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.National Research Council of Canada - Institute for Research in Construction, http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/ctus/ctus-n35.html
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.Described in California Building Code, Section 1207.9, Sound Transmission: Tested Assemblies.
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.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). 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.Long, Marshall. Architectural Acoustics, 2nd ed., Academic Press/Elsevier, 2014 - graduate textbook covering the lab-vs-field IIC/FIIC difference.
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