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Acoustic Control in Sustainable Building Design

With growing acceptance of acoustic control as a key contributor to indoor environmental quality, building project teams are looking for practical, new ways for reducing ambient noise to enhance occupant comfort and concentration.

Acoustical science fundamentals

a. Most architectural acoustic situations consist of a sound source, transmission path, and receiver. Sound originates from various sources outside the building or adjacent spaces inside. Its transmission paths are building elements through which ambient noise from the source travels. The ‘sound receiver’ is a person or group of people who occupy the building space and hear the sounds passing through.

b. Sound intensity, or loudness, is measured in decibels (dB) and can range from very faint to intolerable sound levels. The more intense the sound, the higher the decibel level and the higher the detrimental impact on people. c. All background sound in an indoor environment—including from outdoors, building services, utilities, and conversations or functions of people in adjacent spaces—is generally referred to as ambient noise. While a certain amount of ambient noise in the background is common, an excessive amount can seriously degrade the ability to communicate, making it more difficult for people to hear and speak without raising their voices.

d. Sound transmission class. A project’s acceptable ambient noise level goals are achieved by restricting unwanted sound from entering the spaces being designed. This means creating wall, floor, and roof components or assemblies that

first effectively block the amount of airborne sound transmitted through them. The measurement for this effectiveness is determined by a sound transmission class (STC) rating. A higher STC rating means more airborne sound is blocked by the component or assembly. Lower STC ratings mean more sound passes through the components or assemblies, adding to the background noise level in the space—this degrades occupant ability to hear and understand speech. Contrary to popular notions, airborne sound does not exactly pass through a structural element. Sound generated on one side of a wall energizes the wall structure and sets it in motion, much like a diaphragm. The wall itself becomes the transmitter of the sound energy, which can be heard on the opposite side of the wall by the listener. Hence, the ASTM test methods used to determine STC ratings have focused on this direct transmission process.

e. Impact insulation class. Beyond airborne sound, it is important for multi-story building designs to address the resistance of structure-borne sound, usually created by people walking or creating other impacts onto the floor-ceiling above a space. Similar to STC ratings, which address airborne sound, floor-ceiling assemblies can be evaluated based on impact insulation class (IIC) ratings. f. Reverberation time and speech intelligibility. Reverberations, or echoes, are created when noise reflects off hard surfaces, such as concrete and glass, in interior spaces. Reverberation time (RT) is the acoustical concept measuring how many seconds it takes for a sound to become inaudible in a space. Excessive reverberation can impair speech intelligibility, as the echoes often distort speech and impair verbal communication.

Wall assembly acoustical strategies

Over the years, architects have employed a variety of wall assembly design strategies to meet sound transmission class (STC) rating requirements on projects, some of them being either expensive or complicated to install properly. There have been four methods used for improving the sound containment of wall assemblies—installing additional sound-absorbent material in the wall cavity, adding extra layers of gypsum board, decoupling, and laminated noise reducing gypsum board.

1. Sound-Absorbent Material The most common and affordable way of improving a wall assembly’s STC rating is to install additional sound-absorbent material, usually fiberglass batt insulation, between studs. Of course, all building codes and green building standards already require insulation in wall assemblies to meet their minimum thermal resistance (R-value) requirements. While these base insulation levels certainly contribute to the wall’s STC rating, adding insulation can only help improve the rating, as well as the overall R-value. In addition, combining this simple, economical technique with other acoustical wall assembly design strategies will often lead to an even better STC rating. 2. Extra Gypsum Board Another traditional practice for improving the sound containment of a wall assembly is using additional layers of gypsum board. Though this can be an effective method for improving the STC rating, it’s often a challenging installation, as both layers of gypsum board have to be lined up in a specific way to achieve optimal results. One common error in these installations is the failure to offset the seams on the boards for each layer, as well as from the seams on the opposite side of the wall. Installers should also offset vertical seams whenever the gypsum board is installed in a horizontal position. Offsetting the seams blocks any direct sound transmission path through the wall assembly. Installations with multiple layers of gypsum board also require extra care when making and sealing penetrations, such as electrical outlets, light switch boxes or telephone boxes, which also can create direct paths for sound transmission. It’s also important to remember that, in congruence with the law of diminishing returns, each additional layer of board contributes less to the overall STC rating than the layer installed below it. 3. Decoupling Decoupling reduces sound transmission by increasing the air space of the partition, which allows sound to dissipate within the wall cavity before transmitting to the other side. One way to do this is to build a double-stud wall—essentially two walls in the same assembly. The studs of the second wall are staggered to avoid contact with the studs of the first wall. Though effective, this increases material costs and wall thickness. Another often-used decoupling technique is the installation of metal resilient channels perpendicular to the direction of wall studs or ceiling joists—the gypsum board is fastened to the channels rather than directly to the studs or joists. This technique can produce great results, but it’s effectiveness is easily diminished with even minor installation errors. One common error is using screws that are too long, locating them directly over a stud and making contact with the stud. This gives sound vibrations a direct path to the stud, which will then allow the stud to transmit the vibrations through to the next room. Known as “short-circuiting,” this installation error nullifies the acoustical advantages of the resilient channel and significantly reduces the wall assembly’s STC rating. The sound-dampening abilities of resilient channels can also be compromised after installation, during picture-hanging or whenever heavy objects are pressed against the wall.

4. Laminated Noise-Reducing Gypsum Board Laminated noise-reducing gypsum boards are intended as a more beneficial, easily installed replacement for some of the traditional acoustic control methods used on interior walls and ceilings in residential, commercial, or institutional applications. A factory-made version of a sandwich-style field fabrication sometimes used by drywall contractors; the boards consist of a layer of viscoelastic polymer applied between two specially formulated thin layers of gypsum board. The final product ends up being 1/2- to 5/8-inch thick, or the same as traditional gypsum board. The boards have the ability to impede sound transmission by using the inner polymer layer as somewhat of a shock absorber. The polymer converts the acoustic energy into thermal energy that is dissipated across polymer sound transmission. This type of “constrained layer damping” board product performs well acoustically over an extended range of frequencies and significantly increases STC ratings for the assembly. Wall assemblies that use a single layer of noise-reducing gypsum board have been tested and shown to meet or exceed the acoustic performance of assemblies that use double layers of traditional gypsum board. In fact, these boards have been shown to increase the wall assembly’s STC rating by 10 or more points, while adding an extra layers of traditional gypsum board increased STC by two to three points. Laminated acoustical gypsum board is therefore an excellent acoustic solution for meeting STC requirements without complex techniques, such as isolation clips or resilient channels. Directly applied to framing, they provide more consistent and predictable acoustic performance. The laminated boards can still, however, be used in conjunction with resilient channels or clips in wall assemblies to achieve extra sound transmission control. In these cases, the material would also help reduce the negative effect of any short circuits. Using single-layer noise reducing gypsum board can also help reduce material usage, versus traditional multi-layered gypsum wall. The high acoustic performance of the product makes it possible to build effective noise-reducing walls with less material, conserving valuable square footage, and saving both construction time and material cost. Less material used also means a more sustainable structure.

In conclusion

Acoustic control has increasingly gained recognition in recent years as a key component of sustainable design, due to its role in creating positive indoor environmental quality (IEQ). Its growing importance is evidenced by its inclusion in the requirements of prominent green building programs and codes, such as LEED® for Schools, the Green Guide for Health Care and the International Green Construction Code (IgCC). To create comfortable interior environments with minimal noise and distractions in high-occupancy buildings—especially schools and healthcare facilities—it’s crucial to have a good acoustic control strategy. And wall assemblies are one of the most important focal points of a space when considering acoustic design.

The author Ashwin Himat is the Founder and President of Endurance Growth Strategy Consulting LLC, that provides personalized advisory and consulting service on growth strategies and innovation to the building materials industry. He is a Business and Product Development Executive who has created organic growth through new products, markets and technologies, and inorganic growth through M&A, for 20+ years in the building materials industry at large global and public companies such as Louisiana Pacific (NYSE: LPX, $4B) and CertainTeed (CAC: SGO, €50B).

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