Most people’s knowledge of sound is limited to volume knobs and treble buttons, or whether a restaurant is too loud to talk in.
To Bob Mahoney, the Boulder-based acoustician who helped transform Denver’s dilapidated auditorium into what may be one of the top acoustic performance halls in the world, acoustics is the delicate art of bouncing sound around the audience, of soaking it up, of balancing an orchestra’s sound with a diva’s solo.
To accomplish all of that, Mahoney worries about the angle of seats, the tilt of walls, the slope of the ceiling, the thickness of the carpet, even a toilet flushing somewhere backstage.
“An opera house is the most difficult of all designs,” he said.
Mahoney, 53, studied physics, then piano and French horn at the Julliard School before getting interested in architecture and acoustics. After receiving a master’s degree in architecture from the University of Colorado at Boulder, he began working on the construction of the Boettcher Concert Hall in 1977.
Today his extensive resume includes the acoustics in the Kodak Theatre in Los Angeles (site of the Academy Awards); the Wells Fargo Pavilion in Sacramento, Calif.; and the King Center in Denver. Designing the Ellie Caulkins Opera House is the dream of a lifetime, he said.
As Mahoney walks through the cavernous hall, he talks with ease about the opposing forces of clarity and reverberance (the richness or fullness of the music). He explains the hang time of various reflections, which create clarity and reverberance, about bouncing them out or soaking them up. He must balance the sound-soakers (audience, carpet, seats) against the sound-bouncers (concrete, drywall, wood and metal).
His work begins with the footprint of the hall and the sightlines of all 2,268 seats, which must have an unimpeded view of the conductor and the stage. If sight is impeded, sound is too.
The Ellie’s proscenium, the opening between the stage and the audience, is only 55 feet wide – about 10 feet narrower than a concert hall. This allows the lower walls to be brought in closer, improving the clarity of speech or song.
Clarity comes from reflecting the sounds to each seat, so that they arrive between 50 and 80 milliseconds after the original sound. The early reflections, of substantial strength, help to clarify the original sound. The later reflections, somewhat diminished in strength, blend together, giving the sound its richness and warmth. The sound must be soaked up to make way for new sounds.
Mahoney has meticulously analyzed every surface in the hall.
“We used to fire guns to measure the sounds, but tougher security at the airports has put an end to that,” he said.
Because each reflection is different, Mahoney uses many tricks. For instance, he put corrugated gypsum-board walls along the sides of the orchestra level. The plane of the ceiling rises gently away from the stage, reflecting toward the highest and farthest seats in the back of the house. Ceiling panels made of glass-reinforced gypsum and veneer wood are wrapped on the edges to reflect sound into the side boxes.
“If all the reflective surfaces were mirrors, theoretically you should be able to stand on the stage with a flashlight and shine light into every seat,” said Peter Lucking, principal in charge and lead designer for Semple, Brown Design, which designed the $92 million renovation.
Ideally, sound heard in the very last row, 113 feet from the stage and 67 feet up, should be exactly the same as that heard in the front row. “There are no cheap seats here,” Lucking said.
But clarity is only half the task. Reverberance, the ability of sound to linger in a space, is just as important.
“After the first reflections arrive, you want reflections that are gradually weaker,” he said. “They add the pleasant musical qualities, the warmth and presence that surrounds the audience.”
Mahoney also worries about keeping the music in the hall and the noises out: To that end, he encased the hall in a 12-inch-thick concrete wall looming five stories high.
Another major consideration is the enormous output of an orchestra against a solo singer, a problem that has vexed opera houses around the world. Mahoney and Lucking devised a solution unique in the opera world.
First they designed the orchestra pit on two elevators, front and back. Depending on the size of that night’s orchestra and the sounds needed, the pits can be raised or lowered, which is fairly common in most performance halls.
What Mahoney and Lucking added was a movable wall behind the orchestra and under the stage. This allows the brass and percussion sections to be muted as necessary by moving them farther back under the stage.
“We’re unique with this – it’s the only one in the world like this,” Mahoney said.
There are three basic types of music halls:
Rock-‘n’-roll halls, which need to be very dead acoustically because the loud, amplified sound is controlled electronically and needs to be soaked up quickly.
Unamplified concert halls where the acoustician must worry only about the orchestra.
Opera houses, which must blend singers and orchestras. Opera houses also have sets, scenery and costumes, which affect the sound significantly and over which the acoustician has no control.
