STC and IIC, The Deceiving Metrics of Building Acoustics
Have you ever tried to design or build a room for acoustics? The acronyms can make you feel like you just joined the military, and they don't always make that much sense. Two of the most common metrics for assessing materials in building acoustics are STC and IIC.
STC (Sound Transmission Class) is used to rate the performance of a building assembly to block the transfer of noise. This is commonly referred to as soundproofing. STC is useful for reviewing the acoustics of walls, floor/ceilings, doors, or windows. The single integer represents the decibel reduction that one can expect from the assembly. While STC ratings are determined in a lab setting to rate a single assembly, ASTC testing is completed in situ and assesses the noise transfer from one room to another including any flanking paths the noise can take through the adjacent floor and ceiling. ASTC is a more accurate assessment and is referenced in the 2015 Canadian Building Code.
IIC (Impact Isolation Class) is used to rate the performance of floor/ceiling assemblies to mitigate the noise associated with impact. This could be used to assess the noise implications of weights being dropped in a gym or footfall on a tile floor. Like ASTC, AIIC assesses the noise from impact in a field setting, not a lab, making it a more useful metric.
Both of these ratings are incredibly useful, but when it comes to acoustics the phrase "the devil is in the details" often applies.
Lab vs the Field - Lab tests give us the maximum performance of a product or assembly of materials, not the expected result in the field. This is why ASTC and AIIC ratings are more reliable and are usually 5 points or more lower than their lab counterparts.
Frequency - With just one number to account for the rating, both STC and IIC ratings will be less accurate if the noise is not near the middle of our hearing range. This means that a low-frequency bass sound will typically not be mitigated anywhere near the level you might expect. A dropped weight on a hard floor or a subwoofer in the apartment next door is much harder to reduce than higher-frequency noises. The tests also have a limited range of frequencies that are considered, so noise from outside these frequency ranges are not a part of the rating.
Third band octave results for an STC 32 wood stud wall with 1 layer of 12mm type X gypsum board on each side. Courtesy of National Research Council Canada.
Testing Method - This mainly applies to impact testing. The method for testing floor/ceiling systems for impact ratings uses a specialized tapping machine that generates a standardized series of impacts where the speed, force, and impact are always the same. This is essential for comparison purposes, but the machine is not intended to duplicate the force or frequency of all impacts. Impacts from heavy objects like weights are not represented well by this current IIC test and some manufacturers have started developing their own test methods to generate more meaningful results.
What was Actually Tested? - Last but not least, the ratings you will see in some product literature are not for the product itself but for the assembly it was tested with. This is where people are most often misled and end up unsatisfied with their results. For products that are meant to be just one component of a wall or ceiling/floor, the IIC and STC rating will be based on a complete assembly that may not resemble your construction. IIC tests are generally conducted over a concrete slab for example, so they won't give you the same results if you have wood joists. It may be that it is the other components of the test that were the most crucial to achieving the high values and have little to do with the product displaying the excellent STC or IIC number in their literature.
To achieve the acoustics results you are striving for, you should always look beyond the number and obtain the test results. The test report will show you if the assembly that was used resembles your design. It will let you know if you will require other materials such as a specific underlay or stud size in your design. They also will have the sound level reductions at each octave or third-octave band so you can compare them to your expected noise source frequencies.
Tony Adamson, AScT is a registered applied science technologist in the province of British Columbia and has 25 years of experience in noise and vibration control. He has designed over 50 high STC/IIC floating floor systems in buildings in BC and Alberta.