Acoustical Control Strategies

The main ways to minimize sound transmission from one space to another are by adding mass and breaking the path of transmission materials within the building assembly. Some examples of breaking the path of transmission include decoupling structural components using an air space (e.g. double wall with an air space between), or adding resilient isolators (e.g. isolation rubber beneath wall plates to decouple the wall from the floor it bears on). This latter approach is not as effective as a true separation with airspace, as some vibration may still transfer through the isolating material. However, it is the only way to achieve some isolation of structurally bearing components.

Interior Room Acoustics

When a person speaks inside an empty room finished in drywall with a wood floor, the sound carries very well. All surfaces are smooth and hard, so sound bounces from one hard surface to another until all of the sound energy is absorbed into the structure. Because the materials are relatively dense, this absorption happens slowly. Acoustic engineers call this an acoustically “live” room. It will get noticeably deader (quiet) when carpet, furniture, and acoustic ceiling tile are installed. These finishes cut reverberation by absorbing sound waves into their soft, cellular surfaces.

Carpeting the floor, for instance, will deaden most rooms enough for most applications; however, carpet is not always desirable or practical.

Controlling the acoustics within a room is more of an issue in large spaces, such as schools and hospitals, than it is in homes. In homes, particularly multifamily dwellings, client noise problems are more likely to involve sound transmission from one unit to another, than the deadening of sound levels within the dwelling unit.

Sound Transmission

Simply installing soft, sound-absorbent materials over the surfaces of one room doesn't do much to stem the transmission of airborne sound to another. Acoustical ceiling tile, for example, is often misconstrued as a way to reduce inter-floor sound transfer. It has little effect on sound transmission. It was originally developed as a means to help deaden noises within office spaces, and had some effect on muffling noises from HVAC ductwork passing through the space above a dropped ceiling.

To reduce sound transmission, designers need to focus on sound isolation—breaking vibration pathways through structural materials. Adding sound absorbing materials, such as insulation in building cavities, will also help; but, it's the separation that really counts when blocking both airborne and impact sounds is required.

In multi-family dwellings, the focus is usually on the floor-and-ceiling assemblies and wall partitions. In educational and healthcare facilities, floor-ceiling sound transfer is part of the problem, but special attention to wall partitions for sound-isolated classrooms, libraries, in-patient rooms, consulting rooms, counseling offices, and maternity wards is needed. Sound isolation is also key for intensive care units and surgical rooms where quiet is requisite.

Floor-ceiling assemblies present one of the biggest challenges, because the floor needs to support the live loads within the occupied space. Tools left to the designer include:

• Using soft finish flooring materials that absorb impact vibration (e.g. carpet)

• Adding mass (e.g. using concrete over steel pan or pouring a gypsum-concrete subfloor topping over a plywood subfloor)

• Installing acoustical underlayments beneath the finished floor

• Decoupling the ceiling using resilient channels or isolation clips

When carpet is not an option, unwanted impact sound transmitted through a floor-ceiling assembly can be reduced with the use of an acoustical underlayment. The primary purpose of this material is to isolate the flooring and any flooring impacts from the building structure.

The chief property of an effective acoustic underlayment is resilience. To reduce impact transfer, the material must be able to absorb impacts by compressing and then returning to its original thickness. It does not need to be soft or fibrous; rather it needs to behave like a spring. In fact, soft, fibrous materials tend to absorb water, either from the subsurface below or from wet-use conditions above, making them unsuitable in many applications. A good, resilient underlayment should be water-resistant and capable of supporting the finished flooring.

There are a number of acoustic underlayment materials on the market. The most common materials used in sound-rated building assemblies include:

• Foam

• Cork

• Mass-Loaded Vinyl

• Asphalt Roofing Membranes

• Felt

• Chlorinated Polyethylene Sheets

• Re-bonded Recycled Rubber

Table 3 provides a comparison of attributes a designer can expect from each type of material. Selecting which one to use should be based on a number of factors, including the type of sounds expected in the building and how well the underlayment will support the finished flooring materials. Remember, to be acoustically effective, the underlayment must break the path of vibration. If the finished floor is installed over battens attached to the subfloor, and the material fills the spaces between battens, it won't be effective. Impact vibrations will simply transfer from the finish floor through the battens. Some materials, such as cork, and to some extent felt can be relatively effective at blocking sound, but there is limited structural evaluation of how well it can support tile. The material that is most effective at blocking sound with a proven record of supporting a wide range of finish materials, including tile, is re-bonded recycled rubber. It is also water-resistant, so it can be used in a wide variety of applications.

This table provides a comparison of a variety of floor underlayments commonly used in sound blocking

With all the materials commonly used as an acoustic underlayment the most important thing to review is the published test data. An example is this statement for an acoustical underlayment:

“Test construction included a sound rated ceiling, concrete substrate 6” x 6” quarry tile, and latex modified thin-set... IIC = 62, STC = 59... ASTM E2179 ΔIIC = 12”

While this sounds impressive, there is no mention of the thickness of the concrete substrate or the details of the sound rated ceiling, both of which can have a significant impact on both IIC and STC ratings. The delta IIC rating doesn't specify which floor finish was tested. Quarry tile on a 6-inch slab alone can achieve a 12 dB improvement.

The only reliable criteria designers have to evaluate the acoustical properties of any one material are the IIC and STC ratings. Make sure that the referenced standards match the actual ASTM standards, using Table 4 as a reference.

This table lists the acceptable ASTM standards for evaluating sound-rated materials for floor-ceiling assemblies. Keep in mind that all of the tests rate a building assembly, not an individual material. To be useful, any published reference to one of these Standards should detail the components of that assembly.