Improving Sound Absorption on Ceilings and Walls

The interior design of commercial buildings often means that a variety of space types are created with a range of surface treatments on ceilings and walls. The shape of those spaces and the type of surface treatments all impact the acoustical performance of the space. Large spaces with highly reflective surfaces can create echoes, making it difficult to understand speech. Smaller spaces can suffer too from reflective surfaces with the sense of amplified noise as in a restaurant full of people. The solution to these sound issues is to use or add material to the walls and ceilings to reduce the reflected sound and increase the absorbed or dissipated sound.

Finding the most appropriate material to add to a space typically involves criteria based on three things: sustainability, performance, and design. Each of these is discussed further in the following sections.

Sustainable Materials

Any material used for building interiors needs to meet multiple criteria for durability, workability, fire resistance, and aesthetics. It has also become standard practice for many designers to incorporate sustainable materials into their projects. Toward all of these ends, one material has emerged as a popular and proven choice to create acoustical products from. Referred to as PET felt, this material is a form of polyester (like the clothing fabric) with a technical name of polyethylene terephthalate (PET). The most common use for PET felt in general is for it to be extruded or molded into plastic bottles and containers for packaging foods and beverages, personal care products, and many other consumer products. With a focus on sustainability though, PET felt is an alternative product that is routinely made from 60 percent pre-consumer recycled material. The manufacturing process creates felts that are flat but of varying thicknesses, sizes, colors, and textures. Some manufacturers make it a point to collect any felt waste and transform it to an energy source with lower key emissions than coal, thus avoiding the landfill. Additionally, they are actively researching ways to use waste to create new products. The combination of the material and manufacturing process has allowed some PET acoustical products to earn a Declare label as issued by the International Living Futures Institute, which developed the Living Building Challenge.

Acoustical Performance

PET felt products are very good at absorbing sound, making them a very effective solution for spaces that need acoustical improvement. While such products can contribute to the overall STC ratings of a wall or an assembly, they are more often used to help find the right combination of sound absorption versus reflection for a given space. This is essentially a matter of using several well-developed tools to balance the preferred acoustic characteristics within that space. These tools are known as the noise reduction coefficient (NRC) and Sabin measurements, both described as follows.

• Noise reduction coefficient (NRC): Individual materials can be formally tested according to ASTM C423: Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. This standard is used to measure the rate of sound absorption of materials on a scale of zero to one. As such, NRC ratings of a material are commonly viewed as a percentage. For example, an NRC of 0.75 means that 75 percent of the sound energy that strikes a tested material is absorbed instead of being reflected or transmitted. Note, however, that since the test determines sound-absorption rates at four specific sound frequencies (250, 500, 1,000, and 2,000 Hz), the NRC rating number is actually an average of the results across those four frequencies that are generally in the range of human speech. Hence a material with an NRC of 0 can be presumed to reflect back all of the sound striking it within those four frequencies (i.e., not absorb any), while a material with an NRC of 1 is represented to absorb all of the sound that strikes it in this range. Therefore, NRC is useful for determining the sound-absorbing characteristics of materials in many general building applications, but not for special applications where sound at other frequencies needs to be addressed.
  When PET felt products are tested, they generally achieve very high NRC ratings, including some that demonstrate more than 100 percent (i.e., NRC values greater than 1). These results will vary based on the circumstances of how it is used, the thickness and makeup of the PET felt, and variations between products.
• Sabin: When acoustic materials are placed to hang down in a space instead of on enclosing surface (walls, ceilings, etc.), a different sound measurement is needed. In this case, a Sabin is used as the measure of the total sound absorption provided by an individual sound absorber such as a ceiling baffle when installed within an architectural space. Absorption in Sabin is also measured according to ASTM C423. The number of Sabin per unit is approximately equal to the total surface area of the unit (in square feet) that is exposed to sound, multiplied by the measured absorption coefficient of the material. PET felt products are ideal for this type of application and commonly show test results with a very high absorption coefficient and Sabin ratings.

Based on the available testing data, acoustical products made from PET felt can be selected to reduce echoes and soften the sound in a space. To be the most effective, it is best to locate the products as close to the source of sound as possible. This usually means that applying it overhead (i.e., above where sounds are generated) is typically better than on walls that may be further from those sources. Nonetheless, wall-mounted products are usually designed to complement ceiling products and may be suitable on their own in some circumstances. PET felt products come in a range of types, including ceiling tiles, baffles, and clouds that are installed on supports to allow them to hang vertically or be installed horizontally as needed. Wall products are typically in the form of tiles that can be adhered to a substrate such as gypsum board or other materials.

Integrated Design

Adding acoustical material to a space means finding ways to balance the needed performance with the overall design scheme for that space. This includes finding the right textures, colors, and forms that can enhance or even help create an effective design. In this regard, it is worth noting that PET felt products are available in a wide range of options. Textures range from subtly smooth to more pronounced fabric-like appearances. Some products are intentionally three-dimensional, adding a sculpted or 3-D geometrical look to surfaces. In locations where the thickness is visible, there are choices ranging from thin 3-millimeter (0.11-inch) products up to 12 millimeters (0.47 inch), while some products use multiple layers to create thicker products on the order of more than 2 inches thick. Colors can be selected from a standard palette that includes whites, neutrals, bold, and accent colors in uniform or slightly variegated patterns. All of these attributes will vary by manufacturer and possibly between different product types or thicknesses, so it is always best to review the available options directly with the manufacturer first to confirm what desired products are available. Nonetheless, PET felt products can take on the role of being a functional product that disappears into the ceiling scape without competing against the architecture, or they can become an extension of architecture and make a bold visual statement.

In some cases, custom products may be available, with at least one manufacturer taking the approach that “custom is standard.” In this case, it takes on the challenge of fighting mediocre design by partnering with contractors, architects, clients, design firms, and fabricators. Using computerized parametric design principles, complex patterns or shapes are possible that can be created. Since this parametric approach actually simplifies the fabrication process, lead times can be shortened. At the same time, all products can be designed and fabricated to allow for easy installation, whether being used on ceilings or walls.

Overall, the integration of acoustical products into the interior spaces of a building can improve the general health and wellness of the people who use these spaces. By incorporating ceiling baffles, clouds, or wall tiles, the noise in a space is reduced. This can lead to less distractions, greater productivity and focus for completing tasks, and a more pleasant and enjoyable experience all around.

Photos (from left): © Brandon Stengel; courtesy of Turf Design

PET felt products offer great design flexibility with standard and custom possibilities for everything from bold geometric wall applications (left) to three-dimensional, undulating ceilings (right).

Conclusion

Architects and designers are well served when they take time in the design process to consider the acoustical needs of the building. Addressing the entire spectrum of sound control within individual rooms and spaces in a holistic or comprehensive manner will assure the best performance. By addressing the sound absorption, reflection, and transmission in a given room, the appropriate sound qualities can become part of a successful acoustic design. The multiple options available for materials and finishes allow architects and interior designers to retain control of the visual appearance of the walls, ceilings, and other surfaces. Further, by addressing the control of sound between spaces through walls, ceilings, floors, and specialty areas, each room can be appropriately shielded from background noise, unwanted sounds, and loss of privacy. Overall, this holistic, coordinated approach will yield a design that creates a very positive contribution to the total indoor environmental experience of the building.