Acoustics, Sound Amplification, and Electroacoustics

• By Charles R. Bonner
• 02/01/09

The acoustics of performing arts venues, as perceived by audiences and critics, is a confluence of the “natural” (i.e., non-amplified) acoustics of the spaces and the sound systems serving the spaces (to the extent that such systems are employed). Some events, such as orchestral performances, rely upon natural acoustics, with varying results from venue to venue. Other performance types — such as musicals, plays, pageants, and the like — rely on sound amplification. Such amplification works together with the room acoustics to produce specific results. Touring shows generally bring their own sound systems with them; the same system is used for every venue toured.

“Natural” Room Acoustics Elements
In its most basic terms, the acoustics of a venue are represented by the following elements:

• The loudness of the “direct” sound as observed by the audience (the shortest line-of-sight distance from sound source to listener);
• the timing and loudness of the “early reflected” sound energy from walls and ceilings, ;
• the timing and loudness of the “late reflected” sound energy from walls and ceilings,;
• the ability, or the lack of ability, of the walls and ceilings to reflect low, mid, and high tonesand finally,;
• the feature known as “reverberation time,” which is a confluence of all of the above attributes, and finally;
• the background noise level loudness that is generated by mechanical equipment and air delivery systems.

The ability of the spoken word to be audible and intelligible to the audience is controlled by the loudness of the “direct” sound as compared with the loudness of the reverberant sound energy in the room. If the loudness of the sound source is not audible, intelligible, or pleasing to the listener because of room acoustic effects, sound amplification systems may be and frequently are used. In some cases, the loudspeaker systems may simply increase the “direct” sound loudness as compared to the “reverberant” loudness, with only small amounts of amplification. There are limits to the amount of amplification that can be achieved; the theoretical limit that applies to amplification is reached when the loudness delivered by the source to the microphone is equal to the loudness delivered to the audience by the loudspeaker system.

Twenty-first-century audio technology that comprises sound systems for performing arts venues has developed from meager beginnings during the decades from the 1950s to this day. Development of loudspeaker systems has been advanced, thanks in no small part, by sound equipment manufacturers, acoustical consultants, and the increasingly rigorous demands of the touring sound industry.

Sound systems for the performing arts are broadly classified into two principal categories: sound amplification systems, and electronic enhancement of room acoustics (which is referred to here as “electroacoustic architecture”).

Traditional Sound Amplification Systems
In its basic form, a sound amplification system is a vehicle that is used to amplify a sound source, which may be generated by people, musical instruments, compact discs, DVDs, or other sources. If the loudspeakers are sufficiently “directional,” such systems can partially overcome the effects of excessively reverberant ac oustics. Such systems tend to function within the acoustical environment that is presented to the loudspeaker systems. Reverberation time and other acoustic parameters work together with the directionality of loudspeaker systems to produce a specified and predictable result.

The technological advances that have occurred in most recently include the following elements:

• Loudspeaker systems that provide extended response and narrow beam patterns, reducing the influence of room reverberation upon audio quality: some of them are available in configurations that permit digital control (or “steering”)  of loudspeaker response and beam widths,;
• digital control of such loudspeaker systems,;
• digital routing of audio signals from point point-to to-point within sound system electronics, avoiding cumbersome and trouble-prone manual patch panels;
• digital mixing of audio sources, enabling instant audio “scene” changes at the touch of a button,;
• high-quality UHF wireless microphone systems,; and
• infrared assistive listening devices for the hearing-impaired.

These and other technology upgrades have made sound systems more adaptable to today’s changing requirements for sound amplification. Touring Broadway shows (such as “The Lion King”) can be and are produced with complete (and hands-free) computer-controlled sound systems, and school auditoriums around the country can easily be operated by today’s digitally qualified students.

All of these advances increasingly optimize sound system performance, even in acoustically difficult rooms.

Electronic Acoustical Enhancement Systems
Electroacoustic architecture, on the other hand, uses many (in some cases more than 100) loudspeakers and multiple microphones located strategically to either side and above performers and the audience to create a specific acoustical environment that the venue does not provide naturally.  This technology has been under development in Europe and Asia since the 1970s and has only been introduced in the U.S. within the past five years.

The microphones used in these electroacoustic architecture systems receive the natural sound by orchestras, bands, and choral ensembles, digitally process this sound energy, and reproduce an enhanced electroacoustic signal via the loudspeakers. This energy, in turn, re-enters the same microphones, is processed again and reproduced again. This chain of regenerative signals continues to occur until the perceived acoustical characteristics become inaudible, thus creating reverberation and individual electroacoustic reflections that are unique to the rooms in which the technology is installed.

The loudspeakers and microphones are customarily concealed within the architecture of the room, and are therefore invisible to the performers and audiences alike. Invisible subwoofer loudspeakers can be included as an acoustical “warmth” enhancement tool. These systems include combination or à la carte features (such as the following) that encompass many natural acoustical attributes at the touch of a single button:

• Reverberation time (short, moderate, long);
• acoustic envelopment (off, moderate, enhanced); and
• acoustical warmth settings (off, moderate, enhanced).

The result is an apparent acoustic environment that is not amplified, but rather is an extension of the acoustic capabilities of the theatres and halls that contain these systems. In summary, these systems can make acoustically ordinary theatres, churches, or concert halls into extraordinary acoustic facilities.

Systems of this nature have been installed in numerous states in the U.S. and have been proven to be an effective tool. We have traveled throughout the U.S. and Asia listening and observing these systems, and embrace the technology as being useful in facilities lacking optimum natural acoustical design because of budgetary restrictions. Typical examples of projects are school auditoriums, churches, and university theatres.

Two accompanying photographic examples are depicted in this article. The first is a 2300-seat performing arts center at LeTourneau University in Longview, TX. This hall is an example of a totally “natural acoustic” environment, and was designed from scratch to serve that mission. Adjustable acoustical curtain systems change reverberation time to support symphonies, touring road shows, and speaking events.

The second image is of a theatre renovation now under construction at a major university campus in the southeastern U.S., scheduled for completion in the Fall fall of 2010. Limited available ceiling height, resulting in lower-than-desirable reverberation time for symphonic music performance, made this theatre attractive for electroacoustic architecture. As of this writing, this example is unique in this part of the U.S.

Charles R. Bonner is president of BAI, LLC, consultants in acoustics and audio-video technologies.