US2812825A

US2812825A - Loud speaker diaphragm supporting member

Info

Publication number: US2812825A
Application number: US399906A
Authority: US
Inventors: Charles L Matthews
Original assignee: WM H Welsh Co Inc
Current assignee: WM H Welsh Co Inc
Priority date: 1953-12-23
Filing date: 1953-12-23
Publication date: 1957-11-12
Anticipated expiration: 1974-11-12

Description

Nov. 12, 1957 c, 'nws 2,812,825

LOUD SPEAKER DIAPHRAGM SUPPORTING MEMBER Filed Dec. 23, 1953 Z I l9 16 IMPREGNATE A FABRIC BASE WITH THERMO-PLASTIC l OR THERMO -$ETTINGRESIN WHILE IN FLAT CONDITION.

' PRESS BASE IN 0155 OR INA MOLD T0 IMPART DESIRED CORRUGATIONS AND TO SET THE RESIN DIP 0R SPRAYA THERMO- PLASTIC OR THERMO- SET TING RESIN TO FORMED SPIDER TO R sTmcTPoRosW United States Patent LOUD SPEAKER DIAPHRAGM SUPPORTING MEMBER Chicago, Ill., assignor to Wm. H. Chicago, Ill., a corporation of Illi- Charles L. Matthews, Welsh Co., Inc., nois The present invention relates to electro-acoustical transducers such as loud speakers or the like; and more particularly to an improved supporting and centering member, or spider, for use in such devices and to an improved method for fabricating such a member.

Electra-acoustical speaker devices usually include a moving voice coil that is wound around a cylinder secured to the apex of the speaker diaphragm and disposed in an aperture in the top plate of the speaker coaxial with the pole piece of the speaker magnet. It is desirable to reduce the radial length of the air gap formed in such aperture between the top plate and pole piece to a minimum, and it is essential that the diaphragm and the voice coil attached thereto be accurately centered in the air gap and spaced from the top plate and pole piece so that the coil may have relatively free and unrestricted longitudinal movements with minimum transverse motion in the gap.

Because of the above requirements, centering and supporting members or spiders have been devised which permit free longitudinal movement of the voice coil in the air gap but allow a minimum transverse motion thereof. The preferred form of centering member is a centrally apertured disc which is formed of thin resilient nonhydroscopic material, such as fabric, paper, Celluloid, metal, or the like. These discs may be provided with concentric or spiral corrugations, or may have other suitable configurations, to increase the flexibility of the member longitudinally without unduly reducing the lateral stiffness of the same. The cylinder supporting the voice coil preferably projects through the central aperture of the centering disc and is cemented to the disc and to the apex portion of the speaker diaphragm.

Centering members of the above type formed of a cotton base impregnated with a thermo-setting or thermoplastic resin have gained wide commercial acceptance. This is mostly due to the fact that a centering member of such composition lends itself readily to convenient commercial fabrication in which uniformity of product as to shape, stiffness, resiliency, weight and porosity may be obtained. In addition, such a centering member has been found to have an extremely long life.

It has been found that, for the speaker to exhibit favorable reproducing characteristics over the entire audible frequency range, it is essential that the centering member or spider provide some damping for the voice coil. This damping, in turn, depends among other factors upon the lack of porosity or degree of imperviousness in the spider since this controls the amount of air resistance thereof as the voice coil moves in normal speaker operation. Resin impregnated fabric spiders have been found well suited for this purpose, and the impregnation process can be closely controlled to give just the right amount of stiifness and resiliency to produce a desired amount of damping to the spider in most applications.

It has been found, however, that for certain applications where a relatively high degree of damping is required, the resin impregnated cloth spiders of the prior art are impractical because such spiders cannot be impregnated to the degree of imperviousness required without adversely affecting other desired characteristics, such as resiliency and stiffness, of the spider. Such an application, for example, is in rear seat automobile loud speakers. In most automobiles the trunk compartment extends to the back of the rear seat and, when the usual rear seat automobile speaker is used, the trunk compartment forms a resonant chamber for the speaker. This chamber has an undesirable characteristic of emphasizing the low frequency tones with resulting booming characteristics. To obviate this condition, it is necessary that the low frequency end of the frequency range through which the speaker responds be relatively highly damped. It has been found that it is impractical to produce the required degree of damping for this application by the im pregnation technique. Highly impervious spiders such as those composed of paper, do exhibit the degree of imperviousness required for adequate low frequency damping when a speaker is mounted in the position described above. However, it is extremely difficult to manufacture paper spiders with uniform thickness, resiliency and stiffness and which are not subject to deterioration by moisture.

an object of the present invention to provide an improved centering and supporting member or spider for supporting a moving coil in an electroacoustical transducer, which spider exhibits the desired relatively high imperviousness to the passage of air therethrough so as to provide adequate low frequency damping, and which improves the reproducing characteristics of the transducer when the latter is used in a position where relatively high damping is required in the low frequency end of the audible frequency range.

Another object of the invention is to provide such an improved relatively impervious centering and supporting member that may be manufactured simply and conveniently to exhibit product uniformity as to size, shape, weight, stiffness, resiliency and porosity.

Yet another object of the invention is to provide such an improved relatively impervious centering and supporting member that is constructed to have a relatively long life and which is not subject to deterioration by moisture.

A still further object of the invention is to provide a method for fabricating such an improved relatively impervious centering and supporting member or spider.

A feature of the invention is the provision of an improved relatively impervious centering and supporting member for use in an electro-acoustical transducing device, which member comprises a fabric impregated by a suitable stiffening agent, and which is then coated with a film-like layer to provide a desired degree of imperviousness to the device.

The above and other features of the invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following descrip tion when taken in conjunction with the accompanying drawing in which:

Figure l is a longitudinal central section of an electroacoustical transducer, such as a loud speaker, incorporating a centering and supporting member or spider that may be fabricated in accordance with the present invention;

Figure 2 is a plan view of a centering and supporting member or spider that may be constructed in accordance with the invention;

Figure 3 is a cross sectional view of the member of Figure 2 taken along the line 3-3; and

Figure 4 is a representation of various steps used in the fabrication of the centering spider of the present invention.

The present invention provides a member for supporting a mo '"ig coil in an electro-acoustical transducer and for centering such a coil in an air gap in such transducer. The member comprises a disc of thin, resilient, porous, impregnated material; with a film-like coating on the disc to decrease the porosity of the material and provide damping for the moving coil throughout at least a portion of a selected frequency range.

The speaker illustrated in Figure 1 comprises an electromagnet having a field coil it and a pole piece 11. Field coil ill is surrounded by a metallic housing 12 which includes a top plate T3, the top plate having a central aperture through which pole piece if extends. A centering plate 14 encompasses pole piece ill and is connected by any suitable means to the top plate 13 so as to maintain the pole piece in a centered position. The speaker also includes a diaphragm 15 which is mounted within a casing or basket in. The diaphragm is in the form of a truncated cone and has an outer peripheral rim 17 which is cemented to casing 16. An annular member 13 is also cemented to the basket and to the peripheral rim 17 of the iaphragm. Basket 16 is secured to the top plate 13 by any suitable means.

A centering disc or spider 19 is cemented to the bottom flange of basket 1-5. The central portion of spider 1 .9 is bent upwardly to form an annular flange or collar 2% for receiving a cylinder 21, the cylinder being usually composed of cardboard. The cylinder extends through the aperture in top plate coaxially with pole piece ill, and it has a m ving voice coil 22 wound thereon and disposed in the air gap formed between the top plate and the pole piece. Coil 2-2 is coaxial with the pole piece, but is spaced therefrom and from top plate 13. Cylinder 21 is adapted to receive the lower end of diaphragm l5, and any suitable means such as cement or the like may be employed for attaching diaphragm 15 to cylinder 21, and also for attaching the flange it? of spider 19 to the cylinder so that all these members may move longitudinally as a unit.

As previously noted, coil 22 is disposed in the air gap between core 11 and top plate 13. In order that the speaker may operate efficiently, it is desirable that the radial dimensions of the air gap be reduced to a minimum, and that coil 22 be accurately centered with respect to pole piece 11 while at the same time permitting longitudinal movement of the coil. In order to permit such longitudinal movement to reduce transverse movement of the coil in the air gap to minimum, the coil centering spider 19 is used, as is well-known to the art.

The actual physical shape of the centering and supporting member or spider T9 is more clearly shown in Figures 2 and 3. As shown in the latter figures, the spider has the form of a disc with a central aperture therein, and the disc preferably has concentric or spiral corrugations 23 to assist in the longitudinal movement of the voice coil and diaphragm while maintaining transverse motion at a minimum. As previously noted, it is preferable for spider member 19 to provide sor e damping for the voice coil to enhance the speaker characteristics, and the desired degree of damping is achieved in accordance with the present invention in the manner now to be described.

Spider 19 preferably has a fabric base, such as cotton, and as shown in step A of Figure 4, this base is first impregnated with a suitable stiffening agent such as a thermoplastic or therrno-setting resin. This is accomplished in a manner well understood in the art by dipping the fabric in a resin and alcohol bath, then passing the resin impregnated fabric through a squeeze roll to control the amount of impregnation, and then curing the fabric in an oven to a desired volatile content of the resin. The base is then pressed in dies or in a mold, as shown in step B of Figure 4, to impart the desired concentric or spiral corrugations and to complete the cure or set of the resin. This latter step is also well known, and it previously constituted the final step in the fabrication of the spider. As previously stated, spiders manufactured in this manner have been found to be Well suited for most applications. Moreover, such spiders can be manufactured with product uniformity, exhibit long life, and their stiffness within certain limits can be easily controlled by controlling the amount of resin impregnation.

The present invention contemplates an additional step to restrict air passage through the normally porous impregnated cloth spider so as to increase the low frequency damping of the spider without adversely affecting its other characteristics such as stifiness and resiliency. The restricting material used is preferably a resin coating which fills the pores of the spider, that is the voids or spaces between the impregnated threads of the fabric, and the finished product comprises a fabric spider having all the desirable characteristics of the prior art spiders referred to above, yet having a high degree of irnperviousness for increased damping of the speaker.

it is desirable that the decrease in porosity of the spider be accompanied by a minimum change in stiffness and resiliency so that the desired characteristics of the impregnated spider are retained, and this is achieved in a highly satisfactory manner by employing a film-forming resin filler. The resin filler can be either thermo-setting or thermo-plastic, and it is desirable for commercial use that such filler will not soften at the high temperature end, or become excessively brittle at the low temperature end, of the range at which the speaker is normally exposed.

in accordance with the invention, therefore, and as illustrated in step C of Figure 4, the pressed base is then dipped in or sprayed with a film-forming filler material, such as a thermo-plastic or thermo-setting resin, to produce an impervious film over the surface of thebase further to restrict the porosity thereof.

It has been found that a desirable resin for step C is a modifiedpolyvinyl chloride type. It has been found that this resin produces an excellent film-like coating and does not affect the other characteristics of the impregnated base to any appreciable extent. Moreover, it has been found that such resin does not materially shrink or warp the base upon the formation of the film. This resin is usually supplied in milk-white liquid form and has a water dispersion of approximately 54% solids, a pH' of 8.5, a surface tension of 3447 dynes per centimeter,.a viscosity of 16-36 cps., a specific gravity of 1.17-1.185, and a specific gravity of 1.35 for the resin solids. The resin dries to a white powder and, on heating to 275300 F., the resin particles fuse'together to produce a grease resistant, waterproof, abrasion resistant film.

As previously stated, after-the spider has been impregnated and formed, it is dipped in or sprayed with the resin to form the external film-like coating. This coating, for example, has a thickness of the order of .0005 inch. After the coatinghas been applied, it is dried at an elevated temperature, and it is desirable that the spider then be re-pressed at an elevated temperature of the order of 300 F. before trimmin This re-pressing fuses any unfus ed particles of the resin on the surface so as to increase the uniformity of the film-like coating. Moreover, the re-pressing corrects any slight shrinkage or deformity that might be produced by the film.

When the resin described above was used, a satisfactory coating was obtaiuedwhen the coated spider was dried below 250 F. or the fusing temperature of the resin. It was found possible to decrease at will the porosity of the spider to a value equal to or below that of a paper spider. Resilience of the spider did not show any apparent decrease upon the application of the resin film, nor were any of the other characteristics of the spider adversely affected to any appreciable extent.

The 'i vention provides, therefore, a new and improved member for supporting a moving coil in an electroacoustical transducer and for centering such a coil in an air gap in such transducer. The improved member of the present invention has been found to exhibit the desire l degree of damping required in rear seat speakers and to compensate for the resonant characteristics of the trunk compartment. Moreover, it has been found that the damping characteristics of the spider can be conveniently controlled merely by controlling the amount of spray, or the solids content of the dip solution, in the coating step of the process. Product uniformity has been achieved with the spider of the present invention, and the device has been found to exhibit all the desired qualities of impregnated cloth spiders.

While a particular embodiment of the invention has been shown and described, modifications may be made and it is intended in the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

I claim:

1. A member for supporting a moving coil in an electro-acoustical transducer and for centering such a coil in an air gap in such transducer, said member comprising a corrugated disc of resin-impregnated, thin, resilient, porous cotton fabric; and an external film of modified polyvinyl chloride type resin on the surface of said disc to fill the pores in said cotton fabric and provide damping for the moving coil throughout at least a portion of a selected frequency range.

2. A method for making a spider for supporting a moving coil in an electro-acoustical transducer to provide relatively free and unrestricted longitudinal movement for the coil With minimum transverse motion thereof, and Which spider exhibits a selected air resistance so as to provide low frequency damping for the transducer, which method comprises impregnating a thin, porous fabric with a resin stiffening agent; curing the resin-impregnated fabric and forming a disc of selected configuration; and forming an external resin film over the cured impregnated disc to decrease the porosity thereof.

3. A method for making a spider for supporting a moving coil in an electro-acoustical transducer for providing relatively free and unrestricted longitudinal movement of the coil with minimum transverse motion thereof and for providing low frequency damping for the transducer, which method comprises impregnating a thin, porous cotton fabric with a resin; curing the resin-impregnated fabric; pressing the cured impregnated fabric into a disc of a desired configuration; forming an external coating of modified polyvinyl chloride type resin over the surface of the cured impregnated disc to decrease the porosity of the cotton fabric; fusing the external resin coating to produce a film-like coating; and re-pressing said disc at an elevated temperature to correct any deformity of the disc produced by the film-like coating and to fuse any unfused particles of the external resin coating.

4. A method for making a corrugated spider for supporting a moving coil in an electro-acoustical transducer to permit relatively free and unrestricted longitudinal thereof and to provide low frequency damping for the transducer, Which method comprises dipping a thin, porous fabric in a resin and alcohol bath; rolling the resin impregnated fabric to control the amount of impregnation thereof; curing the resin impregnating the fabric to a desired volatile content of the resin; pressing the fabric so as to impart desired corrugations therein and to complete the cure of the resin impregnation thereof; and forming an impervious resin film over the surface of the cured fabric.

5. A method for making a corrugated centering member for a moving coil in a loud speaker, which such centering member will permit relatively free and unrestricted longitudinal motion of the coil with a minimum transverse motion thereof and provide damping at the low frequency end of the range of operation for the loud speaker, Which method includes impregnating a thin, porous fabric with a resin stiffening agent, squeeze-rolling the resin impregnated fabric to control the amount of impregnating material therein, heating and pressing the fabric to impart the desired centering member corrugated configuration thereto, and providing an external resin film over said corrugated impregnated configuration of the fabric to decrease the porosity thereof and affect the damping characteristics of the centering member.

References Cited in the file of this patent UNITED STATES PATENTS