A lot of controversy surrounds product performance in noise control. Some suppliers will actually publish exceptionally high field numbers, implying that the performance is standard. Architecturally, this can be very dangerous, since one reference point in the field does not equate to consistent performance. Remember, products themselves do not have an IIC, FIIC, STC or FSTC performance number – whole systems have performance ratings.
Sound Mat Qualities
There are three things that make a good sound mat: resilience, thickness and void space. Resilience helps to absorb sound waves, void space limits touching and the conduction of sound waves, and a greater thickness in sound mats is more useful in that there is a longer wave for lower frequencies. What that means is that thin, rigid and solid mats are less functional than resilient, airy, thick mats. The physics behind the results do not lie. Do not be fooled by a manufacturer stating, “we were surprised by the results.” There are no surprises – performance can be explained.
What we measure
We measure impact noise from 100 to 3150 Hz in 13 separate octave bands. This means that we measure the level of filtration that occurs from the noise control system at 13 reference points. A standard level of performance is established by ASTM, and deviations from this standard are then calculated. Any total deviation level greater than 32 moves the IIC level down a point, and any octave band deviation greater than 7 points also moves the IIC level down. It’s that simple. The test means that we have to perform adequately across the entire frequency range, and high or low frequency performance can limit our results. The system has to perform across a relatively broad spectrum.
Humans can hear in a greater frequency range; typically between 20 to 20,000 Hz. This means that our test does not indicate soundproofing, but instead reflects sound control.
We also adjust the results for the size and sound absorption of the room, and the background noise of the environment. Testing small bathrooms can be deceiving, since the adjustment made for room size has a positive impact on results. Occupants do not complain about what is heard in the bathroom as much as they complain about bedroom and living room noise – enough said.
So here is a radical thought about field performance evaluation. In my old job, we used to test the flow of grout for setting machinery beds. We did this in side-by-side competition with other brands. It was like watching dueling blades of grass leaves grow on steroids. But why not take this approach with noise? Remember, this does not apply to airborne noise control, but we can develop a pretty accurate test with a small sample in the field.
Here are the criteria:
1. Perform the test after drywall is installed to limit flanking noise.
2. Perform the test in a bedroom or living room at least 100 square feet in size, but preferably 200 square feet.
3. Perform without any contact with walls.
4. Use 16 square feet of mockup with finishes.
a. Pre-pour the assembly either at the site or offsite.
b. Compare mats of similar thicknesses with similar underlayment thicknesses.
c. Use the same finishes on all samples (brand, type, etc.).
5. Test in the exact same room area with all of the competitive materials.
6. Only perform impact testing – NO AIRBORNE testing or FSTC.
Viola! The results will show which products have the most impact noise control performance in the field in a relative sense. They will also indicate if the rest of the system (joist assembly, resilient channels, gypsum board installation) is performing adequately. There is a lot of value in this for everyone from the GC, flooring contractor, material supplier and owner. Everyone gets on the same page with expectations. Everyone understands the performance limitations or value of the system. I’ll go one step further and suggest that one 16-square-foot segment be tested without mat and underlayment to see an overall value.
Remember that noise control results from a system and not from a product. So this is not just a reflection of the products called “sound mats;” it is a reflection of the assembly that was designed. Do review laboratory tests prior to building the structure, since modifications to the building after gypsum board installation will be costly. The U.S. code calls for two things: an assembly that has been tested and shown to perform above 50 STC and IIC in a laboratory test, AND compliance within 5 points of that in the field. This field tests get the owner over the final hurdle.
Sounds good, doesn’t it?