Colorado Real Estate Journal - April 20, 2016
With all the multiresidential apartments, condominiums and townhomes being built, one of the biggest acoustical design challenges is impact sound isolation. The trend or desire to use thinner and lighter construction materials often result in complaints from the resident. On top of it, the national standards that apply do not account for low frequencies or people’s satisfaction. From our work on many of these projects, we know the International Building Code Sound Transmission Class and Impact Isolation Class minimum required rating of 50 does not provide the privacy that most people desire. This may be connected to the fact that you can hear low-frequency impact noises or “thumps.” The type of structure used directly affects the level of sound isolation that can be achieved. Wood structures are lighter than steel or concrete. They are not as stiff, so when your neighbor is doing jumping jacks over your head, you may not hear his voice, which is in the mid- to high-frequency range (500 hertz to 8,000 hertz) but you can hear the low-frequency (31.5 Hz to 125 Hz) impacts. The light structure gives and transfers the energy to the space below. To add to the challenge, fitness spaces are included as an amenity for the residences but they are located above the units. Here’s the rub. ASTM standardized tests for Impact Isolation Class require the use of a special tapping machine and include testing for frequencies between 125 Hz to 4,000 Hz. The tapping machine is not effective in generating impact noise at frequencies lower than 125 Hz. The hammers of a standardized tapping machine that impact the floor surface weigh a little over a pound each are dropped a height of a little more than 1½ inches. While this provides a standard method to compare the assemblies, it does not mimic the impact of a person walking in hard-sole shoes. This means you can test and pass a floor-ceiling assembly as compliant with the requirements but the occupants can still hear low-frequency impacts, thuds or thumps from footfall. Unfortunately, the Impact Isolation Class ratings for what are considered to be low-, medium- and high-quality apartments or condominiums are not in sync with people’s satisfaction – most notably for wood structures. To address this issue, the structure needs to be stiffer and mass added to the floor-ceiling assembly. This is why we review the octave-band transmission loss at the frequencies below 125 Hz when comparing the acoustical performance of assemblies. We also have conducted nonstandardized tests dropping larger objects like a medicine ball to measure the low-frequency impact isolation. Bottom line, reducing low frequency impact noise is very challenging, particularly in wood structures. There is no silver bullet (i.e., the paper-thin underlayment that allegedly increases the impact isolation by 25 points or more).Carpet and pad used to provide good levels of impact isolation but are now being replaced by wood, tile and vinyl toppings. For this reason, it is now more important than ever to provide structural stiffness and a decoupled mass layering to isolate residences from each other and reduce that thump, thump, thump in the middle of the night.