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SINCE SOUND Is essentially the movement of air at a frequency and amplitude sufficient to be heard, the basic job of a loudspeaker is to cause the air to move within the nominal audio range (20 Hz to 20,000 Hz) and with enough "thrust" to create sound waves. You can demonstrate this basic principle for your own edification: Grasp a piece of stiff cardboard and wave it back and forth near your ear. Note that the faster you wave it, the higher the pitch (frequency); the more forcibly you wave it, or the longer the distance of each wave, the louder the sound level (amplitude). So much for lesson 1 on how speakers work.
For lesson 2, ask someone (who won't think you're mad) to drone a simple monotone like a drawn-out "ah-h-h" and then hold cupped hands over the mouth while continuing to drone at the same pitch and loudness. The sound will immediately become louder and likely clearer, or at least easier to hear.
Lesson 1 shows how speakers generate sound.
Lesson 2 demonstrates how sound is coupled to the environment. The whole technology of speakers sizes, shapes, materials, enclosure designs, and so on-is built on these two basic ideas.
Their first commercial application was Alexander Graham Bell's telephone "receiver" in 1876, essentially a prototype of the headphone as well as of the loudspeaker. It was, in fact, a "speaker"-the "loud" part came a bit later with the development of the phonograph. The essential difference, in socio-cultural terms, was that the telephone receiver was (and presumably still is) a private listening de vice, while the loudspeaker is a group-listening de vice. To put it in technical terms, the telephone receiver (and its modern counterpart, the head phone) is designed to fill a very small "cup" with sound that is directly coupled to the ear. The loud speaker is designed to fill a large space with sound that gets to the ear via the normal air and environment of a room.
The first loudspeakers were small discs that were attached to the vibrating stylus of the phonograph.
The low-amplitude sound thus produced by this "diaphragm" was amplified in megaphone fashion via a horn. A marvel in its day, this design was crude and severely limited in response.
Attempts to build a better loudspeaker culminated in 1925 with the development (by two General Electric researchers, Chester W. Rice and Ed ward W. Kellogg) of a moving-coil or "dynamic" loudspeaker that was powered ("driven") from a 1-watt amplifier. This design, which marked the end of the era of the "Morning Glory" type of horn, coincided with the rise of radio sets powered from line-voltage rather than from batteries. Its development paralleled the work of the notable audio pioneer, P.G.A.H. Voigt who, at Edison Bell in Eng land, was pioneering such (then) novel notions as free-moving diaphragms, huge magnets, aluminum voice-coils, and horn-loaded enclosures.
Today, speaker design is still very much an "open-ended" affair, with continued experimentation as characteristic as adherence to any established rules. Indeed, we seem now to be in a period of re-examination of basic principles, not only those of conventional speaker design and performance but of the very methods for generating sound.
This ferment is seen in the recurrence of such phrases as "true piston action," "omni-directionality," "matching the speaker to the room," and so on. More to the point, it is evident in the new diaphragm materials that replace the older paper cone; new suspension techniques; new diaphragm shapes, including the oval and the rectangular. No less important is the employment of older techniques newly refined. Finally, there are designs that ignore the moving coil altogether and offer alternate methods of generating sound, such as the electrostatic and other types of speakers.
Dynamics Dominate
The prevailing design, however, remains the dynamic speaker, in which a moving "voice-coil" is made to vibrate in a magnetic field when fed with amplifier signals. One end of the coil is attached to the apex of a thin diaphragm, or cone, which vibrates with the coil to produce sound. Many design features contribute to the accuracy with which the vibrations correspond to the signals from the amplifier--i.e., to how good the sound will be. Among them are the efficiency of the magnet, the total inertia of the moving elements, the inherent resonances of the parts, the shape of the diaphragm, and so on. These features vary from model to model, but a paramount aim-whatever the design approach--is to get the diaphragm to behave like a true piston. The cone, made of felted paper and flaring out from an apex, lends itself to piston action, but such action--or the speaker's response--is limited by a number of factors. For example, the cone must be rigid or stiff enough to vibrate like a solid piston rather than like a piston-in-parts, lest breakup and its attendant distortion occur. At the same time, the cone should be light enough in weight to respond readily to the voice-coil's vibrations, thus avoiding another form of distortion. All other things being equal, the desirable cone is one that has a given amount of stiffness for the lowest possible weight, or what engineers call a favorable "stiffness-to-weight" ratio.
Another problem of the cone speaker is that its shape-particularly when it is fairly large--naturally produces good bass response, but fails to pro duce as good treble response. The deep cone shape, for one thing, tends to beam the highs instead of dispersing them evenly and smoothly. For another, the large cone is not as readily controlled, and has inherently lower resonances and slower movement potential than the smaller cone. Yet, for treble response, a diaphragm that can move rapidly is a prime requirement.
Obviously, the features of a cone that make for good bass response do not make for good treble response. One widely adopted solution to this di lemma has been to treat a single cone in such a fashion that it acts in two different ways, acoustically speaking. The center area of the cone, ex tending two or more inches beyond its apex, is stiffened in order to raise its resonance to a frequency higher than that of the cone as a whole. A ridge circling this area separates it from the rest of the cone. which retains its original, lower resonance and is--by comparison with the center portion-less rigid. though stiff enough to respond to bass frequencies. This technique, known as "mechanical crossover," effectively separates the speaker's response to highs and lows between two different portions of the cone. Instead of one dividing ridge, some cones are made with a series of corrugations designed to have the same effect. One popular refinement of the technique is the addition of an auxiliary stiff, very small cone to the center of the main cone. Known as a "whizzer," it helps spread the highs and smooth the response. Speakers of this kind are known as "twin-cone," or "full-" or "wide-range," and are, as a group, generally suitable, though by no means the best available, for reason ably clean sound.
Less of a compromise is the speaker system in which separate drivers are employed to handle the highs and the lows. A crossover, or dividing net work, is used to separate the signal from the amplifier and to channel correct portions of it to each speaker or "driver." Freed from the necessity of re producing the full range, a woofer can be built to be a specialist at low frequencies. Similarly, the design of the tweeter can concern itself with the special requirements of treble reproduction, such as ex tended response for the highest overtones and wide-angle dispersion of them by means of small, specially shaped diaphragms, as well as by special structures fitted to those diaphragms. An advance over the two-way system is a three-way reproducer, in which the woofer and tweeter have less work to do individually, inasmuch as they are aided by a midrange driver, again designed specifically to cover a limited portion of the total audio spectrum.
The most elaborate speaker systems employ four drivers, assigned to the deepest bass, the lower mid range or "upper bass," the upper midrange, and the extreme highs.
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The heart of the dynamic speaker is an electromagnet
surrounded by a moving coil popularly termed the voice coil. It receives
signals from the amplifier which vary the magnetic field causing it to
move in and out like a piston. The diaphragm attached to one end of the
moving coil thus vibrates to generate sound.--- ELECTROMAGNETIC (DYNAMIC)
SIGNAL FROM AMPLIFIER MAGNET CONE DIAPHRAGM VOICE COIL SUSPENSION
----------- In an electrostatic speaker, the "diaphragm" is
a thin movable plate that is spaced closely to a fixed plate. An initial
electrical attraction between them is set up by a polarizing voltage, and
the effect is similar to that of a huge capacitor. Signals from the amplifier
then vary this attraction so that the movable plate vibrates to produce
the sound. Electrostatic speakers require no enclosure as such, only a frame
to hold the structure in place.
-------------- The ribbon speaker is an ingenious variation on the electromagnetic
principle. The ribbon itself is a thin, flat conductive metal section that
receives signals from the amplifier. It functions, in effect, as both "voice-coil" and "diaphragm." The
sound energy it produces is quite low in amplitude and must be coupled via
a horn to be heard in the listening area.
The ionic speaker uses no diaphragm at all. Instead, the air in a small chamber is electrically charged so that it becomes ionized. This tiny "cloud" then is impressed with the audio signal from the amplifier so that molecular vibrations of air, corresponding to the signal, are set up. Because the amplitude level of these vibrations is very low, a horn is needed to couple them to the listening area.
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A recent solution offered to the problem of piston motion vis-a-vis weight and stiffness is the use of cone materials other than, or in addition to, the traditional paper. One very widely used is expanded polystyrene foam, alone or as part of a laminate with paper, aluminum, or aluminum and paper. Other materials gaining favor include rubberized plastic and specially prepared papers. The new materials provide a better stiffness-to-weight ratio in the interests of good woofer design, and lend themselves to the manufacture of smaller diaphragms more suitably shaped for good treble response.
"Loading" the Sound to the Room
Whatever the design, a speaker represents--to the amplifier driving it-an electrical load or impedance. The input impedance to a speaker is very low because the voice coil has an innately low electrical resistance, generally about 8 to 16 ohms and of ten lower than that. Ideally, this impedance should remain constant over the speaker's range to permit an optimum transfer of power from the amplifier.
Variations in impedance during use can cause peaks and dips in the response (amplitude distortion), as well as changes in the time sequence of the related parts of musical signals (phase distortion).
They also can contribute to whatever other forms of distortion may be present for other reasons in the speaker's output. Some impedance variation is in evitable; the impedance of a speaker, being an inductive effect, naturally rises with frequency. This small increase, however, is not serious. More germane is the effect of reflected sound, a phenomenon caused by room dimensions as related to cone dimensions. Instead of the speaker wafting sound into the room, sound-wave patterns in the room push against the cone, causing distortion. To a great degree, though, their effects can be minimized by a speaker system designed so that its drivers will respond only over their most constant impedance ranges-again, a factor in the superiority of the multi-driver reproducer to the single-cone speaker.
If the speaker presents an electrical load to the amplifier, the room acts as an acoustic load for the speaker. When a cone "looks" directly into the room it is said to perform as a "direct radiator." The load on such a speaker is the air on its surface, and thus its acoustic impedance is determined by the area of the diaphragm. At high frequencies, this load is modified by the distribution pattern of the sound, which itself is determined by the shape, size, and number of tweeter diaphragms. The methods that make for a wider distribution pattern of highs also help match them to their acoustic load.
At low frequencies, the acoustic load is modified-usually adversely-by standing waves set up by a room's dimensions. These variations in turn modify the electrical load presented by the speaker to the amplifier. As suggested above, a well-designed woofer whose own impedance remains fairly constant in the bass range will minimize these effects. But an equally, if not more, serious problem in bass reproduction is the tendency for bass tones to interfere with each other at their source. While high frequencies tend to form a beam, lows radiate much more circularly. Thus, the bass sound produced by one side of a cone can very easily "meet" the bass sound produced by the other side. Inasmuch as these two sounds are exactly out of phase with each other, they will negate each other when they do meet and thereby considerably reduce bass output. This phenomenon, known as acoustic short circuit, can occur at the outer rim of any speaker.
To prevent it and permit a fuller bass response, the rear and front waves must be "baffled" from interfering with each other. Just how to baffle the cone speaker is probably the most controversial single is sue in the whole field of sound reproduction.
Enclosures Are More Than "Boxes"
Although speaker enclosures are legion-in terms of size, style, shape, operating principles-there are some general factors relevant to all of them.
To begin with, when the rear of a speaker is enclosed in a finite space, such as a box, the space it self influences the speaker's response by limiting its cone movement at low frequencies. The exact degree of limiting depends on the size of the box and the cone's natural resonance-that is to say, its resonant frequency when un-baffled or in "free air." However, an auxiliary opening in the box--a "tuned port," or duct, or a labyrinth passageway can be used to change the effect of the. finite space so that it will aid, not restrict, the cone's movement at low frequencies. Enclosures of this general type, known as "bass-reflex" systems or resonators, are fairly efficient in that they actually make use of the speaker's rear sound waves, adding them to the front radiation to enhance the total output. A resonant enclosure, correctly "tuned" for a given speaker, can provide very acceptable sound and, indeed, such systems have been long-time favorites among many high fidelity enthusiasts.
On the other hand, if the rear of the speaker could "see" an infinite amount of space, that space would no longer be a factor in the speaker's response. Then, the speaker would respond as smoothly as its inherent quality and its front-loading allowed down to its natural resonance, below which the output would drop rapidly. Obviously, the speaker would have to have a very low natural resonance. Too, inasmuch as only its front radiation would be heard (half of its total sound output being dissipated), such a speaker would require proportionately more amplifier power to produce a given volume of sound and would therefore have to be of sufficiently robust construction to accept that power without difficulty. A truly "infinite" baffle is patently impossible, and for most people, its practical equivalent has taken the form of a simple box.
If large enough, and acoustically "dead" inside, the space in that box will not raise the speaker's resonance to degrade its bass response.
An ingenious variation of the infinite baffle concept is the acoustic-suspension system, or the infinite-baffle-turned-upside-down. The speaker, to begin with, is designed to have a very low natural resonance, below the audible range. This unusual characteristic is achieved by special techniques, of which the most conspicuous is a very loose, floppy suspension of the cone in its outer frame. To get such a speaker to respond from any bass frequency and upward into the audio range, the cone must be stiffened. This stiffening is accomplished by installing the cone in a small, heavily padded, tightly sealed box. The air trapped within the box acts as an acoustic spring against the cone, stiffening it to the point at which it will respond to amplifier signals. It should be added that the progenitors of this system emphasize that it was developed primarily because they found it to produce cleaner bass and less distortion than other speakers. The compact size, they point out, was a by-product of the design rather than a motivation for it.
One type of speaker system in which "designing for dimension" does seem to be a major consideration is the doublet, in which sound is made to radiate from front and rear, but without having to be baffled completely or otherwise necessitating a large box. In a doublet design the baffle area employed and the response of the drivers in it are calculated to provide different outputs from front and rear of the cone, or cones. The difference is obtained by such methods as using cones of varying resonant frequencies, or by mounting them so that the distances between the front and rear of any cone to the edge of the baffle are unequal. Such techniques can "frustrate" that acoustic short-circuit tendency and still permit good bass response.
Doublet design has been used mostly to produce "slim-line" systems, although a few full-size and experimental systems have been made on the same principle. (The full-range electrostatic speaker, more of which presently, is a doublet by nature inasmuch as both of its sides radiate sound. Its bass response, however, is accomplished by techniques that have little to do with conventional enclosure features.) All of these general types are direct radiators.
That is to say, the sound produced depends directly on the amount of air on the surface of the cone.
Power at the bass end depends additionally on the ...
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BASIC ENCLOSURE TYPES
20 A: INFINITE BAFFLE
B: BASS –REFLEX
C: AIR-SUSPENSION
D: ACOUSTICAL LABYRINTH (left) and one possible transmission-line variation that enables "tower" design.
E: THREE TYPES OF HORN-LOADING
Front-horn-loading (left) uses speaker as compression driver. Design shown is simplified folded-horn design; "granddaddy" of folded horns, Klipschorn, is far more complex. Center drawing shows partial front-horn-loading combined with bass-reflex, while right drawing shows one section of double slot-loaded conical horn by Hegeman.
The infinite baffle (A) is a large, sealed box de signed to completely "baffle" the speaker's rear wave from interfering with its frontal radiation. The box must be proportioned with respect to the size of the driver and its characteristic low-frequency resonance so that diaphragm motion is not adversely affected. If correctly dimensioned, the infinite baffle enclosure permits a speaker to respond smoothly down to its own natural resonance, be low which frequency the response rolls off rapidly.
This type of enclosure traditionally is associated with sturdy woofers of low resonant frequencies.
Since only half of the speaker's total output is utilized, the system is of low efficiency and thus re quires relatively high-powered amplifiers for appreciable sound levels.
Infinite-baffle-loading can be accomplished by mounting a speaker in a large closet (if a clothes closet, so much the better since the clothing will help absorb the back wave), or in a wall between two rooms. In fact, the wall between two rooms provides the "most infinite" baffling since the large volume of air behind the speaker cone will never have a degrading effect on bass response.
The bass-reflex enclosure (also known as a Helmholtz resonator, or a phase-inverter), has an auxiliary opening, called a port, which permits most of the speaker's rear energy to emerge in phase with the front radiation (B). To function correctly, the bass-reflex box must be built and pro portioned with respect to the woofer's size, and the port must be critically dimensioned or "tuned" with respect to the woofer's resonant characteristics in that box. The bass-reflex tends to reduce the low-frequency resonance of the woofer and effectively extend its range. Since energy is available from both front and rear of the diaphragm, this sys tem has relatively high efficiency and requires less amplifier power than an infinite-baffle system for a given sound level.
The air-suspension system (C) uses a relatively small enclosure that is tightly sealed and stuffed with sound-absorbent material in order to confine a given amount of air behind the woofer cone. The cone itself is specially made to be floppy in free air, with a low-frequency resonance that is below the normal audio range. The very loose suspension of this speaker becomes "tightened" by the trapped air so that the speaker can respond within the audio range. Because only half of the speaker's total output (its front radiation) is available, this sys tem is of low efficiency and, like the infinite-baffle system, requires relatively high amplifier power for given sound levels. Note too that the comparatively small size of the a/s system (vis-a-vis the infinite-baffle or the bass-reflex) is not a matter of expediency or convenience, but is a direct consequence of its acoustic design, since the amount of trapped air is quite critical.
The tuned-column enclosure (D) covers another general class of speaker-loading, also known by such names as the "acoustical labyrinth" or the "transmission line." The earliest examples used the speaker as a direct radiator, but loaded a critically dimensioned passageway or duct behind it for improved low-frequency response. More recently, this passageway has been seen loaded to the front too in certain designs. It also has been heavily "damped" with sound-absorbent material so that the speaker's response is less influenced by the resonance of the column and aided more by a "smoothing" effect that avoids peaks in the sound. Efficiency of this general class of enclosure varies from model to model. The less-damped resonant types have an efficiency similar to that of the classic bass-reflex, while the more-damped types have less efficiency, about the same as air suspension designs. Note too that while the earliest duct-loaded enclosures had proportions not unlike those of bass-reflex or infinite-baffle en closures recent models have taken advantage of the possibilities inherent in this design to assume a more columnar shape, popularly known as the "tower" design.
Horn-loading (E) may be applied to the front of a speaker or to its rear. When a speaker is front horn-loaded, it becomes an indirect radiator-that is, its energy is transferred via the horn into the listening area and not directly from its diaphragm. As a result, rear baffling becomes less critical; there is little or no need to be concerned about improving response through influencing cone resonance.
Despite the ignoring, in this design, of the speaker's rear-wave energy, front-horn-loading is the most efficient way of coupling a speaker's out put to a listening area (the horn acts as an "acoustic transformer"), and such a system requires the least amount of amplifier power for a given sound level.
A variation on front-horn-loading is partial front and-rear horn-loading; another would be direct radiation from the front and horn-loading at the rear. Designs of this type appear from time to time and are too varied to categorize definitely.
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Two versions of the fiat induction type of speaker have been proposed although none is commercially available. In the Orthophase version, audio signals are fed to conductive tapes fitted over ridges behind the diaphragm. In the Isophase version, the conductive paths for the signals, as well as the magnetic field, are laminated to both sides of the diaphragm, itself housed within a perforated cover.
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.... amount of excursion of the cone; the longer the excursion, the louder the bass. The "long-throw" woofer refers to bass speakers designed to have longer excursions than older models. Just how much longer entitles any woofer to be called "long-throw" never has been established, but most experts agree that a quarter-inch to one-half-inch excursion, without distortion, is long enough. Claims of excessive cone excursion, especially in low priced speakers, should be viewed skeptically. Of ten such excursion can be accomplished only with high distortion. Sometimes, too, the figure quoted is the extent to which the cone was made to move by an unusually strong signal just before it was torn loose from its suspension.
Indirect Radiators (Horn-Loading)
Another major class of dynamic s) stems is the "in direct radiator." As the name suggests. the driver is not loaded directly to the air but rather through an intermediate device, something that can be regarded as an "acoustic transformer" in that it expands the acoustic load seen by the speaker. In practice, an acoustic transformer is represented by a horn that matches the relatively small and stiff diaphragm of the speaker to the larger, more compliant load of air in the room. Inasmuch as a horn changes the pressure relationships between a diaphragm and the air, its use makes the amount of rear baffling much less critical: that is, if a speaker is horn-loaded at its front, an enclosure behind it has much less effect on its performance than if the same cone were used as a direct radiator. Any box used at the rear of a speaker that is horn-loaded at its front serves only as added assurance against an acoustic short circuit rather than as a means of im proving response by influencing cone resonance.
The bass response of a horn is related to its size, and for deep bass a horn becomes quite large; something like a structure about twelve feet long and with a mouth diameter of seven and a half feet would be needed to reproduce a 40-Hz tone.
Such horns have been built experimentally and for theater use, but the benefits of horn-loading did not accrue to home music systems until the horn was folded on itself and arranged to form a not impossibly large structure intended to stand in a corner where it used the walls of the room as part of its mouth opening. This system has been adapted and modified for use in many fine-sounding reproducers since it was first introduced in the early 1940s. The "full-horn treatment" renders the woofer useless above a few hundred Hertz and consequently involves the use of additional drivers to cover the rest of the audio range. The complex structure and the number and quality of drivers needed make it an expensive system. Efforts to pro duce simpler and less costly systems. still embodying something of the horn concept. have resulted in partial front-horn-loading as well as rear-horn-loading in which the woofer is permitted to respond more into the midrange, with only one tweeter and a simpler dividing network needed to round out the system.
Horns for tweeters can be quite small inasmuch as tweeter response not only can cut off well above the bass range but actually should do so. A horn-loaded tweeter, combined with a direct-radiating cone woofer, is a very popular combination found in many "coaxial" speakers, where the tweeter nestles very conveniently at the apex of the larger woofer.
Electrostatics and Other Types
A fairly serious challenge to the dynamic speaker has come from the electrostatic speaker. This type operates on an entirely different principle, and the conventional laws of baffling and so on simply do not apply to it. In an electrostatic speaker, sound is generated not by current flowing near a magnet but by the attraction and repulsion between two very closely spaced metal plates that are initially impressed with an electrical charge. A major advantage of the well-designed electrostatic is the tendency of its movable plate, or diaphragm, to move as a complete unit, that is to say, as the much desired piston. The "single-ended" electrostatic has one fixed plate and one movable plate that serves as the diaphragm. This type has been used for producing some very clean-sounding tweeters, but its mechanical limitations render it virtually useless for bass reproduction. The "push-pull" electrostatic, how ever, overcomes these limitations rather handily. In this type, the diaphragm is centered between two fixed plates. When one "pushes," the other "pulls." The resultant action greatly reduces distortion and, if the plates are made large enough, permits full bass response.
Another, entirely different, kind of speaker is the ionic, in which molecules of air, confined within a tube, are first charged electrically or "ionized" and then impressed with an audio signal from the amplifier. The vibrations of the air, rather than any moving member, produce sound. To be heard, the sound must be coupled to the room by a horn.
Credited with producing very clean and undistorted sound, the ionic speaker has been employed so far only as a tweeter, to be used with more conventional midrange and bass drivers.
The "ribbon" speaker actually is a variation of the dynamic idea. A thin metal strip, or ribbon, suspended in a magnetic field, receives signals from the amplifier and vibrates in step with them. The ribbon itself serves as both "voice-coil" and diaphragm. In common with the ionic speaker, ribbon design has been limited to tweeters that require horn-loading to be heard.
Another type of speaker is best described as a flat induction transducer. Introduced in 1961 in Paris and dubbed the "Orthophase," this speaker employs a thin, rigid diaphragm, on the back of which is a series of projecting ribs. Fitted over the ribs are magnets and under them, fastened to the ribs, are conductive tapes that carry the signal from the amplifier. The signal activates the ribs, which in turn move the diaphragm. Very fine piston action is claimed for this design, and those who have heard it attest to its clean, smooth sound-at least as a tweeter. Lowering its response, to include the bass range, would mean enlarging it, presumably by adding more sections to the basic diaphragm, which is itself about thirty-two inches square. The exact array needed for full-range reproduction has not yet been determined, and the cost has been estimated at perhaps $1,000.
Somewhat similar, but having features unique enough to have earned a patent for its inventors at the Weitzman Institute in Israel, is another flat induction speaker. Known as the Isophase, this speaker employs-instead of actual magnets-numerous magnetic strips that are coated onto both sides of the diaphragm. A printed circuit-on a Mylar backing, and also laminated to the diaphragm carries the signal from the amplifier. The diaphragm itself is enclosed in a thin, perforated housing, and the system may be used as a doublet or not, as the listener chooses. No one seems to know, or is willing to say, whether this design eventually will be released commercially.
The Walsh Driver
A radically different new loudspeaker is the Ohm A. Invented and patented by the late Lincoln Walsh, famous in high fidelity history for his Brook all-triode amplifier, it has no direct antecedents in the art (though some claim the Hegeman tweeters of the Fifties worked on the same principles). The Ohm A driver looks like an inverted funnel, the large end of which is fastened to an infinite baffle box. The funnel, or cone, is made of copper and titanium, forming a composite cone of rather large size and heavy mass. The theory of operation is, for the bass below 200 Hz, that of mass-loading; and for the midrange and treble, high-velocity wave-train propagation down the cone, with radial propagation of all frequencies of musical interest.
The Ohm A has been publicly demonstrated and is in limited production. It is very inefficient, but when driven by amplifiers of sufficient power seems to give a good account of itself, according to many auditioners. Traditionally the British metal-cone speaker designs of the past (G.E.C., Jordan-Watts, and Jordan) have been lauded for their clarity while drawing some complaints of a metallic edge to high-frequency sounds. None has been precisely of the Ohm A shape, of course, and it will be ...
----------- Diagram from Ohm Acoustics indicates how Walsh driver works. Conventional magnet-and-coil driver (top) produces wave in cone, which in turn excites air in contact with it. As wave moves down cone (second and third diagrams) it stays synchronized with acoustic wavefront (dotted vertical lines), progressively reinforcing it.
... interesting to see what the final evaluations of the Ohm A will be.
Infinity's "Plucked" Tweeter Very similar in design and built under the same basic Walsh patents-but intended only for treble propagation-is the Infinity Wave Transmission Line tweeter. The tweeter also resembles a funnel.
but with the large diameter upward. This cone is made of plastic with a thin aluminum skin-a laminate that, it is stated, will support a sound trans mission speed of 11,000 feet per second (about ten times that of sound in air). A voice-coil at the cone apex "plucks" it, causing it to emit waves orthogonally; i.e., in circles, spreading outward from the cone surface.
This tweeter is rated to handle up to 200 watts of program input and is said to display a flat impedance characteristic to 100 kHz; the designer says it can be driven at living-room level with a 25-watt amplifier-transistorized or tubed.
The Heil Air Motion Transformer
The firm of ESS has offered the Heil Air Motion Transformer as "the loudspeaker of the future." In vented by Oskar Heil, the unit is a midrange and treble driver whose corrugated plastic diaphragm (with imprinted voice-coil, called a "conduction cortex") folds on itself, reducing and expanding the volume of the "multiple interfacing cavities" presented by the magnet's vaned pole pieces and projecting sound outward with an "almost perfect transfer of kinetic energy." Dr. Heil further claims near-instantaneous acceleration of the diaphragm, very low distortion, and omnidirectional dispersion in the horizontal plane since sound is "squeezed" out from both front and back of the driver.
Very likely, the Heil unit will be endlessly discussed and debated by sound enthusiasts. Among other things it claims to be "the first new principle of sound propagation in fifty years." Various as pects of the design suggest past products such as the Kelly ribbons of the Fifties, the compression-throat tweeters of the Twenties, the perennial acoustic lens, and so on-all of which principles seem to be amalgamated in the Heil.
More Flat Diaphragms
Another company claiming to make obsolete all electrostatics is Audio Research, famous for its all-tubed amplifiers. The Magneplanar loudspeaker is offered as a replacement for free-standing, full-range electrostatic loudspeakers, intended to solve their inherent problems (particularly the need for a power supply) and to improve their quality and performance.
The Magneplanar stands six feet tall, four feet wide, and one inch thick! Each speaker is hinged twice like a folding screen, forming three panels that are set up in zigzag fashion: two with woofers, the other with the tweeter. Each woofer or tweeter diaphragm is made of thin Mylar (as in electrostatics), to which are glued closely spaced vertical wires. The diaphragm is stretched over a frame; bar magnets are attached to the same frame and inter leaved with the wires, which make up the voicecoil. A crossover operates at 3,200 Hz, though there is provision for bi-amping if you prefer.
The Magneplanar bears a strong family resemblance to the short-lived Ge-Go Ortho-phase from France a few years back, though in modern dress. It sounds like no other loudspeaker, and is thus (again, as with the Heil) the center of brisk debate.
One valid criticism is acknowledged by the manufacturer: its lack of extreme bass. A new add-on flat-panel subwoofer now is available.
Another flat loudspeaker is the Fisher Sound Panel. While not claimed to be state-of-the-art, the unit is offered as an alternative to bookshelf loudspeakers. A single flat slab of acoustic polymer has two voice-coils fastened to it. Because of the panel's physical design and the placement of the two coils, one acts as a woofer and the other as a tweeter.
Sound is produced equally from front and rear.
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The ESS Air Motion Transformer has an accordion-pleated diaphragm (removed, foreground) with conductors running in folds.
Large magnet structure (background) has pole pieces at center that focus magnetic field in conductors, alternately squeezing and opening pleats when alternating audio current is fed to it.
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New System Approaches
Many recent speaker systems represent not necessarily new ways of generating sound as such, but rather new ways of using existing sound-producing elements to overcome many problems and deficiencies of older design approaches. One very well known and widely acclaimed design in this general class is the Bose "direct-reflecting" system. As em bodied in the Bose 901, it consists of nine medium sized speakers mounted in a five-sided enclosure so that one speaker radiates directly into the listening area while the other eight radiate against rear and side walls for a "bounce-and-reflect" acoustic effect. The acoustic perspective may be varied by the distance chosen between a stereo pair as well as by their relative angles to the listening area. The 901 system is used with an electronic equalizer inserted before the power amplifier stages.
Other recent system designs include the use of a rotating driver (the Leslie system); the use of a movable treble section (as in the DVX system); the use of drivers facing upward (e.g., the Hegeman); the revival and improvement of the old "acoustical labyrinth" or "tuned column" type of loading, now renamed "transmission line" and prominently featured in many of the "tower" systems.
Yet another approach is the Dahlquist system, the product of a group headed by Saul Marantz and named after its designer, Jon Dahlquist. The Jon Dahlquist Phased Array speaker is planar.
(The first samples looked like the Quad electrostatic.) It is not, however, a dipole (or doublet), radiating front and rear; Dahlquist strongly rejects such concepts. Rather he states that the flat shape is a device to avoid the diffraction distortions common to conventional enclosure loudspeakers.
Mounted on the flat baffle are five dynamic speakers, each chosen for a special range of frequencies.
These are joined by a complex crossover network, which equalizes their on-axis response with special compensation for on-axis time-delay distortions.
The purpose of all this is to keep all phase relationships coherent-that is, in step with each other at all frequencies-just as they would be in radiating from a live source.
Dahlquist believes that a good loudspeaker should only operate on the frontal hemisphere, and never backward; that good dispersion forward is a virtue; but that it is better to have poor dispersion than to let any signals be reflected. Indeed, the de sign concept of the loudspeaker is to keep from "wasting energy" in other directions and to keep from confusing the stereo image. Mr. Dahlquist, it might be noted, speaks from a vast background of research and development on other kinds of loud speakers-and sounds like a spokesman for the English (BBC) school of speaker research, or the corresponding French (ORTF) school. His ideas and his patented speaker represent a divergence from the prevailing U.S. school of wide, or even omnidirectional, dispersion. The design is a refreshing restudying of the principles of sound propagation and of the relationship of the speaker to the room and to the listener.
The new designs and the improvements in older designs add up to one general conclusion: In loud speakers, nothing is sacrosanct. It seems there is no rule that cannot, or has not, been broken. Indeed, the only rule that would seem to apply is that of cost: In speakers you generally get what you pay for.
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(High Fidelity, 1976)
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