Threshold Acoustics


Architect William Rawn is often asked about the 85-foot-long undulating glass facade at his recently completed Ruth Caplin Theatre on the campus of the University of Virginia (UVA), in Charlottesville. People wonder, he says, if it was inspired by Thomas Jefferson’s serpentine brick walls that are part of the so-called Lawn—the complex of 18th-century structures and grounds at the university’s historic heart. But Rawn insists that the ribbonlike curtain wall enclosing the 300-seat performance space, which is partially submerged in the steeply sloping terrain of the school’s arts quad, takes its cues from the topography of its site rather than Jefferson’s garden walls. “It isn’t that literal,” he says.

Instead, the Jeffersonian elements that most informed the new thrust theater were the tall triple-hung windows found in many of the pavilions surrounding the Lawn and at Monticello—the home that Jefferson built for himself just a few miles from UVA. “Transparency was one of the lessons we took from Jefferson,” says Rawn.

Transparency, and accompanying daylight, may seem like odd features for a theater. Yet much of the time that the space is in use—for rehearsals, dance classes, or when sets are being built—controlled theatrical lighting is not necessary, points out Clifford Gayley, a principal of William Rawn Associates. What’s more, with students passing Caplin on the new set of stairs that serves as one of the main pedestrian routes through this part of campus, the glazing provides a direct view of the stage. It “puts theater on display,” elevating the drama program’s presence, says Gayley. And for those times when darkness is required, the daylight coming in through the north-facing curtain wall is easily blocked with motorized black-out shades.

One aspect of the space that was trickier to control was its acoustics. “Glass is specular,” and not just in the visual sense, says Carl Giegold, a partner at Threshold Acoustics, the project’s acoustician. “It is almost purely reflective of sound,” he says. Used in the right place, glass can be advantageous, since reflection is important for musical clarity and the intelligibility of speech, especially consonants. But if used incorrectly, it can garble sound, he says.

At Caplin, where the glazing is at the perimeter of the 5,500-squarefoot, semicircular room, it could send the sound back toward the stage with a delay, causing an echo. To mitigate this undesirable effect, the project team devised an inner wall of laminated glass canted at 2 or 4 degrees, depending on location, and set a few inches away from the exterior insulated glazing units (IGUs). The tilted glass directs the sound toward the underside of the roof deck or ceilings, which are covered in absorptive materials such as sprayed-on cellulose or stretched-fabric panels. “The goal is to send the sound somewhere where it doesn’t do harm,” explains Giegold.

The extra layer of glass has additional acoustical benefits. Its thickness—¾ inch—keeps low-frequency sounds, like those from the bottom registers of a typical male voice—from escaping. It also helps isolate the room from outdoor noise, such as the rumble of the freight trains that pass Caplin on tracks that are only about 150 yards away. This ability to keep unwanted noise out is enhanced by the cavity between the laminated glass and the IGUs, according to Giegold. In addition, the cavity’s varied depth, which ranges from 7¾ to 2 inches, prevents the curtain wall system from resonating at a uniform frequency. Otherwise, says Giegold, sounds like the drone of a lawnmower could cause the assembly to vibrate, producing a discernible hum.

Naturally, unwanted sound can also be generated from within. A typical culprit is mechanical systems. But at Caplin, the ducts are all oversized, so that air moves through them slowly and quietly. The room’s silence, along with its intimacy (no seat is farther than 25 feet from the stage), allows the audience to focus on the performers. The acoustics are so good, says Gayley, that “you can even hear the actors’ slightest whispers,” without amplification.