US2458158A - Magnetically shielded electrodynamic sound reproducer - Google Patents

Description

Jan. 4, 1949. w. E'. GILMAN MAGNETICALLY SHIELDED ELECTRODYNAMIC SOUND REPRODUCER 3 Sheets-Sheet 1 v Filed Nov. '25, 1942 J/Zorrzeys jh/fifarx ZI/AJZerE 5177/2412 o a 6 0 Q o 0/ o o v 0 0 0: 0 0 0 0 0 0 Jan. 4, 1949. w, GILMAN 2,458,158

MAGNETICALLY SHIELDED ELECTRODYNAMIC SOUND REPRODUCER Filed Ngv. 25, 1942 3 sheet s-sheet 2 F I 5110: I F 5'. f1

116 @zfiM- 104 97 r U Patentecl Jan. 4, 1949 MAGNETICALLY smnnnnn srlcptrno- DYNAMIC soUNn BEPRGDUCER Webster E. Gilman, Maywoed, Ill, assignor to Per-moflux Corporation, Chicago, 111., a corporation of Illinois Application November 25, 1942, Serial No. 466,903

u ta -v 1 This invention relates to transducers and particularly to transducers of the dynamic type as contrasted with the magnetic type thereof.

In dynamic transducers as heretofore con-.

structed it has :been recognized that in order to obtain the desired sound transducing capacity it has been necessary to resort to relatively large dimensions in the transducers, with the result that the weight of such transducers has been excessive when compared with the output capacities thereof. Where size and weight were deter mining and vital factors, it has been customary to resort to the use of magnetic type transducers, even though fidelity and range of frequency response have been sacrificed through this procedure, and accordingly it is an important object of this invention to enable dynamic transducers to be employed even where weight and size are determining factors in the selection of the type of transducer to be used; and further objects related to the foregoing are to enable dynamic transducers to attain a substantially flat frequency response curve and a substantially fiat impedance curve over a wide range of frequencies such as those frequencies utilized in ordinary practice and particularly over the range of frequencies used in conversation; and to enable an output capacity to be attained in dynamic speakers which is comparable to the output capacity of other types of transducers of comparable size and weight.

One factor which has contributed in a large measure to the foregoing disadvantages and limitations inherent in prior dynamic speakers has been found to be in the relatively complicated manner of construction and assembly of the elements of the magnetic circuit, and with such prior constructions the elements of the magnetic circuit have been enclosed. within a housing which has added objectionable size and weight to the transducers. A further object of this invention is therefore to simplify the construction of the magnetic circuits of dynamic transducers, and an object related to the foregoing is to accomplish this simplification in such a manner that an element of the magnetic circuit may function as the housing of the transducer. Further and related objects are to form the magnetic yoke of a dynamic transducer with an annular or circular transverse cross section, to incorporate positioning means in such an annular magnetic yoke whereby mounting of the outer pole piece or plate may be readily and accurately accomplished, to provide a transducer wherein the operative elements are fully shielded, and to ac- (c1. ire-115.5

complish such shielding through use of the housing not only as a housing but also as a magnetic shield.

In accordance with prior practice it has been customary to provide an independent diaphragm support which, after assembly upon the magnetic yoke, has been used to support and position the diaphragm and the voice coil. Through the exercise of proper care and skill in the formation and assembly of the parts it has, of course, been possible to attain accuracy of relationship of the parts in such prior transducers, but even then, the cost and added size resulting from the use of such prior diaphragm supports have been objectionable. In view of this it is a further object of the present invention to enable means formed integrally with or as a part of an element of the magnetic circuit of such transducers to function as a diaphragm support. According to one aspect of my invention it is a more specific object to enable the diaphragm to be supported by means formed integrally with the magnetic yoke; and a further object related to the foregoing is to form the magnetic yoke in a generally cupiike form such that the diaphragm may be supported upon the rim of the cup-like magnetic yoke while the outer pole piece or pole plate may be located Within and supported by the cup-like yoke in spaced relation to the rim thereof.

A further object of this invention is to simplify the mounting and location of the outer pole piece or pole plate in dynamic transducers and a more specific object is to accomplish this through the provision of means formed in the cup-like magnetic yoke in spaced relation to the bottom wall thereof and cooperating with the separately formed outer pole piece to locate the same accurately With the relation to the magnetic yoke.

A further object of this invention is to utilize the outer pole piece or pole plate to support the diaphragm, thereby to eliminate the cost of production and assembly which has heretofore been involved in the production and assembly of a separate diaphragm support.

A further object is to enable a relatively small permanent magnet to be used in a transducer in such a relation that an extremely 'high flux den- Sity is produced in the magnetic gap of the transducer; and objects related to the foregoing are to enable a transducer to be so constructed that the space within the housing is efficiently utilized, thereby to produce maximum flux density in the magnetic gap and to attain this result in relatively small transducers; to so mount the per- 3 manent magnet that the magnet serves as the inner side of the magnetic gap; and to utilize permanent magnets of cylindrical form which may be economically produced and which serve the function usually performed by a separate inner pole piece.

It has been pointed out hereinbefore that dynamic transducers embodying this invention may be utilized either as microphones or speakers, and When a transducer is used as the speaker or earphone of a headset, many difficulties have heretofore been encountered which have objectionably increased the cost of production of such speakers and have limited the usefulness of the speakers. One objection which has been found in prior speakers of the dynamic type has been that in order to utilize such speakers or earphones with remotely located receiving apparatus it has been necessary to utilize voice coils having a relatively high ohmic resistance, and. in order to provide such a high resistance it has been necessary to employ numerous layers of relatively fine wire, with the consequent difficulties in winding the coils. Accordingly it is a further object of this invention to enable voice coils having a relatively small ohmic resistance to be employed in speakers which are to be used as parts of a headset, and a further object related to the foregoing is to accomplish this result through the incorporation of a transformer within and as a part of the speaker so that the length of the leads from the speaker to the receiver is rendered immaterial in a practical sense.

Speakers which are adapted for use as parts of a headset are ordinarily arranged for cooperation with a headband of a particular size and construction, such headbands being in many instances provided with a clip assembly at the ends thereof adapted to be snapped into association with cooperating means provided on the speaker. Such clip assemblies are relatively well standardized as to their dimensions, but in certain classes of work it has been found that clip assemblies having two different dimensions are frequently encountered, and as a result it has been found that speakers or earphones are often unsuitable for use with the headbands which are available. Therefore it is an object of this invention to enable speakers or earphones to be readily utilized in connection with clip assemblies of different dimensions, and an object related to the foregoing is to accomplish this in a simple and expeditious manner. More specifically, it is an object of this invention to utilize a cup-like magnetic yoke which has the side walls thereof so formed as to provide different engaging means of different dimensions whereby clip assemblies of different sizes may cooperate with either one or the other of such engaging means.

A further object is to simplify the assembly of the diaphragm and voice coil element so that accuracy of interrelation of these elements may be readily attained.

A further object is to enable a transducer to be rendered waterproof in the sense that the internal electrical elements cannot be reached by water or moisture.

A further object of this invention is to simplify the assembly of the cap plate upon the open end of the speaker housing, and an object related to this is to employ interlocking means for rotatively relating the cap plate to the housing and to employ a simple clamping means to hold such interlocking means together.

Other and further objects of the present irivention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments and the principles thereof and what I now consider to be the best mode in which I have contemplated applying those principles. Other embodiments of the invention embodying the same or equivalent principles may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention and the purview of the appended claims.

In the drawings? Fig. 1 is a front elevational view of a transducer embodying the features of Fig. 2 of the invention;

Fig. 2 is a sectional view taken substantially along the line 2-2 of Fig. 1;

Fig. 3 is a sectional view taken along the line 3-3 of Fig. 2;

Fig. 4 is a plan view looking downwardly in Figs. 1 and 2;

Fig. 5 is a view showing one face of the outer pole piece;

Fig. 6 is a sectional view of the diaphragm and voice coil assembly;

Fig. 7 is a front elevational view illustrating another alternative embodiment of the invention;

Fig. 8 is a side elevational view of the transducer shown in Fig. 7;

Fig. 9 is a rear elevational view of the transducer shown in Fig. '7

Fig. 10 is a cross sectional view taken substantially along the line i0lll of Fig. '7;

Fig. 11 is a cross sectional-view taken substantially along the line ll-ll in Fig. 10;

Fig. 12 is a fragmental sectional view taken along the line l2-l2 of Fig. 11;

Fig. 13 is a cross sectional view similar to Fig. 10 and illustrating an alternative embodiment of the invention;

Fig. 14 is a cross sectional view similar to Fig. 10 and illustrating an alternative embodiment of the invention;

Fig. 15 is a sectional view taken along the l5-l5 of Fig. 14; and

Fig. 16 is a sectional view taken along the line |6I6 of Fig. 15.

In the form of the invention illustrated in Figs. 1 to 6 inclusive of the drawings, the invention is embodied in a transducer l0 adapted for use either as a microphone or as a speaker, this particular transducer being intended and adapted primarily for use in portable radio broadcasting and receiving units so as to avoid duplication of transducers therein. In accordance with the present invention the transducer in has its elements so formed and related that the magnetic circuit is formed in part by a cup-like member I I made from a ferrous metal such as steel which constitutes the magnetic yoke and embodies a bottom wall I2 and an annular side wall l3. Thus the cup-like magnetic yoke ll may function as ahousing for the operative elements of the transducer. The side wall I3 is tapered from its juncture with the bottom wall l2 to a somewhat larger diameter at the outer end thereof, and the bordering edge of the outer end of the annular side wall I3 has an outwardly projecting annular flange l4 formed integrally therewith and parallel to the bottom wall 12 to facilitate mounting of the transducer III in the unit in which it is to be employed. At a point spaced from therear or bottom wall l2 line and from the flange I4, the side wall I3 has an annular rabbeted groove I8 formed therein to receive the outer peripheral edge of a soft iron disc I9 which serves as the outer pole piece or pole plate of the transducer, and it will be observed that when the pole piece I9 is mounted in the rabbeted groove I8, the pole piece I 9 is spaced a substantial distance from the bottom Wall I2 of the yoke to afford space within which a permanent magnet 26 and a soft iron inner pole piece 2! may be disposed. It will also be observed that when the pole piece I9 is thus mounted it is spaced from the end flange I4, thereby to afford space within which a diaphragm 25 may be disposed.

The permanent magnet 20 is in the present case frusto-conical in form and has its larger end seated against the inner face of the bottom wall I2 of the cup-like magnetic yoke, thereby to establish an eflicient magnetic contact with the magnetic yoke when the magnet 2!! is drawn firmly against the bottom wall I2. The permanent magnet 26 has a recess 26 extending therethrough which may be in the form of a lateral slot or in the form of a bore, and a screw 21 is extended through a relatively large opening 28A formed in the bottom wall I2 so as to project through the recess or bore 26 and into threaded engagement with the inner pole piece 2 I. The inner pole piece ZI has a relatively wide and thick flange 28 formed on one end of a cylindrical body 29 so that when the screw 2'! is tightened a relatively large area contact is established between the small end of the magnet 20 and the flange 28 of the inner pole piece 2i. The cylindrical portion 29 of the inner pole piece 2I is arranged to extend into a central circular opening 30 formed in the outer pole piece or pole plate I9 centrally thereof, and thus an annular magnetic gap is afiorded between the inner and outer pole pieces, and a voice coil structure 3| carried on the diaphragm 25 is arranged to extend into this annular magnetic gap.

In order to assure accuracy of formation and dimensions of the magnetic gap, the pole piece I9 has a locating plate 35 formed from a nonmagnetic material such as brass and secured in position on the inner face of the pole piece I9 by rivets 36. The locating plate 35 has an offset portion 31 spaced from the plane of that face of the pole piece I9 upon which it is mounted. The portion 37 of the locating plate 35 has a circular opening 38 formed therein arranged to snugly embrace the periphery of the cylindrical portion 29 of the inner pole piece 2|, and in practice the openings 30 and 38 formed respectively in the pole plate I9 and the locating member 35 are simultaneously cut to their final size b a suitable trimming tool so as to insure concentric location of the two openings 30 and 38.

In the assembly of the transducer ID, the permanent magnet 20 and the inner pole piece 2| are first placed in position within the cup-like magnetic yoke II and the screw 21, bearing a lock washer iii, is inserted through the opening 38 in the wall I2 and through the recess or bore 25 of the permanent magnet 20 and into threaded engagement with the inner pole piece 2|. The screw 21 is not tightened at this time, however, for it will be recalled that the locating ring 35 is relied upon to accurately position the pole piece 2i. The outer pole piece or plate I9 is then placed in position in the groove I8 with the eating plate 35 engaged with the cylindrical portion 29 of the pole piece 2|, and the pole piece I9 is fixed into the rabbeted groove I 8 with a press nu While such a press fit in many iristances affords the desiredmechanical and magnetic connection between the pole piece I9 and the annular side wall I3 of the magnetic yoke, I prefer to swage the metal of the wall I3 as at 42 in order to firmly connect the side wall I3 with the pole piece I9. Such a swaging operation may be conveniently performed along theedges of air relief slots or openings 43 which are formed in the side Wall I3 intermediate the flange I4 and the rabbeted groove I8, and to facilitate this swaging operation, the edges 43' of the openings 43 are preferably spaced slightly outwardly from the outer face of the pole plate I9. Thus after the outer pole piece I9 has been secured permanently in position within the cup-like magnetic yoke II and in the accurately determined location afforded by the rabbeted groove I8, the pole piece 2! will also be located accurately through the action of the locating plate 35. Thus after this has been done, the screw 21 may be tightened to produce the desired magnetic connection between the pole piece 2| and the wall l2 of the magnetic yoke and the opposite ends of the magnet 20;

The diaphragm 25 is preferably provided with an auxiliary supporting structure operable to damp the vibratory movement of the diaphragm and thereby maintain the voice coil 3I in its optimum relation to the magnetic gap, and this end is attained in the transducer of Figs. 1 to 6 through the provision of a flexible suspension member 44 mounted on the outermost face of the pole plate I9 and engaging the diaphragm just outwardly of the voice coil structure. Thus the suspension member 44 is generally disc-like in form and is made from a flexible sheet material such as paper impregnated with a material such as a thermo-setting plastic. The suspension member 44 has a central opening therein defined by an integral cylindrical portion 45 which is adapted to surround the voice coil structure 3| in spaced relation thereto so that the free edge of the cylindrical portion 45 may engage the inner face of the diaphragm 25. At the other end edge of the cylindrical portion 45 the suspension member extends outtvardlyto afford a flexing portion 44 which preferably includes a plurality of concentric corrugations, and at the outer edge an angularly disposed portion extends'from the portion 44' to merge with an outwardly projecting flat mounting flange 46. The suspension member 44 is mounted on the pole plate I9 with the cylindrical portion 45 concentric with the annular magnetic gap 30, and this may be accomplished through the use of a thermo-plastic adhesive which is applied to the flange 46 thereof, and when the suspension member is to be secured in position, the flange 46 thereof is pressed against the pole plate I9 under considerable heat and pressure so that the thermo-plastic material bonds the flange 45 to the pole plate l 9.

After the foregoing assembly operations have been performed, the diaphragm 25 may be placed in position and the leads from the voice coil may be connected to the respective terminals #8 and 49 which are carried on an insulating'terminal plate 50 secured on the outside of the wall I3. It should be observed that while the elements of the terminal block might be put in place on the wall .13 prior to the assembly of the elements of the magnetic circuit, I prefer to do this surface of the housing H may be freely used for centering and supportingpurposes duringassembly of these parts. 49 are secured on the terminal plate 50 by rivets so that connecting arms 52 of the terminals 48 and 49 project beyond one edge of the terminal plate; and the terminal plate 50 is positioned on the housing I l with an interposed layer 53 of insulating material, and is secured in place by screws 54 extended through the terminal plate 5|] and threaded into the wall of the housing II.

In preparation for its mountingin position in the housing I i, the diaphragm has a pair of small metal rivets 55 secured therein as shown in Figs. 1, 2 and 6 and the ends of the voice coil, are connected thereto, as by solder, and at the same time flexible woven lead wires 56 are soldered-to the respective rivets 55 on the rear face of the diaphragm 25. The lead wires 56 are extended loosely and in a radial relation toward the open lugs 43 which are adjacent to the terminal block 50 and are drawn through openings in opposite ends of a relatively stiff insulating sheet 51 which is secured by arivet 58 on the inner face of the wall [3 so as to extend at its opposite ends partially across two of the openings 43. The'openings in the insulating sheet 51 through which the lead wires 56 are extended are so positioned as to maintain the lead wires out of contact with the metal of the housing II, and preferably a drop of solder is placedon the wires 56 outside of the insulating sheet 5'! to prevent undue looseness of the lead wires between the sheet 51 and the rivets 55. Short insulating sleeves 69 are then placed on the extended ends of the wires 56 which are then threaded through outwardly extending openings in the arms 52 of the terminals 48 and 59 and are soldered thereto at 52' on the outer surfaces of the arms; The lead wire 56 which is thus connected to the terminal 48 is extended as at 56' and is soldered to the rivet 58 to afford a ground connection inaccordance with usual practice.

The diaphragm and the cylindrical Voice coil support, in this embodiment of the invention, are separately formed as shown in Fig. 6, and are secured together by means such as adhesive, and it will be observed that thediaphragm 25.has a central opening therein which in the finished transducer is closed by a cap 65 as shown in Figs. 2 and 4. When the leads 56 are being-connected as just described the diaphragm 25 and the voice coil structure are located loosely in position, and after the connections between the wires 56 and the terminals 48 and 49 have been completed a plurality of relatively thin shims made from paper or like material are placed in position between the cylindrical portion 29 of the inner pole piece and the inner surface of the voice coil form, thereby to accurately locate and center the voice coil structure in the annularmagnetic gap The bordering edge ofthe diaphragm 25 is then secured in position in a rabbeted annular groove formed in the flange l4, and this may be accomplished through utilization of a thermo-plastic adhesive, the procedure being the same as that. described hereinbefore in connection :with the mountin of the suspension member 44. The shims are then removed and the cap is se.-- cured in place by adhesive to complete the assembly of the transducer.- I 1 In the transducer 10 which is thus provided the space between the diaphragm 25 and the suspension member 44 is freely vented by the openings 43, while the space behind the suspension Thus the terminals 48 and- EDI member 44 is vented into the relatively large V01?" ume chamber afforded behind the pole plate 19 by a plurality of openings 51 formed in the pole plate l9. In some instances the damping of the diaphragm 25 may be improved by restriction of the air flow through the openings 61, and this may be accomplished by gluing or otherwise for the principal elements of'the speaker and also serves as a magnetic yoke for the magnetic circuit. lhe housing H is formed as by suitable stamping and drawing operations to provide a bottom wall l2 and a generally annular or cylindrical side wall l3 which is flanged outwardly at Mat its outer end. In this embodiment of the invention the outer pole piece or pole plate 79 is mounted on the flange M rather than directly uponthe side wall of the housing, and is secured in place thereon by rivets extended through openings appropriately spaced about the flange i l and the mounting edge of the pole plate ill. The pole plate :9 is dished inwardly as shown in Figs. 10 andlZ so that the central portion thereof in which a circular opening 8! is formed is spaced inwardly of the housing H, and through this arrangement space is afforded for the mounting of a diaphragm 82 between the pole plate 19 and a cap plate 83 which closes a substantial portion'of the open end of the housing H and affords an open central area 85 through which sound energy may pass.

In the assembly of the pole plate 19 and the cap plate 83-011 the housing ii, the flange M is provided with a plurality of openings Bil of a size adapted to receive the rivets 8B, but such rivets 8d are employed only in alternate ones of the openings Bfiflthe others of such openings being engaged by projections 85 formed at appropriate points about the mounting edge of the pole plate incorporate a foraminous iron or steel plate 89 through substantially the entire area of the cap plate. In the molding operation the openings 90 throughout the central area 85 are maintained free of the plastic material, but throughout the balance of the area of the plate 89 the plastic extends through the openings 90 so as to be interlocked with the plate 89. The outer face of the plate 89 is preferably covered and concealed by the plastic material except in the central area 85; and the plastic material is formed to provide clearance sockets ill for the heads of the rivets 8B and to-provide projections 88 for engagement with the sockets 8'! in the pole plate l9.

After the diaphragm 82 has been secured in place by adhesive or the like on the outer face of the mounting edge of the pole plate '79, the cap plate may be put in position, suitable notches In this embodiment of the i preferably being formed in the edge of the diaphragm to aiford clearance for the rivet heads and the projections 88. To secure these parts together, a clamping ring 93 is put in position with one flange 94 engaging the outer face of the cap plate 83 and another flange 95 is formed at the other edge of the ring, as by spinning, to engage the rear face of the flange I4. Thus the housing II is effectually closed and the internal elements of the speaker are shielded by the plate I9 which is incorporated in the cap plate 83. These internal elements of the speaker are, of course, mounted within the housing II prior to the final closure of the housing as aforesaid, and these elements include means cooperating with the cup-like magnetic yoke afforded by the housing II and with the pole plate I9 to afford an annular magnetic gap within which a voice coil structure 96 carried by the diaphragm 82 may operate.

Thus, as shown in Figs. 10, 11 and 12, a cylindrical permanent magnet 91 is disposed in the housing ll substantially coaxial therewith, and 2f screw 98 is extended through a longitudinal bore 99 in the magnet 91 and into threaded engagement with a stationary nut provided by a plate I80 which is Welded to the bottom wall I2 of the housing. The head of the screw 98 is seated in the counterbored central opening of a washer [III and the screw is initially left fairly loose until the proper centered position of the magnet 97 has been determined. In accordance with the present invention as illustrated in Figs. 10, 11 and 12 the outer end of the magnet 91 is utilized as the central pole piece of the magnetic circuit, and hence this end of the magnet extends into the opening 8I formed in the outer pole piece or pole plate I9. To insure an accurately formed magnetic gap the pole plate I9 has a locating plate I93 secured by rivets I04 on the rear face thereof, this locating plate being formed from a non-magnetic material such as brass to afford an offset annular portion I95 arranged to snugly embrace the magnet 91. Hence when the pole plate '19 is secured in place on the housing H the locating plate I03 acts to center the magnet 91 in the opening ill of the pole plate I9, and thus an accurately formed magnetic gap is afforded. The screw 98 may then be tightened to produce an effective magnetic path between the bottom wall of the housing and the adjacent end of the magnet ill.

The voice coil structure 96 is preferably formed with its cylindrical voice coil form molded integrally with the flexing portion of the diaphragm as described in detail in my Patent No. 2,440,439, issued April 2'7, 1948, and this voice coil form includes an annular external shoulder IIlI facing away from the diaphragm 82 as to serve as a guide for locating the voice coil on the form. This external shoulder I9! is formed by offsetting the material of the voice coil form, and hence a corresponding internal shoulder is also formed so as to face toward the diaphragm. Thus when the screw 93 has been tightened, a diaphragm button I98 having a cylindrical flange I99 thereon may be mounted in closing relation to the outer end of the voice coil form merely by inserting the flange I 99 into the form until it engages the internal shoulder, in which position it is secured by means such as adhesive. The diaphragm button I538 in this instance constitutes a part of the loading portion of the diaphragm structure and is slightly domed to impart strength thereto.

With this arrangement the head of the screw 98 and the washer IIlI are shaped to provide a domed form generally complemental to the internal form of the diaphragm button I I18 and spaced uniformly therefrom. The open central area of the foraminous plate 89 is also slightly domed as shown in Fig. 10 to conform with'the shape of the diaphragm button I08.

The ends of the voice coil, formed by wires I I 9, are preferably led through the diaphragm 82, near the voice coil form, as shown in Fig. 12, and on diametrically opposite sides of the form and are led outwardly in generally opposite directions along the forward or outer face of the diaphragm, to points substantially aligned with openings II I formed in the pole plate I9. At these points the wires IIO are led back through the diaphragm. The portions of the wires III) which are thus located on the forward or outer face of the diaphragm are preferably covered by a water proof surface coating. The ends of the wires III] are led through the openings II I where they are soldered to the free ends of connecting clips M2, the other ends of the clips II2 being secured to the rear face of the pole plate I9 by rivets H3.

The voice coil lead wires III] must, of course, be electrically associated with a receiver or other source of signal current, and this is accomplished by means including a terminal block I I5 mounted within the housing II. As shown in Figs. 10 and 11, the terminal block I I5 is molded from insulating material to include a neck portion I I6 which projects outwardly from the housing II through a complemental slot II I formed in the side wall '3 thereof, and a screw IIB extending through the wall I2 and a bore in the terminal b ock H5 is threaded into one arm of an angle bracket I I I, which is in turn he d in place by a rivet 8' which passes through the s de wall I3 and the other arm of the angle bracket. This serves to hol the terminal block firmly in position.

The terminal block H5 has two parallel bores or sockets IZEI formed therein and opening outwardly through the neck portion H6, and. these sockets are arranged to intersect with and extendthrou h metallic s eeves I2I which are mo ded in the terminal b ock in spaced and insulated relation to each other. The sockets IZE! are of a size adapted to snugly receive metall c connectorthe outside of the back wall I2 of the casing II.

The tips of the connecting wires may thus be held in position in the sockets I20 and in electrical association with the respective metal sleeves I2I; and to provide for connection of the metal sleeves I2I to the voice coil, each sleeve I 2! has a metallic ear or clip I23 extending therefrom into an accessible position within the housing II.

It has been pointed out hereinbefore that earphones or speakers are often used in remote locations with respect to the receiver with which they are associated, and in the past it has been the practice to employ high resistance voice coils in such earphones in order to produce a properly matched impedance in the plate circuit of the amplifier. This prior practice has resulted in anexceedingly high cost and large weight and size in dynamic speakers, but in accordance with the present invention these objections are avoided through the provision ofa transformer I25 included within the housing II and employed as an electrical coupling between the metal terminals I21 and. the lead wires l in of the voice coil. Thus, as shown in Figs. 10 and ll, the transformer SE5 embodies mounting bracket are which is fixed upon the pole plate 74 9 by a rivet I2? or the like. The transformer I25 is so disposed on the pole plate IQ that when the pole plate is put in position in the housing I! the transformer will be disposed within the housing a position diametrically opposite from the terminal block IE5. Prior to the mounting of the pole plate IS in the housing the lead wires 28 from the secondary winding of the transformer E25 are connected to the respective connector clips H2, there being insulating sleeves surrounding the intermediate portions of the wires I28, and the ends thereof,

being connected to their respective clips H2 as by soldering. The lead wires I29 from the ends of the primary winding of the transformer I25 are relatively long, and prior to the mounting of the pole plate '19 in the housing ll, insulating sleeves are placed on the wires i253 and the extreme ends of the wires I29 are connected by soldering to the respective clips E23 of the terminals IZ'I. Thus when the assembly of the parts has been completed the speaker of this invention. embodies means for matching the impedance of the receiver with which it is to be used, and hence. the voice coil may be of a relatively small size and resistance and the other elements of the speaker. may be correspondingly reduced in size. the present speaker is adapted for use under all of the conditions encountered in practice. and particularly Where long leads are to be interposed between the receiver and the speaker.

When speakers of the kind shown in Figs. 7 to 12 are in use they are ordinarily mounted on the ends of a headband of conventional construction, and such headbands embody clip assemblies matched with the headbands available, thereby causing objectionable delay in setting up or repairing a communication system. However, in accordance with this invention, the casing II is so formed as to afford two different sets of bearing sockets spaced different distances apart so as to enable different styles or sizes of clip assemblies to be employed therewith. In accomplishing this desirable result the wall 13 of the housing II is ofiset inwardly at diametrically opposite points to form substantially parallel wall sections 1.3. These wall sections I3 are disposed in symmetrically related positions on ops posite sides of the opening II"! and the terminal block H5, and as shown, in Fig. 8, the wall sections 13 extend from the bottom wall 12 toward the flange M for a distance slightly less than one-half the distance between the bottom wall and the flange M. Thus the wall I3 is of cylindrical. form between the. ledge 113av and the flange I4. Av pair of aligned bearing sockets Hill are formed in the wall sections I3 substantially centrally thereof for cooperation with clip assemblies of one predetermined dimension or standard, while similar aligned bearing sockets I3I are formed in the areas of the wall 13 sub- Thus:

I vention has been embodied [ill stantially midway between the ledges 13a and the flange I4 for cooperation with clip assemblies of a difierent predetermined dimension or standard. Thus the speaker III of this invention may be readily put to use with headbands of two different standards to thereby facilitate set-up or repair oi radio and like communication systems.

As pointed out hereinbefore, the total weight of a transducer is ofter a controlling factor in determining whether or not it is suitable for use in a particular situation, and while the, embodiments illustrated in Figs. 1 to 12 have maintained the high and desirable performance characteristics usually associated with dynamic transclucers While materially reducing the weight of such transducers, an even greater weight reduction may be attained through the use of the embodiment of the invention illustrated in Fig. 13. In this form, the inin a transducer III which is most of its structural characteristics is similar to the transducer I0 hereinbefore described, and in all instances where such correspondence exists the same reference numerals with a prime suffix have been applied in Fig. 13.

Thus the transducer 10' has a housing II which is cup-shaped and is flanged at its open end as at I4. In this form of the invention the housing II also serves as the magnetic yoke of. the device, and a pole plate 19A is mounted on the open end of the housing as by rivets 80. The pole plate 19A is dished inwardly in the same general manner as in the transducer I0, thereby to locate the central opening 8i thereof about one end of the permanent magnet 97 and in an inwardly spaced relation to the cap plate 83. In this embodiment, however, the pole plate ISA is formed so that it is relatively thin near its outer edge, as indicated at I33, and gradually increases in thickness toward the central opening 84. With this arrangement an effective magnetic path is afiorded from the outer edge or flange I34 to the magnetic gap, and yet a substantial reduction in material and in weight is efiected. It will be observed that the flange I34 of the housing may be minimized, a relatively wide and flat nut I36 is utilized to receive the end of the screw 98'. The nut I36 has a lug I31 thereon adapted to engage a socket formed by a half-punch I38 in the bottom wall of the housing, thereby to hold the nut against rotation as the screw 98' is tightened. The nut I36 is so formed as to bear firmly against the outer face of the bottom wall I2 over a wide area, thereby to afford an eihcient magnetic path from the magnet 91. through the Wall, I2 and to the nut I36, and because of its thickness the nut I35 serves to afford ample cross sectional area in the magnetic path near the central axis of the housing- The nut I36 is. tapered or rounded to a thin outer edge, as indicated at I39, but such taper is gradual so that the nut extends outwardly to point. where the efiectiv-e cross sectional area, of the. wall 12. is suflicient. to provide an efilcient magnetic path.

With this, arrangement it is in many instances possible. to somewhat reduce the Wall; thickness of the housing 'II', and as a result, the weight of the transducer as a whole may be minimized while maintaining efiicient operation and per-v formance thereof.

In locations adjacent to magnetically sensitive apparatus such as magnetic direction finding or indicating equipment. it is. esential thatv stray magnetic fields adjacent to such sensitive apppae. ratus be eliminated or at least minimized. With the transducers illustrated in Figs. and, 13 it will be. evident that the housing 'II or 'II'. serves as the primary path for the magnetic flux: so that stray magnetic fields will be present about such transducers. Where such stray magnetic fields are objectionable as in the above cited instances, the present invention may be embodied in the alternative form shown in Figs. 14 to 16 which retains the general form and size of the trans.- ducers of Figs. 10 and 13;. but is so constructed and arranged that stray magnetic fields are avoided. Thus the transducer 10a of Figs. 14 to 16 has a generally cup-shaped housing 'I.Ia which is flanged at Ma at its outer end and is formed with sockets 130a. and I3Ia so as to be adapted for mounting in clip assemblies of different sizes or designs in the same manner as the transducers of Figs. 10 and 13.

In this alternative form of the invention the housing II (1 serves as a path for conducting the magnetic flux to the annular magnetic gap of the transducer, but in this form of the invention this function of the housing Ila is accomplished. in such a manner that the. housing acts as a shield rather than as a primary part of the magnetic circuit, and means is provided within the housing I I a to afford a primary magnetic circuit. To this end a magnetic yoke I40 is formed from a relatively heavy strip of soft iron or other magnetically permeable material t afford a bottom member I4I, two side members I42 disposed at right angles to the bottom member MI, and a top member I43 which is provided by bending the ends of the side members I42 inwardly toward each other. These ends may be joined together as by welding, and the two ends joined are punched to aiiord an opening BIa which forms the outer side of the magnetic gap. of the transducer, and hence the member I43 constitutes the outer pole piece in this embodiment of the invention. On its inner face the top member I43 has a locating member I 05a. made from nonmagnetic material and held in place by rivets at I04a.

member I4I of the magnetic yoke. I40, and a screw 98a extended through an axial bore 99a. in.

the magnet is threaded into the bottom member I4I to hold the magnet 91a. in position.

The magnetic yoke I40 is supported in position within the housing 'IIa by a mountingplate 19a which has a central opening therein somewhat larger than the opening Bla so that the central portion of the plate 19a may be secured tov the outer face of the top member I43 in a central relation to the. opening 8Ia, and such a connection is in the present instance afforded by the rivets I04a. The mounting plate 19a is dished inwardly so that when its outer. flange or edge portion 19b is placed against the outer face of the flange 14a, the magnetic gap afforded by the opening 8Ia will be disposeda substantial distance inwardly of the open endof. the housing Thus a permanent magnet 91a of cylindrical form may be extended through the. locating member IflEa and into contact with the bottom Ha. A. diaphragm 8.2a of a. generally cone.--

shaped form has a voice coil structure 96a supported thereon in such position that it may be centered Within the magnetic gap afiorded bebetween the opening 8| a and the magnet 91a. The outer edge of the diaphragm 82a is disposed in abutment with. the outer face of the mounting flange I911, and a magnetic shield in the form of a metal plate 8511; has its peripheral edge 85b disposed against the outer mounting flange portion ofthe diaphragm 82a. The plate 850., the diaphra-gmBZa; and the mounting plate 19a. are held in position on the flange 14a of the housing by a plurality of rivets 80a which are extended through theseveral flanges, and in the present instance a fibre gasket I45 is also secured in place on the rear face of the flange 14a by means of the rivets a. After the shield or plate a has been secured in position as just described the outer cap plate 83a; may be placed in position over the plate 85a, suitable interlocking means bein provided so that openings a. in the cap plate 83a may be accurately aligned with corresponding openings 90b in the shield plate 85a. After the cap plate 8341 has been placed in position, it is secured in this position by an annular clamping ring 93a. In the present instance the shield plate 85a is offset in its central portion as indicated at I41, and a sheet of fabric I48 is secured as by adhesive to the plate 85a in the space afforded by the offset I41. Through this arrangement the reproduction characteristics of the present transducer may be governed to produce the desired frequency response characteristics.

The transducer of Figs. 14 to 16 has a terminal block II5a which is substantially similar to the terminal blockshown in Fig. 10 and which is secured in position by means such as a rivet M. In connecting the terminals I2Ia of the terminal block to the coil of the voice coil structure 96, a transformer I25a is utilized in substantially the same manner as in the embodimentshown in; Fig. 10, and wires I 290. are extended from therespective terminalsv I2la to the opposite terminals of the primary of the transformer I25a. The transformer [25a is supported on the plate 19a by means including rivets IZIa. Wires I-28aare extended from the secondary of the transformer to rivets I49 which are mounted by means ofinsulating -sleeves I50 in the mounting plate 19a as shown in- Fig. 14, and wires IIlla are extended from the rivets I49 through the diaphragm 82a and to the opposite ends of the voice coil in the same manner as in Fig. 12.

It will be.- observed that the bottom member [4.1 of the. ma netic yoke I40 is supported out of contact with the wall- 'I 2aof; the housing, and a similar spaced relation is afforded between the side members I412 and: the side walls of the housing. Since the sockets. I3IJ'a are formed in the oifset portions of the side wall, clearance slots I52: are formed in; the members I42 so that the inwardly projecting bosses, formed in the wall 73a. when the sockets tim are stamped, do not contact the sidemembers I42. clear that any stray magnetic fields about the magnetic-yoke I46 wil'l" pass to the walls 12a. or 13a of the housing Ha and will be conducted to the mounting platev 79a.- and thence to the member M3: which constitutes the outer pole piece and" defines the outer side of the magnetic gap. Hence. the casing; Mar in this, embodiment of the invention serves asa path formagnetic flux, but

' since the primary magnetic path afforded by the Thus it will beyoke I40 provides the best and most efiective path to the magnetic gap, the flux density in the housing Ha is maintained relatively small at all times, such flux density in the housing Ila being so small that objectionable stray fields outside the housing are avoided. Where the walls of the housing are relatively thin as in this embodiment of the invention, the bottom Wall 1211 may be strengthened by the formation of an embossed panel 12b therein.

In securing the desired frequency response in the transducer the back loading of the diaphragm. 82a is preferably reduced through the provision of a plurality of openings 19c in the mounting plate 79a, this arrangement being such as to enable substantially the entire interior of the housing to function as a back chamber. Further improvement of the frequency response is also attained by forming the diaphragm 82a with an annular bead or corrugation 82b in the outermost portion of the free area of the diaphragm, and with this bead it is found that the flexing of the diaphragm isrelatively free under all conditions. The mounting of the textile member I48 between the cap plate and the shield 85a also contributes to the attainment of the desired frequency response, for by selection of the fabric it is possible to eliminate or reduce undesirable components of the voice energy.

It will be evident that the embodiments of this invention illustrated in Figs. 7 to 16, as thus described, are waterproof in character since the diaphragms are of waterproof construction and are arranged to cooperate with the housing in excluding water from the internal electrical elements, and in furtherance of this purpose a suitable lacquer may be applied within the housing about the terminal block and at other points where leakage might occur. When, however, the transducers are employed for some uses, as for example, in aviation work, it is desirable to provide for equalization of the pressure in the back chamber, and where this is desirable and where absolute protection against moisture is not vital, this may be accomplished by providing relatively small pin holes in the diaphragm adjacent to the edge thereof.

Under other circumstances, where the water proof character of the transducer is to be preserved. the present invention enables the desired equalization of the pressure in the back chamber to be attained through-means which'prevents entry of water. While such means might be afforded in any of the embodiments shown in Figs. '7 to 16, it is shown only in Fig. 14 where an opening I54 is formed in the panel 12b of the housing and a resilient sheet or membrane I55 of rubber or the like is secured on the inner face of the bottom wall 12a so as to span the recess afforded by the panel. The sheet I55 is secured to the bottom wall 12a by adhesive so as to exclude water from the electrical elements within the housing, and yet when the pressure within; the housing exceeds that outside the housing, the resilient sheet I55 may be stretched into the recess afiorded by the panel 121), thereby to reduce the pressure in the back chamber to correspond with the external air pressure which is applied to the sheet I55 through the opening I54.

Where the back chamber is thus completely closed or sealed, the air confined within the back chamber is of course compressed as'an'incident to the operation of thediaphragm, and because of the elastic properties of "such air an undesirable variation' in the frequency response. of the transducer may result at some frequencies. but in accordance with this invention, such an undesired variation is avoided by the provision of means operable to dissipate the energy of the air which is thus compressed within the back chamber. As shown in Fig. 14, such means are afforded by providing a fibrous filler material in a substantial portion of the back chamber, and while this construction may be employed in any of the illustrated embodiments, it is herein shown in but one of such embodiments. Thus, as shown in Fig. 14, the rear portion of the housing is filled with a fibrous material I60 such as kapok or wood fiber. ably compressed into blocks of the shaperequired to fit about the elements of the magnetic circuit and the transformer so that the preformed block of fibrous material may be readily placed in position, but if desired the material I60 may be packed in place bit by bit while the mounting plate 1% is held in a slightly separated relation to the open end of the'housing. Where the material is thus put in place bit by bit it has been found desirable to place a protective'shield of textile fabric across the openings 190, Fig. 16, or the openings III, Fig. 11, for by such an expedient the filler material I60 is maintained out of contact with the diaphragm at all times. It will be understood that for clarity of disclosure, the fibrous filler material I 60 is shown in but a small portion of the back chamber in Fig. 14, but in practice the filler material is placed in position in all or a substantial portion of the back chamber, and is compressed or packed relatively firmly so as to place a high resistive load on'the back of the diaphragm and have an efficient dissipating action with respect to the energy of the air as such air is compressed. When the back chamber has been filled to a relatively great extent with wood fiber it has been found that the present transducers of Figs. 7 to 16 have a frequency response which conforms more closely to the desired fiat response curve than any prior transducer, and such close conformity has beenfound to extend through the range of frequencies from below one hundred cycles to above four thousand cycles. Moreover, it has been found that with the transducers of this invention, the variation from the normal frequency response which is usually experienced at high altitudes is reduced to a substantial extent, thereby to produce highly improved operation in high altitude aviation uses. Thus these transducers of Figs. '7 to 16 insure maximum intelligibility of transmission under all noise conditions, and such intelligibility materially exceeds that attained by prior transducers.

From the foregoing description it will be evident that the present invention enables dynamic transducers to attain .the desired transducing capacity while maintaining the size and weight thereof at the minimum, and hence the transducers of this invention may be utilized where such considerations are vital factors in determining what type of transducer is to be'selected. The use of a cup-shaped housing which also functions as a magnetic yoke contributes in a large measure to the attainment of this desirable result, and it will be observed in this connection that the cup-shaped housing affords a magnetic path of large cross sectional area so as to increasethe' effectiveness of the magnet in produc ing maximum flux density in the magnetic gap. Moreover, the use of a cylindrical magnet extending into the outer. pole piece and serving as The fibrous material ISO is prefer-- 17 the .inner pole piece enables the maximum .of magnetic force to be supplied in a transducer of a lgiven'size. These novel features, of course, enable maximum magnetic flux to be attained at the magnetic gap to thereby contribute to the efficiency of the transducer as a Whole, and par ticularly it has been found that the high flux density attained in the magnetic gap of the transducers of this invention enables the voice coil to be reduced radically insize and resistance to thereby correspondingly contribute to the desirable characteristics of the transducers of this invention. The cup-shaped yoke and housing employed in the present transducers is so related to the form and location of the outer pole piece or pole plate that the yoke may also function as the primary supporting means for the diaphragm, and hence the transducers are simplified as to construction and assembly.

When the present invention is embodied in a speaker or earphone the arrangement is such that the cup-shaped housing provides means for mounting the earphone on headbands having different sizes of clip assemblies, and hence the usefulness of the earphones is greatly increased. In this embodiment of the invention the efiiciency of the cup-shaped magnetic yoke is such that the magnet may be relatively small, and hence the location of the coupling transformer Within the housing of the earphone is attained in accordance with this invention Without sacrifice of compactness in the structure asa Whole.

In the illustrated embodiments of the invention it Will be evident that unusual simplicity of structure and assembly has been attained so that the transducers may be produced Wi'thgreat economy of time and material. Moreover, it will beseen that the construction and arrangement of the diaphragm and voice coil assembly contributes materially to the economy of the present construction, particularly in the mounting of the diaphragm button on the diaphragm in the form shown in Fig. 10, for it Will be clear that the diaphragm button may be quickly located in its final position merely by insertion of the flange of understood that these are capable of Variation and modification and I therefore do not wish to be limited to the precise details set forth, but desire to avail myself of such changes and alterations as fall Within the purview of the following claims.

I claim:

1. In a transducer, means includin a permanent magnet and an outer pole piece of magnetically permeable material arranged to cooperate to define an annular magnetic gap adjacent to one end of the magnet, a diaphragm having a voice coil structure disposed in said magnetic gap, a cup-like housing formed from magnetically permeable material and disposed about said magnet in such a relation that said annular magnetic gap is located inwardly of the open end of said cuplike housing, said pole piece being secured to said cup-like housing to afiord .a magnetic path between the pole piece and the housing, and means completing a magnetic circuit from the other end of said magnet to said outer pole piece to enable at least a portion of the magnetic flux to flow through said housing to said pole piece.

2. In a transducer, a cup-like housing formed from magnetically permeable material and having a bottom wall and a generally annular side wail,:a mounting plate extended across .the open end of said housing and secured to the side Wall of said housing to provide a magnetic path fromsaid mounting plate to said side Wall, a ma netic yoke formed from magnetically permeable material to provide a bottom member, ide members and atop member apertured to constitute said top member as an outer pole piece, said outer pole piece being permanently secured to said mounting .rnember to support said magnetic yoke Within said housing with said bottom member and side members thereof in spaced relation to said bottom- Wall and sidewall respectively of said housing, a permanent magnet mounted on said magnetic yoke With one end of the magnet in flux transmitting contact with said bottom member of the yoke and'vvith the other end disposed Within said aperture in said pole piece to define an annular magnetic gap, and a diaphragm having a voice coil structure disposed withinsaid annularmagnetic gap and having "the outer edges of the diaphragm mounted on the outer edges of said mounting plate.

3. In a transducer, a magnetic yoke formed from magnetically permeable material to afford a bottom member, two side member and a top member .having a central aperture therein, a perma nent magnet secured to said bottom member and extended-into said aperture in said top member to afiord'an -annular magnetic gap, a mounting plate formed from magnetically permeable .material and secured .to said top member, a cup-like housing formed from magnetically permeable material "andhavin'g said mounting plate secured across the :open 'end thereof to support said magnetic yoke within said housing in spaced relation to the Walls'thereof, said mounting plate :being secured to said housing at the open end thereof to afford a magnetic path between said plate and said housing whereby stray magnetic flux passing from said yoke to said housing is transmitted through said mounting plate to said top member of said magnetic yoke.

4. In a transducer, a cup-like housing formed from magnetically permeable material and having a bottom Wall and a generally annular side Wall, a mounting plate extended across the open end of said housing and secured to the side wall of said housing to provide a magnetic path from said mounting plate to said side wall, a magnetic yoke formed from magnetically permeable material to provide a bottom member, side members and a top member apertured to constitute said top member as an outer pole piece, said outer pole piece being permanently secured to said mounting member to support said magnetic yoke Within said housing With said bottom member and side members thereof in spaced relation to said bottom Wall and side wall of said housing, a permanent magnet mounted on said magnetic yoke with one end of the magnet in flux transmitting contact with said bottom member of the yoke and with the other end disposed within said aperture in said pole piece to define an annular magnetic gap, a diaphragm having a voice coil structure disposed within said annular magnetic gap and having the outer edges of the diaphragm mounted on the outer edges of said mounting plate, a shield plate of magnetically permeable material positioned across and in spaced relation to the movable parts of said diaphragm, a cap plate of insulating material positioned against the outer face of said shield plate, said cap plate and said shield plate having a plurality of correspondingly positioned perforations therein through which voice energy may pass into contact with said diaphragm,means for locating said cap plate and said shield plate in such a relation that said perforations are in registry, and means for holding said cap plate and said shield plate in position on said housing.

5. In a transducer, a magnetic yoke formed from magnetically permeable material to afiord a bottom member, two side members and a top member having a central aperture therein, a permanent magnet secured to said bottom member and extended into said aperture in said top member to afford an annular magnetic gap, a mounting plate formed from magnetically permeable material and secured to said top member, a cuplike housing formed from magnetically permeable material and having said mounting plate secured across the open end thereof to support said magnetic yoke within said housing in spaced relation to the walls thereof, said mounting plate being secured to said housing at the open end thereof to afford a magnetic path between said plate and said housing whereby stray magnetic flux passing from said yoke to said housing is transmitted through said mounting plate to said top member of said magnetic yoke, a diaphragm having a voice coil structure disposed within said annular magnetic gap and having the outer edges of the diaphragm mounted on the outer edges of said mounting plate, a shield plate of magnetically permeable material positioned across said diaphragm, a cap plate of insulating material positioned against the outer face of said shield plate, said cap plate and said shield plate having a plurality of correspondingly positioned perforations therein through which voice energy may pass into contact with said diaphragm, means for 20 locating said cap plate and said shield plate in such a relation that said perforations are in registry, a sheet of fabric disposed between said shield plate and said cap plate, and meansfor holding said cap plate and said shield plate in position on said housing.

WEBSTER. E. G'ILMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 966,247 Schwarz Aug. 2, 1912 1,115,519 Edwards Nov, 3, 1914 1,546,567 Childress July 21, 1925 1,586,391 Timmons May 25, 1926 1,702,434 Hanna Feb. 19, 1929 1,792,300 Hanna Feb. 10, 1931 1,901,898 Clarke Mar. 29, 1933 1,960,449 Midgley May 29, 1934 1,961,665 Holly June 5, 1934 1,987,413 Olson Jan. 8, 1935 2,010,537 Emmert Aug. 6, 1935 2,020,211 Quam Nov. 5, 1935 2,029,282 Serge Jan. 28, 1936 2,037,185 Thuras Apr, 14, 1936 2,058,555 Betts, et a1 Oct. 27, 1936 2,085,721 Warnke June 29, 1937 2,119,272 Engholm May 31, 1938 2,155,474 Creager Apr. 25, 1939 2,162,270 Mott June 13, 1939 2,180,615 Serge Nov. 21, 1939 2,202,886 Abrahams June 4, 1940 2,216,961 Samial Oct. 8,1940 2,293,078 Proctor Aug. 18, 1942

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