Sound System Components

A sound reinforcement system consists of three parts: the input, the amplifier and controls, and the loudspeakers. The input can be a microphone, any of a variety of types of commercial broadcasts, or one of many means of reproducing recorded material in common commercial formats, for example, compact discs. In sophisticated systems, the input is connected to local computers and computer networks.

Microphones convert acoustic energy into electrical energy. They convert sound waves into electrical signals, which are further amplified, transmitted, and processed as required in the sound system. Microphones may be hand held or on stands, or miniature lavaliere types that allow more movement by the speaker. Small wireless transmitters are available for any type of microphone, and are especially helpful for theatrical plays.

Preamplifiers reinforce the signal of the microphone for further processing and feed it to the other components of the sound system. A control console or mixer does the same job as a preamplifier, but is more complex and allows for more flexible control of the sound.

Power amplifiers provide a signal output with sufficient power (voltage and electrical current output) to feed the loudspeakers connected to the system. The amplifier is rated to deliver sufficient power to produce 85 to 90 dB for speech, 95 dB for light music, and 100 to 105 dB for symphonic music, in situations where the background sound level is 60 dB. Systems for rock bands produce sound pressure levels above 110 dB, and exposure to this level of loudness for extended periods will damage hearing. The amplifier's power can be reduced in quieter spaces. An acoustic specialist or a sound engineer usually specifies the amplifier. Amplifiers have controls for volume, tone mixing, and input-output selection.

Signal processing equipment includes equalizers, limiters, electronic delays, feedback suppressors, and distribution amplifiers. Equalization controls allow the signal to be shaped to increase or decrease specific frequencies. After the system is installed, portions of the overall audio frequency spectrum are adjusted. This process tailors the system to the acoustical properties of the space. Without equalization controls, the system will howl, sound rough, or give insufficient and poorly distributed sound. A competent sound engineer should do equalization after completion of installation and construction.

Limiters protect the amplifiers and speakers from overload by restricting the level of the signal applied to the system inputs. Electronic delays retard signals to amplifiers serving supplementary loudspeakers closer to the listener, and are also used with distributed loudspeaker systems to delay the sound to distant seats. This delay is necessary in a large space, where the electronic signal, which travels at the speed of light, may have to be delayed on its trip to the loudspeaker to match the speed of the direct sound, which travels at approximately one-millionth that speed. Feedback suppressors are used with weak-voiced speakers or with long-range microphone pickups in the theater. Distribution amplifiers are used in large systems to provide division of signals to many amplification tracks while isolating the source from the receivers.

Loudspeakers convert the electrical signal supplied from the power amplifier into air vibrations that the ear perceives as sound. The design and placement of the loudspeaker system must be coordinated with the architectural design. The speaker placement of conventional distributed loudspeaker systems has to be coordinated with the locations of lights, sprinklers, and air-handling system diffusers. Central loudspeaker systems require architectural enclosures and adequate supports. Loudspeakers must be of the same high quality as the rest of the sound system.

It is not physically possible to produce a simple loudspeaker with a satisfactory reproduction over the entire audio-frequency range from a single unit. Usually, different speakers reproduce separate frequency bands. Manufacturers of loudspeaker equipment offer packaged modular loudspeakers systems designed to meet the requirements of a range of typical, frequently encountered applications. This simplifies installation and integrates more easily with the interior design.

The most common loudspeakers are electromagnetic direct-radiator types that range from 7 cm (2f in.) in diameter for high frequencies to 45 cm (173 in.) in diameter for low frequencies. They are housed in cabinets to cancel the radiation of sound from the back of the cone. Smaller speakers are not as efficient as larger types at low frequencies. Directional speakers focus high-frequency sounds into a narrow beam, whereas coaxial designs offer a better high-frequency dispersion angle, and work better for distributed loudspeaker systems. Systems are available with two loudspeaker arrays with controlled directivity, a bass enclosure, and an electronic controller that shapes frequency response of the system. Only two loudspeaker arrays are needed for speech, with the bass speaker added for music. Recently, much smaller units have become available for highpower, high-quality low-frequency sound reproduction.

Depending on the application, loudspeakers are arrayed in either a centralized system or a distributed pattern. Centralized systems are used in large spaces with high ceilings to project sound with strong directionality from a focal point such as a stage or pulpit. Distributed loudspeaker patterns are used in spaces with lower ceilings where the sound is distributed evenly and without a strong sense of source through many smaller speakers, as in offices or restaurants.

A conventional central loudspeaker system (Fig. ^ 55-1) uses a carefully designed central speaker array of high-quality, sectional (multicell), directional, high-frequency horns and less directional low-frequency large-cone woofers to provide directional realism with a relatively simple design. The array is placed slightly in front of the primary speaking position, or above the source of the live sound. In most theaters, this is just above the proscenium arch on the centerline of the room. These components are very large, and the architect and interior designer must be aware of the dimensions to be accommodated. Smaller units are available with folded horns, but they are less responsive in low frequencies, making them acceptable for speech but not appropriate for music. Supplemental speakers may be added for coverage of the balcony and under-balcony areas.

Central loudspeaker systems provide directional realism for medium to large auditoriums. Where the audience is deep, the coverage of the front and distant parts of the audience is likely to be uneven, and supplemental speakers are used. Deep balconies and areas under balconies, as well as wide first rows, often need supplemental speaker coverage. Delayed signals help preserve the sense of direction of the sound.

Figure 55-1 Auditorium with central loudspeaker system.

In large meeting, convention, and exhibition halls, distributed loudspeaker systems (Fig. 55-2) offer flexibility in seating arrangements and reinforce sound from any position in the room, even when the room is divided by movable partitions. Distributed systems provide flexibility in spaces where the source and the listener locations vary according to the use of the space. They don't provide directional realism, but offer very good clarity and intelligibility. Distributed systems are not appropriate for rooms with very high ceilings.

Distributed loudspeaker systems use a series of small low-level speakers, 10 to 31 cm (4-12 in.) in diameter, located throughout the space. These are often ceiling mounted or recessed in the ceiling and send sound directly down, so that each speaker covers a small area. Speakers may also be located in the backs of seats or pews. Distributed loudspeaker systems are used in areas with low ceilings where a central loudspeaker cluster can't provide the proper coverage. They are also used for public address functions if directional realism is not essential, as in exhibition areas, airline terminals, and offices. In public areas with highly reflective surfaces, careful speaker positioning and volume levels are critical, or the result is extremely loud but unintelligible speech.

Individual loudspeakers in a distributed system can be easily switched on or off for proper coverage. Ideally, a listening position gets sound from only one loudspeaker. If the speakers overlap, loudness and garbled speech results. Another significant advantage of a distributed system is that the speakers nearest the microphone can be switched off, which is important in deep rooms with low ceilings. Speakers should not be placed on each side of a proscenium opening, and rows of speakers should not be located on one or both sides of a room.

Whatever the loudspeaker arrangement, the sound system operator should be within the covering pattern of the speakers. This means that space must be provided within the audience seating area or in a control room with a large opening located at the rear of the space. The best location for the sound control room or position is in the rear of an auditorium, where the operator can diB H H H H H H H H

Figure 55-2 Distributed loudspeaker system.

Figure 55-2 Distributed loudspeaker system.

rectly hear the sound and follow activity. The sound control room should either be fully open to the volume of the space, which is practical only for small systems without much equipment, or in a separate sound control room with a large operable sound control window. Using a monitor loudspeaker or working from a remote location control room produces unsatisfactory results for live performances. Many performing arts sound reinforcement systems use control facilities located entirely within the audience area.

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