April 15, 2014

The sound(proofing) of music

By Marianne Goss

In the early days of construction of the new Music and Communication Building, senior project manager Chris Erickson of Power Construction would hear students practicing at adjacent Regenstein Hall of Music. As mellifluous as the sounds may have been, they weren’t music to a contractor’s ears. Soundproofing, Erickson observes, has come “light years” in the less than four decades since Regenstein was built. “The new building has some exotic materials, but what sets it apart are the acoustic features,” he says, meaning not only how performances sound to listeners but especially how well each of the 99 practice rooms, 35 teaching studios, 12 classrooms, 4 music technology rooms, and 3 performance halls trap sound.

Joseph Myers of the acoustics design firm Kirkegaard Associates is principally responsible for the building’s soundproofing. Before describing the how of the sound isolation, he describes the whys and wherefores: “Sound is vibration in a medium—air or a solid surface. When the energy of vibrating molecules hits a wall, the wall moves, and, like a loudspeaker diaphragm, recreates this sound on the other side. To make a sound inaudible outside the room, we need to get rid of 99.9 percent of the energy. We strive for three things: airtightness—no holes or cracks; mass—the thicker and heavier the surface, the more it resists vibrating; and separation—inefficient contact between what’s vibrating and what it’s in contact with.”

To minimize sound vibration, Myers specified multiple layers—each room a box within a box, with the inside box not rigidly attached to the outside box. Though scarcely simple, the construction of five sides of each box followed Kirkegaard’s standard practice for a music building. But the sixth side posed a unique challenge: except for three large performance spaces, which have an outside cladding of Indiana limestone over concrete, most of the building’s exterior is a glass “curtain wall.”

The glass exterior was the architect’s response to a request from Dean Toni-Marie Montgomery. Inspired by Jazz at Lincoln Center’s Appel Room—where a glass wall behind the stage overlooks New York City’s Columbus Circle—she proposed a glass backdrop for the recital hall stage to provide an inspiring view of the Chicago skyline. The final design uses glass not only for the recital hall but for most of the building’s outer walls—a much bigger challenge.

“How do you keep sound from going through relatively thin glass?” asks Myers, who’s been in the business 26 years. “I’ve never seen it done before on a music building. In most music schools, if you’re lucky you may have some punched-out windows, and many music buildings have no windows in practice rooms because of sound transmission.”

But Myers embraced the challenge. “I wouldn’t have tried if I thought it couldn’t be done,” he says. “I would have told the architect it was impossible if I thought so. I thought the aesthetic value of more light and connection to the exterior was considerable and made it worth the acoustic challenge.”

the opera theater during construction

With his three-pronged formula for isolating sound, Myers could get airtightness but not mass from glass, so “we went for more separation than normal. Essentially the approach is to have a series of inner windows—the architect calls them ‘jockey sash’ windows, a term that was new to me—that are resiliently separated from the outer curtain wall. The inner windows kiss up to the exterior with rubber gaskets but are structurally separate from it and from the concrete slabs above and below them.”

The jockey sash windows are made of laminated glass that Myers describes as “like windshield glass but much heavier. It doesn’t like to vibrate.” A perforated aluminum shadowbox with fiberglass insulation sits behind some of the outer glass, covering the edges of the walls and slabs. “If a sound wave strikes the jockey sash window, some sound will get into the cavity,” says Myers. “To get out, it would have to travel through the heavy aluminum mullion through fiberglass through more aluminum back through the jockey sash into the adjacent room. If I were an eight-foot sound wave, I’d be intimidated.”

Project architect Patrick Loughran of Goettsch Partners notes that the jockey sash window system fulfills aesthetic as well as acoustic requirements. “It is virtually undetectable from the exterior,” he says. “This allows the facade to appear uniform, even though the rooms behind the facade have varying acoustic requirements.”

The other five sides of the box may be standard practice for Kirkegaard Associates’ design of a music building, but they aren’t standard practice for most new construction. Instead of a single layer of gypsum board on either side of a wall, there are two separated studs with several layers of gypsum board on each side and two fiberglass insulating blankets between them. Two tracks on the floor replace the typical single track for studs, and felt separates the outer track fastening to the concrete from the inner one holding the studs. The studs are also farther apart at the bottom than the top, producing slightly sloped walls that push the sound up toward the absorptive ceiling. A squishy acoustic sealant makes for an inefficient sound-transmitting connection of walls to floor and ceiling.

For the floors, rubber pads—which do not transmit sound efficiently—are laid on top of the concrete slab, covered by two layers of ¾-inch plywood and then the top layer of flooring. For the ceiling, gypsum board and fiberglass blanket layers separate the concrete slab from the visible wire-hung tile.

The teaching studios, where faculty will work one-on-one with students, will have the highest level of sound isolation, the practice rooms slightly less. “A little awareness of low-level background sound we think is desirable in the practice rooms so that students won’t feel they’re in bunker-like isolation from their community,” explains Myers. “And you will hear low-level sound in the corridors. The administration has been very clear that it wants to hear music within the corridors and public spaces—they don’t want it to be sterile.”

Because the rooms aren’t sealed up yet, Erickson is able to point out the multiple layers on a walk-through. He compares the sound isolation techniques to a cellphone on a table: “If you put a cellphone without a case down on a hard surface, it vibrates when it rings. If it’s in a soft case, it’s resiliently isolated. You may not hear it vibrating.”

All of this layering of course means that the building will cost more and take longer to build than an office building or a college humanities building. Projected to cost $117 million in all, the building has been under construction since July 2012 and is expected to be finished in spring 2015. “It’s certainly premium cost,” says Erickson. “The materials alone—there are 70 sheets of gypsum board, about 4,000 pounds, in each practice room. The average single-family home has about 150 sheets in the entire home. That’s an immense amount of gypsum board.”

“The practice rooms have four times as much building material in the same space as a normal office, which would have a single-stud wall and a layer of gypsum board on either side,” adds Myers. So, will four times the usual amount be enough to achieve his goal of isolating 99.9 percent of the sound?

“I wish I were answering when it’s finished,” says Myers, “but, yes, I believe we will achieve it.”

Marianne Goss is a senior editor in the Department of University Relations.

This article originally appeared in the spring 2014 edition of Fanfare