Acoustical Metrics: Sound Transmission Class and Ceiling Attenuation Class
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Buildings often are constructed with little attention to the acoustical environment. It may be a matter of cost; acoustically engineering a building properly may mean spending a bit more upfront. In many cases, however, facility executives simply lack a strong understanding of the role of acoustical engineering.
As a result, facility executives often are forced to solve acoustical problems after a building is already constructed, when fixes typically are more expensive and the construction budget depleted, says Bennett Brooks, president of Brooks Acoustic Corp.
While not easy, late fixes can often be done. A first step is a bit of detective work to identify the source of the noise, says Brooks. For example, by listening closely in a room, it’s easy to figure out that an unwanted sound is coming from the air vents. Next is figuring out the path the noise is taking.
Having completed this analysis, the next step is identifying possible solutions to the problem and choosing the one that fits within the organization’s budgetary constraints.
Although it can take time and some investment to address acoustical problems, the effort can bring important benefits. “Good acoustical design is an integral factor in creating a pleasant, efficient work environment,” says Michael Cain, acoustics consultant with JaffeHolden.
One of the more common acoustical challenges is ensuring that conversations held within private, enclosed offices remain confidential, says Pablo Daroux, principal with Wilson, Ihrig & Associates. Sound, for example, travels through a facility’s HVAC ductwork, allowing others outside the office to overhear conversations.
One way to impede the sound is to line the interior of the duct with sound absorbing material, such as fiber panels, says Daroux. Or the ductwork itself can be extended and arranged in a labyrinth design, so that sound can’t travel in a straight line between offices.
Another contributor to the problem may be ceiling tiles that fail to act as sound barriers. Although it seems counterintuitive, a ceiling tile that absorbs sound isn’t necessarily an effective sound barrier, Daroux says.
Measuring the Ceiling
Two metrics measure how well a ceiling reduces sound transmission. One is ceiling attenuation class (CAC). CAC indicates how well the ceiling acts as a barrier to sound transmission when the wall between adjacent offices goes only to the ceiling, and stops short of the underside of the floor above, Daroux says. While this was intended to replicate real world conditions, the laboratory tests in which the ratings are determined fail to consider lights, HVAC systems and other elements in an office ceiling.
Another metric is sound transmission class (STC). STC applies to more than just ceilings, as it measures the ability of a material to block noise, regardless of how it’s installed.
Noise reduction coefficient (NRC) should also be a factor in choosing office materials. Ceiling tiles with a score of 1 absorb 100 percent of the sound that hits them. Quality ceiling tiles, for instance, have an NRC of 0.7.