Because multi-family has been one of the strongest areas of growth in housing construction in the last several years, there has been increasing focus on acoustics and noise abatement.

There are three basic types of noise transmission from one living space to another:

1. Airborne—voices or music
2. Impact—footsteps or dropped objects
3. Structural component movement—resulting in squeaking or creaking floors.

Most local building codes have standardized on two methods of measuring noise: Sound Transmission Coefficient (STC) measures the airborne sound moving through a floor, and Impact Isolation Class (IIC) measures sound from impact on the floor. Both can be measured in a laboratory as well as on site. In both the STC and IIC measurements, the higher the number, the better the performance of the sound abatement solution being used.

The Uniform Building Code (UBC) requires lab measurements of 50 dB and field measurements of 45 dB for both IIC and STC. While not all local building codes are the same, the UBC is the most common benchmark in the U.S.

After local building codes are consulted and IIC and STC requirements are determined, the next step is to identify the best method of achieving or surpassing these requirements.

Cork, rubber or composite underlayments are effective barriers to airborne or impact noise transmission. For hardwood flooring, new developments in adhesives with exceptional sound abatement properties are rapidly growing in the market. The most premium products even incorporate recycled rubber spacers within the adhesive to increase sound abatement performance and ensure that the specified membrane thickness is maintained throughout the entire installation.

Drop ceilings below a 6-inch concrete slab are used to achieve a significant improvement in acoustic performance, but in many cases are beyond the scope of the project.

Each method has benefits, and various methods used together can further reduce acoustic transmission between floors.

The best solution or system of products depends on the specific application, materials, budget and project time constraints. When considering any system of products, it is critical to read and understand the manufacturers’ requirements and installation instructions of each product. Any conflicting requirements will require sourcing of alternate materials to ensure that the entire system works together.

For instance, if an acoustic underlayment manufacturer requires that the flooring be floated, but the hardwood flooring manufacturer requires the wood to be bonded, the system will not work together and new compatible products must be found.

Because there are so many different products with different requirements and installation methods, developing a one-size-fits-all approach is next to impossible. However, there are some common considerations during installation that will apply to most applications.

One of the most common mistakes is to nail hardwood flooring through acoustic membranes. This significantly reduces effectiveness as sound wave vibration is transferred by the nails through the acoustic membrane and into the substrate below.

The products selected also impact project timelines. Advanced adhesives that combine sound abatement can be installed in the normal time required to install a glue-down hardwood floor. Installation of cork, rubber, or composite membranes often require that the membrane be glued down first, allowed at least a day to fully cure, and then crews can come back in to finish the hardwood installation.

In renovation projects, care must be taken to ensure that the increase of height of the flooring because of any acoustic solution does not cause other issues. The added thickness may require modification to cabinets, base molding or other built-ins to accommodate this increased height, which also increases the cost of the project. Quarter-inch-thick cork is generally effective to achieve UBC requirements but increases the height of the flooring by more than just the additional ¼-inch, as you also need to factor in the extra height from two layers of adhesive. Adhesive systems that are typically only the thickness of two credit cards have a distinct advantage here by eliminating the ¼-inch thickness of the underlayment.

Two considerations regarding floating floors are acoustics within the room and extra expansion/contraction requirements. Using floating hardwood floors on acoustic membrane underlayments does usually result in the targeted IIC and STC values; however, they often do not significantly reduce the sound of footsteps within the room. When there is an impact on a floating hardwood floor, the boards vibrate. Downward motion is absorbed, but the upward rebound is not. This results in a hollow sound in the flooring. Adhesives with sound abatement characteristics work to reduce the vibration of the boards in both directions resulting in a quieter floor.

Expansion and contraction of floating floors is also a concern—especially in new construction. Care must be taken to assure that the flooring has adequate room to move around built-ins, baseboards and other fixed items, or the flooring can “walk,” showing gaps around perimeters or, in severe cases, can be pulled apart.

Adhesive acoustic membranes are emerging to be the fastest growing segment of the sound abatement systems market, especially in specifications for high-rise residential construction. Other, more established products, can be used in conjunction with the high-performance adhesive membranes to further increase noise abatement, moving from a solution that simply passes code to a system that dramatically increases the comfort of the space and virtually eliminates one of the most common complaints of multi-family living or multi-story homes.

Eric Kurtz leads Bostik’s Hardwood & Resilient Flooring Systems as market manager with over 20 years of experience in product management and development. He also holds the special designation of Engineer-in-Training (EIT) from the National Council of Examiners for Engineering and Surveying and is a LEED Green Associate. You can reach him at (414) 607-1685 or eric.kurtz@bostik-us.com.