US2295483A - Loudspeaker - Google Patents

Description

Sept. 8, 1942.

H. s. KNOWLES LOUDSPEAKER 4 She'egs-Sheet 1 Filed June 4,. 1954 w W J w 9 w I K S p 3, 4 H. s. KNOWLES 2,295,483

LOUDSPEAKER Filed June 4, 1934 4 Sheets-Sheet 2 A714 72 Knowles. 131/ @W I P 1942- H. s. KNOWLES 2,295,483-

LOUDSPEAKER I Filed June 4, 1934 4 Sheets-Sheet 3 jni/erzi'on @5711 z 5y SKM *w P 1942- H. s. KNOWLES 2,295,483

LOUDSPEAKER Filed June 4, 1934 4 Sheets-Sheet 4 Q fi I f2 ,flfi'nowlas. 35 MMQW Patented Sept. 8, 1942 LOUDSPEAKEB Hugh S. Knowles, Chicago, 111., assignor to Jensen Radio Manufacturing Company, Chicago, 11L, a

corporation of Nevada Application June 4, 1934, Serial No. 728,993

7 Claims.

This invention relates to loud speakers, and is more particularlyv directed to loud speakers or telephones of the electro-dynamic or moving coil type.

A speaker of this type, when operating in open 5 or unconfined fluid, radiates little P wer at low frequencies, because the compression wave from one side of the moving system is almost completely neutralized bythe rarefaction wave on the other side. The use of an infinite baflle for separating the front and rear radiation of a moving diaphragm system will effectively overcome this difliculty, but such a construction is, of course, not of practical utility. In attempting to provide means for performing the function of an infinit baflle while yet remaining within practical limits, the ,use of. various types of enclosures for the back side of the moving system has been resorted to in order to accomplish this result. However, certain inherent difiiculties and disadvantages have occurred in the use of such enclosures, which has rendered them unsatisfactory in speakers of this type.

For example, there is considerable power loss or loss of effective sound radiation with the ordi-' nary type of enclosure due to the resistance caused by forcing the air in and out through the annular gap between the top plate of the coil assembly and the voice coil bobbin, and between the voice coil bobbin and the core. At low frequencies, below the fundamental resonance of the system, the resistive component reduces the velocity of the diaphragm with a corresponding decrease in effective sound radiation, which may be termed a power loss, due to this highly resistive impedance. Also, the enclosure produces constructive and destructive interference at various frequencies, causing changes in the velocity of the moving system, or resonance. Particularly is this true when the enclosure is made large enough to provide low stiffness reactance at low frequencies in order to avoid any objectionably high fundamental resonant frequency of the I system.

In attempting to overcomethese difficulties, enclosures have been provided which have a sink or the like for absorbing all back side radiation, this sink having sound absorbing material therein. Sound absorbing materials of the type now known, however, inherently have poor absorbing characteristics at low frequencies. In order to increase the effectiveness of the material, therefore, and to reduce pressure on the back side of the cone due to stiffness, vents are ordinarily manner.

provided. These vents and the sound absorbing material introduce resistive components into the mechanical impedance of the vibrating system.

Also, with the use of the electro-dynamic type of speakers having, a moving voice coil disposed in the annular gap between the core and the top plate, there is a tendency for any paramagnetic particles Suspended or moving in the fluid in which the unit operates to be drawn into this gap, which is of relatively small area, and consequently disturb the electrical and mechanical relationship of the movable parts interfering with effective operation of the unit. While such units ordinarily operate in air, I employ the term fluid to include both gases and liquids, since such sound producing means might be employed for sub-aqueous transmission of sound, if so desired.

Th present invention has for its primary object the elimination of the undesirable features heretofore, mentioned, as well as providing a speaker of this type which has certain mechanical andacoustical features not present in the present type of unit with which I am familiar.

A still further advantage secured by th present invention is the provision of a centering member for the inner end of the diaphragm which may be employed as a closure member for entirely excluding air flow therethrough, thereby preventing dust or the like from entering th air gaps, or may form a filter or screen excluding the dust particles while allowing the fluid to filter therethrough. When imperforate, this member provides one defining surface of a volumetric fluid displacement chamber employed in th overcoming of the resistance loss through the air gap. Further, this dust excluding screen or cover is employed to reducev the resonance of the spider which forms the centering means for the diaphragm. This is accomplished by forming the centering member in such manner as to provide an eccentrically corrugated surface, which produces a non-resonating centering support, and which may be suitably supported at its outer end by means 01 damping members mounted upon the spider for the cone. The eccentric surface may be formed by providing eccentric rings, if desired, which may be fibrous rings which add mass, stiffness and resistance so proportioned as to break up the natural modes of vibration of the spider or centering member, which rings may be formed by means of felting or the like, why eccentrically disposed corrugations, variations in cross-sectional area, or in any other suitable Another object secured by the present invention is the provision of a vibrating cone or diaphragm which possesses relatively great mechanical strength to static-deflection, and which is so formed as to have eccentric radial sectors for decreasing or distributing the resonance at higher frequencies. This eccentricity may be produced in any desired manner as will be pointed out hereinafter. Further, the outer peripheral mounting for the diaphragm preferably is supported by non-uniformly spaced damping supports, which break up the. resonance or natural modes of vibration of the diaphragm itself. The eccentric rings, corrugations or the like, may be formed in the cone, or may be of material such as fibrous rings, which add mass, stiffness and resistance to the diaphragm.

This diaphragm itself is preferably disposed in eccentric position with respect to the enclosure, in order to break up the sound radiation reflected from the interior of the enclosure, the eccentric position of the diaphragm preventing any two paths from the enclosure walls to a point on the diaphragm being equal.

A still further object of the present invention is the provision of an enclosure which is substantially weather-proof and air-tight, and which is internally provided with a plurality of dissimilar baifle means for still further breaking up sound radiation reflected from the walls of the enclosure, and to prevent internal resonance in the enclosure. The construction of the enclosure is for the purpose of serving as an equivalent of an infinite baflle, to avoid back side radiation, and to add distributed stiffness to the vibrating system which improves high frequency radiation thereof. In addition, novel means are provided within the enclosure for conducting the heat generated in the coil assembly of the speaker to the outer surface of the enclosure, which heat conducting means is so constructed as to eliminate any internalresonating chambers therein. Another feature provided by the enclosure is the utilization of a high resistance vent for equalizing low pressure differentials between the interior of the enclosure and the front side of the diaphragm, but preventing rapid equalization of pressure differentials at an audible frequency rate. Further, the walls of the enclosure are pro vided with stiffening channels or members which prevent resonance thereof, and which still further tend to break upsound radiation within the enclosure. The dispersion vanes or baiiies are preferably perforated, and, in addition, have damping material of low sound absorbing characteristics disposed on the surfaces thereof With the present enclosure construction, I am able, if desired, to eliminate the use of sound absorbing material, such as formerly used, since the construction itself efl'ectively prevents resonances in the enclosure and takes care of back side radiation, while adding distributed stiffness for improving high frequency radiation of the moving system.

Other objects and advantages of the present invention, such as certain details of construction and operation which present novel features not,

to my knowledge, heretofore provided, will be described in detail in connection with the following description, which, taken in connection with the accompanying drawings, will disclose to those skilled in the art a preferred embodiment of the present invention.

In the drawings:

Figure 1 is a sectional elevational view of a 75 Figure 6 is a sectional elevational view of a V modified form of speaker unit;

Figure 7 is a sectional view taken substantially on the line 1-1 of Figure 6;

Figure 8 is a sectional view taken on line l. of Figure 6.

Referring now in detail to the drawings, in Figure l, I have provided an enclosure indicated generally at I, comprising a top wall member 2 formed of wood or the like, a rear wall member I formed of wood, a lower wall 4 preferably formed of metal or a heat conducting material, and a front wall 6 formed of metal or a heat conducting material, the front wall I being provided with the opening 6 disposed below the center of the enclosure for centering the speaker unit with respect thereto. An annular outwardly flaring horn I is mounted about the defining edge of the opening 8 and is secured in position by means of the annular angle bracket member 8.

Within the enclosure I, I preferably provide irregularly shaped rigid dispersion plates or vanes, which may be made of metal, wood, thin fibrous material, or any rigid dispersion surface which preferably occupies small volume so that the total effective volume of the enclosure is not appreciably reduced. These surfaces are so shaped and mounted that adjacent elements of area on the plates differ in distance from some element of area on the cone. Optionally, the surfaces of the vanes may be coated with material for damping their self-resonance.

Their effect on the wave front produced by the vibrating systems, since the enclosure 1 adds only a resistive component of approximately ten per cent or less of the total effective resistance of the vibrating tem, including the resistance due to the electrical circuit, may be further modifled by the use of perforations I0 varying in size and location in such manner as to break up the wave front. Since their eflect on the wave front is a function of the wave length of the incident sound, this construction produces a different pattern and different frequencies and increases the random phase relation of the sound striking the back of the diaphragm.

The enclosure I need not be completely fluid tight, but when a horn is attached to the front of the diaphragm, as shown in Figure i, it may be desirable to have the enclosure weather-proof. To provide for such changes in barometric pressure which otherwise might produce a difl'erential pressure between the front and back sides of the diaphragm, a high resistance vent or tube I! may be provided, which extends through a suitable cpening in the rear wall 3 of the enclosure. This tube oflfers a very high impedance to the motion of fluid at a high velocity corresponding to sound frequency, but will permit a very slow flow of fluid such as would be required to equalize a quasi-static pressure differential between the two sides of the diaphragm. While the vent to the axis of the speaker.

wardly,

desirable with a smaller type of enclosure.

The speaker unit, which, in this particular embodiment, is of the large diaphragm electrodynamic type, has a field structure It, comprising a substantially cup-shaped housi which is secured to a top plate l4, and has mounted therein the fleld coil 13. This coil, when energized.

produces the desired magnetic flux in the annular air gap It. The vibrating assembly consists of the diaphragm H, the voice coil and bobbin assembly II, the flexible centering member ii, the dome-shaped cover 23, the flexible annular diaphragm support 22 and the self-damping strips 23. It is of course to be understood that the various speaker units shown in the several embodiments of my invention as disclosed herein will all be mounted in an enclosure in an offset position with respect to the center front wall of the enclosure, and the outer peripheral edges of the, various diaphragms may be supported as hereinabove described, and provided with irregularly spaced damping strips corresponding to the damping strips 23 of Figure l.

The rings orcorrugations 24, since they may comprise rings felted or otherwise mounted on the diaphragm, or corrugations formed directly in .the diaphragm, are preferably so made that the annular ring sections of the diaphragm between them are not of constant width. The rings may be disposed to lie in the position as determined by the "intersection of the surface of the diaphragm and a plane not quite perpendicular The radial distance between different sections of the rings and between the rings and the annular flexible support ,22 should preferably not be a constant.

As will be noted in Figure 3, the damping support strips 23'may be irregularly spaced about the periphery of the diaphragm i1, or may be unequal in surface extent, in order to break up the natural modes of vibration of the diaphragm.

The fleld assembly I3 is held in position by means or a pair of strip supporting members 25 which are formed of heat conducting material,

and which are spaced apart to provide a through air space therebetween in order to eliminate any resonating effects from the provision of a conflned chamber formed by these conducting strips.

These strips serve to conduct the heat generated in the coil l5 from the housing 13 to the metal wall 4 of the enclosure, and, in combination with the metallic conehousing 28 which extends between the top plate l4 and the outer peripheral edge of the diaphragm l1, serves to conduct this heat away from the coil assembly and to the bottom and front, walls 4 and 5 of the enclosure.

The wall members 2, 3 and 4 are provided with stiffening channels or ridges 21. formed therein, which, in addition, serve to prevent self-resonace of these wall surfaces.

1 In the conventional type of speaker, the movement of the voice coil bobbin assembly it into and out of the air gap 16 between the core indicated generally at 23 and the"defining edgev of the opening in the top plate l4 results in move- .ment of the diaphragm l1.- This produces an increase in the pressure of fluid on the back side of the diaphragm as the diaphragm moves inwhich ordinarily results in'some flow of fluid from the back of the diaphragm through the air gap between the top plate l4 and the voice coil assembly l3, and between the voice coil.

assembly and the core tip 23 of the fleld structure.

10 closed at its outer end by means of" the closure plate 23 which is substantially fluid-tight. Thus, when the voice coil assembly l3; with the dome Ill, moves inwardly with respect to the fleld assembly, there is compression of fluid in the space 16 33 of the core assembly. At the same time, due

to the volume of the defining end of the voice coil assembly, thereis compression of fluid in the annular chamber 32, surrounding the core tip 23, and sealed from the coil assembly I! by means of the annular shield member 33. The volumes 30 and 32 are preferably so chosen that, with the proportionate displacement of the end of the voice coil assembly I3 and the displacement of the dome 20, the fluid is compressed by an equal amount in these two chambers so that there is no diflerential in pressure therebetween, which prevents the flow of fluid from the chamber 32 past the outer surface of the tip 23 and within the voice coil assembly i8 into the space 30, or vice versa.

The flexible centering member l3, which is, in this embodiment of the invention, composed of an imperforate, eccentrically-corrugated flex- I ible member, is substantially fluid-tight, being 35 mounted upon a shoulder 34 formed 'on the spider 26, by means of an annular channel member 35, which allows tightening of the outer deflning edge of the member l9v into sealing engagement with the shoulder 34 without producing any torsional stresses therein. .As the diaphragm,

therefore, moves inwardly, the flexible member l9 compresses the-air in the chamber 31, which air may be allowed to escape to some extent through the ports 33 formed in the spider 23, and which lead to an annular chamber 39 therein. Preferably, the volume of the chambers 31, 33 and 39 is so proportioned with respect to the chambers 30 and 32 that, upon inward movement of the voice coil bobbin assembly I3 and the dome 23, the compressioneffected in the chamber 31, or in the chambers 31, 33 andv 3!, is substantially equal to the compressio existing in the chambers 30 and 32, which means that the pressure upon opposite sides of the gap between the top plate l4 and the outer surface of the voice'coilassembly II is substantially equalized, so thatno flow of fluid through this gap is permitted. This provides for inward movement of the voice coil assembly without producing -a flow of fluid through the annular gap between the core and the top plate, which means that the resistive component or friction loss is eliminated, and thus provides for effective sound radiation at low frequency. The pressure provided in the chambers 30, 32 and 31 produces a pure stiflnws for the vibrating assembly, which stiffness is relatively low at low' frequencies,

since the three chambers may be made as large as desired, so that the unit increase in pressure 0 is considerably reduced.

To further reduce the stiffness contributed by the volumes 30, 32 and 31, it may be desirable to substitute a substantially air-tight bulbous enclosure for the cover plate 29, in order to increase the total volume of the chamber 33. It

is to be understood that these three volumu are not independently variable since the requirement is that the instantaneous pressure in all three be substantially the same at low frequencies. If desired, the entire volume of the enclosure may be used for one of the chambers, the other two volumes being proportioned in accordance therewith to produce the same instantaneous pressure in each of the chambers upon vibration of the system at low frequencies.

The cone housing 28 is centered on the top plate H by the shoulder 42, which shoulder is formed concentric with the gap and the voice coil assembly. It is important in an assembly of this type in which the voice. coil is completely enclosed, to provide me for completely exposing the voice coilto permit repairs, adjustments, or the removal of foreign particles inadvertently introduced during assembly. The shielding member 33 fits snugly on the core, but may be removed from it and from the top plate II to permit the removal of particles which inadvertently get into the enclosure 82. This construction makes all parts of the enclosure readily accessible. By the use of eccentric rings or the like, in the imperforate spider or centering member It, the resonating effects are broken up, so that the natural modes of vibration of the centering member itself are not added to the vibrating system.

In this embodiment of the invention, the dome 20 is a relatively light rigid member which may have embossings thereon to increase its rigidity, and is connected to the voice coil bobbin I. by means of a stifl annular ring or shoulder which is centered on the bobbin and helps to keep the voice coil round. The stiffness of the ring and the mass of the dome are normally so chosen as to give a rise in velocity of the dome at high frequencies. This frequency maybe slightly above the frequencies at which the sound output of the speaker without the dome would begin to fall. This extends the frequency range of the speaker.

The use of the imperforate spider or centering member I! prevents paramagnetic dust particles or the like which may be suspended in the fluid surrounding the magnetic circuit of the speaker from being drawn past the dome 20 aroundthe outer periphery of the diagram i1 and into the annular gap about the core tip, since the introduction of such particles thereinto will destroy the operating efficiency of the speaker.

Considering now in detail the centering member is, it is to be understood that at any mode of vibration of a flexible member, such as the member is or the diaphragm H, such a member offers a large mechanical impedance when driven from the vibrating system. In other words, each mode'of vibration may, to an approximation, be represented by an anti-resonant circuit, the impedance of which is usually large enough to reduce appreciably the velocity of the moving system. When a'wide and flexible annular ring is .used to reduce the stiffness of the ring and to permit large excursions of the voice coil, the first or fundamental mode of the ring driven from the diaphragm lies in the middle or lower voice frequency range, and appreciably alters the velocity of the mechanical system near the resonance frequency of this mode.

Likewise, when the dimensions of the centering member I 9, are made sufliciently large so mamas that the volume of the enclosure 81, under this memberismadelargeenoughtokeepthestiiiness low, and when the centering member is large enough to reduce the total stiffness added to the moving system, the fundamental mode lies roughly in the middle of the voice frequency range and introduces appreciable impedance at the frequence corresponding to this mode. In the case of both the annular supporting ring I! and the centering member ll appreciable impedance is also introduced at the higher modes of vibration, although these are, in general, less pronounced and more highly damped.

Since it is important that the device have a smooth velocity frequency curve, these resonances should be eliminated, and one method of reducing the same is shown in Figure 1. The centering member II is a complete, circular, imperforate member. It will be noted from the sectional view, however, that the ridges or grooves are not symmetrical and are so designed that the eifective mass and stiffness varies continualLv from radial section to radial section around the centering member at high frequencies but provides substantially uniform stiffness at low frequencies. At low frequencies the distributed mass has negligible effect (i. e., negligible mass reactance). As the frequency is increased and approaches the first or gravest mode of vibration the eiiective stiffness reactance decreases.

At frequencies at which the centeringv member is antiresonant (viewed from the voice coil) the mechanical impedance may be very high and reduce the velocity of the voice coil and hence the sound output oi' the speaker. To prevent this resonance occurring at the same frequency ineach radial sector the shape and distributed mass are so chosen as to prevent their resonating at the same frequencies. This m nimizes changes in impedance to motion of the voice coil and of the spider and therefore to motion of the diaphragm to which it is attached. Since the distributed stiifness and mass control the natural modes of vibration of a mechanical system of this type, the higher modes of vibration will therefore vary from one radial sector to another so that no large increase in the velocity of the centering member can occur.

The anti-resonant eifect of the centering member may be reduced as shown in Figure 2. In this embodiment, the centering member consists of two or more radially extending members II and 82, which have ridges or grooves formed in such a manner that the natural modes of vibration of the various radial centering members differ. The radial members may also be tapered longitudinally or shaped in such fashion that their cross section varies, for example, in the manner as determined by the tapered shape shown in Figure 4. The principal feature of this construction is that it avoids duplication of the modes of vibration of any of these members. This may also be achieved by changing the thickness of the various radial members, or tapering them in thickness or in section.

The fundamental modes of the centering members are damped by comparatively thin strips ll of material such as rubber, felt or the like, which introduces a large resistive or damping component. This material, if it does not adhere directly to the centering member, may be held in place by thin flexible spring clamps 8! which are mounted in position about an an- .piece cemented or enemas nular supporting member II by means of the mounting channel 81 and bolts 22.

While in Figure l the mechanical resistance added to the vibrating system by the fiow of fluid through the air gap is reduced by the proper selection of enclosed volumes so that no pressure differential occurs across the gap at highly resistive fluid flow path shown at ll between the core tip and the voice coil bobbin II is shunted by the low impedance paths made up of the passages 82 and 82, which passages may be drilled or cast in the core and should be limited to the size which will provide a low resistance path without unnecessarily increasing the reluctance of the magnetic circuit. Moreover, the high resistance path between the voice coil bobbin BI and the top plate 25 is bypassed through slots in this plate through the chamber 08 and the vent or orifices 21 into the chamber 22 in which the field coil 99 is disposed. This chamber, in turn, may be vented by means of the screen I and the port I02 in the top plate I! into the annular space surrounding the diab Where the centering means ll comprises a continuous circular centering member, the air space IIII may be vented through fine holes in a perforate centering member, such as shown and described in Figures 4 and 5. which holes should be disposed away from the curved surfaces of the member. If the centering member is made up of radial arms 80 and 82, then it is desirable that a thin perforated screen, such as a very fine or light wire mesh or cloth should be cemented or held in contact with the centering means by means of the springs 25 in order to exclude foreign particles. Such a screen is shown in detail in Figure 4 and indicated at I.

The 'diaphragms I1, 52 and the diaphragm I05 of Figure 2 may be supported at the opening in the enclosure In the manner described in connection with Figure 1, where the annular ring member 22 may be formed as an integral part of the diaphragm, or may be a separate otherwise attached thereto. The felt damping strips 22 should preferably be secured in position over the ring 22 by means of light spring members, such as the spring members 85, described in connection with Figure 2, and should preferably not be distributed symmetrically, or should vary in width in order to break up or damp the natural modes of vibration of the supporting rings 22.

These supporting or centering means, such as the member I9 of Figure 1, may be constructed in a number of ways. Preferably they are formed of some fibrous material, such as paper or cloth, which may be impregnated with a lacquer, varnish, phenolic compound or other sub-- stance which increases the strength and stiffness to the desired degree.

To increase the strength of this centering member when paper pulp is used, a light, coarse mesh cloth screen may be used on which the paper pulp is felted or deposited from the pulp bearing solution. If the paper is already fabricated, a separate cloth may be cemented to it,

, conical\ urface I I2.

preferably at the time the paper is formed, to increase the strength to a considerable extent. In Figure 5 is shown another form of centering member in which the supporting radial arms 82 and 22, which are mounted by means of the annular channel mounting ring 81 upon a supporting member, serve in turn to hold a circular eccentrically corrugated flexible cloth mesh or screen indicated at I06. This member is clamped by means of spring members, such as springs 86, to the radial sectors or arms, and at its inner end is extended down and flanged, as shown at III, to engage the inner flanged edge of a diaphragm such as the diaphragm I05.

In Figure 2, the dome member for the central portion of the inner end of the diaphragm is replaced by the annular member IIII of the shape shown in Figure 2. This member has secured thereto a substantially conically extending surface of revolution, comprising the member II2, which at its outer end is closed by the closure plate III having the reenforcing ribs III thereon, the center of the closure plate II3 being mounted by means of the stud II! to the core tip Hi. If desired, the diaphragm I05 may be centered by a member such as II3 secured to the inner surface thereof and supported eccentrically or otherwise upon tip IIB.

Thus, in Figure 2, an annular air space H6 is provided between the diaphragm I05 and the Upon movement of the dia- Jphragm, I05 at low frequencies, the air chamber volume I It provides a low stillness reactance to such movement, which does not appreciably damp the sound radiating effect of the diaphragm I06. However, when the frequency is increased, an increasing stiffness reactance or component in the space H6 is provided, which is, of course, desirable, as heretofore pointed out. If desired, the member IIO may be replaced by a single flanged member, and the member II2 may be joined directly to the juncture of the member Ill and the diaphragm I25 at the inner end of the centering means.

In the usual type of speakerwith which I am familiar, the voice coil bobbin is provided, between the voice coil anrLthe dome portion, with vents which are placed to relieve the pressure under the dome. However, these have always been so placed that if the excursions of the voice coil were large enough, the cross-section of the vents would be reduced when the voice coil moves into its innermost position within the gap. To overcome this difliculty, the voice coil was ordinarily made long enough and the vents were placed at the upper end of the neck thereof so that the area wasnever reduced. However, this increased the mass of the voice coil and consequently reduced its high frequency response.

In Figure 6, I have provided means for overcoming these difilculties. In this figure, reference numeral I20 indicates a field coil assembly, to which is secured the top plate I22, the coil being shown at I23. The core tip is shown at I24, and has a central cyllndrically recessed portion I25 which is provided with a plurality of laterally extending radial slots I26 opening into the air gap between the top plate I22 and the over which is secured an annular supporting member I32 for supporting the flexible eccentrically corrugated member I33, the member I33 having an inwardly extending flange portion I34 which is adapted to extend over the inner flanged edge I35 of the diaphragm and has engagement with the outer peripheral surface of the voice coil bobbin I36. Suitable damping members, such as felt strips I33, may be provided at spaced intervals, and may be cemented or otherwise suitably secured to the member I33. The voice coil bobbin I36 is provided with a plurality of prick punches I37 for rigidly securing it to the flanged end I35 of the diaphragm I29.

Spaced substantially symmetically about the cylindrical portion of the bobbin I36 are a plurality of openings I38, which are adapted to vent the space I39 within the dome I40 upon inward movement of the voice coil assembly with respect to the core tip I24. The inner end of the voice coil bobbin I36 is slotted, as shown at I42 in Fig. 10, to provide for resilient tensioning of the voice coil conductor I43 thereon, and to reduce eddy currents.

Upon inward movement of the dome I40, with a consequent tendency to compress the air or fluid above the tip I24, this fluid will pass outwardly through the vents I38 to the space I44 between the supporting member I30 and the centering means I33, and may then be vented through openings I45 and the ports I46 in the top plate I22 into the field assembly, through the screen I41. As the vents I38 pass beneath the outer surface or end of the member I24, pressure of the fluid in the space I39 compresses the fluid in the recess I25, and this compressed fluid passes outwardly through the radially extending slots I26 and through the vents I38 and the corresponding slots I 21 into the space I44, and inasmuch as the center of the radial slots I26, the vents I38 and the slots I21 is at a common point in the same plane, the impedance to the flow of fluid is not appreciably altered during movement of the bobbin, and therefore the fluid is not strained through the vent, and no objectionable noise, or what is commonly termed wheezing, results therefrom. In addition, this component of the mechanical resistance of the vibrating system, which is a function of its displacement and leads to distortion, is reduced.

The voice coil bobbin I36 is formed of a light weight metal, such as aluminum, beryllium or magnesium alloy, or a similar material, with the dome I40 preferably formed integral therewith to increase the strength and to insure good thermal conductivity. The voice coil bobbin has shoulders at the defining end thereof to retain the voice coil conductor in position. The pin pricks or embossings I31 engage the cone to increase the stress that can be applied without failure of the assembly.

Further, heat conduction may be provided by making the flexible support member I33 out of some light flexible metal with an extension to take the place of the reenforcing portion I34. If desired, a thin metallic film may be deposited either on the front or the back side of the diaphragm, near the apex, so as to provide thermal conduction with the dome I40 and the bobbin neck I3E, this metal being sprayed on in a molten state, cemented or electrolytically de-, posited. It will be noted that the supporting spider I28 has an integral portion thereof forming the support for the flexible centering member I 33, and is also provided with an additional inwardly extending flange portion I43, whiclrportion lies on top of the top plate I22 and forms an integral part of the magnetic circuit. This increases the cross section of the top plate at the section near the voice coil where the flux density is a maximum, and therefore reduces the reluctance of the magnetic circuit. The voice coin leads, as indicated at I50, are run up between the flange portion I34 of the centering member I33 and the flange portion I35 of the diaphragm I29, and extend along the outer surface of the neck portion I36 of the bobbin to the voice coil itself. The lead I50 is extended through a grommet I52 secured in the diaphragm I29, and from the grommet a flexible connection I53 is extended to a terminal member I54 carried upon the support for the centering means which is carried upon the angular supporting ring I32.

An advantage secured by the construction shown in Figures 6 to 8 is the provision of the head bolts or cap members I56. which secure the cone housing I28 to the top plate I22, and also secure the field housing structure I20 to the top plate. By removing the bolt members I 56, the entire cone housing I28, together with the diaphragm I29, the flexible centering member I33, the voice coil bobbin I36, the voice coil itself, the leads therefor and the like can all be removed from the top of plate I22, and the fleld assembly housing I20 can also be removed therefrom, if so desired. This permits the voice coil to be inspected, and provides for removal of foreign particles, without disturbing the alignment of the voice coil withrespect to the cone housing and the flexible centering means.

Having described my invention in accordance with the patent statutes, what I claim as new and desire to secure by Letters Patent is:

1. In combination, an outwardly flaring main diaphragm, means for supporting the outer end of said diaphragm, eccentricalhr corrugated flexible centering means engaging the small end of said diaphragm, said means having substantially uniform stiffness against static deflection, and irregularly shaped damping members engaging said centering means.

2. In combination, a loud speaker housing substantially closed to atmosphere and having wall ends intersecting to form a single compartment, a speaker unit having a cone diaphragm. and means for supporting said unit in of! center position with respect to one wall of said housing to provide a non-uniform sound path from any point on said cone to the walls of said housing.

3. In combination, an enclosure having substantially smooth walls normally reflecting sound waves therefrom, said enclosure having an opening disposed eccentrically in one wall thereof, a speaker unit within said enclosure having an outwardly flared diaphragm cone secured to the defining edge of said opening, and dissimilar dispersion vanes separated from said walls and within the space bounded by said enclosure for breaking up sound radiation therein.

4. In combination, an enclosure formed generally of material having poor heat conductivity and being substantially closed to atmosphere, a speaker unit having means normally generating substantial heat within said enclosure, thermal conducting means supporting said speaker unit in position within said enclosure and connected to the walls of said enclosure, at least one of the walls to which said thermal conducting means is connected having at least part thereof formed ph a said diaphragm, and a centering disk engagin of good heatgconducting material -and forming substantially continuous good heat conducting paths from said speaker unit directly to the atmosphereoutsid'e of said enclosure.

5. In combination, an outwardly flaring diameans ior supporting the outer end oi the small end of said diaphragm, said disk having non-uniform corrugations located asymmetrical with respect to the disk axis.

6. In combination, an outwardly flaring diaphragm, means for supporting the diaphragm outer end, a centering disk engaging the diaphragm smaller end and extending outwardly therefrom, and vibration damping means carradially thereon, said disposed around said ried on said disk extending damping means being so disk to obtain irregular damping angularly around said disk axis.

'1. In combination, an outwardly flaring diaphragm, means for supporting the outer end thereof, a voice coil at the small end thereof, a centering disk for said small end, said diskbeing formed at least in part of flexible metal having good heat conductivity and extending outwardly from the diaphragm small end, and metallic means for engaging the outer peripheral portion of the disk for anchoring the same HUGH S. KNOWLES.

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