Subtle Magic

IN RECENT YEARS, performance spaces and audio systems have increased in complexity and capability. Audiences and performers expect intelligible speech,
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Subtle Magic

Apr 1, 2002 12:00 PM, By Sam Berkow

IN RECENT YEARS, performance spaces and audio systems have increased in complexity and capability. Audiences and performers expect intelligible speech, tonally balanced music, even coverage of the audience seating area and full audience envelopment. Meeting those expectations has become important for successful venues of all types, and sound designers and acousticians are finding electronic acoustical enhancement systems highly effective tools for doing so.

Electronic enhancement systems are designed to electronically alter the acoustics of a space by providing extra sound energy in the form of reverberation or reflections to improve sound quality. These systems can work with traditional sound systems or in natural acoustic environments. It is important to note that enhancement systems do not generate audio signals on their own, rather they pick up sounds already within a space, process them and feed them back into the acoustic environment.

Enhancement systems are sometimes called electronic architecture systems, because they are not traditional sound reinforcement systems used to amplify a performance. Instead, these systems are meant to improve or alter the natural acoustics of a space, enhancing the performers' and listeners' experience. At first, the idea of adding energy to a space might seem strange; that extra energy causes reverberation and reflections, two of the most common acoustical problems in facility design and operation. So, how can adding sound energy actually help overall sound quality? In many cases, the traditional physical architectural solutions to these problems, which reduce reverberation or control reflections, leave facilities either lacking in some acoustical property or acoustically dead. Electronic enhancement systems can compensate for those effects. Furthermore, in many situations electronic enhancement systems can provide a level of flexibility that is impractical — or impossible — with regular architectural solutions.

Enhancement systems have existed for many years; however, the complexity of the systems, the lack of early system transparency and high costs often made them less than desirable. But with recent advances in digital signal processing, these systems are becoming increasingly effective in addressing acoustical issues. Newer systems that use DSP for acoustic performance and stability control far exceed earlier ones. And the recent use of common DSP platforms within these systems to provide equalization, delays and matrixing is making them increasingly cost effective.

▪ Past controversies. Because the enhancement systems incorporated into a facility are often concealed from the audience, their use can be controversial. The clandestine approach has created some tension for traditional audiences and engineers who are uncomfortable with the use of electronics to repair or alter the natural acoustics of a space. For many people, this reaction is based upon early experience with enhancement systems that were unable to alter natural acoustics without significant tonal coloration or artificial-sounding tones. But there is no question that current enhancement systems are effective and sound natural. Still, the remnants of controversy and the predisposition against them often cause the technical and popular media to overlook the current proliferation of enhancement technology.

▪ Enhancement Basics. At the core of all electronic acoustical enhancement systems is a series of strategically placed microphones, which pick up sound generated within a space. The sound is processed and then fed back into the space as additional reverberation and reflections. It's the processing of the signal that forms the critical step. This processing allows the characteristics of the signal fed back into the space to be controlled and altered advantageously. It addresses both time and frequency domain characteristics of audio input. It also ensures that the enhancement system remains stable and does not feed back.

The ability to provide sufficient gain without feedback has been the major design challenge for enhancement technology. This is also where the commercially available electronic enhancement systems differentiate themselves, using very different schemes to provide maximum flexibility while remaining stable. The choice of feedback-control schemes also helps determine which measurement techniques can be used to optimize these systems.

SUCCESSFUL IMPLEMENTATION

enhancement systems are used for two main purposes: to correct a specific acoustical problem and to provide acoustical flexibility that is not possible or practical to achieve architecturally. Also important is a system's ability to enhance the time, level and tonal balance of room reverberation.

Today's enhancement systems can add energy to a room, resulting in natural-sounding decays and allowing the existing acoustic decay rates to be increased as much as 300 percent without noticeable tonal coloration. Conversely, the largest caveat about using enhancement systems is that they require the system designer to have a thorough understanding of the acoustic conditions and of what the systems can and can't do. The following examples illustrate situations in which electronic enhancement systems have been well used.

▪ Performing Arts Spaces. Many theaters use enhancement systems either to increase or improve the tonal balance of reverberation and provide additional sound reflections to increase intelligibility and envelopment. Enhancement systems have also been used to allow theaters that are too acoustically dry to support unamplified symphonic performances without a traditional sound reinforcement system. In theaters and concert halls, enhancement systems have been used to improve the sound in orchestra pits and concert shells, which benefits both the performers and the audience.

Opera houses pose several challenges to achieving optimum acoustics and illustrate some best-practice uses for enhancement systems. Opera orchestras traditionally perform in a pit, beneath the stage. The singers perform above and behind the orchestra. In addition, there is often a cover or flap at the top of the orchestra pit that prevents direct sound from reaching the stage. Acoustically, the goal is to provide the audience with adequate impact and envelopment for the orchestra without overwhelming the performers or providing excessive reverberation that will render singing unintelligible. In larger opera halls, the greater distance between the singers and the audience reduces the level of direct sound as well as sound that returns to the stage. In such venues, electronic enhancement processors can be dedicated to both early and later energy fields. This split system configuration enables precise optimization of direct, early and later energy throughout the venue. Electronic enhancement can lift voice levels dramatically, restoring impact and improving clarity and intimacy throughout the house. Electronic enhancement can also radically improve the strength and envelopment for the orchestra without loss of intelligibility.

▪ Worship Spaces. Many worship spaces have diametrically opposed acoustical requirements: intelligible speech and sufficient reverberation to properly support choral and organ music. Enhancement systems have been used to allow the acoustics of a space to be altered as the program changes from speech to music. Electronic enhancement systems can increase intelligibility of choral singing and congregational response and give acoustic support to church organs.

Let's examine a hypothetical 1000-seat worship space. The sound committee plans to add a large pipe organ and increase its choral singing program. The existing space in this example provides excellent acoustics for speech reinforcement and some light music, but the facility has a relatively low mid-frequency reverberation time (1.3 seconds), and choral singing lacks bloom and fullness. Furthermore, this facility has 300 seats located underneath a balcony. Those seats feel acoustically separate from the main room and suffer tonal imbalance and spatial problems common to under-balcony spaces. In this case, an electronic enhancement system can increase the overall reverberation levels and decay rates both in the main seating areas and in the under-balcony spaces. Using microphones located in the upper portion of the room, the system can be used continuously without degrading the intelligibility of any existing sound/speech reinforcement system. The system can be further tailored to provide a decay that is appropriate for the type of music being performed by the organ, ranging from short decay times to gothic-sounding long decays. In order to improve the acoustics of both the main space and the under-balcony space, the enhancement system would be zoned, providing different output characteristics for each area.

▪ Sports Facilities. As arena sizes expand, particularly upward to accommodate luxury boxes, the roar of the crowd has become less dense. Even high excitement can seem dull when these large facilities are less than full. Electronic enhancement has been effectively used to address this issue.

PREPARATION AND SETUP

although each manufacturer provides different hardware and software, a system designer must take several critical steps to implement an enhancement system successfully, independent of the brand chosen.

▪ Get Quality Mics and Loudspeakers. As a rule, enhancement systems must be as resonance-free as possible. That means that the response of both the input and output stages (microphones and loudspeakers) must behave as linearly as possible. Even small resonances in these components can result in tonal coloration, creating an artificial-sounding affect.

▪ Position and Group Loudspeakers Carefully. Even with the availability of inexpensive DSPs, it is critical to carefully group loudspeakers. A system requires thorough calculation of the power handling requirements and polar response of loudspeakers in order to determine how the enhancement energy will behave.

▪ Fix the Fixable. Enhancement systems are the best choice when adding energy to improve sound quality. In cases where there is a strong slab back-style reflection or other major acoustical challenge, enhancement systems can be used to help hide the problem, but the overall sound will be much better when the problems are addressed architecturally.

Addressing these issues means days of fun — and anguish. Setting up an enhancement system often requires experience, so most of the major manufacturers will provide staff to help get the system running. As a rule, the following setup procedure is common to most enhancement systems:

MEASURE THE SPEAKERS

With the enhancement system off and the microphones muted, measure and optimize the transfer function between the loudspeakers and the room. Set gain levels and equalization.

SET UP TRANSDUCERS

Unmute the microphones and set the gain levels for them. Next, with the enhancement system bypassed and the microphones in line, measure the loudspeaker-room-microphone transfer function. (Take this measurement at low level to avoid feedback.) Use filters to dampen any resonance.

ADD THE ENHANCEMENT SYSTEM

Put the enhancement system in line and generate a source at typical operating levels. Optimize the gain structure of the system for maximum performance and stability.

Repeat this process for each zone. Once these steps have been completed, the distribution of early reflections and decay rates of late-energy reverberation can be set through the enhancement systems interface.

It is common for enhancement systems to use a large number of loudspeakers of varying power handling and frequency ranges. Systems with 50 to 100 loudspeakers are common; some use hundreds of loudspeakers. Therefore, cabling and installation costs are a major factor to consider.

There is no doubt that the role of enhancement systems is growing, as demand for sound quality increases and DSP power becomes increasingly affordable. Understanding these systems can provide one more valuable tool to help improve audio in a wide range of venues.

Sam Berkow is an acoustical consultant based in New York City. He designs performing arts, production and other sound facilities. Berkow is a partner in the Walters-Storyk Design Group and the founder of SIA Acoustics and SIA Software Company, where he developed the SIA-Smaart meas-urement system. Berkow can be reached at siasoft@aol.com.

Sources:

Four companies' electronic enhancement systems

LARES ASSOCIATES

The maker of the Lexicon Acoustic Reverberation Enhancement System is located in the Boston area. LARES began as a Lexicon product developed by Dr. David Griesinger before being developed and managed by LARES Associates. The system uses a time-varying technique to maintain stability by shifting the output in time enough to maintain stability but not enough to introduce tonal coloration. The use of this time-varying filter is an ingenious use of DSP; however, it limits the ability of traditional measurement systems such as MLS and dual-channel FFT techniques to directly measure the resultant impulse response. For DSP hardware, LARES uses hardware developed by Lexicon and BSS.

LEVEL CONTROL SYSTEMS

Based in Sierra Madre, California, and Western Canada, LCS makes the Variable Room Acoustics System. VRAS was developed by Dr. Mark Poletti of Industrial Research Ltd., Wellington, New Zefter, who selected LCS as the worldwide licensee of this technology. The VRAS system acts similarly in acoustically coupled spaces. The processing hardware is the LCS matrix system with an interface written in the BeOS operating system.

ACOUSTIC CONTROL SYSTEMS

Based in Garderen, Netherlands, the ACS system uses a large number of mics — typically 12 or more — in mic arrays. The arrays create specific coverage patterns for the input side of the system. The ASC hardware system is based on a card-cage configuration.

SYSTEM FOR IMPROVED ACOUSTIC PERFORMANCE

SIAP, developed by SIAP B.V. at Uden, Netherlands, and distributed in the United States by RPG Diffusor Systems, typically positions mics close to the source of the performance (within 26 to 32 feet). SIAP offers only enough energy to make up deficiencies in the natural sound. The hardware system is a remotely controlled card-cage configuration.

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