Happy New Year! As we roll into 2026, we want to bring things back to basics and discuss two of the more recognized, but often misunderstood, sound isolation performance metrics. Sound Transmission Class (STC) and Impact Insulation Class (IIC) ratings for interior partitions are used in IBC, LEED, FGI, and many other codes and guidelines; however, both ratings have limitations. Read on to discover if your project can rely simply on these metrics or if a more comprehensive sound isolation performance evaluation is needed.
Loosely speaking, the STC rating of a wall or floor/ceiling assembly describes its ability to block airborne sound. A higher rating means the assembly can provide a higher degree of sound isolation. However, the STC rating only considers frequencies between 125 and 4000 Hz (speech frequencies for the most part), meaning that the partition’s ability to isolate airborne sound from low frequency sources, such as subwoofers, is not considered in the rating. It is actually possible for a wall with an STC rating in the 40s to significantly outperform a wall with a rating in the 50s at frequencies lower than 125 Hz. Small subwoofers with 6”-8” driver diameter (e.g., Sonos Sub Mini, Audioengine S6) are typical for popular bundled audio systems commonly found in residential living rooms. Even though these subwoofers are small, they can still generate low frequency signals at 50 Hz and even lower. Therefore, your STC 50 or better wall meets IBC compliance for a residential separation wall, but the tenant may still find their neighbor’s small Sonos audio system objectionable.
The IIC rating intends to quantify the ability of a floor/ceiling assembly to control impact sound, such as footfall. Like STC, a higher rating means a higher degree of isolation. And similar still to STC, IIC is limited in the frequencies it considers; the rating evaluates frequencies down to 100 Hz (clicking from hard-soled shoes). One common issue is footfall in wood-framed residential buildings. Achieving IBC compliance with an IIC 50 or better assembly does not necessarily mean that low frequency sound, such as thumps from running children or a heavy walker, is adequately controlled. There are many flooring products on the market that may show IBC compliance in laboratory tests, such as MgO boards and integrated LVT planks, when used in wood-framed construction; however, the low frequency control of these products may not have been thoroughly vetted. In general, in addition to a resilient layer, an element of mass and stiffness, such as a 1” cementitious topping at a surface weight of approximately 10 PSF, is needed in the wood-framed floor construction to adequately control low frequency sound from typical occupant impacts.
The discussion above describes just a couple simple examples of the limitations of the STC and IIC rating systems. We did not even touch upon heavy impact and high sound energy sources, such as those from fitness centers and wedding reception halls, for which STC and IIC are not the most appropriate evaluation tools. Don’t let STC and IIC ratings get the best of you – hire a trusted acoustician for your project to determine when the sound control evaluation needs to extend beyond these commonly used (and frequently misunderstood) metrics.