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Sound Isolation and Noise Control

Tips on controlling sounds and noise in metal buildings

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Whether you're in an office building or a hospital room, a classroom or your home, acoustics play a major role in how a building or space functions. When designing rooms or buildings, it's important to take into consideration how sound waves act in a space. Depending on the type of surface it strikes, sound waves are absorbed, reflected or transmitted. Sound waves reflect off of hard surfaces, such as walls, roofs and floors, which can cause an interior that echoes. To create a space that doesn't echo, you want to absorb the sound waves that bounce around a room, using materials that are light and fluffy. To block sound from travelling to other rooms, a heavier, dense material is required. Often, this means adding additional layers of drywall to make the walls or ceilings thicker.

Benjamin Markham, LEED AP, director, architectural acoustics at Cambridge, Mass.-based Acentech, an acoustical consulting firm, says the nature of the required communication within a space is what dictates the requirements for noise control and sound isolation. "The starting point is what kind of goals are there in terms of communication, on the one hand, and privacy or confidentiality, on the other hand," he explains. "Once we have an understanding of the building program, we set goals for noise control and sound isolation."

For metal buildings, Markham says there are specific constructions that help achieve those goals, starting with the walls and floor/ceiling assemblies. And within those assemblies, there are three tools designers can use to isolate sound from one space to another: mass, separation and insulation.

 

Controlling sound and noise in metal buildingsMass and Separation

Metal buildings, which are lighter in weight than a concrete building, for example, have to use different strategies to control sound than mass alone. "Unlike a really heavy mass building, you don't have the benefit of having an exterior which is not going to be as willing to conduct sound as a metal building is," explains Lucas Hamilton, manager of building science applications at CertainTeed, Malvern, Pa. One way to help with sound and noise in a metal building, he says, is by putting insulation between the surface of the building and the occupied space inside.

"In a metal building, we can design buildings that are more structurally efficient: the walls don't weight as much, and we get the sound isolation benefit from the fact that we have two masses rather than one," Markham explains. "You build a stud and there's drywall on either side of that stud.

There's a mass, and then there's a separation--an insulated separation ideally-and then another mass on the other side. And that separation is what allows us to design constructions that block just as much sound as other constructions that are much, much heavier. And we're able to do it at a lighter weight."

The same is true in floors and ceilings. "If you have a concrete slab, the sound isolation that you get is from the mass of that concrete slab," Markham explains. "With a steel building, most of the time, there is a ceiling. So when you have a steel building, you have a lighter weight building; it doesn't weigh as much as that cast-in-place concrete slab. But, with one mass at the floor and a second mass at the ceiling, we can, with a lighter structure, block just as much sound."

 

Controlling sound and noise in metal buildingsSeparation Strategies

In most projects, the type of structure and how much it weighs is determined before noise control and sound isolation are considered. "The mass of those assemblies, in terms of the basic structure, is usually set and established by other priorities," Markham says. "And that's OK, because we have other tools that we can use by taking advantage of the separation strategy."

If a floor assembly, such as lightweight concrete on a metal deck, is a given in the structure, Markham says that still leaves the ceiling as an option. Things to look at include how heavy the ceiling is, how it is suspended from the deck, and how deep the ceiling plenum is. "Those tools have an enormous impact on how much sound is isolated by the floor/ceiling assembly," he says.

Markham explains that the first thing you want to set is how heavy the ceiling needs to be. How many layers of drywall does it have on it? Is it suspended concrete panels or a lightweight acoustical ceiling tile?

Second, how is the ceiling suspended? If it's furred down with something rigid, such as a ceiling furred down with studs, it doesn't offer much true separation since there is a rigid connection that ties one mass to the next. Instead, if the ceiling is suspended with a wire hanger, it provides better separation. Better yet, Markham says the ceiling can be isolated with a resilient isolator that works a bit like a shock absorber, such as a resilient hanger.

Third, how deep is the ceiling? "The more space there is between those two masses-between the deck and the ceiling-the better the sound isolation is likely to be," Markham explains. "Especially in low frequencies. That depth needs to be coordinated with the rest of the design--HVAC, plumbing, and all sorts of things. But the depth is an important factor in how much sound gets isolated as well."

And, once the depth of the ceiling is set, insulation should be added between the deck and the ceiling. For wall assemblies, Markham says having a stud wall with drywall on either side of it provides some separation. The first upgrade is to add insulation to the cavity, which provides additional benefits. To improve the separation further, he suggests adding mass (e.g., two layers of drywall on each side instead of one). For the best acoustical improvement, introduce separation by creating either a double-stud or staggered-stud wall. Another option for providing separation is a resilient channel or clip, similar to ceiling assemblies.

Once the ceiling and wall assemblies are in place, Markham says it's important to maintain their integrity. For walls, that means not having outlets back to back. For ceilings, it's not putting in a bunch of recessed lights and HVAC connections that poke a lot of holes in it. "Coordinating ceilings in a way that is in balance with your acoustical goals with the other building systems, building needs--lighting, HVAC, sprinklers, and so forth--is quite an important step, to make sure you are getting the most sound isolation benefit out of the ceiling that you've designed."

 

Controlling sound and noise in metal buildingsInsulation and Energy Efficiency

In general, energy-efficient metal buildings tend to be very quiet metal buildings, since sound and heat flow are closely tied together in regards to what types of materials they are conducted by. "Thermal insulation plays this dual role of reducing the acoustic energy as well as the thermal energy flow," Hamilton says. "Because it's playing both of these roles simultaneously, that's why you get this quiet and comfortable building."

"Insulation is useful because it absorbs sound between those two masses," Markham says. "It can also provide some structural damping of the skins, such as drywall. But what it's mostly useful for is absorbing the sound that's inside that cavity, and that boosts the sound isolation performance of the overall wall."

There are many types of insulation available--mineral wool, fiberglass, cellulose, open-cell foam, Icynene, etc.--and there are minor differences between them and how they absorb sound. Note that closed-cell foam is not acoustically absorptive.

For thermal control, Hamilton says you also want an insulation that is appropriate for sound absorption. "The ones that absorb sound best are soft and fluffy, rather than rigid board insulations, which may be good thermal insulations but are not good at controlling sound," he explains. "You want something that absorbs sounds, similar to if you were to stick your head under a pillow. That's why fluffy insulations work better than rigid insulations."

"What's key is that the building insulation itself have a high noise reduction coefficient (NRC)," adds Hamilton. "You want this material to be one that absorbs sound as opposed to conduct sound." NRC expresses the ability of materials to absorb sound at multiple frequencies, with the higher the number, the better the performance.

 

Final Touches

Once the floor, ceiling and wall assemblies are set up for sound absorption and blocking, it's important to avoid penetrations and use acoustical sealants to get the most benefit. "Providing acoustical sealants at the penetrations and at the perimeter of these assemblies, is a way of preventing leaks and gaps that would undermine the performance of the assembly," Markham explains.

Different kinds of structures provide different degrees of damping on the skins that are applied to them. Markham says this is true of drywall and windows, and as such drywall that is not well damped will not provide as much sound isolation benefit at certain frequencies. "There are products out there that introduce damping to the mass layers," he explains. "There are windows that have internal lamination, for example, and that internal lamination introduces damping to that glass, and that means that at the frequencies where this particular issue is relevant, the introduction of that damping improves the sound isolation."

Markham goes on to say that there is drywall with a viscoelastic damping layer inside, which provides more damping than traditional drywall. This provides better performance at the frequencies that are most effected by damping.