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The goal is not to create a library-quiet space, but rather a comfortable environment that's not noticeably noisy, says Berens. A sound-masking system and furniture systems both come into play with open plenum acoustics.
"A soundmasking system does not make the guy in the next cubicle speak any quieter," he says. "What a soundmasking system does is it reduces the differential between quiet and the speaker." That cuts down on the distraction caused by sounds like coughing or pages turning, and it also makes it more difficult to pick out individual words in overheard conversations, improving speech privacy.
Traditionally, sound masking systems are mounted above the ceiling tile, with the speaker pointed up to the deck and the masking sound filtering into the space through the acoustical tile. If an open plenum space uses clouds, these can be used to house a sound masking speaker. Otherwise, system options exist to position speakers to "downfire" directly into the space. This strategy requires the use of more speakers, at a lower volume, says Doggett. In addition, it requires a speaker designed for the task — you can't just take an existing system housed above the ceiling tiles and merely flip the speakers over if the design changes to open plenum.
"You want an even volume," she says. "You don't want to walk into hot spots or dead spots of soundmasking. The whole trick is that you want it to seem that it's not even there."
Trends in furniture design contribute to the challenge of forgoing an acoustic ceiling, as cubicle walls trend lower and lower or are absent altogether. This trend is great for many things — daylight and access to views, collaboration, space efficiencies — and not so great for speech privacy or the ability to focus on the task at hand.
Acoustics experts' recommended minimum barrier height is 48 inches from the floor, the line of sight when seated. If that is absent, "That's obviously not great for acoustics because you're talking right at the person, if you're facing them," says Doggett. While not as beneficial as a cubicle partition made with acoustically absorptive materials, one solution she has seen is glass partitions at the sight line, which at least block some of the sound.
Sound-Masking, Furniture Systems Improve Open Plenum Acoustics