AV Acoustics: Collective Wisdom
Four experts, including Acoustical Design Collaborative's Neil Thompson Shade, offer their advice on working in four different environments.
Stadiums and Arenas: One Ain't Like the Other
In Oriole Park at Camden Yards, high frequencies (yellow) come from three speakers, while low frequencies (red) come only from the large, front, forward-firing speaker (projection patterns are approximate).
Credit: Courtesy EAW, SIA Acoustics
Modern sports facilities generally come in two flavors: outdoor stadiums and indoor arenas. It seems that everybody–pro sports teams, colleges, municipalities–wants either a new or better one. While they're both challenging to design, they're also very different.
"Arenas tend to have an abundance of mid/low reverberation and plenty of large curved surfaces that create reflections," says Sam Berkow, Pro AV columnist and founding partner of SIA Acoustics, which recently handled the sound updates to Oriole Park at Camden Yards. "In stadiums, the overall reverberation time is low but you have pockets of poor sound environments. Sound system coverage is always the biggest problem, whether it's overlap zones or not enough coverage." However, trying to get stadium owners to integrate acoustical treatment on large surfaces is hard. "It's tricky because sound-absorptive material tends not to survive in high traffic areas," Berkow says. "And covering a large surface area increases the costs dramatically."
One technique that Berkow uses to add absorptive material to an indoor arena is to festoon mid-density fiberglass banners in a shallow U-shape on 60 to 80 percent of the arena's ceiling. This technique can also be used in stadiums with domed ceilings, but the downside is the expense for covering a large area.
Glass balcony railings and corporate suite windows are popular in indoor arenas, but they also create acoustical problems. "We try to slant the glass up and away from the seating area, or we minimize the energy going towards it by not aiming a loudspeaker there," says Berkow.
In outdoor stadiums, the SIA Acoustics team will work with the architect from day one to try and create underbalcony spaces with a tall ceiling and advantageous angle to direct sound. "A tip is to angle some of the underbalcony surfaces or create surfaces that scatter sound. As a result, you end up with less reflection," Berkow says.
For existing balconies, he observes, "It is harder and harder to get retrofit space on an overhang because dimensions are so tight. You could use the space on the front of balconies, but they are dominated by signage. Another alternative is to work with the signage people to create microperforated signs with acoustical batting behind them."
For both types of venues, it's important to control off-axis energy from loudspeakers. When it comes to outdoor field seating in particular, the trick is to roll off the low frequency at the underbalcony loudspeakers but pair them with a full-range loudspeaker firing in the opposite direction.
"The idea is to limit the bandwidth of the speaker in the underbalcony space to control coverage. Single speakers often become omnidirectional below 250 Hz and now that energy is bouncing all over the concrete, which takes away from articulation," Berkow says.
He warns that ultradistributed systems, in which architects or owners try to hide extra-small loudspeakers in signage or railings to bring direct sound closer to the seats, are gimmicky. "You pay more for packaging than performance," he says. "There is always another alternative to improve the listening experience. The key is to EQ the speaker so that the interaction with the room is optimal. The computer is there to quantify the interaction and response, but critical listening is always part of the process."