How To Beat A Highly Reverberant SpaceSound experts share their tips on tackling some of the most difficult acoustical challenges. 9/25/2006 10:56 PM Eastern
How To Beat A Highly Reverberant Space
Sound experts share their tips on tackling some of the most difficult acoustical challenges.
Specializing in worship facility planning, sound system design, acoustical analysis, and AV systems design, Vance Breshears, president and principal consultant with Sound Technology Consultants in El Cajon, CA, is shown at the First Presbyterian Church of San Diego, preparing to conduct impulse response measurements.
Credit: Dennis Davis
“The most important aspect is why the space is too reverberant,” says Sam Berkow, founder and principal consultant of SIA Acoustics in New York, noting that the need for silence in a reverberant space is equally important. “There are two types of problems — the decay time and the tonal balance of the reverberation.”
And you can't beat that reverberation with “brute force,” maintains Joel Lewitz, president of Lewitz & Associates in Sausalito, CA. Instead, you must control it, which doesn't mean going overboard and creating a dead space.
In the following four examples, expert acousticians reveal their tips for beating reverberation in some of the most common and frustrating AV venues imaginable.Church acoustics
— Vance Breshears, president and principal consultant for Sound Technology Consultants, El Cajon, CA
With their widely varying program content and acoustic requirements, “churches are one of the most difficult acoustic spaces,” Breshears says. “To be successful at the acoustical design of a worship space, you not only must understand the physics of acoustics, but also have a good understanding of what ministries want to accomplish.”
A church's sanctuary may need to accommodate the spoken word, dramatic presentations, and highly reinforced music. “You want a space that is live enough to work well for acoustic music as well as congregational participation, but also controlled enough to work well for highly reinforced music, which presents opposing acoustical requirements,” Breshears says. “For congregational singing, the goal is a ‘singing in the shower' appeal.”
Good sound is expected by church clients because the use of audio technology is an important way to effectively communicate. “With today's church moving from presentation style toward participation, they want to make it more of an interactive experience and less of a show,” Breshears adds.
Fan-shaped seating areas are typically found in the contemporary church environment. Breshears and his design team have been using intermediate vertical surfaces or seating offsets to help modify the acoustical environment. By breaking up the seating plane, the room performs like a smaller acoustical space. The overall room shape has a tremendous influence on acoustics, so the key is to control the direction of any reflection with shaping.
For churches on a budget and going through a retrofit, changing surface finishes offers a cost-effective solution to acoustical problems. “Your choices are absorptive, reflective, or diffusive materials,” Breshears notes. “It's typical to add absorptive materials in key locations to address both excessive reverberation and problematic reflections. I advise measuring what frequencies are causing the problems, and then choosing materials based on their performance at the problem frequencies. If you just add absorption everywhere, you can either make the room too dry or expose reflection problems that were previously masked by reverberation.”
Credit: Photo courtesy of SIA Acoustics
For the new Grace Community Church in Noblesville, IN, Breshears and his team employed several methods in their design to keep acoustics under control. Grace Community Church's architectural design uses a unique combination of concrete, steel, glass, and wood. The acoustical design included shaping in both the horizontal and vertical planes while using some key finish materials in specific locations. In addition, they added both fixed and movable ceiling clouds so that church staff can change the acoustic characteristics of the room depending on the program.
Another tool in the church acoustics toolbox is electronic acoustic enhancement systems like Level Control System's Variable Room Acoustics System (LCS VRAS). For rooms that are less reverberant than desired, or a room where it's advantageous to change the acoustical characteristics, systems like VRAS are used to modify the natural acoustical environment at the touch of a button. “The primary advantage of electronically variable acoustics is that changing the acoustics of the room can be accomplished instantaneously, even during a performance or worship service,” Breshears says.Performance hall acoustics
— Sam Berkow, founder and principal consultant of SIA Acoustics, New York
When Sam Berkow and his team at SIA Acoustics begin work on a performance hall project, they first determine the type and shape of the space. “There are typically four types of halls, each with its own way of affecting sound,” he says.
Perhaps the most typical is a hall with a stage at one end and a fan-shaped seating area. In this design, the walls splay outward, sending the sound energy from the stage to the back wall. The acoustical flaw is that the audience members seated in the middle don't experience much lateral sound. Next is the shoebox hall with rectangular seating and straight walls like the Boston Symphony Hall in Boston. This type of space has a bit more uniform reverberance than the first example. The problem is that people seated in the front rows don't get any lateral sound. Then there's the modified shoebox with reverse fan-shaped seating. This shape forces energy back in the audience and is an acoustically friendly design that has been used over the last 20 to 30 years. And don't forget about a venue that brings the orchestra out into the seating area like the Berlin Philharmonic in Berlin. In this case, one-third or more of the audience surrounds the orchestra. From an acoustic standpoint, the audience and orchestra share the same space.
“When we walk into a space, we answer two questions: What is the volume of the space, and will sound energy excite the entire space or not,” Berkow says. “In a traditional concert hall where there's usually a uniform space, the answer is simple.”
Much like sports facilities, performance halls have become much more use-specific in recent years, Berkow maintains. Such is the case when his firm was tasked to produce a special event in a concert hall designed to support full orchestral music. The client, however, wanted a small string quartet.
The rise of high Q (very directional) sound systems in the last few years has given acousticians another tool to shape the acoustics of a room. EAW's Digitally Steerable Array (DSA) and Renkus-Heinz's Iconyx are just two examples of this new product category. “Using such a system in a reverberant space means there's an increased importance of aiming,” Berkow says. “You must be careful not to excite more of the space than necessary.”
In an extreme example of what can be done to control the acoustics for a performance or event, Berkow points to his firm's work on the Mount Sinai Medical Center's Crystal Ball fundraising event in New York, which was held in the atrium of Mount Sinai, a creation of glass and steel with a terrazzo floor. The high-end, high-profile event included dignitaries and Nobel prize winners as attendees. To achieve the perfect environment for both music and speech, SIA used two types of loudspeaker systems: several arrays of Meyer M1Ds along with Renkus-Heinz Iconyx boxes for additional speech reinforcement. In addition, the sound system was hung along with 680 acoustical banners wrapped in sail cloth. The banners were used to absorb off-axis energy. In total, 61 chain motors were needed for the entire truss for this six-figure event. The budget included technical systems for the event as well as technical direction, lighting design, acoustics, and structural engineering.
Another way of addressing the acoustics of a performance space is to look at other sources of sound energy beyond the obvious solutions. Berkow notes that low-frequency rumble from HVAC systems is a common problem and can mask consonant sounds, especially for older listeners.
In spaces with smaller budgets, the padding or bracing of the stage is often forgotten. The acoustic result is like the beating of several drums as performers walk across the stage. This problem can be found in projects with larger budgets, too. When Berkow was asked to help with the acoustics for the Republican National Convention in New York's Madison Square Garden, he immediately recognized that the temporary raised flooring would present a problem. “Instead, we did a multi-layered floor with a honeycomb pad as a vibration damper to keep from exciting the reverberant space below the floor,” he says.
Credit: Tim Griffith
— Tom Young, director of electroacoustic design for SH! Acoustics, Milford, CT
Similar to performing arts spaces, all museum spaces aren't created equal. “There are two types of museum projects, either built into an existing building or designed from the ground up,” Young says. “Many museum projects are housed in large, warehouse-like spaces in which there are smaller exhibit spaces that are often semi-enclosed. Reverb time at lower frequencies is horrendously long and not at all acoustically friendly.”
To help tame the overall space, acousticians use a variety of treatments depending on the client's budget. Perforated metal accents with fiberglass treatment hidden behind it are cost effective. A spray-on treatment like K-13 is also a flexible option because it can be tailored to specific project requirements for insulation (R value), noise reduction coefficient (NRC), and color, among other attributes. Pyrok acoustical plaster, or an acoustical system called BASWAphon manufactured by RPG, that use rigid boards installed on walls or ceilings are more expensive options for absorption. Diffusing elements, available from many manufacturers of acoustic treatment materials, are used when pure absorption isn't appropriate for the programming of the space.
The major challenge in a museum environment is sound containment, specifically with individual exhibits. In this type of space, the visitor is often facing or interacting with the exhibit, and focusing the sound on the very small listener area helps to not excite the greater reverberant field. Young usually works with exhibit designers who fabricate custom exhibits using video screens embedded in the wall or into interactive tables.
Instead, Young focuses on exhibit isolation. “I'm interested in targeting the person who's listening to the exhibit, regardless of the space,” Young explains. “To do so, I use higher Q devices like traditional horn-loaded speakers or the newer technologies such as steerable arrays.”
For the redesign of the Liberty Memorial Museum in Kansas City, MO, scheduled to open on Pearl Harbor Day 2006, Young was faced with several acoustical challenges in his sound system design. The theme of the museum is “America at War” with the focus on World War II. The museum itself is built like a large concrete bunker with exhibits on an elevated floor.
One exhibit resembles a trench in a battlefield, and others feature walls of video displays or interactive tables. In one area, visitors can listen to actual dialog from different nations involved in the war. Overall sound effects mimic warfare, with the requisite low-frequency boom of distant bombs. “Due to its longer wavelengths, the low-frequency energy needs careful design to be contained, so we used smaller woofer devices developed for home audio,” Young says. “These, along with the full-range loudspeakers, will be locally placed within a foot or two of the listener and are band limited to approximately 50 Hz.”
Often within the museum environment, an acoustician will also encounter a small theater space. Theaters are used to show content about the museum and to welcome visitors to the area. The Liberty Memorial Museum features two enclosed theaters, one of which is the unique Horizon Theater. The theater walkway is well above the main floor; below and out front is a tableau of a battleground with a large 80-foot-wide by 25-foot-high video screen above and to the rear of it. This theater isn't entirely enclosed, so sound containment is still an important issue.
Young specified EAW Digitally Steerable Arrays as the primary source for dialog. Directly overhead, several subwoofers were flown so that the low-frequency energy will be aimed at the pedestrian floor and not travel out the open end into the museum. Two-way speakers and acoustical treatment are to be installed behind the projection screen to give the audience more directional cues to the visual images.Stadium acoustics
— Joel Lewitz, president of Lewitz & Associates in Sausalito, CA
Most people are familiar with stadium or arena acoustics from attending sporting events. But when dealing with acoustics, Lewitz notes that reverberation is a common problem in enclosed stadiums. “There's the potential for poor intelligibility from the audio system because of excessive reverberation,” he says. “The key is to find a good balance between the acoustical design and the sound system design — and each stadium is different and unique from the other.”
Take for example, the now-defunct Kingdome in Seattle and its scalloped dome ceiling. The design used Tectum acoustic material that served as the form for the concrete dome.
“This worked physically, but the material didn't have a high enough absorption coefficient,” Lewitz says. “Coupled with the large internal volume, there was still quite a bit of reverb.”
The Kingdome also suffered from a low-frequency reverb time of greater than 10 seconds. To address that, EQ was used to electronically control the low-frequency reverberation. Frequencies below 250 Hz were rolled off to limit the amount of low-frequency energy from the sound system that would excite the reverberant field. Because human speech contains very little intelligibility below 250 Hz, the amount of low cut was a balance between maintaining clarity of speech and naturalness for music.
In contrast, the nearby Tacoma Dome has a suspended fiberglass acoustical ceiling. Accordingly, the Tacoma Dome has less reverberation than the Kingdome because the fiberglass ceiling has more absorption. “Cost is always a factor, so different stadiums and arenas may have different options for adding absorption,” Lewitz adds. “The challenge is finding the balance between the cost and amount of absorption, and available surfaces on which to put the absorptive materials.”
So when enough surface areas aren't available, the challenge is to find the most efficient absorption and the most appropriate surface areas to treat. In large stadiums, a bigger problem is sometimes discrete echoes across the stadium and reflections from the direct sound aimed at seating. The aspect of occupied seats versus unoccupied seats as it relates to acoustics is also, in and of itself, another topic. “You want to maintain a constant level regardless of occupancy, but this may be difficult if seats aren't upholstered,” Lewitz adds. “Sometimes absorption can be added to the seat risers to make up for an unoccupied seat.”
Typically, stadiums use fixed acoustical treatment on walls and ceilings, acoustical wall panels, and/or some form of suspended banners. To try and maximize intelligibility, the electroacoustic design brings the loudspeaker closer to the listener. “Stadiums are also using loudspeakers with tighter pattern control similar to the JBL Precision Directivity (PD) line of speakers,” Lewitz says. “But remember, there are some times when you don't want to beat the reverberation. You want there to be some energy to reflect the excitement from the crowd.”
Linda Seid Frembes is a freelance writer and PR specialist for the professional AV industry. She can be reached at email@example.com.