ONCE upon a time, a music lover who wanted to play records in his home merely waltzed into a furniture or appliance store, said, "Sell me that phonograph over there, the one with the vermillion doors," went home, plugged his purchase into the wall socket, and voila!, music (of a sort) permeated his premises. There was no muss, no fuss, no installation problem. Unfortunately, often there was not very much real music either, at least not for the discerning ear used to the sonorous basses, the bite of real brass tone, the tinkle of the triangle and the kwis-s-shhhl of a pair of 16-inch cymbals in the concert hall. Frequency range was low (which often mercifully concealed some screeching upper-harmonic distortion); dynamic range was modest; hum, scratch and wow were abundant, and pickups wore shellac records into spirals of black thread in heaps of gray groove dust--unless the abrasive in the shellac wore down the stylus first.
The advent of high-fidelity reproduction of recorded sound changed this simple picture drastically. Confined at first to a small, dedicated group of technical and musical aficionados who built their own equipment with loving care, the taste and demand for at least tolerable quality in home reproduction were given a mammoth boost by a number of factors. Among these were the musical film "Fantasia" co-starring Stokowski, Deems Taylor and Mickey Mouse (a most unlikely and unusual trio); the introduction of plastic materials without abrasive content for pressings, and the development of the LP record by Columbia Records.
Since these primitive early times (primitive, at least, for the average music. lover who was not also an electronics engineer), there has been a steady advance in the art of recording and reproducing sound. Today, the quality of reproduction in even a modest home can substantially exceed that available to even the wealthiest 15, or even 10 years ago. In fact, the art has grown, improved and proliferated into different techniques so rapidly that the major decision for the modern music lover may well be between competing techniques of recording and reproduction rather than between items of equipment. The improved quality of reproduction is great from the point of view of the consumer, but the diversity in recording and playback techniques adds somewhat to the problems of the installer, be he do-it-yourselfer choosing and putting in his own system or a full-time professional advising clients and making complete installations. Before we get into the nut-and-bolt details of installation problems, it will be helpful to have an overall look at basic techniques of recording and reproducing sound currently of commercial importance.
Monophonic sound reproduction systems
The old standby of sound reproducing systems and still the most popular is the misnamed "monaural" system. It is misnamed because monaural, in a fairly literal translation, means "one-eared" or "one-heard," whatever that is. Monophonic is preferable. Actually, monaural refers to the recording of music, with the output of all microphones used for picking up the program mixed into a single composite signal (Fig. 101) and amplified through the same amplifying stages. The amplified signal is then applied to a single recording transducer, which puts the signal on the recording medium (tape, disc, or motion-picture sound track) as a single signal. When reproduced, the recorded program plays back as a single signal. This single signal may be reproduced as a sound wave by a single speaker or by many speakers, either clustered or widely spaced, or even by headphones. But no matter how it is reproduced, the program is still heard as a single signal, and the apparent source will be the general area of the reproducer.
Fig. 101. Essentials of a monophonic reproduction system.
However, even in the monophonic technique, advances have been made in achieving a more spacious, concert-hall effect, as well as in reducing distortion and increasing dynamic range. In the best monophonic systems, a respectable feeling of presence and immediacy can be achieved by use of reproducers consisting of a considerable number of good speakers, so arranged physically that the "listening-through-a-knot-hole effect" that a single small speaker often produces is substantially removed. A good monophonic system, using a fairly large total radiating surface distributed over an area perhaps 6 or 8 feet wide and 4 feet high, and with two or more good squawkers and tweeters properly positioned and phased with the woofer assembly, can still compete in overall listener satisfaction with most good stereo systems, in the opinion of the authors. This is not to say that stereo does not have something special to offer, for it does have. But plenty of stereo systems now installed and operating don't provide anything Tike the advertised effect of spatial location, perspective and presence actually possible in the present state of the art. And, in direct comparison with many a simple stereo system, a monophonic system of the sort described will need no apologies.
Perspective and sound-source spatial distribution
Experiments in reproducing music in a way to give the same effect of spatial perspective and distribution of the sources of sound that a listener at the live performance out very early in the development of the audio art, so early that they are not easy to trace and date. Some of the most basic research and experiments were conducted by the Bell Telephone Laboratories as far back as 1933. In an historic experiment, music and other audible program material were performed on the stage of the American Academy of Music in Philadelphia, picked up by separate microphones, transmitted over separate telephone lines to Constitution Hall in Washington, D.C., and there reproduced through separate speakers.
Properly speaking, the system used in this early experiment was of a type that is now called stereophonic. Another experiment made in the preceding year used two separate microphones and audio channels, and reproduced the separate signals in two separate earphones mounted to form a single headset. Such a system differs somewhat from the type used in the Academy of Music experiment. It is called a "binaural" system, although this, too, is a misnomer in its way, as we shall see.
Binaural reproduction
The term binaural (which might be better written "bi-aural''). has come into general acceptance in the audio field as meaning a method of reproduction in which two complete audio channels are provided, with complete separation of the channels from the two microphones to the two ears of the listener. A direct translation would be "two-eared," but, since most people listen to any kind of sound system with both ears, the term is not accurate without some explanation. As now used, the important distinguishing feature of a binaural system is that none of the signal entering the left microphone reaches the right ear of the listener, and none of the tight-microphone signal reaches the left ear (Fig. 102).
In practical applications, this can be done effectively only by using a separate earphone for each of the listener's ears, with earphone cushions that provide a very effective seal against sound leakage. Because of these requirements, binaural systems have so far had a limited popularity, since most people find the wearing of headphones physically distracting and visually unaesthetic, especially for the length of time required for a symphonic performance. An approximation of the binaural effect can be produced by mounting two small speakers in the wings of a well-padded wing chair, but the effect is limited to the occupant of the chair, and the low-frequency response of small speakers suitable for this application is limited.
From the installer's point of view, the rare occasion when he is called on to install a pure binaural system is practically a holiday, for the problems of speaker placement and room acoustics vanish when only headphones are used for listening. From the user's point of view, this characteristic of the binaural system removes any possibility of an irate neighbor or spouse (if he happens to like loud music at 3:00 am), and it also cuts the cost of high-quality reproduction drastically. This is true because of the low cost of the highest quality headphones compared to quality speakers, and the very low power requirements for head phone reproduction.
Stereophonic reproduction
Of the various means of providing an effect of spatial distribution in the apparent source of the sound, the "stereophonic" method is by far the most popular. The term is generally accepted in the audio field as meaning a system of recording and reproduction in which the audible program is picked up by two separate microphones and recorded as two distinct sound signals. In playback, the two separate signals from the recording medium are picked up and amplified separately, and reproduced through two separate speaker systems (Fig. 103). The two speaker systems are so positioned that the sound signal picked up by the micro phone at the right, as seen by an observer facing the sound source, is reproduced through the speaker at the right, as seen by the listener facing the speakers in his living room. Similarly, the signal picked up by the left microphone in the recording studio is reproduced through the speaker at the left in the stereo reproduction setup.
BINAURAL PICKUP (DUMMY HEAD) SOUND-SOURCE
Fig. 102. Essentials of a binaural reproduction system.
This scheme closely resembles the binaural method, but there are significant differences, some of which are obvious while others are rather subtle. The unaesthetic aspects of wearing headphones are removed, along with the physical discomfort and the restricting effect of the headphone cords. The apparent sound source is given a stable orientation in the room instead of turning with the headphones when a listener turns his head, an effect some listeners find weird. However, the portion of the room in which the maximum stereo effect is heard is fairly limited, and complete separation between the sound signals at the ears of the listener is impossible in a practical situation. To be more specific, some of the sound from the right speaker always reaches the left ear of the listener, either directly or by reflection from the walls of the room, and vice versa. Also, the sound-absorbing and/or reverberant qualities of the listening room now enter the picture, in ways that may either enhance or conflict with the best stereo effect.
In spite of these complicating factors, a good stereo system, properly installed, can give a very realistic effect of presence and sonic perspective. The result is particularly good for sound sources that were distributed over a large stage, or moved about on the stage during the recording session. Thus, a symphony orchestra presenting a selection in which a phrase played by the string section at one side of the stage is answered by a following phrase from the reeds at the other side can be reproduced by a stereo system with a startlingly realistic effect. Similarly, an actor speaking lines while walking across the stage is heard by the listener as apparently crossing the room from one side to the other as the lines are spoken.
All in all, stereo appears to be on its way to dominance in the high-fidelity field, although it is unlikely that the monophonic type of system will ever entirely disappear. From the installer's point of view, stereophonic systems usually pose the more difficult problems. This is true because of the necessity for achieving the full stereo effect, in addition to satisfying the other requirements, such as visual appeal, full frequency range under room conditions, and freedom from echo, standing wave, and sound leakage difficulties. In almost any home listening-room situation however, the skilled installer can accomplish the desired result without serious trouble. The toughest problems arise when the stereo effect must be achieved in a large public room, such as a restaurant dining room. There, some compromises must be made, and it is certain that the effect will not be good in all parts of the room.
Such systems are rare however, and a careful explanation of the physical limitations of stereo to the client will usually prevent misunderstanding.
Other sonic perspective systems It is unlikely that the average installer of hi-fi systems will encounter any type other than those already mentioned. Nevertheless, there are some other schemes for producing the sonic perspective effect, in which the apparent position of the source of the reproduced sound corresponds to the position the sound source occupied with respect to the recording microphones. It is worth knowing about some of these other methods, because they are an aid to understanding the general sound principles involved, and also because they may become more common in the future, particularly for large public rooms.
One method used occasionally for sound reinforcement at large outdoor concert shells and similar places is called the "curtain-of-sound" method. A row of microphones, usually at least four and often six or more, is spaced across the front of the stage.
Each is connected to a separate amplifier which drives a separate speaker positioned with its back to the microphone, so that its output is directed to that portion of the audience directly in line with the microphone and the center of the sound source (such as a symphony orchestra). If the microphones and speakers have approximately similar pickup and radiation patterns, the effect for the audience can be very good to excellent. Such a system practically eliminates the typical public-address-system effect that many simpler sound reinforcement systems give. Yet it makes it possible to present the program at a high enough average sound level to cover a very large audience effectively, despite the inevitable ambient sound from outside sources, such as aircraft overhead.
However, because of the number of separate channels involved, it is very unlikely that the curtain-of-sound idea will be used in recording and playing back sound programs.
Another system that has found limited usage is called coded stereo. It has been used in "road-show" presentations of some motion pictures in which the sound track was of special interest, as in the case of "Fantasia." In this system, a single audio signal is usually recorded, but the playback system has two, three or more amplifiers and speaker systems. All amplifiers are fed the audio signal, and the speakers are dispersed across the stage, usually behind a sound-transparent screen or curtain. In addition to the sound program on the recording medium, coded signals that control the gain of each amplifier are recorded. These signals are usually a sine wave at a frequency below the audible range, around 12 to 15 cycles, or perhaps 20. The coding can be arranged to control the gain of the various amplifiers in such a way that the sound of a marching bland, for example, is heard at first only in the speaker at stage right. As the volume gradually rises (con trolled by the coded signals), the next amplifier and speaker to the left are brought into action and, as this channel level rises, the output of the first one is reduced. The apparent source of the sound can thus be caused to move across the stage to correspond with the natural movement of a marching band.
The system can be quite effective for certain kinds of programs, (such as the marching band), but it cannot produce the effect of a duet between separate instruments located at two different points on the stage, since only a single audio channel is actually used. It is unlikely that a private installer will encounter such a system professionally, since they are practically limited to special shows that carry their own crew of sound technicians.
The installer and system types
Fig. 103. Basic stereo system consists of stereo program source, dual
control amplifier, power amplifiers and two speaker systems. More speakers
may be used, but placement is critical. (Photo: Allied Radio Corp.)
Practically all installation work at present involves one or the other of the monophonic and stereophonic types of system.
Essentially, a monophonic system is simpler, both because less equipment is involved and because the acoustic factors are less complex. In making a monophonic installation, however, it is well to double-check the possibility that the client will want stereo at some time in the future. If so (and the present trend is in that direction), it may be possible to simplify the later job by a little forethought, such as running an extra pair of speaker wires from the amplifier position to the probable speaker location.