Haas, the acoustical engineer, says that the new panels have made a big difference in sound control projects. He estimates that the real world (or “field”) performance of a wall assembly using standard drywall with resilient channels drops between 8 decibels and 12 decibels compared to its (higher) performance in the lab. When sound-absorbent panels are used, however, performance dips between 4 decibels and 6 decibels, he says.
Of course, the new panels do cost more than conventional materials. Depending on the product, the retail price for a half-inch sheet of 4×8 drywall would be around $50 before any volume discounts. For maximum efficiency, performance, and value, though, manufacturers recommend looking at a project’s entire wall assembly. For example, the new sound-absorbing drywall is effective enough [in terms of sound control that a builder can downgrade a wall assembly from double to staggered studs, while also increasing its acoustical performance. (Quiet Solution and Supress both have extensive technical resources on their Web sites, including sample wall assemblies.)
For those worried about the price of such materials, offer it as an upgrade to condo buyers, Doty suggests. For instance, sound-absorbing drywall could be a standard feature in party walls separating units, but offered at extra cost in other places in the unit, such as between a master bedroom and other rooms.
SOUNDPROOFING STRATEGY As helpful as sound-deadening drywall and plywood can be, though, these materials don’t reduce the need for a comprehensive strategy to block sound paths. Most developers are aware of the need to seal around penetrations like electrical boxes and HVAC ducts, but those penetrations aren’t the only culprits. For instance, Schwob has seen problems in high-rise buildings where curtain walls are hung from the slab with a gap in between. “Sound goes around the edges of the slab and into the unit above or below,” he says. Unless these gaps are properly sealed, then even the best floor and wall designs will be ineffective.
The other drawback is that these new panels don’t absorb different types of sound equally. The ability of a wall assembly or material—including the new sound-absorbing panels—is expressed as its sound transmission class, or STC rating. “STC ratings were designed to measure sound transmissions at middle frequencies: people talking, pots and pans banging around, or perhaps a single console radio,” says Brandon Tinianov, an acoustical engineer and Quiet Solution’s chief technology officer. They don’t indicate how well a material or an assembly will block sound at very low frequencies. That was fine in the 1960s, Tinianov says, but not so helpful today, thanks to sub-woofers that transmit the low bass sounds of an action movie to a resident and all the neighbors.
Despite this drawback, experts say that STC ratings are still useful as a relative measure and that a resident will be happier with a wall rated at STC 50 than with one that gets a 30. Research backs this up. The National Research Council of Canada recently surveyed multifamily residences to determine residents’ satisfaction with different ratings. The result was that complaints were reduced for walls with a rating of 50 or better—which is the minimum for party walls—and almost nonexistent for walls with STCs of 60 or above, “where you can’t hear neighbors voices, sound of pipes, music at reasonable levels, or sound of computer video games,” according to Donaldson of Supress Products.
Of course, that still doesn’t predict the behavior of a wall at extremely low frequencies, such as those broadcast by home stereo and home theater systems. Today’s subwoofers can produce sounds in the 40-hertz range. Even the new panels don’t claim to be effective at blocking sounds at frequencies that low.
That’s a problem, because low frequency sound issues arise in all types of construction, thanks to the ability of such frequencies to travel through concrete as easily as wood framing. Because of this, Tinianov knows one consultant who advises all apartment owners to prohibit residents from using subwoofers. (Most consultants say this isn’t practical, although an owner could specify what type of stereo equipment a resident can have or how it must be mounted in the unit.)
Design offers a partial solution. In a high-end project, some rooms might be designated as media rooms, which can then be designed with the goal of keeping the low frequencies from entering the building’s structure in the first place, Haas suggests. “We end up using a combination of sound-absorbing drywall and resilient attachments to decouple structures in these rooms,” he says. This includes putting a resilient pad between the subfloor and finish floor, as well as an isolation material around the edges of the subfloor, to acoustically separate it from the adjoining rooms. (Kinetics makes a roll-out floor material for this purpose. It has pre-positioned isolation pads to support the plywood.) The slab or floor framing in this room needs to be about two inches lower than the rest of the unit to maintain a flush finish floor. Penetrations such as sprinklers and lights need to be sealed for air transfer. Recessed ceiling lights should be surrounded with sound-absorbing drywall.
Overall, while controlling sound is a tough challenge, noise issues can be minimized. But that requires willingness to pay attention to the hidden details and spend some extra money. For high-end projects, such cost and effort are probably worth it. “The competition is pretty fierce among developers, and this gives them a hook to differentiate themselves,” Haas says. Still, it may not be the easiest sell. It’s a challenge to present a good acoustic design, but once buyers and renters move into their units, solid sound control is something they will appreciate daily.
Especially when their building’s new bowling alley opens on the first floor.
Charles Wardell is a freelance writer in Martha’s Vineyard, Mass.