Sound Transmission and Metal

Sound control is crucial for many building applications. An acceptable level of noise for residential construction, according to the Uniform Building Code, is 50 decibels for both airborne sounds (voices, music) and impact sounds (walking, moving furniture). Excessive noise in the classroom contributes to lower test scores for students and in the workplace leads to lower productivity.

Tips on soundproofing and its installation

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At work, obnoxious coworkers who scream into their phones are especially nettlesome. Soundproofing an office can:
• Improve worker concentration
• Lower work errors
• Reduce employee stress
• Eliminate conversational distractions

There are two types of noise problems to be solved: sound leakage and sound transmission. “The solutions to these two problems are easy to imagine: seal all air leaks and solidify all building materials,” says John Calder, director of marketing and communications, Acoustical Surfaces Inc., Chaska, Minn. “Isolation and mass are the two keys to noise control. Lowering problem noises to acceptable levels,whatever acceptable means to you, can usually be accomplished with existing acoustical products, though custom-designed solutions are always an option.”

Two Classifications

There are two classifications of sound isolation and noise control measures: room acoustic control treatments (absorptive/reflective/diffusive panels); and sound isolation treatments (resilient channels, privacy boards, etc.). The former are relatively easy to install, as they usually are simply screwed directly or indirectly (on furring channels or other spacers) onto walls or ceilings. The latter are often installed incorrectly, and require frequent inspections by a trained acoustician. According to Michael Wesolowsky, Ph.D., P.Eng., associate principal, Swallow Acoustic Consultants Ltd., now part of Thornton Tomasetti, Mississauga, Ontario, Canada, the most common mistake made during installation of noise control devices is incorrect installation of resilient channels. He contends they are installed incorrectly approximately 90 percent of the time.

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“The basic premise of these devices is to introduce a flexible component of a wall (or ceiling) assembly that significantly reduces the sound-induced vibrations transferred across the assembly (which reconstitutes as noise on the other side: the speaker effect),” he says. “They are meant to be screwed to the studs, and then the drywall is in turn screwed to the flexible portion of the channels. The main installation error encountered on our inspections is that the drywall screws are driven through both parts of the resilient channel, directly into the studs, thereby creating a short-circuit of the flexible portion of the channel through the screw, rendering them useless. Contractors often mistake them for common spacers.

”Another common issue is that they are installed upside-down. “They are intended to be installed with the flexible side of the channel pointing upwards, such that the weight of the drywall pulls them away from the wall (thus allowing a more ‘springy’ assembly),” Wesolowsky adds. “When installed upside-down, the weight of the drywall pushes them towards the stud, which stiffens the entire assembly and allows soundinduced vibrations to transmit more easily across the assembly.”

Connection Importance

Metal framing has a bad reputation when it comes to sound transmission. Ted White, president of The Soundproofing Co., Bay City, Mich., says yes, compared to wood, metal is more conductive, but insists that metal itself isn’t the problem, but rather the assembly technique is. “The focus is primarily the drywall-to-steel framing connection,” he explains. “Steel studs provide higher isolation than their wood counterparts. This is due to the slightly greater flex of 25-gauge steel. The wall is improved by moving to 24-inch off-center studs: more flex. After that, it’s all about adding more mass: 5/8- inch drywall, rather than 1/2-inch. Decouple the drywall from the framing using a clip-and-channel array. You create a spring system on the ceiling, comparable to a suspension on a car. The spring resists the energy transfer; it’s crazy simple.”

Sound-attenuating Curtainwalls

An alternative to rigid, permanent walls is sound-attenuating curtainwalls. Acoustic curtainwalls have Sound Transmission Class ratings up to 23+, making them an effective alternative. In a typical application of fabric curtainwalls, a metal mounting angle is installed across or along a run of the roof supporting bar joists (welded, clamped or bolted), and the insulated curtain is simply screwed to this angle along its top edge using self-drilling screws. The curtains are normally made of 5-foot-wide panels and have vertical Velcro attachment hems to join multiple panels creating a full-width curtain.

“In a fabric curtainwall installation, the loaded vinyl side of the core is typically positioned facing the offending noise source, between the source and the desired ambient sound area,” says Chuck Ashelin, engineering manager, Rite-Hite Environmental Enclosures, Milwaukee. “In a completed installation, the wall acts to trap the sound in a specific area and limit the migration of the sound to the ambient employee areas. Fabric curtainwalls can be configured to fit virtually any interior space and require significantly less time to set up than to build permanent walls. Curtainwalls also can be easily re-configured to adapt to changing production demands and floor layouts. It’s easy to add to or enlarge an initial curtainwall installation. If source sound levels increase for any reason, it is very simple to add a second layer of flexible sound curtain to an existing one, or even to augment an existing rigid wall, a fairly common application.”

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The floor-ceiling assembly must not be ignored with discussing noise transmission. “On thinner steel/concrete floors, or those constructed using lightweight concrete, there is not enough mass to provide adequate sound isolation for many occupancy adjacencies,” Wesolowsky says. “Often it is necessary to install a drywall ceiling below such assemblies. In more extreme cases¾very loud noise sources or very sensitive noise receptors¾these drywall ceilings should be hung on resilient spring hangers.”

Another consideration should be that of Impact Insulation Class (IIC). While many floors are adequate for airborne-sound isolation, they may be inadequate for impact sound created by occupants above walking around or moving furniture (scraping chairs). Wesolowsky says to provide adequate IIC ratings, it is often also necessary to introduce some sort of resilient underlayment under the finished floor.

Cambridge, Md.-based GKD Metal Fabrics sells ceiling products that are acoustically absorbing. SilentMesh uses a panel system attached to a typical drop-in ceiling grid. Panel sizes vary and a variety of woven mesh can be used. It is secured with a clip system. “Any ceiling installer that has done a drop ceiling and knows how to snap a grid together will be comfortable with our product,” says Tom Bialk, LEED Green Associate, engineering manager, GKD Metal Fabrics. “In addition, this type of installation is compliant with seismic ceiling category and can be upgraded with components to address higher seismic ratings. When it comes to wall panels, we use z clips that are simple and as uncomplicated as possible for an easy-yet-secure installation.”

Talk to the Sound Experts

Controlling noise is labor-intensive process, one that may take several steps to lower different sound sources to an acceptable level. While there is some reliable, and some conflicting, information on the Internet showing how to do this, noise control is frequently produced from a complex set of interlocking and difficult-to-identify causes. Consulting an acoustical expert may be the best solution, as experience is nearly always the most valuable part of the soundproofing process.

Also, the consequences of not performing field inspections with qualified experts during construction of floor/ceiling and wall assemblies in order to save on the project budget carries with it the high risk of installation errors. “Other errors occur during the installation of building mechanical, electrical and plumbing equipment, where they are not properly isolated,” Wesolowsky says. “Common mistakes include improperly installed spring isolators, improperly installed flexible piping and the absence of spring hangers for pipes and ducts near the equipment. The costs associated with tearing out and replacing improperly installed wall or floor/ceiling assemblies can outweigh the cost of a single acoustic inspection by several orders of magnitude.”

A resource, “Facts for Steel Buildings #4: Sound Isolation and Noise Control,” available as a free download from American Institute of Steel Construction, provides building owners and designers with a basic understanding of these issues and useful tools for addressing them efficiently and economically.

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