Sound Reduction Solutions:
What the Ratings Mean, and How to Select the Right Products to Get the Results You Need
March 30, 2007
Earlier this year, I took my wife to see the movie “The Devil Wears Prada” with Meryl Streep, because this is what a man in love does. I have to say that I was amused by one scene, where fashionable young women in high heels walking across the marble floors of a cavernous office building are labeled “clackers.” I now refer to my wife as a clacker – not in a derogatory manner, of course, but because when we are getting ready to go out, I shower and dress much faster than she does. So, I head downstairs to my office and do a few things while she finishes. I always know when she’s ready to go…see, I installed beautiful travertine over a cement backerboard unit, on top of a plywood subfloor. The last thing she does is put on her high heels, and I can hear their impact on the hard surface transferring down to my office, which is directly below the master suite.
This sound is music to my ears, because it signals that my lovely wife is ready to spend the day with me. But coming from anyone else – say, a neighbor in an upstairs apartment or condominium – it would be downright annoying and disruptive.
Hard Floor Construction and Sound ControlSound control is an extremely important issue in construction, especially with the growing trend of multi-unit residential development where it’s essential to minimize or eliminate sound intrusion between floors.
Because hard floors such as wood, tile or stone do not absorb sound and vibration, special spongy or absorbent materials (usually mat underlayments) are often installed beneath the floor surface to provide acoustical control. These underlayments can vary widely in terms of materials, effectiveness and ease of installation. Cork was traditionally the material of choice, but new research and technology have given us materials like expanded polypropylene, which is not susceptible to mold, mildew and decay, weighs less than cork, and performs as well or better in sound control tests.
Understanding Sound Control Measurements and RatingsSo, with new sound control products on the market, how do we evaluate them and make the right selection? You’ll probably find a broad range of numbers and “sound reduction” claims assigned to these products, so it is important to know how to read the labels and understand what the numbers mean in order to select the materials that will perform best for your project.
There are two general categories for measuring sound transmission: sound transmission class (STC) and impact isolation class (∆IIC). STC measures sound that is transferred through the airwaves, such as music or speech. ∆IIC measures sound that is caused by impact, such as walking or dropping something on the floor, and transmits through the structure to a space below. ∆IIC class is the most relevant for hard flooring, so we will examine ∆IIC measurements and ratings, and what they mean.
Most of us are familiar with ASTM International (originally the American Society for Testing and Materials), which develops testing procedures and measurements that are accepted as the industry standard. A complete ceiling assembly has a ∆IIC rating. The International Building Code (IBC) has a minimum requirement of ∆IIC 50, meaning that the ceiling, made up of the sum of all materials used in the floor-ceiling construction, should equal ∆IIC 50 or higher in order to provide adequate sound control. The higher the ∆IIC value, the higher the level of sound control.
Sound reduction materials used in the construction, as well as different types of ceiling constructions, like a drop ceiling, will contribute to the ∆IIC rating of the total assembly. This contribution is also assigned a rating value based on that product’s performance in one of the approved ASTM tests. These tests are independent laboratory measurements of sound impact through floor-ceiling assemblies using a tapping machine.
Measuring Individual Products vs. Complete AssembliesSounds simple, right? But it can be misleading if you don’t understand the testing methods behind the ratings. There are several different ASTM tests that manufacturers may use to measure either a total assembly’s performance, or a product’s individual ∆IIC performance, and knowing what they are makes a big difference when it comes to evaluating and comparing specific products.
Some tests, such as ASTM E-492, measure the performance of a complete resiliently suspended ceiling system, including drop ceilings, air cavity insulation, resilient channels and double-layer gypsum boards. The resulting ∆IIC rating, usually ∆IIC 50 or higher, actually measures the effectiveness of all components working together. For example, a manufacturer might label an underlayment “∆IIC 70.” Sounds impressive, but this really means that the underlayment was one part of a complete system that achieved a rating of ∆IIC 70. The performance of that single material cannot be extrapolated from this number, and you should not assume how effective that material will be in reducing impact sound solely based on this rating.
Comparing Sound Control CapabilitiesInstead, the best indicator of a product’s impact sound reduction capability is a product’s ∆IIC rating. This refers to ASTM E-2179, a test that isolates the acoustical performance of a single material within a complete ceiling assembly, and therefore allows “apples to apples” comparison between products. For example, a mat underlayment rated ∆IIC 23 can be accurately compared to another underlayment that is rated ∆IIC 13. The ∆IIC 23 product would be the better choice for an installation that needed a higher overall ∆IIC rating – like my travertine floor above my workspace – while the ∆IIC 13 product would be sufficient for a project with less extreme needs. Comparing the ∆IIC is the only way to truly discern the differences in performance, and select the right mat underlayment for your hard floor installation.
While my travertine floor installation clearly has a less-than-ideal ∆IIC rating, I wouldn’t change a thing since it lets me hear the sweet sound of my own dear clacker.