Canada - Institute for Research in Construction, http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/ctus/ctus-n35.html Deprecated link archived 2013-02-19 at archive.today
- Described in California Building Code, Section 1207.9, Sound Transmission: Tested Assemblies.
- ASTM E1007-21, Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission Through Floor-Ceiling Assemblies and Associated Support Structures, ASTM International — consensus standard; authoritative defining document (WP:RS).
- Long, Marshall. Architectural Acoustics, 2nd ed., Academic Press/Elsevier, 2014 - graduate textbook covering the lab-vs-field IIC/FIIC difference.