IN THE WORLD OF ACOUSTIC MIRAGES

IN THE WORLD OF ACOUSTIC MIRAGEEpisode about twenty years ago, which reminded of the old, accidentally fell into the hands of the magazine, is perceived today with a smile. One of the American cities came to tour the well-known orchestra. The best the city concert hall that evening was Packed. The concert began. The inspiration was reflected in every gesture of the conductor, the faces of the musicians; the audience enjoyed the art of the performers.

 
But suddenly, at a sign from the conductor, the orchestra stopped. Bows hung in the air, the pianist’s hands fell to his knees, the drummer, left standing with arms raised over liteframe sticks…
 
And the music was stopped. It was implausible: still the orchestra continued to sound as if nothing had happened. Surprise present in the room was hard to describe.
 
Everything became clear only after he explained to the audience that she, herself without knowing it, was involved in the experiment, organized by one of the gramophone companies. Demonstrated new stereophonic system of sound recording and sound reproduction, providing the natural sound of the music.
 
The orchestra that night, did not utter a sound: the program was pre-recorded and played through the system covertly installed in the hall of speakers. All that was done at the same time on the stage was only thinly held by dramatization, pantomime, imitating a live performance, to more effectively and convincingly to show the advantages of the stereo system audio in front common mono.
 
In the years since the events described, the technique of sound engineering has leaped forward. And now, when millions of copies are produced stereo records, when in many concert halls such as the Kremlin Palace of congresses in Moscow, La Scala in Milan, Palais de Chaillot in Paris, there are stereo amplification system, when at last the use is increasingly implemented in broadcasting, one can only smile, remembering that twenty to twenty — five years ago, the session of the stereo playback was seen just as a technical trick, sensational.
 
The use in our time has become an open page in the history of electroacoustics, an integral part of the complex of technical means, which is provided by the so-called “high fidelity” playback.
 
What is the attractive force stereo? Why listeners prefer stereo playback monaural sound normal? What technical means are implemented this way sound transmission with a new higher quality?
 
Imagine for a moment that in the concert hall separates you from the stage thick, even transparent soundproof wall in the middle which left a hole diameter in normal room speaker. Would you prefer to hear and see the performers directly, without any obstacles, as the sound quality and the aesthetic pleasure you receive will be much higher in the natural perception than described, contrived way of listening.
 
Unfortunately, this method is not entirely far-fetched. It reproduces the conditions in which we usually listen to the radio or monaural recording.
 
“Image” sounds, which “draws” the microphone installed in the hall, where the obtained point. The sounds from the instruments of the orchestra, as well as sound reflections from the ceiling and walls of the hall will come to sitting at the dynamics of the listener from one point in the same direction. We are accustomed to this “radioscan”, sometimes simply learned to ignore these distortions are fundamentally inherent in the normal monophonic transmission.
 
At the concert, for example, a person watching all the nuances of sound and light dynamics of performance, sees it as a whole and at the same time can highlight one of the many simultaneously sounding instruments in the background of the orchestra. This can be done even when sound energy accompanying instruments predominates over the energy of the soloist.
 
A person’s ability to be adjusted on perception of the most important for him at this point, to distinguish it from secondary is one of the amazing abilities of human hearing. Moreover, this selectivity is largely due to the ability of the ear to perceive the difference of the directions of arrival of sound waves from different sources and to focus on one .these areas.
 
To listen often include multiple speakers at once, placing them in different spots of the room. So artificially create a distributed in space sound radiation. Acoustic listening environment and, of course, improved, but no significant changes of the transmission quality occurs. This is to be expected, as transmitted on a single channel monaural transmission does not contain any information about the dimensions of the area occupied by the on stage orchestra, nor about mutual position of individual instruments in the orchestra. Naturally, to recreate the thus three-dimensional, spatial sound image is impossible in principle.
 
Technically, to implement a system capable of transmitting sound panorama in its entirety and naturalness, scientists, physiologists and scientists, the acoustics could have many years ago.
 
It is well known the ability of the human ear to determine the direction of the sound source. This effect is called binaural, is that sounds from the same source (unless it is right in front of the listener) are coming to the right and left ear simultaneously. In the Central nervous system occur at the same physiological effects that create judgments about the direction of the sound. The sound seems to be coming from the right or the left, depending on which ear, right or left, the sound vibrations are coming with greater intensity or ahead in time. This physical factor is, in fact, the main reason for the psychological possibility of a person to produce, say engineers, location sound image, that is, to determine the location of the sound source in space.
 
For the first time stereophonic sound transmission took place in 1881 at the Paris Opera. Left and right sides of the stage were installed two microphone. From them electric oscillations were reported separately to the several pairs phone headphones (from the left microphone to the left the numbers from the right — to the right). Listeners who used headphones, I got the impression about the spatial arrangement of the performers on stage.
 
Later began to use two of the same MIC, mounted in the “ears” mastoid model of the human head life-size papier-mache. The left and right microphones were contacted via separate channels with a pair phone headphones. This system of transmission was known under the name of binaural.
 
This system provides the spatial pattern of the sound, but because of the inconvenience of using headphones, the practical application is not received.
 
In the 30-ies was the first study of three-channel loudspeaker system for stereo playback (And, Horon in the USSR, G. Fletcher and L. Stokowski in the US), which confirmed the possibility WITH’ building the room sound field perceived by the listener as natural.
 
However, serious prerequisites for the practical implementation of stereophony was created only after the invention of magnetic and improvement of the phonograph record.
 
First develop stereo systems intended for wide usage, belong to the early 50-ies. Numerous experiments and theoretical studies conducted by this time in the Soviet Union, and abroad, have determined the most economical and relatively easy to do practical system of stereo sound reproduction and recording. Since then, speaking of the stereo, usually mean dual channel system. From the above-mentioned binaural dual channel use differs only in that the transmitted left and right microphones, electric oscillations, showing how the left and right halves of the “sound frame”, after amplification, are fed not to the headphones, and two located at a distance of about three meters from each other and the speakers. If the listener is centered in front of these two speakers, it quite clearly distinguishes between areas in which, as he heard sounds coming from different artists.
 
There is an interesting effect, which makes two-channel stereo system is quite reasonable and can compete in quality with systems containing three or more transmission channels. If both microphones in the Studio were equidistant from the contractor and the signals reproduced by two loudspeakers, exactly the same volume and are emitted strictly at the same time, the artist seems to lie in the center between the speakers, creating a fictitious, apparent source of the sound.
 
Thus, having only two channel and two speakers, you can get a full sonic panorama, limited only by the width of the distance between the speakers.
 
At first, dual use was most widespread in the recording industry. Now, as we know, most records are produced by the Soviet Melodiya, stereo.
 
The use is being implemented gradually and in broadcasting, the Introduction of a new method of radio broadcasting should not be associated with complete replacement of the existing transmitting and receiving equipment. After adding to the equipment of some units and restructuring it should be suitable for transmitting and receiving stereo programs.
 
It is important that a system of stereo broadcasting is compatible with the monaural broadcasting. This means that the listener has not yet had time to purchase a stereo receiver, needs to take a stereo program usually in the form of a full mono transmission.
 
In different countries at different times were tested several systems for stereophonic broadcasting; many of them provide high quality sound, were well compatible with monodrama. However, comparing these systems between them, the experts focused on the two most favorable technical and economic indicators.
 
This, in particular, a system developed by American firms “Zenit” and “General electric” and it was accepted to regular broadcasting in 1961 in the USA, and then spread in many other countries under the name of “pilot-tone”.
 
Finally, developed by our scientists in Leningrad system “from the polar modulation”, authorized to broadcast in the Soviet Union.
 
And both involve the maintenance of stereophonic broadcasting in the range of ultrashort waves. VHF FM transmitters are adapted to simultaneously transmit on the same wave full stereo information, i.e. “left” and “right” stereo signal.
 
Both of these systems provide high-quality reception of stereo programmes on special receivers and however, give the opportunity to listen to stereo transmission and conventional VHF FM receiver: of course, in mono.
 
Despite the fact that the systems differ little from each other, some technical details do not allow you to use one receiver system for the reception of the transmission on the other.
 
The use does not limit the technical capabilities of the transmission. Currently, the search for scholars aims to create a system of sound transmission that would be able to create in any room acoustic environment exactly simulated the atmosphere of the concert hall. It is difficult to overestimate the value of such work to meet the growing spiritual needs of the Soviet people who live far from major cultural centres and deprived of the opportunity to attend the best concert halls and theaters in the country.
 
To further improve sound transmission — design chetyrehkantnyj, or so-called Quad systems. The principle quadraphonia and how it is implemented — a topic of much conversation.
 
B. MEERSON, chief technologist of the State House of broadcasting and recording

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