the increase of the loudness of natural sounds by an electroacoustic system. Sound-amplification devices are used to amplify speech and music in concert halls, theaters, and auditoriums, on open stages, and in stadiums. In addition to stationary systems, mobile systems (sound trucks) and portable systems (megaphones) are also used. Under high noise conditions (in industrial enterprises and in transportation), sound-amplification systems have special apparatus to enhance the intelligibility of speech. Hearing aids are used to provide sound amplification for the hard-of-hearing.
Monophonic sound-amplification devices consist of one or more microphones, electrical amplifiers of audio-frequency oscillations, and loudspeakers. An amplifier with loudspeakers makes up a so-called sound irradiation system. According to the arrangement of the loudspeakers with respect to the listeners, sound irradiation systems are classified as centralized, zonal, or distributed. In a centralized system, which is used on a stage, the weak sound is picked up by the microphone and converted into audio-frequency electric oscillations. The oscillations are then amplified to the required power and fed to the loudspeakers, which convert them back into loud sound. The loudspeakers are usually located on either side of the stage. With such an arrangement the listeners have the impression that the sound is coming from the middle of the stage.
Centralized systems make possible the creation of stereophonic sound amplification. In this case there are two or more independent channels in which audio-frequency electrical signals are produced, amplified, and distributed to the corresponding loudspeakers. When properly adjusted, a stereophonic sound-amplification system provides good coincidence between the visual and aural images.
In a zonal sound-amplification system the irradiated surface is divided into a number of zones, each of which is irradiated by its own loudspeaker. A disadvantage of this system is that the visual and aural images are separated, and an echo may be produced near the zone boundaries.
In a distributed amplification system, sound reaches the listener from many loudspeakers. Distributed systems are possible with chains of loudspeakers located on the walls or ceiling of a room, as well as on the backs of seats in an auditorium. In the latter case the loudness of the sound is equal for all listeners.
Acoustic feedback is present in any sound-amplification system, because the radiation from the loudspeakers impinges on the microphone. If the magnitude of the feedback is greater than a critical value, the system passes into the generation mode—that is, it begins to radiate sound spontaneously. A characteristic distortion of the sound, which is called regenerative reverberation, appears when the amplification is close to the critical value. The effect of feedback on the sound amplification in open air can be substantially diminished by the use of directional microphones. In rooms where the feedback is due mainly to reflected sound, acoustic wall and ceiling coverings are used to reduce it. The acoustic characteristics of multipurpose halls may be controlled by providing so-called ambiophonic installations, which can alter the resonance of a room, thus creating favorable conditions for the sound of various programs.
REFERENCESFurduev, V. V. Akusticheskie osnovy veshchaniia. Moscow, 1960.
Dreizen, I. G. Sistemy elektronnogo upravleniia akustikoi zalov i radioveshchatel’nykh studii. Moscow, 1967.
Papernov, L. Z., N. T. Molodaia, and Ch. M. Meter. Raschet i proektirovanie sistem oivucheniia i zvukousileniia v zakrytykh pomeshcheniiakh. Moscow, 1970.
L. Z. PAPERNOV and N. T. MOLODAIA