acoustics, revisited These sacred spaces present a host of acoustical challenges, depending on their layout and design.
Shallow versus deep spaces If your auditorium hasn’t been built yet, you can build a room that’s more conducive to intelligibility, with close to a three-to-one ratio of depth to width. If you’ve seen and heard a good-sounding room, it can be copied, if not necessarily scaled. By definition, scaling a room to a smaller size produces shorter wavelengths between walls and a different set of reinforcing and canceling frequencies. If the balcony is lowered, it creates an “acoustic shadow” below, requiring delayed sound reinforcement. Any artificial delay should always be at a lower volume and slightly later in arrival time to keep attention focused toward the natural sound source. (Like a well-designed subwoofer, it should only be obvious when it’s turned off.) With the balcony lowered, sound may now bounce off its face, which is now in the path of the primary speaker array.
The value of speed bumps If you already have a shallow room — or one that’s circular or nearcircular, such as an octagon or hexagon — acoustic soundness is more of a challenge. In some large dome venues, sound can be heard racing around the perimeter. This “race” can be slowed by installing acoustical speed bumps, such as hollow, half-round broadband absorbers. They are also effective on balcony facings to abate slap-back to the stage. If you have a dome-shaped sanctuary, ceiling clouds might be appropriate to reduce focus. The dome was an effective sound reinforcement technique and can be found in early 20th-century venues for acoustical performances.
Spread the sound around Diffusion used to be built into rooms; now, acoustical devices can “fix” an existing space. For large spaces, diffusers — of the poly-cylindrical, barrelshaped variety, for example, which also serve double-duty as bass traps — might be best for redistributing energy throughout the room and extending low-frequency absorption of fabric-covered, sound-absorbing wall panels. Without them, intelligibility might improve. But, an uncomfortable “boom” in the bottom range could remain.
(Photo courtesy of Acoustics First Corporation/The National Theater)
It has been said that whenever someone invents a better mousetrap, a better mouse isn’t far behind. To that end, this author has noticed that whenever something is “common knowledge,” some uncommon problems soon follow. Case in point: It’s well-known that an auditorium works best when it’s deep rather than wide. This allows the room sound to develop and envelope the audience while using the reflectivity of the side walls to engulf them with sound. With the walls far apart and the rear wall closer, two things happen: Sound returns from the back wall at higher intensity — and usually out of sync with the music — while overlapping the spoken word. When the reverend repeats, it reinforces his message with added emphasis, while any room repeats usually overlap and obscure the message.
BY NICK COLLERAN
The National Theater balcony face provides a good example of half-round, acoustical speed bumps placed along a concave, curved surface.
Sound judgment Two significant points influencing amplified sound are centered on the ability to hear the sound accurately. First, a reflective, flat surface behind the mixing position will color the sound and falsely influence decisions on equalization, and so on. The phenomena can be corrected by adding binary-array wall diffusors behind the mixing position. Second, placement of the sound mixing console below the balcony (or otherwise distant from the listening audience) can lead to excess listening levels in more remote areas of the sanctuary. If the mix area must be obscured, a visit to these areas by the audio minister during the performance should be routine. An acoustic performance area that’s over-amplified to the point of having the balcony shake is now a common scenario. CE Nick Colleran is former president of Society of Professional Audio Recording Services (SPARS) and Virginia Productions Services Association (VPSA), a former recording artist and recording engineer. Today, he is a principal at Acoustics First Corporation [acousticsfirst.com] in Richmond, VA.
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