Good Vibrations

Sept. 29, 2005
Understanding the importance of acoustics

An ideal acoustical environment is not a matter of soundproofing a space, says Kim Graaskamp, director of sales and marketing at Hunter Douglas Contract Ceilings in North America, Thornton, CO, as she points to the difference between a distracting noise in an office setting vs. the exciting buzz of a lively restaurant space; rather, it’s knocking down the reverberation time of a sound.

In fact, achieving acoustical goals requires an understanding that noise comes from three places: the outdoors, in terms of street and traffic noise; mechanical noise, such as the sounds from an operating HVAC unit or beeping security system, all of which are inherent in any building; and people-related noises, which might include conversations among coworkers or the sound of footsteps across hard-surface flooring.

“Just as there are many things that cause sound, there are many ways to stop and/or absorb sound,” says Graaskamp, noting that key to acoustical control is a true understanding of the use of a space and how speech intelligibility - frequencies of sound that usually comprise a two-octave range - comes into play. “For instance, in a classroom, you may want to absorb disturbing noises, but you don’t want to deaden the room so much that the kids in the back row can’t hear the teacher’s voice,” she cautions. “Conversely, in a call center environment, you may want to absorb the speech sound range quickly and effectively; otherwise, you might hear all 85 people on the phone at one time.”

Fortunately, everything absorbs sound - even concrete (albeit not very well) - so materials in a space do help. A simple tool in helping users prioritize how certain products and systems can optimize the acoustical quality of a space is readily available, explains Graaskamp. “The Sabine formula is a basic reverberation calculation in which the dimensions of a space are plugged in along with [the materials] that are [within or] on that space. Let’s say a space is 40- by 40- by 25-feet in size, and it includes a marble tile floor, hardwood walls, and a drywall ceiling. Once [these elements] are plugged into the Sabine formula, the result is a very rough calculation of the reverberation time in a space. If [the result] is ‘X’ and what you really want is ‘B,’ based upon the use of a space, you need to determine how to get from ‘X’ to ‘B.’ By adding acoustical treatments - wall panels, fabrics, ceiling panels, etc. - and their absorptive ability into the mix, the formula will tell you how many square feet of those types of sound-absorbing materials are needed in a space to adjust that reverberation time.”

Not surprisingly, since sound travels on air waves and most air moves up, the biggest bulk of absorption occurs at the ceiling, says Graaskamp. Ceilings that offer a Noise Reduction Coefficient (NRC) of 0.75 or better are ideal, she explains, noting, “If the ceiling can carry more of [an acoustical] load, then [facilities professionals] are free to use other products elsewhere.” Additionally, ceilings can be the easiest, quickest fix in instances where an overly distracting space can be improved through the installation of a higher NRC-rated ceiling.

Linda K. Monroe ([email protected]) is editorial director at Buildings magazine.

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