The Dynamic Loudspeaker
Many different types of loudspeakers go into the making of high fidelity reproducing systems. However, the differences between the characteristics of various speakers are mostly a matter of degree. The actual functional differences between loudspeakers will be treated in section 3. In the present sectionwe shall deal with the basic theories of loudspeakers in general, in terms that will help later in judging their qualifications for specific hi-fi applications.
Perhaps the most popular type of loudspeaker today is the permanent magnet dynamic type. Because of its comparative simplicity of construction and design, the precision that may be built into it, the ease with which it is coordinated with other equipment, its easy adaptability to many different applications, and its comparative freedom from electrical trouble, the dynamic loudspeaker has found acceptance in all kinds of reproducing systems. It is found in the smallest pocket radios and is a major component of the most elaborate theater systems. Figure 2-1 is illustrative of the scope of applications of the dynamic loudspeaker.
One of the major factors contributing to the popularity of the permanent magnet dynamic loudspeaker is the fact that it has its own powerhouse. It requires no external source of power other than the signal power to make it operate. This powerhouse (the magnet) has a virtually eternal life. The permanent magnet of the dynamic loudspeaker, once it has been charged to full capacity by the manufacturer of the loudspeaker, retains its magnetomotive power almost unaltered forever, and unless it is subjected to severe mechanical shock, it cannot be drained or run down. It is always ready for action.
The first elements seen by the eye in the usual dynamic loudspeaker are the basket, or housing (with some sort of rear structure attached to it, which encloses the magnet circuit) and the paper diaphragm (or cone) supported on the front of the basket. But there is much more to a loudspeaker than this. Let us dissect such a loudspeaker assembly and lay bare the various components to get a working knowledge of the roles they play in the operation of the loudspeaker.
Basically the dynamic loudspeaker is made up of the following components:
a. The voice coil
b. The voice coil former
c. The centering spider
d. The magnet
e. The magnetic circuit
f. The diaphragm (cone)
g. The apex radiator (and dust cap)
h. The basket (or housing)
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POCKET RADIO DYNAMIC SPEAKER DYNAMIC IS. ' WOOFER FOR NI.FI MULTI-SPEAKER SYSTEMS. THEATER INSTALLATIONS THE DYNAMIC LOUDSPEAKER DESIGN LENDS ITSELF TO APPLICATIONS IN ALL SOUND REPRODUCING SYSTEMS COMPRESSION DRIVER TYPE OF DYNAMIC LOUDSPEAKER FOR HORN LOADED MID RANGE, TWEETERS. AND PUBLIC ADDRESS SYSTEMS
Fig. 2-1. Typical dynamic loudspeakers. ( Courtesy University)
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Although many dynamic loudspeakers have other important components as integral parts of their design, these are the basic elements of the simplest type. Yet when properly designed this same group of elements, with no additions whatsoever, may constitute a high quality loudspeaker of special application. Figure 2-2 shows a full cutaway of a typical loudspeaker in which all these elements are clearly marked and arranged in a somewhat exploded view.
The dynamic loudspeaker is a device that functions with other electronic equipment, but in itself it has little that is of an electrical nature. The loudspeaker is simply an assembly of mechanical parts - a cloth spider, a paper cone, a metal basket, a magnetic piece of iron -- to which is added a few turns of copper or aluminum wire (the voice coil).
The Voice Coil: Sole Electrical Element in the Dynamic Speaker
This voice coil is the only thing within the speaker that carries any electrical current or signal. It is energized directly from the amplifier, as illustrated in Fig. 2-3. The voice coil, as its name implies, is the part of the loudspeaker that does the "talking" by virtue of the fact that it is energized by the signal or "speech" currents fed to it by the amplifier. The voice coil consists of several turns of wire wound on a supporting bobbin. Depending upon the functional design of the loudspeaker, the coil itself may be either copper or aluminum wire, although insulated aluminum ribbon is also used. (In the case of the latter, the ribbon is wound on edge with the flat surfaces of neighboring turns adjacent to each other and all the turns held together by a binding cement.) The bobbin upon which the wire is wound (the voice coil form, or former as it sometimes called) may be made of a strong grade of thin paper, wound around on itself several times to provide a rigid cylinder. Sometimes the voice coil is wound on aluminum or dur-aluminum forms, and in some designs the forms are made of rigid paper, reinforced by an aluminum ring around the outer edge.
Voice Coil Must be Immersed in Fixed Magnetic Field
Fig. 2-2. An exploded and cut-away view of a dynamic loudspeaker. ( Courtesy
University loudspeaker)
Fig. 2-3. The only electrical element in the basic dynamic loudspeaker is the
voice coil, with its connecting leads to the basket terminal posts.
The voice coil doesn't do the amplifier's bidding all by itself.
If this voice coil were isolated in free space it wouldn't make the slightest sound, even if the signal current flowing through it represented the full power output of a 50-watt amplifier. In fact, not only would the coil be speechless; it would probably turn red and burn to a crisp in utter silence. It is not the input power alone that makes the loud speaker work, nor is it the voice coil, or even the combination of ...
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Fig. 2-4. The complete magnetic circuit consists of the magnet, the
return keeper circuit, and the air gap in which the voice coil rides.
The illustration shows cross-sections of three common arrangements.
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... the two. We have to add our "powerhouse" to the system, for it is the magnetic field of the permanent magnet that makes the voice coil speak. The voice coil of a dynamic loudspeaker can "speak" only when it is immersed in a magnetic field. Such a magnetic field may be produced by constructing an iron "loop" with the magnet in one section and an air gap in another, as illustrated in Fig. 2-4.
Once the magnet structure has been charged there is immediately formed in the magnetic circuit a magnetomotive force which drives lines of force across the air gap. The strength of the magnetic field set up by the magnet is naturally dependent upon the quality and size of the magnet, the size of the air gap, and the amount and quality of iron disposed about the magnet. These items are all factors that deter mine the figure of merit of the magnetic circuit, which plays a great part in the efficiency of operation of the hi-fi loudspeaker. Accordingly, the more specific and specialized details concerning such magnetic circuitry will be reserved for section 3. However, we cannot continue without discussing magnetic circuitry in general, because this factor is fundamental to basic dynamic loudspeaker design.
The effect of the permanent magnet upon the performance of the loudspeaker depends upon the magnet composition, weight, and physical shape. There are several grades of magnet material, all possessing different magnetic properties. Magnet weight, without giving the grade of magnet specified, is a meaningless figure. Nearly all loud speakers use magnets made of "Alnico." There are many grades of Alnico, each quite different from the others in magnetic properties.
Very often a one-pound magnet of "Alnico V" may make a much better speaker than a five-pound magnet of "Alnico II." One must be careful, in examining the specifications of a loudspeaker, to consider the combination of grade of magnetic material and weight of the magnet, rather than either factor by itself.
Magnetic Circuit Plus Moving System Determine Performance
The truth of the matter is that not even these two factors give the complete picture as far as the entire function of the magnetic circuit is concerned. Our British cousins make an effort to give a figure representative of the "gap flux" - the strength of the magnetic field in the air gap - which signifies how well the magnet is working in its completed iron circuit. This figure, indicative of the strength of the magnetic field in which the voice coil rides, is intended to convey some idea of the loudspeaker performance. In a sense it does, but it does not describe total performance. As we shall see in section 3, the actual level of performance of the loudspeaker depends on a complete inter relationship of magnetic circuit with diaphragm weight, composition, and shape; the size of the voice coil and whether it is made of copper or aluminum; and other mechanical features of the assembly. There fore, rather than give some figure of magnet worth that is only partially descriptive of speaker performance, American manufacturers have limited themselves to the statement of the basic facts of magnet weight and type.
The various types of magnets classified as "Alnico" all contain iron mixed with various amounts of aluminum, nickel, and cobalt.
(It will readily be seen that the term "Alnico" is made up of the first two letters of each of these elements.) The proportions of these elements, in conjunction with the other basic magnetic iron with which they are compounded, form the Alnico series. In general terms, the heavier the magnet and the higher it is in the Alnico series, the better the loudspeaker will function.
Magnets may be cast in many shapes, depending upon the design of the equipment in which they are to work. There is no standard or best shape of magnet, other than one that fits best what the design engineer is trying to build into his item as far as performance and cost are concerned. Figure 2-5 shows some representative shapes of magnets commonly found in today's loudspeakers. The simplest of these shapes is the slug magnet. The cored slug and the ring are a little more difficult to manufacture, and the W magnet is the most intricate of the lot. It is apparent that for a given weight of magnet material the cored slug, ring, and W types will be more expensive than the plain slug (and also larger in overall dimensions). The choice of a magnet for a particular speaker depends upon the use for which the speaker is designed and the particular properties required in the speaker. Figure 2-4 illustrates the typical magnetic circuits for the types of magnets shown in Fig. 2-5.
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1/2 OUNCE SLUG 8 OUNCE SLUG FROM 2" SPEAKER FROM 12" SPEAKER 12 OUNCE CORED SLUG 6 OUNCE RING MAGNET FROM COMPRESSION DRIVER UNIT 24 OUNCE "W"- MAGNET FROM COAXIAL TYPE SPEAKER
Fig. 2-5. The shape of the permanent magnet is determined by the performance
and cost objectives of the loudspeaker.
THIS GAP SECTION SHOWN ENLARGED
VOICE COIL FIELD DIRECTION OF GAP FLUX DIRECTION OF MOTION LIKE FIELDS REPEL
Fig. 2-6. Current flowing through voice coil sets up magnetic field
around coil. Interaction is set up between this field and the permanent
magnetic field, forcing it to move,
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The magnetic circuit, comprising the magnet, the return circuit, and the gap, is completely dead magnetically until the magnet is charged.
In practice it is never charged unless it is in its completed mechanical structure. As soon as the magnet is charged, it sets up a complete magnetic field, which reaches out from the top surface of the magnet, goes through the iron and through the air gap interposed in the iron circuit, and then returns to the other side of the magnet. The magnetic field (the lines of "flux" the magnet has been able to push out) is the "force field" of the magnet, for in this air gap rides the voice coil, ready to interact with the magnet field. For a fixed amount of magnet, the amount of flux that actually gets into the gap will depend upon the size of the gap and the quality of the iron from which the magnetic return circuit is designed. With high grades of irons, like "Armco," high values of magnetic flux may be obtained. Cheap grades of iron are easily saturated; that is, they are not capable of carrying the full charge of the stronger magnets, thus the air gap flux is not as strong as in Armco circuits. Once the flux gets into the air gap, the magnetic circuit is completed and ready to make the voice coil do some useful work as soon as it gets an electrical "go-ahead" from the amplifier.
Voice Coil Field Reacts with Fixed Magnetic Field
This "go-ahead" is the electrical signal current, flowing through the voice coil from the amplifier. With current flowing through the voice coil, what we actually have achieved is the formation of a varying magnetic field, caused by the signal current flowing through the voice coil, in close proximity to the magnetic field of the permanent magnet, as illustrated in Fig. 2-6.
In the region where the fields are alike, there will be a strong concentration of magnetic flux, which tends to exert a repelling force.
In the region where the lines of force are in opposite directions, there will be a cancellation and reduction in total magnet flux, which tends to exert an attracting force. Motor action will thus be developed be tween the field of the magnet, which is stationary, and the field of the voice coil. Because of the interaction of these two fields, one fixed and the other movable, the voice coil tends to move one way or the other (up or down in the figure), depending upon the direction of the current through the voice coil. (The direction of the current determines the direction of the voice coil field.) The direction in which the voice coil travels is, in general, parallel to the length of the gap in which it is balanced. This problem of balancing the voice coil in the magnetic gap is an important one in loudspeaker design. The coil must be balanced both magnetically and physically within the gap. The importance and the manner of these balancing techniques will be discussed in subsequent chapters. For our present purposes, however, we may examine these balancing means with an eye to how, rather than to how well.
Voice Coil is Aligned in Gap by "Spider"
The voice coil is balanced in the gap by means of a device usually referred to as the spider. The spider, or centering device, as it is more accurately called today, holds the voice coil centered in the gap radially as shown in Fig. 2-7. The voice coil must be centered radially so that in vibrating in and out of the gap it does not strike the metal walls of the gap. To obtain high magnet efficiency the gap clearance between the coil and the iron is usually very small. (In the case of tweeters it
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Fig. 2-7. The centering device (the spider) must keep the voice coil
travelling in an unswerving axial direction in and out of the gap. It
must keep the voice coil radially centered so that it does not hit the
iron side walls of the magnetic circuit air gap.
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may be only 3 to 4 thousandths of an inch; in the case of larger speakers, it may be 10 to 15 thousandths of an inch.) Thus the spider or centering device plays a very precise role in keeping things lined up as the voice coil vibrates in and out of the gap.
The centering device made up of the long twisted "legs" that attach to the voice coil ( from which the name "spider" was derived) was one of the earliest types. Figure 2-8 shows some spiders commonly in use today. The open spider shown has a shortcoming in that the open area between the legs permits foreign particles to get into the small gap spaces. This may seriously hamper loudspeaker operation, and often causes considerable damage to the coil, especially if these particles are magnetic in nature, or large, rough, and coarse. In order to mitigate this defect of the open spider, fibrous material (commonly cotton) is frequently placed behind the spider, between it and the all-important gap area.
Another means of overcoming these problems in the cloth spider.
This is made of a loosely woven material, impregnated in a thermo setting resin, such as a phenolic, and then formed under heat and pressure into the flexible structure shown in Fig. 2-8. The outer edge of this spider is permanently affixed to the basket, and the inner diameter suspends the voice coil in the gap. There is remarkable radial stability to this type of spider, and the built-in springiness of the waves molded into it allow for precision of motion of the voice coil in the
Fig. 2-8. Some typical voice coil centering devices (spiders).
axial direction. By means of devices like these spiders, the voice coil is kept to a true and rigidly aligned direction of motion in and out of the magnetic gap.
Voice Coil Moves Diaphragm
To the voice coil is attached the diaphragm, which actually "fans" the air into motion. In Fig. 2-9 are shown some typical loudspeaker diaphragms. These are usually made of special paper or impregnated cloth, although metal has also been used. Because the diaphragm is directly fastened to the voice coil, it must follow in a rather exact fashion the vibrational pattern of the voice coil itself. A great deal of modification of this original vibratory motion may actually be introduced between the voice coil and the diaphragm, for reasons we will discuss later. These modifications fortunately give rise to designs for specialized speakers for hi-fi reproducing systems. We may nevertheless accept the proposition that in general the diaphragm (or cone)of the speaker moves back and forth, impelled by the voice coil to do
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Fig. 2-9. Typical diaphragms with specialized shapes for different
applications.
12" CURVILINEAR DIAPHRAGM FOR WIDE RANGE USE (PAPER MOLDED) 2" PHENOLIC IMPREGNATED CLOTH DIAPHRAGM FOR USE IN COMPRESSION DRIVERS AND HORN SYSTEMS 15" CONICAL DIAPHRAGM FOR WOOFER APPLICATION (PAPER MOLDED)
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Fig. 2-10. Forms of diaphragm suspension.
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...so, in a manner representative of the actual signal current. As the diaphragm vibrates back and forth, it causes the air with which it is in contact to vibrate in exactly the same manner, and sound waves are then created.
In order to allow the diaphragm to vibrate back and forth freely, it is necessary to provide it with some sort of flexible support that will allow it to have motion and yet keep it vibrating in a true axial direction. The diaphragm is provided with a flexible area at its outer edge sufficiently compliant to allow the diaphragm to flex in and out.
In the great majority of present-day speaker structures, this flexible area is actually part of the diaphragm itself, and it acts as a spring that suspends the diaphragm in a state of equilibrium. This area of the diaphragm is usually referred to in acoustic terminology as the rim compliance of the loudspeaker; it plays an important part in controlling the characteristic performance of the loudspeaker. In Fig. 2-10, this integral rim compliance is illustrated along with other means of accomplishing the diaphragm suspension.
Sometimes the rim compliance is accomplished by providing an annular ring of soft chamois leather, which is cemented both to the basket edge and to the paper diaphragm. When made in this way, it is sometimes called a skiver, since skiver, according to Webster, means "a piece of sheepskin." Sometimes the term skiver is improperly used to signify any type of rim compliant material cemented to the cone, such as a loose cloth (which, although as compliant as the chamois, may have undesirable side effects). A general term sometimes used for this edge compliance is the "surround," which is quite obvious in its meaning. It literally surrounds the speaker.
Many types of diaphragm go into the making of the dynamic loud speaker. The diaphragm may be made of paper, wool, and other fabric components. This type of diaphragm is usually found in the "cone" type speaker, a type of speaker in which the large radiating diaphragm is clearly visible to the naked eye. This is the type most commonly used as a general purpose speaker. However, for the type of speaker in which specialized performance may be required, such as the compression type driver unit, the diaphragm may be made of molded phenolic, or even dur-aluminum. This type of diaphragm is seldom seen by the user, because it is usually sealed into a comparatively small head structure that contains other important acoustic elements requiring it to be sealed off acoustically except for its throat. Such a unit may usually be recognized by the fact that it is connected at its throat to some type of horn. However, despite the fact that in external appearance there may seem to be no similarity between the open cone type speaker (the direct radiator) and the closed compression type (to be used with horn radiator), the mode of operation is exactly the same for both. They differ in degree of performance, in frequency coverage, and in several other ways; but the manner in which they set the air in motion (through their respective magnet structures, voice coils, and diaphragms, along with auxiliary devices such as centering means) is functionally the same for both types.
Diaphragm Apex Important for High Frequencies
Due to manufacturing contingencies peculiar to the means of assembling the various parts of the speaker, the apex area of the cone is usually open. Before the speaker may be considered completed, this open area must be closed off for several good reasons. From this apex area of the cone comes the greater proportion of the high frequencies of the cone. Every possible surface in this section should be utilized as an active part of the diaphragm; every square inch in this area should "push air." If this apex were left open, there would be less active diaphragm area in the apex, and less air would be "pushed." Therefore, in the general type of loudspeaker, there is usually added some sort of apex "dome" to the cone. However, in some instances, this apex "dome" consists simply of a loose piece of felt across the apex opening placed there to keep dust particles out of the sensitive gap area, and yet allow the speaker to "breathe." Free flow of air in and out of the rear of the apex of the cone is necessary in some speaker designs to release the captured air cushioned in these rear areas, which might restrict the free rearward motion of the diaphragm. This situation is usually found to be the case in small general purpose utility type speakers, and usually a piece of felt is placed across the apex to provide the breather.
In the more specialized speakers, where large magnet assemblies are the rule and large internal volumes exist behind the voice coil, this air cushion in the rear sections is greatly minimized, so that "breathers" as such are relatively unnecessary. More functional devices may then be designed into the apex areas to provide better high fidelity performance. These specific details will be covered in section 3.
Basket Supports Entire Structure
The various elements of the loudspeaker are all supported by the housing, or basket. Depending upon the size of the speaker and its weight, this basket may be made either of cast metal or of stamped sheet metal. Whatever it is made of, however, it must be rigid in construction so that it may adequately support all the parts of the speaker without deforming under the weight of the magnet or by being screwed to irregular or warped baffle boards. Any mechanical weakness of the basket structure will in time result in damaged voice coils, distorted acoustic performance, and shortened life. These factors will be treated in more detail in section 4.
Dynamic Speaker is Current Sensitive: Low Impedance
Because the dynamic loudspeaker is actuated by current flowing through the voice coil, this type of loudspeaker might be called current sensitive. For an electrical device to be current sensitive it must pre sent very little resistance to the flow of current through it. More specifically, it must have low impedance. Therefore, the dynamic loud speaker falls in the low impedance class. Typical impedance values range from 2 to 16 ohms.
The Electrodynamic Loudspeaker
Although we discussed the permanent magnet dynamic loudspeaker first, it did not come first in technological development. The reason for this is that magnet materials available in the early days of audio devices were not very efficient in comparison with other means of obtaining a magnetic field within the gap. With the advent of better alloys of permanent magnet materials, these deficiencies in magnetic ...
SOFT IRON MAGNETIC
Fig. 2-11. Field coil, carrying current, sets up magnetic field, which
magnetizes iron circuit and produces flux in the air gap only while
current flows through coil.
... circuits were gradually overcome. But until they were, the type of loudspeaker that had widespread popularity was the "electro-dynamic," a type of speaker that even today enjoys widespread use in certain applications. The electrodynamic speaker and the permanent-magnet (p-m) dynamic speaker function in exactly the same manner. The parts are identical with one very important exception.
Magnetic Field is Electrically
Energized In place of the permanent magnet found in the p-m dynamic speaker, there is instead a completely "soft" iron circuit. Around the center leg of this iron circuit is wound a large multi-layer coil, as shown in Fig. 2-11. When a direct current is made to flow through this "field" coil, it sets up a magnetic field about itself, which magnetizes the iron circuit within the coil. A field of magnetic flux is thus set up across the air gap of the return keeper. The strength of this field is a direct function of the strength of the current that flows through the coil, and the design of the iron circuit. With proper magnetic circuit design it is possible to get large values of gap flux, which means a more powerful speaker. The current through the coil does not permanently charge the magnetic circuit, however, since the circuit has no permanently magnetizable material in it. The "soft" iron of the magnetic circuit becomes magnetized and stays magnetized only while there is ...
Fig. 2-12. The electrodynamic speaker must have its field coil connected
to a source of direct current, such as the power rectifier that feeds
the amplifier, or to a separate power supply.
FIELD COIL CONNECTION
DC RECTIFIER AMPLIFIER AUDIO O
VOICE COIL CONNECTION
... magnetizing current present. Once current is shut off, there is no magnetism. The energizing current is obtained from the same power supply that powers the associated amplifier or radio equipment.
The manner of connecting an electrodynamic loudspeaker is shown in Fig. 2-12. This type of speaker is in widespread use in auto radios even today because of some economies it effects in actual power supply equipment for the dashboard radio. In cases where it is desirable to eliminate bulky filtering elements from large power supplies, such speakers may find usefulness, because of the filtering action of their field coil upon the power supply.
Electrodynamic Loudspeaker Requires External Power Supply
The speaker does have disadvantages, obviously. It must be rather close to the amplifier with which it is working so that it may be efficiently interconnected with the power supply. An even more serious fault is that only one such speaker may usually be connected to one combined power supply and amplifier. The continued efficiency of the loudspeaker is naturally dependent upon the efficiency of the power supply itself. As power rectifier tubes lose their output, the energizing current drops, and the speaker output falls off. Furthermore, special hum bucking coils are required in the structure to eliminate residual hum in the speaker that cannot be completely filtered out by the field coil. And finally, the field coils themselves may burn up or short out due to various operating malfunctions of the system, or because of the drying out of the coil insulation due to heat and age.
It should be realized that in the electrodynamic loudspeaker the field coil has absolutely nothing to do with the voice coil. The two are distinctly different both in function and in design. The field coil produces the steady magnetic field, the voice coil produces the varying signal field. Connections from the field coil are made directly to the system power supply, while the voice coil connects to the amplifier signal output terminals exactly as in the permanent magnet dynamic loudspeaker. Because of these problems, because of the dependency upon the efficiency of the power supply, and because of the possible inherent failures of additional electrical components, the electrodynamic speaker is seldom found in hi-fi installations.
The Electrostatic Loudspeaker
Of recent interest but ancient vintage is the "electrostatic" speaker, also called the condenser (capacitor) loudspeaker. Actually, the con denser loudspeaker was first pioneered after it was noticed that poorly made glass plate condensers had a tendency to "sing"; to actually make sound in some likeness to the signal they were carrying (in the same manner as transformers, which have a tendency to "sing" when their laminations are loose). Very recently, the electrostatic loudspeaker has attracted some attention. In the form in which it has been popularly offered, it affords a means of providing a rather economical tweeter (high-frequency speaker). This puts the smaller, more economical "ready-made" systems into the "two-speaker" class. The word system is used quite advisedly, because as we shall soon see, not only is the electrostatic loudspeaker integrally married to the power supply of its driving amplifier, but it must be connected to the amplifier output tubes through specific coupling circuits that best match the electrostatic speaker to the particular tubes in the output stages of the amplifier; furthermore, it must be matched with auxiliary speakers of comparable levels of efficiency.
In the case of the dynamic loudspeaker, electrical energy is trans formed to mechanical motion by the forces of magnetic attraction and repulsion. In the case of the electrostatic speaker the electrical signal is ...
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Fig. 2-13. Basic structure of the electrostatic speaker.
FIXED ELECTRODE PLASTIC SHEET INSULATION WITH METALLIC PLATED SUR FACE HELD UNDER TENSION CLOSE TO THE FIXED ELECTRODE. ALTERNATING VOLTAGE APPLIED TO THESE ELECTRODES CAUSES THE PLATED SURFACE TO BE ATTRACTED OR REPELLED FROM FIXED ELECTRODE
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Fig. 2-14. One form of electrostatic speaker.
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... transformed into mechanical motion through the forces of electrostatic attraction or repulsion. Basically, the electrostatic loudspeaker consists of a movable electrode held very close to a fixed electrode, but insulated from it. A simplified structure of such a speaker is shown in Fig 2-13.
A unidirectional "polarizing" voltage, fixed in magnitude, is applied ...
--- [1] Although it has been used most widely in low-cost commercial units, the electrostatic speaker is emerging as a high-quality "custom" component in several versions. One of these, the Janszen tweeter, uses a push-pull arrangement (one movable electrode between two fixed electrodes) to overcome the nonlinearity inherent in the two-element type.---
... to the two plates, and the signal voltage is superimposed upon this polarizing voltage. The purpose of the polarizing voltage is to prevent "frequency doubling" by the loudspeaker. Frequency doubling is what occurs when a note is reproduced an octave higher by the loudspeaker, which would hardly be hi-fi! Because of the necessity of providing this high biasing voltage, it becomes mandatory to tie the speaker into some already existing power supply, or to provide it with its own power supply.
In Figs. 2-14 and 2-15 are shown the actual internal construction of two typical electrostatic tweeters. In the former (Fig. 2-14) the movable electrode (the thin piece of plastic coated on one side with a conductive material) is held in direct contact with an arched and perforated screen. Against the back of this screen is applied the pressure of two springs. This spring pressure, applied to the perforated screen, which is the fixed electrode of the speaker, keeps the electrode taut against the plastic sheet, which carries the movable electrode. Thus the tautness of the perforated screen is imparted to the plastic film and its superimposed electrode. Stretched across the whole assembly is a fine piece of gauze, which covers the movable electrode and holds the whole assembly in place.
Fig. 2-15. Another form of electrostatic speaker.
The speaker shown in Fig. 2-15 is, in concept, essentially the same device shown in Fig. 2-14, but with a different mechanical arrangement.
In this case, the plastic film is forced against the perforated fixed screen by the pressure of a somewhat resilient pad of felt, which is in turn backed up by a bakelite wafer supporting plate. On the felt side of the plastic sheet, we again find the thin metallic plated surface, which is the movable diaphragm.
Variable Electrostatic Attraction Between Electrodes Causes Diaphragm Motion
The basic principle of operation of the electrostatic loudspeaker is as simple as that of the dynamic type. If we let the matter of the polarizing voltage go for the moment, we may examine the way the speaker operates in its simplest form. Consider the two plates of the capacitor of Fig. 2-16 uncharged at the start. Now let us apply a slowly varying alternating potential to the two plates. As the difference of potential between the two plates increases from an initial value of zero, there will be a force of static attraction between the two ...
Fig. 2-16. When electrostatic speaker is not polarized the movable
electrode is attracted to the fixed electrode twice for each cycle of
signal voltage. This produces "doubling" of the signal.
... plates, growing larger in value as the voltage difference between the two plates grows larger. The movable plate will consequently be attracted to the fixed plate and tend to move toward it. When the voltage reaches its crest and starts to decrease in value, the electrostatic force between the two plates decreases, and the movable plate returns to its original position as the voltage reduces to zero. Thus one pulse of air motion has been produced in an inward direction for a positive alternation of the signal.
However, the audio signal is an alternating voltage that has a negative half cycle along with the positive half cycle. Thus, after the applied signal voltage has returned to zero following its positive half cycle, it goes into its negative half cycle, again imparting its charges to the two plates but in opposite polarity. The plates, however, will again be attracted to each other because, despite the fact that the charges on them have been reversed, they are still opposite in polarity to each other and the force between them is still one of attraction. Therefore, the movable plate will again be pulled in toward the fixed plate, and will subsequently return to its equilibrium condition when the signal voltage has again reached zero. Thus, for one complete alternating cycle of electrical signal voltage consisting of a positive and negative alternation between the two plates, two identical pulses or cycles of air motion have been produced, with the air pulses moving in the same direction. This means that if a signal of 1000 hz were applied to the unpolarized electrostatic speaker as shown, there would be a sound output at 2000 hz. This phenomenon is called frequency doubling.
Polarizing Voltage Needed to Prevent Frequency Doubling
Frequency doubling is of course highly undesirable in sound producing devices. Naturally, we want to hear middle C if the piano is playing that note and not a note one octave above middle C. Frequency doubling also may exist in other types of reproducing systems. How ever, in such cases, it is usually corrected through proper initial design of the components and is not an inherent characteristic of the speaker.
The electrostatic speaker system is theoretically and essentially a voltage doubler device, and no amount of proper design of the speaker itself can remedy it. What must be done is to provide some external factor that will prevent the doubling effect; this is accomplished by the polarizing voltage. The polarizing voltage (in the order of several hundred volts) is a fixed unidirectional voltage applied to the two plates of the capacitor to provide a sort of mechanical bias, as shown in Fig. 2-17.
This fixed high voltage between the two plates exerts a steady electro static force of attraction between the two plates so that, even with no signal voltage applied to them, the movable plate is already attracted to the fixed plate to a considerable degree.
As the positive alternation of the signal is applied to the two plates, the movable electrode is drawn in to the fixed plate beyond that position where it was held simply by the polarizing voltage. As the signal voltage returns from its positive value toward zero, the attraction between the two plates is reduced to the original pre-strained position. As the signal voltage reverses its direction, an entirely different situation comes into being. This time the polarity of the signal voltage, which assumes a direction opposite to that of the polarizing voltage, has the effect of reducing the polarizing voltage between the two plates, and the force ... of attraction between the two is now decreased below the value produced
Fig. 2-17. When polarizing voltage is applied between plates of electrostatic
speaker, the movable plate takes an initial set. As the signal voltage
is applied this set is increased or released depending upon the signal
polarity fluctuation, and doubling is eliminated.
... by the polarizing voltage. The amount of release from the pre-strained condition that the movable diaphragm experiences follows precisely the swing of the signal voltage. The end effect of such action (positive signal alternation working in the same direction as the polarizing pull and negative alternation working in a direction opposite to the polarizing pull) is to pull the movable plate in beyond its fixed pre-strained position and then allow it to swing out away from that position. Thus one alternating cycle of air pulses is produced by the movable electrode, following the original single electrical cycle - voltage doubling has been eliminated by the polarized voltage.
Electrostatic Speaker Not Suitable for Low Frequencies
Because of the inherent mechanical limitations of the movable diaphragm of the electrostatic speaker, this type of speaker is not adaptable to wide range reproduction by itself. A self-sufficient speaker must produce both low and high frequencies. For the reproduction of reasonable low frequencies, the vibrating diaphragm must experience large motions, and must be large in size. Such large diaphragm motions are not possible in the typical structures illustrated.
Close spacing between the electrodes is necessary to obtain maximum signal sensitivity from the device and to get reasonable polarizing effects from the voltages found in the average power supply of the amplifier or radio. These conditions of intimate contact and close spacing between the electrodes place natural restraints upon the motion of the movable diaphragm, and low frequency response is entirely eliminated.
Consequently, the electrostatic speaker cannot be used as a single all purpose speaker, but only as an upper range speaker to supplement a speaker capable of adequate bass reproduction.
There is another fundamental difference between the electrostatic speaker and the dynamic type of speaker. The dynamic loudspeaker has its own "powerhouse" (the magnet), which gives the weak voice coil signal a powerful ally in getting its message across. The electro static speaker has no such ally. It must broadcast its message entirely by the strength of the signal applied to it. Having no reservoir of auxiliary "push" power upon which to draw, the output level of the electrostatic speaker seldom reaches that of the dynamic type. How ever, there are features that may be built into the electrostatic speaker that in some measure overcome some of its limitations, and make it suitable for hi-fi systems.
Electrostatic Speaker is Voltage Sensitive: High Impedance
Since the electrostatic speaker is a true capacitor and is energized by voltage rather than current, it is a "high impedance device." This means simply that for optimum performance it must be connected to a source that is also of high impedance. Amplifiers, however, have out put transformers that are low impedance devices designed to match the low impedance dynamic loudspeaker, a current fed device. This output transformer must still be employed for the woofer (low-frequency) speaker in the system employing the electrostatic speaker as a tweeter, as shown in Fig. 2-18. However, the electrostatic speaker cannot be connected to the low impedance output transformer. It must be connected to the high impedance circuit of the output tubes of the amplifier. Since some amplifiers use pentode and some triode output, the impedance the electrostatic speaker sees is different, from amplifier to amplifier, depending upon what tubes are used in the output stages.
Fig. 2-18. Since the electrostatic is a high impedance, voltage operated
device, it must be connected to the source of high signal voltage at
the plate of the output tube, from which it may also obtain its polarizing
voltage.
Its output efficiency and frequency response will accordingly change from amplifier to amplifier. This factor makes it difficult to properly match the electrostatic speaker to an amplifier, unless the manufacturer of the speaker gives the user the circuit coupling information necessary to enable him to properly couple the speaker into the high impedance side of the set, while the woofer is still connected to the usual low impedance side.
Because of these "matching" problems (the dependence of the electrostatic speaker upon the existing power supply and the necessity of coupling it with a woofer low enough in efficiency to correspond to that of the electrostatic speaker), the electrostatic speaker has found widest acceptance as a tweeter in ready made small radio, phonograph, and television equipment, where it may be integrally designed into the system by the manufacturer.
The Crystal Loudspeaker
There is another type of loud-speaking device, which, although voltage actuated, is not of the electrostatic type. This is the piezo electric, or crystal loudspeaker. The principle behind the crystal loud speaker is one of contraction and expansion of a certain crystal material (usually Rochelle salt) under the influence of an alternating electric field applied to the surfaces of the crystal. No polarizing voltages are necessary for the crystal loudspeaker as the flexural motion of the crystal follows directly in step with the polarity of the applied voltage. How ever, crystals are rather fragile devices, and it is not possible to drive them sufficiently hard to obtain useful output power for loudspeaker performance, especially on the low frequency end. Therefore, they are at present used only for earphones and for pillow speakers. In this latter application the listener lies with his ear pressed down upon the pillow to hear the loudspeaker.
The low frequency response of this type of loudspeaker is limited because any reasonable excursion of the crystal is not possible without fracture. For general communication or restricted range music it is perfectly satisfactory, however. A bank of crystal loudspeakers could be used as a tweeter to supplement a woofer, but the woofer would have to be low in efficiency to properly balance the output of the crystal bank. Like the electrostatic loudspeaker, the crystal unit must be tied in directly to the high impedance tube circuit ahead of the output transformer; however, no polarizing voltage is necessary for the crystal speaker. It encounters the same complications, however, of proper matching to the output tubes and of affecting the loading of these tubes, and the same integral amplifier connection problems that con front the electrostatic speaker.
The Ionic Loudspeaker
The newest working addition to the loudspeaker field is the Iono-phone. However, a study of patent history will show that loudspeakers working on the principle of ionized air particles are not at all new. In fact this type of loudspeaker is an offshoot of another phenomenon, just as the electrostatic loudspeaker was an offshoot of the singing capacitor.
Actually, the first "ionic" loudspeaker was the "singing arc." In the early days of wireless transmission, radio waves were generated by electric arcs jumping across gaps in high voltage circuits. It was soon noticed that these arcs would "sing" as the voltage producing them ...
Fig. 2-19. A dielectric field placed around the quartz cylinder produces intense
beat in the quartz, which in turn heats the high emissivity electrode. The
ionic atmosphere boiled off from the electrode is modulated by a surrounding
modulated radio frequency field and pulses accordingly to put the air into
motion.
... changed. Thus was born the "Ionophone" - a loudspeaker utilizing the principle of an ionized (electrically charged) gas. Today, however, the Ionophone has come far from the days of Nernst, Duddel, DeForest, and the "singing arc."
Charged Air Particles Vibrate as a Diaphragm
In its present form it consists of a mechanism by which air particles are given an electric charge. After the air molecules have been charged, they form an "atmosphere" of ions, the ions in this case being, of course, charged air particles. A modulating voltage is then applied to this atmosphere, making it pulsate in accordance with the signal-modulated voltage. This pulsing gives rise to an actual sound wave (in the air), which is then propagated in the usual manner. This mode of operation makes the Ionophone truly a "diaphragm-less" loudspeaker.
The air molecules are actually agitated electrically without anything physical pushing them.
As shown in Fig. 2-19, the heart of the system is the small, narrow, slowly expanding quartz tube located within another quartz tube, with the space between them evacuated. There is no electrical function served by this evacuated area. It is a heat barrier to keep the inner section of the quartz cone reasonably protected from outside temperature changes that might work their way into the apex area of the inner section of the ionizing chamber through wall conduction. In the apex of the quartz tube is fixed an electrode of very high ionic emissivity character.
This ion-producing electrode is made of a special platinum compound that, when heated, throws off large amounts of charged particles. The ions bubble away from the electrode surface and impart their charge to the air particles in the immediate vicinity of the heated electrode. Thus is formed an "atmosphere" of charged air particles around the emissive electrode.
The method of heating the electrode is dielectric heating, as used in diathermy equipment. A high voltage of very high frequency is applied to the quartz capsule that supports the electrode. By diathermy principles, the high frequency high voltage induces heat in the outer and inner quartz tube, which closely surrounds the electrode. This pocket of heat, in turn, causes the electrode surface to be heated, and the charged particles "boil off" forming the "ionic" atmosphere.
The voltage required to produce the necessary dielectric heating effect is quite high, in the order of several thousand volts. It is obtained from an external power supply and high frequency radio modulating device. This power supply also serves as the modulator unit, which receives the audio signal and converts it into the radio frequency field that surrounds the charged atmosphere.
Under the influence of this field, the atmosphere of charged particles is pulsed back and forth to form the genesis of the sound wave. A horn system must be coupled to the ionic generating element to provide an acoustic load for the small sound source.
Ionic Loudspeaker Requires External Power and Modulator
The Ionophone requires its own booster amplifier to feed the modulator device so that the performance of the system may be balanced with the auxiliary woofer that must be used with it. The Ionophone is not a complete system by itself, but forms a high frequency adjunct to existing systems. The life of the system is limited by the life of the ion emissive element, which (like a vacuum tube) may be replaced when deteriorated. The high frequency range of the Ionophone is theoretically unlimited, because there is no motion of anything physical in the system. The low frequency range is specifically limited by the nature of the horn load applied to it. In its present prototype form, the low frequency limit is about 500 hz.
Summary
In this sectionwe have covered four basic types of loudspeaker: the dynamic, functioning by interaction of magnetic fields; the electro static, functioning by means of electrostatic fields; the crystal, functioning by stresses produced within crystal formations; and the ionic, functioning by the electromagnetic pulsing of a gaseous atmosphere.
We have attempted to carry this discussion only far enough to indicate the general modes of operation of the various types, so that we may, in the next chapter, use these introductory principles to deter mine what modifications must be made for the hi-fi variations within the basic types.
There are several schools of thought concerning high fidelity - the "play it exactly as performed" school, the "play it so it sounds real" school, and the "play it the way I like to hear it" school. There is logic in all three approaches. Someday, perhaps, they will all be synthesized into one, but until such time as the controversy is resolved we must accept the proposition that there are as many types of high fidelity as there are listeners. In order to satisfy these many concepts, it is necessary to provide a sufficient variety of component parts to make possible many different kinds of system. Accordingly, we shall discuss speakers from the point of view of their general application to the hi-fi field and then concentrate on the more specific and specialized units that become component parts in the more expanded hi-fi reproducer systems.