ALTHOUGH "OMNIDIRECTIONALITY" has been espoused of late with considerable gusto, it is neither a new feature of loudspeakers nor the sole criterion of speaker performance. It is, however, an important feature-some contrary opinion notwithstanding.
The term omni-directionality refers to a broadening of the sound waves produced by a speaker and more precisely, of the treble portion, inasmuch as bass tones naturally radiate in a circular or 360-degree fashion. As frequency rises, however, wavelengths shorten and tend to form into a beam.
The beaming tendency increases with the size of the speaker diaphragm, so that a relatively large-diameter speaker will "want to" beam high frequencies more readily than a smaller-diameter unit. This is one major reason for tweeters being traditionally smaller than woofers.
Which, of course, points up the obvious:. that some effort to avoid beaming of the highs always has been a part of speaker design. Lately this one aspect has loomed quite prominently as a feature of speaker systems. Why? To begin with, an overly directional speaker is a distorting speaker: midrange and highs that are concentrated in a beam sound unnaturally bright, edgy, harsh. Moreover, such sound reproduction, besides being unpleasant, tends to obscure the inner detail of musical textures and is especially degrading of complex instrumental passages. An up per string, for instance, playing in close harmony with a woodwind, might sound strong over a beaming speaker, but chances are you would not be able to recognize that you were indeed listening to the two distinct instruments playing together. Finally, such a speaker tends to sound like a hole in a box instead of a transparent "window" (or indeed an open window) on the performers; it imparts a constricted sense of eavesdropping instead of an unimpeded feeling of clear, open sound.
For years, designers of quality speakers have at tacked this problem (along with others, such as wide frequency range, low distortion, smooth and linear response, power-handling ability, and so on) in terms of tweeter designs that sought to achieve wide sound dispersion instead of narrow beaming.
One of the strongest impetuses to this trend came from the movie sound people, at the time that they attempted to solve the sound-dispersion problem in theaters when the first talking pictures were shown some forty years ago. Moviegoers sitting in the extreme side seats of the theater couldn't hear the dialogue on the soundtrack nor distinguish sounds clearly because the upper-middle and treble tones (the frequency range that carries all the important overtone structures that define different sounds and voices),were not getting to them. The solution to that problem--the sectoral- or multicellular-horn type of tweeter-proved something of an engineering coup. Not only did it disperse the sound over a very wide angle, but it was inherently a device of great efficiency, since a horn loaded to a smaller driving element acts as an acoustic transformer by helping to match the high-pressure/high impedance sound energy from the vibrating element in the horn's throat to the low-pressure/low impedance energy at its mouth. This efficiency in turn meant that these horn systems could be driven to high output levels (big, room-filling sound) with the use of amplifiers that were themselves relatively modest in terms of size, power, and cost. The horn tweeter, in one style or another, has since found its way into many excellent home speaker systems.
Not to be outdone, advocates of cone tweeters have steadily improved their designs to achieve although with the concomitant need for somewhat higher-powered amplifiers to properly drive them pretty much the same acoustic results. An early design effort in this direction resulted in the whizzer a small auxiliary cone or flat ring projecting near the apex of the speaker's main cone. Such a device can serve as an inexpensive way of extending, or at least of smoothing, the frequency response of a full-range speaker by decoupling the outer surface from the throat so that the former handles only the bass.
Its shape also helps to spread the high frequencies by diffraction: the effective treble aperture of the speaker becomes the diameter of the whizzer cone, while bass is produced over the remaining area of the diaphragm.
above: Omni speaker variety by Bose.
Many fine speaker systems use separate cone tweeters (which superficially resemble small woofers), and they achieve wide dispersion in several ways. They can use a really small diaphragm, two inches or less in diameter. Properly made, and backed by a hefty magnet structure, such a tweeter can produce excellent highs. In the last ten years the convex hemispherical dome tweeter has been used extensively. Because the sound tends to go out at right angles to the plane of the diaphragm, the outward-curved dome spreads the highs over a very wide angle. Some tweeters are found in many bookshelf speaker systems.
Using more than one tweeter to form a divergent array that covers a wide angle is another way to spread the highs. Yet another is the slot-loaded tweeter: the highs emerge from several tweeters radiating into a narrow slot that characteristically has a wide projection angle. Specially shaped cone units and plug-loaded midrange and treble cones have also been devised for fanning out the sound.
Acoustic lenses-small slats or vanes that help spread the highs-have been used on both cone and horn tweeters.
All of these speaker types, which can achieve wide-angle dispersion of high, are based on fairly popular design approaches. One other that until recently received relatively little attention outside of engineering circles was investigated some years ago by Dr. Harry Olson, head of RCA Laboratories and a pioneer audio designer. He mounted the same loudspeaker in a series of enclosures, each of a different shape (though all having the necessary internal cubic volume to baffle the speaker correctly), and then tested each for dispersion characteristics. The best of those he tested was a design in which the usually flat mounting board (baffle) had been cut and reshaped to form an enclosure that approached a sphere.
This shape of course has appeared at times in the form of loudspeakers of the ball type and others in which the surfaces of the enclosure are used to mount an array of multiple drivers.
Many conventional speaker systems spread strong highs over 120 degrees; some do even better.
A pair of stereo speakers, each having 120- to 150-degree dispersion, will spread the highs throughout any typical living room. If the speakers are set in the corners, 90 degrees of dispersion in each speaker will be enough to cover the entire room.
Only when a speaker system is placed mid-wall in a very wide room would 120-degree dispersion fail to cover most of the listening area.
Omnidirectional dispersion allows greater flexibility in speaker placement. For example, multidirectional systems need not--in most cases should not--be placed against a wall. Moving a conventional speaker out into the room would almost certainly produce dead spots in its coverage.
Though the wide-front dispersion commonly found in many of today's better speaker systems will produce a satisfactory stereo image, some speaker systems are aiming at even wider dispersion--up to 360 degrees, or omni-directionality--at least in the horizontal plane. And a few are going after dispersion in more than just the horizontal plane by spreading part of the vertical plane in an umbrella effect. Indirect radiation, most prominently espoused by Bose, uses the room walls to reflect the sound. According to Bose about ninety per cent of the sound reaching the listener from the Model 901 has been reflected one or more times from the walls inasmuch as eight speakers face toward the wall in back of the speaker system and only one is in the front, pointed toward the listener. The one direct-firing speaker, triggering our hearing's precedence effect, is enough to establish a stereo image amid such a high preponderance of reflected sound.
Dispersion and Stereo
How does dispersion relate to stereo? Actually, good stereo depends not only on a sense of left-to-right breadth, but also on the stability of that directionality. The localization of sound depends on our ears' ability to sense the relative strengths and timing of the sounds from the two speaker systems. Each of the two separate sounds that enter this comparison must seem to come directly from the relevant speaker. The mid-highs and highs are especially important. If the strength of the treble changes markedly as you move around the room, stereo localization will come loose from its moorings. Of course there always will be some change with motion in two-loudspeaker stereo. But if the highs are evenly spread, the stereo image should be at least reason ably firm--without jumps or reversals-throughout the listening area.
Furthermore, as long as some direct sound is still produced by a speaker, that direct sound will reach our ears a split second sooner than the indirect sound. For this reason omnidirectional speakers can indeed preserve the directional clues needed for stereo, some British commentators to the contrary.
But there's more. Wide-angle dispersion relates closely to "broad-source" sound which seems to emanate from an area larger than the size of the speaker system itself. Inasmuch as good stereo coverage intrinsically presents an apparently broad source to the listener, the question of opening up or spreading out the sound from the individual speaker systems received less attention in stereo's first years than it did in mono days. But the concept of the broad source has invariably implied a more natural kind of sound: a sense of the proscenium, an ambience that suggests depth as well as breadth in order to achieve greater realism in music.
What does seem certain in the general omni-directionality thrust is that stereo is being helped, not hindered. Since the direct highs from an omnidirectional speaker reach all parts of the listening area relatively un-muffled, the stereo image has maximum stability and it can be perceived from a greater number of listening spots in the room than is possible when using directional speakers.
This feeling, and one's evaluation of it, may be purely personal-but it has become a vital part of today's speaker designing. Indeed, when Quad and other companies first introduced these large, full range electrostatic speakers the claim was that they gave the sensation of hearing the music as if it were coming through a large doorway. This broadening of the apparent sound source is also implicit in the Bose concept and other multi-radiating systems-re produced sound reaching the listener not only from the speaker directly but from the surrounding walls too by reflection.
Note, however, that a large radiating surface in itself does not guarantee wide-angle dispersion.
Precisely the reverse would happen, for instance, in a large electrostatic panel for the same reason it happens in a large cone diaphragm; as wavelengths shorten with respect to the dimension of the surface producing them, they tend to form a narrow beam.
Today's large electrostatics overcome this tendency in several possible ways. The Quad has a curved front which helps fan out its sound. The big flat electrostatics have frequency-sensitive sections which make sure that the extreme highs are reproduced only by a relatively small area. And an electrostatic-by being located at some distance from the wall behind it-can be made to function as a di pole or doublet so that a large portion of the rear radiation is reflected and mixed with the front radiation. This effect, of course, varies with the relative positioning of the speaker and so some experimentation is required for optimum results in a particular room.
The prediction and evaluation of reflected sound effects is, of course, a complex subject. A few British commentators have taken the position that sound reflection equals sound interference, and thus these effects should be avoided at all cost by designing a loudspeaker to beam, thereby minimizing reverberation and concentrating sound energy in the directly propagated signal. American designers for the most part reject this concept.
In any event, it is obvious that poorly designed speaker units will not sound pleasing just because they put out highs in all directions. Thumping bass: honking mid-range, and screechy or edgy highs are not more acceptable when they come at you from all directions than when they hit you in a straight line. The over-all smoothness, transparency, and naturalness of a speaker system remain basic criteria. If a system has these virtues plus that of omni-directionality-and it pleases you-then it is a speaker for you.
Dispersion and Four-Channel Sound
To date, the accumulated insights-based on listening to avail able quadriphonic material played on a variety of systems and auditioned over various types of speaker systems--add little to what already has been stated, pro and con, on the subject of "omnidirectionality." It is true, of course, that the very adding of two more loudspeakers to a listening situation loads more sound into the room, and sets up new sound-radiation sources which in turn aid in the general dispersion of sound while simultaneously tending to further "wash out" deficiencies in room acoustics. And if the particular four channel material itself contains deliberately re corded "ambient information," that ambience will be reproduced as part of the signal rather than induced as a condition of the listening room's acoustical character. And so, to that extent, the use of "omnis" for the rear channels would not appear to be as critically needed as for normal "up-front" two-channel stereo reproduction.
However, if the four-channel material is the kind that presents "primary signal" information on all four channels, then the desirability of maximum dispersion on all four channels becomes as important a factor as it might be for two-channel stereo.
And, with any kind of material, the other advantages of ample dispersion still pertain. They would be, of course, an effective "enlarging" of the listening area, and a reduction in the system's phase distortion brought about by the "anti-beaming" effect of multi-angular dispersion of middles and highs.
Finally, the question of balance on all four channels is paramount to optimum quadriphonic reproduction, and so if you have already installed speakers for stereo of a given dispersion pattern, ideally they should be duplicated for the added rear channels in a quadriphonic setup.
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(High Fidelity, 1976)
Also see:
GLOSSARY--Definitions widely used in speaker systems.
BUYER'S GUIDE TO SPEAKER SYSTEMS--A complete listing of available models from various manufacturers giving important design and performance features, sizes, prices.