US2085721A - Sound translating device - Google Patents

Description

June 29, 1937. H, R WARNKE 2,085,7'21

- SOUND TRANSLATING DEVICE Filed Oct. 29, .1932

Patented June 29, 1937 SOUND 'rnaNsLa'rrNG DEVICE Herbert R. Warnke, Camden, N. I., assigner to Radio`Corporation of America, New York, N. Y., a corporation of Delaware Application october 29, 1932, serial No. 000,171

10 ciaims. (ci. 17a-115.5)

My invention relates to sound translating devices and, more particularly, to loudspeakers of the movable coil or dynamic type having a permanent magnet field and a large direct-acting conical diaphragm.

In the construction of dynamic loudspeakers having a permanent magnet eldwith which I have been familianlarge unwieldly eld yokes, usually comprising a multiplicity of bars, have been employed and the construction has been lacking in the simplicity, compactness, and ruggedness characteristic of certain makes of dynamic speakers of the electromagnetic type.

In these permanent magnet field dynamic speakers, it has been the practice heretofore to secure the parts Vtogether by means of bolts, clamps, or the like. In such structures, the field magnet has usually been of extremely hard chrome-steel and it has been necessary to soften the metal by special heat treatment prior to forming a large hole in the eld yoke for the purpose of securing the center pole piece thereto. It has been found, however, that this operation weakens the material appreciably and, upon subsequent hardening of the metal, considerable trouble has been experienced because of a tendency of the yoke to crack and deform from its previous shape. y

InV other respects, prior constructions of the above character lacked certain desirable features, particularly in regard to cost of manufacture and magnetic efciency considered from the standpoint of magnetomotive force in relation to weight and bulk of material used in the eld structure.

Itis, accordingly, an object of my invention. to provide a dynamic speaker of the type having a permanent magnet field and a diaphragm of the direct acting type having appreciable active area andY depth, that shall be compact in form and particularly adapted for use in automobile installations where space is quite limited.

A further object of my invention is to provide a permanent magnet field structure for a dynamic loudspeaker, having considerably increased magnetic eciency, with a minimum of Weight and bulk of magnetic material used.

A still further object of my invention is to provide a permanent magnet field structure for a loudspeaker of the dynamic type, whereby the manufacture and assembly thereof may be facilitated and the operation and adjustments incident thereto requiring great skill, may be simplified,

thereby resulting in a device that may be manufactured in quantities with a high degree of` uniformity and low cost.

The invention will, however, be better understood when taken in connection with the accompanying drawing, and its scope will be pointedv out in the appended claims. Y

Referring to the drawing, Fig, 1 is a rear view in perspective and on a reduced scale,` of adynamic loudspeaker embodying my invention; Y

Fig. 2 is aside YView in section, slightlyenlarged of the deviceshownA in Fig. 1and includingin addition, a housing structure; and f Y Fig. 3 is a side View of a modification of the eld yoke structure of the device shown in Figs. 1 and 2 and embodying my invention.`

According to one phase of my'invention, a permanent magnet eld structure for a cone type dynamic speaker. is constructed, preferably of a single yoke bar, to produce a magnetic field of great strength by means of a field yoke structure u so formed and arranged with respect Vto the back of the'conediaphragm that a permanent magnet yoke oi' relatively greatlength isrpossible ,with--V outany substantial waste of available space from front to back `of the speaker, thereby producing a compact, Simple, and highly eiflcient structure. I am aware of the fact that compact dynamic cone speaker constructions have been made in the past, e. g. by placing the dynamic eld structure entirely or partially within theconcave side of the cone diaphragm, e. g. in the manner disclosed in application of A. Ringel, Serial No. 515,190, `filed February 12, `1931` and assigned to RCA Victor Company, Inc., Camden, New Jersey.

Referring to Figs. 1 and 2, the sound translating device shown is a dynamic loudspeaker of the permanent magnet eld type, representing-a preferred embodiment of my invention, and in-l cludes a permanent magnet in the form of-a wide U-shaped yoke member l of chrome-steel in conjunction with a relatively short inner pole piece 3 of ferro-magnetic material, e.. g. cold rolled steel, secured at its lower end tothe yoke, pref-k erably by electric arc llet welding. The lower end of the po-le piece 3iis preferably enlarged or hanged to-form a flux plate 4 which provides a better support and gives aV larger volume of ma-V terial as well as a greater area of magnetic contact to thereby reduce the reluctance. One of a pair of diametrically opposite lllet welds is shown at l! along a lower edge of the flange. or the ux plate d. The upper end of thecenter pole'pieceterminates at a point appreciably below the level of the ends-of the U yoke member I.

An inwardly bent yoke plate 5, which bridges the yoke magnet I, is made of ferro-magnetic material, e. g. soft iron of high permeability, and serves as a pole piece for the permanent magnet eld yoke I. The yoke plate 5 is provided with a central opening 'I in the inwardly directed portion for cooperating with the center pole piece 3, and is secured at its ends, preferably on opposite sides thereof, to the outer ends of the U-shaped yoke, preferably by electric arc llet welding, as shown at 9 in Fig. l. During welding, the parts are held in such manner that the circular opening is in concentric relation with the pole piece, thereby forming an annular air gap. The abutting faces of the yoke plate 5 and the ends of the U, as well as the bottom face of the flange I on the inner pole piece and the face of the yoke in Contact therewith, are all surface-ground prior to assembly and welding. 'I'he parts are assembled by means of a special form that is adapted to hold the parts in assembled and spaced relation during the Welding process, the novel structure being particularly adapted for assembly by the foregoing operations. A voice coil and form I3 attached to the small end of a frusto conicaldiaphragm I5 is supported for vibration in the air gap, a resilient spider I6 being secured to the cone and attached to the magnetic structure, preferably to the free end of the inner pole piece for centering the coil in said gap.

For the purpose of supporting the base of the diaphragm, a bracket structure is provided therefor by securing, as by welding in similar manner, an annular metallic ring II preferably to the outer ends of the yoke plate 5. A treated fabric structure I9, for flexibly supporting the base of the cone, and attached thereto, is firmly clamped between this ring l1 and an adjacent metallic ring 2l adapted to be held together, as by rivets 2,3.

The ends of the permanent magnet field yoke I are preferably joined to the ends of the yoke plate 5 adjacent to or substantially in the same plane with the base of the cone. The yoke plate 5, which constitutes part of the cone supporting bracket structure, is inwardly bent substantially in conformity with the contour of the conical diaphragm and into operative association with the shortened inner pole piece. While I have shown, as my preferred embodiment, a bracket structure formed by securing the ring II directly to the ends of the yoke plate 5, for supporting the cone, it should be obvious that a separate bracket structure not associated with the ends of the yoke plate could be provided for the diaphragm. If desired, the ring l1 and yoke plate 5 could be integrally formed out of a single plate.

The foregoing field structure is especially adapted to be magnetized after the assembly has been completed. It is particularly necessary to magnetize after assembly because of the fact that, if the field yoke is magnetized beforehand, as has been previously, of necessity, done in many cases, by reason of the construction, the strong magnetic field tends to attract stray particles of iron to the air gap, and it has been found quite dicult and impractical to free the gap of such particles after they are once there.

While the yoke bar I appears to be one magnet, and is so structurally, magnetically it is two magnets in parallel, each being constituted by the portion from the Amiddle of the bar at the inner pole piece to an outer end of the U. The ends of the U are of one and the same polarity, While the middle portion is of the opposite polarity. This arrangement has resulted in a highly eifrcient structure, giving a strong intense magnetic field, and it has been particularly well adapted to available materials. It has been found, for example, practicable to harden magnet chromesteel bars of a thickness of only about 5/8 inch, the thickness of the yoke bar used in speakers built in accordance with my invention. -In order to obtain the necessary flux at the air gap for proper operation, it has been found necessary, when using magnetic material of the above nature, to employ a plurality of bars physically, as Well as magnetically, in parallel. In one construction with which I am familiar, a pair of bent magnet bars were used in parallel in a superposed co-extensive relation, with their opposite ends attached to the two pole pieces respectively. The resulting structure was quite unsymmetrical and proved difficult to manufacture.

As shown in Fig. 2, the speaker assembly is mounted in a sheet metal housing 21, preferably of non-magnetic material, such as aluminum, by means of bolts 29 that pass through the annular rings ll and 2l on the assembly and an inwardly turned flange SI on the housing. The rear end of the metal housing is open except for a cover-r ing of fabric 33 in order to prevent the entrance of foreign particles into the assembly while not interfering 'with the free passage of sound Waves. The front of the housing is likewise covered with a protecting layer of fabric 315. The housing is supported at its rear by means of a bracket 35 adapted to be attached in automobiles to a surface of substantial area, as to the dashboard 31, and is preferably spaced therefrom, as shown, for desired acoustic effects.

Referring to Fig. 3, I have shown a modified structure that makes possible a still more efl'lcient arrangement wherein the permanent magnet field yoke is lengthened by bowing outwardly the sides 2 of the U, thereby giving a higher magnetomotive force. The ends of the U extend` inwardly into engagement with the yoke plate 5 at points spaced a short distance from the ends of said plate. This arrangement permits a shortening of the magnetic path through the iron yoke plate in the manner shown, which is desirable. The yoke plate 5, preferably dropforged, is preferably provided with offset portions 43 havingparallel ground faces for contacting with the angularly faced ends of the field yoke at 4I. The angular face formed on the ends of the inwardly bent U at lll results in a greater area of contact between the ground surfaces, and thereby appreciably lowers the magnetomotive force loss at the juncture between the pole portions.

Several features of my abov'e construction have been shown! and/or described, but not claimed, in the application of Creager et al,

Serial No. 589,824, filed January 30, 1932, assigned to the same assignee, and now U. S. Patent No. 2,027,473.

By means of my above described novel arrangements, the eiciency of the field structure of the permanent magnet dynamic device has been greatly increased over prior constructions by reason of. the construction that permits the employment of a yoke of relatively great length and cross-section. At the same time, a compact arrangement, particularly in the direction from front to back, has been effected by reason` of the fact of the novel combination or association of the magnetic field and conical diaphragm structure. Furthermore, my novel structure has made possible the production or" a relatively inexpensive construction that is particularly adapted to large quantity production with a high degree of uniformity in the output.

I claim as my invention:

l. In a permanent magnet eld system for a sound translating device of the dynamic type, a U-shape bar of extremely hard ferro magnetic material, the sides of said U being inwardly bent, an inner pole piece attached at one end to the back of said U-shape yoke, and an inwardly directed yoke plate adapted to form a magnetic circuit and an air gap between the ends of the said U and the other end of. said inner pole piece, said yoke plate having offset portions adjacent its ends for attachment to the endsof said U-shape yoke.

2. In a permanent magnet loud speaker of the electrodynamic type, the combination of a Wide permanent magnet of rectangular cross section, a cylindrical pole piece secured to one side of and extending at a right angle to the intermediate portion of said magnet, and a ux plate disposed between said pole piece and the magnet, said flux plate extending at both sides of the pole piece to the edges of said magnet to provide a low reluctance magnetic flux path between the magnet and said pole piece.

3.Ir1 a permanent magnet loud speaker of the electrodynamic type, the combination of a wide permanent magnet of rectangular cross section having its ends of the same polarity and its intermediate area of another polarity, a round pole piece secured to said magnet at its intermediate area and projecting outwardly therefrom, and a ux plate disposed between said pole piece land said magnet, said ux plate extending from the pole piece to the edges of the magnetV whereby substantially all of the magnetic flux at the intermediate area of theV magnet will be directed into the pole piece.

4. In an electrodynamic loud speaker, the com- V bination or". a U-shaped permanent magnet of rectangular cross section having ends of the same polarity and an intermediateA area of another polarity, a round pole piece of magnetic material secured upon said U-shaped magnet at its intermediate area and projecting between and parallel with its ends, a bridging member of soft iron welded to the ends of said U-shaped member having an aperture into which the end of said round pole piece projects to form an annular air .gap of magnetic iiux, and means between said round pole piece and the intermediate area of the magnet for directing the magnetic flux at the intermediate area of the magnet into said pole piece,

5. A permanent magnet field structure for an electrodynamic loud speaker, comprising a permanent bar magnet of rectangular cross section having ends of the sam-e polarity disposed in spaced parallel relation and its middle po-rtion of another polarity, an iron pole piece secured across the ends of said bar magnet and having 'its middle portion disposed inwardly between the ends of the magnet in spaced parallel relation with the middle portion of the magnet, a cylindrical iron pole piece mounted upon and projecting from the middle portion of the magnet for completing a magnetic circuit between the first pole piece and the middle portion of the magnet, and means engaging the full width of said bar magnet at its middle portion for directing the magnetic flux of the magnet into said cylindrical pole piece.

6. A permanent magnet eld structure for an electrodynamic loud speaker, comprising a permanent :magnet of rectangular cross section having its ends of the same polarity and its intermediate portion of another polarity, the ends of said permanent magnet being disposed at a right angle to the intermediate portion of the magnet, a short pole piece of round cross section secured between the ends of said permanent magnet and projecting in the samedi-Y rection as said ends, said short pole piece having a length less than that of theends of the permanent magnet,V a magnetic flux conducting member secured across the ends of said v72 In an electrodynamicloud speake'rgfthe combination of a U-shaped member of magnetized steel having ends of the same polarity, a pole piece of magnetic material secured upon said U-shaped member intermediate and parallel with the ends thereof, f a bridging member Yof soft i iron secured across the ends of said U-shaped member having an aperture into which the end of said pole piece projects to form an annular air gap, and a flux directing member of magnetic material intrposed between saidpole piece'and the intermediate portion Yof said U-shaped member.

8. In an electrodynamic sound reproducing device of the character employing a permanent magnet for maintainingia magnetic flux Vin its air gap, the combination of a. U-shaped member of rectangular cross section formed of. magnetized'steel, an outer pole piece member having an aperture therein secured upon said U-shaped member at a point of one polarity, an inner pole piece member of round cross section having one end disposed within the aperture of said outer pole piece member to form an air gap and the other end adjacent a point of another polarity on said U-shaped member, and means at the latter end of said inner pole piece engaging the full width of said U-shaped member for providing a magnetic path of low reluctance between said inner pole piece and said- U-shaped member.

9. The invention set forth in claim 2 characterized in that said cylindrical pole-piece and said flux plate are constituted by an integral structure and characterized further in that said iiux plate is Welded to said magnet.

10. The invention set forth in claim 6 characterized by the addition of a substantially conical diaphragm provided with a voice coil, said voice coil being disposed in said aperture andlabout the adjacent end vof said short pole-piece, and characterized further in that said magnetic ux conducting member is disposed inwardly substantially in conformity with the contour of. said 70 diaphragm into cooperative association with said short pole-piece.

HERBERT R. WARNKE.

Images