US1830401A - Electromagnetic sound reproducer - Google Patents

Description

Nov. 3, 1931. B. F. Mlr-:ssNER 11,830,401

l ELECTROMAGNETIC SOUND REPRODUCER l Filed March '7. 1929 am/WM? @w www 2W spea C0 tofore realized.

Patented Nov. 3, 1931 UNITED STATES PATENT ori-ICE BENJAMIN r. umssNni, or excam HILLS, Naw JERSEY; vaasIeNoiz. 'ro mmssima 1N- VENTIONS INC., OF SHORT HILLS, yNEW JERSEY, CORPORATION 0F NEW mmc'rnoncNnTIc soUND nnrnonucnn i appnmon mea laren 1, 1929. serial No. ''ilagsii;

Theipresent invention relates 'to electro-l magnetic sound reproducers, and more particularly to those reproducers including provision V'for maintaining therein an intense `5 magnetic field.

The invention contemplates in particular the so-called Ielectrodyna'mic type of sound reproducer in which an intense magnetic field is maintained for .interaction with a dynamic coil excited with the sound representing currents.

An object of the invention is the r'ovision of an arrangement whereb the eld producing coil of such a repro ucer can be excited with a uni-directional pulsating current without disturbing effects on the dynamic coil, and in particular excited with incompletely filtered rectified alternating current, as is had in the now usual practice ofenergizing the three-electrode vacuum tubes of amplifying systems from commercial alternating current sources.

In the usual mode of construction of electrodiynamic reproducers as so-called loud ers for radio broadcast receivers and electrical `phonographs the field coil is made of such electricalV dimensions that its inductive impedance is of suitable value as to be well adapted to function as the high inductive impedance in a filter system associated with an alternating current rectifier for su plying the plate and grid energy for amp ifier tubes.

ployed, of such filter systems, thereby saving weight, space and cost of material. However, the incompletelyfiltered current at'the Vpoint in a filter system where the field coil must be used in orderto replace a choke coil A l undesirably intense. l

Endeavor has heretofore been made to neutralize thel effect of the fluctuating field on the dynamic coil of the speaker-with but little success, and one feature of my invention is the provision of means for neutralizin the hum'effect to a degree of perfection not erey Such use eliminates the usual choke coil, or one coil if two or more are em- My invention will be readily understood from the following description in connection with the figures of the accompanying drawings in which like reference characters represent like parts in the several figures.

Fig. 1 shows in a cross sectional view the usual form-of construction of so-called dynamic speakers arranged to include features of my present invention. v

Fig. 2 diagrammatically illustrates suitable electrical connections for taking advantage of the features of Fi 1.

Fig. 3 shows a fragment o the device of Fig. 1 sufiicient to illustrate a modified form of my invention.

Fig. 4 shows in detail a useful manner of mounting a dynamic coil and attached reproducing element.

Referring to Fig. 1, a magnetic circuit of the usual form of electrodynamic reproducer is shown to includea cylindrical element 1 of cast iron or other magnetic material having a closure at one end by an integral part of the casing, and a closure at the other end comprisl ing a disk 4 of soft iron or other suitable magnetic material secured to the wall of the cylin- .der 1 by means of a suitable series of screws 9. A ceni ral corel 2 of soft iron or other suitable magnetic material is bolted to the base of the c lindrical element 1 by a bolt 3' as'shown, tliecore 2 extending throu h a circular opening in the front disc 4 as s own, the opening in the disc 4 being of sli htly larger diameter than the diameter of t e core 2 in order to accommodate a coil 6 secured to a cylindrical projection 7 of a conical sound producing element 7. f

`A coil 5 usually comprising many turns of fine wire encircles the core 2, and is arranged with in ut terminals 13 and 14 through which't e felddproduciiig or energizing current is supplie With the arrangement shown, the energizing of the coil 5 with a unidirectional curcylindrical element 1, and thence'through the 10c the gap portions. so that When sound representing currents are applied to this coil through the terminals 15 and 16, there is a motion producing reaction set up betweenthe coil and the intense magnetic field of the gap, `resulting in sound reproducing motion being imparted to the cone 7 or other associated sound reproducing element. The usual construction of electrodynamic reproducer endeavors to make the air gap as short as the structure of the coil 6 and its mounting on the cylindrical element 7 will permit while leaving sufficient clearance to prevent frictional contacts, this in order that the flux in the gap may be maintained as intense as possible for a given structure ofmagnetic system and field producing coil.

If the coil 5 is energized fromA a source. such as rectified and filtered alternating current, having a pulsating component, obviously the resulting flux through the gap will be of unidirectional pulsating character. and Will react With the coil 6, with or Without the signal representing current fioul. to proradio broadcast reception.

duce motion of the cone 7 that will cause a hum characteristic ofthe lpulsations in the field exciting current, an undesirable feature in sound reproducing devices in gencral` particularly those used for entertainment purposes such as phonographic reproduction or Because of convenience, the/distaste of the public for batteries and the characteristics of the electrical energy required. it is almost compulsory to employ energy derived from .commercial alternating current sources for energizing the field coil of current designs of electrodynamic reproducers. Further, the high inductanee available in the field coil of commercial electrodynamic reproducers makes these coils particularly suitable as the inductive impedance element of the filter systems commonly usedy in 'connection with filtering rectified alternating current for energizing the plate and grid electrodes of three-elecl trode vacuum tube amplifiers used in radio receivers and2 electrical phonograph reproducers, and such use saves the expense, Weight and space of additional coils for filter purposes in addition to avoiding the employment of a. separate source of' current for energizing the field coil.V For these reasons a satisfactralization.

tory arrangement vfor preventing the production of hum when energizing the field coil with uni-directional pulsating current is of great importance, and the features ofmy present invention permit of this hum elimination to most effective degree. y

It has heretofore beenv proposed to provide means, commonly known as a bucking coil, in an effort to neutralize the effect of the fluctuating field on the dynamic or movable coil and I show a coil 13 in Fig. 1 located adjacent the field coil 5 as illustrating the usual practice towards this end. In this practice the coil 13 is connected in series with the dynamic coil 6, and of such polarity that the induction by the fluctuating field opposes the induction by the fluctuating field in coil 6, the connection being made through terminals 19 and 20.

Experience has shown however that such a bucking coil has given very little improvement in the matter of hum production, it

being usualto find the hum when the coil isused about 1/3 of the intensity of the hum u'ithout the coil, but still sufficiently intense to be unsatisfactory in the usual entertainment providing apparatus. v

On mere casual consideration it would seem reasonable to assume that a bucking coil 13 could be so proportioned, or have its output so controlled, as to mjake it possible toarrive at a proper degree of' neutralizing effect to satisfactorily opposethe hum producing effect in the dynamic coil 6, but the fact is that no manner of skilled proportioning of the bucking coil 13, or its output, has heretofore succeeded in arriving at the desired hum elimination results, Whereas with the features of my present invention I am enabled to so far reduce the hum that for all prac-4 tical effect on human audition it may be said to be Wholly eliminated.

In arriving at this result, I have found that the practice of locating the bucking coil with respect to the magnetic system in the usual Way as shown by the coil 13 in Fig.- 1

apparently permits of some induction in the coil by components of magnetic flux not linking -With the dynamic coil 6, probably resulting in a disturbancev of the induction inV the bucking coil to give a different Wave form therein from that in the dynamic coil, so that the mere poling of the coil 13 to be generally opposed to the coil. 6 does not satisfy all of the requirements for complete neu- As a result of this appreciation of a diHerence in the induction effects I have sought andfound a location of the bucking coil with respect to the dynamic coil and its relation to the magnetic flux of the system that marked improvement results, and have .further foundl that by including a shielding effect in a Way to govern the magnetic induction in the bucking coil, I can control the production of a bucking induction in such a Way as to ,bring about most complete neutraliz'ation. In Fig..1, I show 'my location of a bucking coil 8, of so-called pancake form, in very close relation to the gap in which the dynamic coil 6 is located, the bucking coil being shown to encircle the opening in the disc 4, on its inner side, and as lclose to the disc surface as practical considerations. permit. With this location I found in experimenting with a commercial form of reproducer that the hum could be reduced to the order of ,1f of that had without the use of a bucking coil, as compared with the reduction to 1/3 for the location previously discussed., I further find that by covering the coil 8 with a shield of magnetic material 10, magnetically'secured to` the disc 4 by the screws 12 for instance, that I was enabled to reduce' the hum to the order of 1/500th of that hadwithout any hum' vbucking co'l,

andcompared with the 1A, had with coil 3,

and the 1112 hadl with the coil 8 without the shielding, so much so that the hum is almost .humanly inaudible and the electrical energy of it almost impossible to measure with available measuring apparatus.

Heretofore it has been usual to employ a disc somewhat of .the structure of that of the disc 10 secured tothe inner side of the disc 4, having a central hole therethrough of substantially the same diameter as the diameter of the core 2 in order'to provide means for accurately centering the core 2 in the opening of the disc 4 to provide a gap of permanent dimensions for the dynamic coil 6. In order vto prevent this supporting and aligning disc from short-circuitng the inagnetic flux from the coil 6, it yhas been usual 'to make this disc of non-magnetic material such as brass. In order to use the disc as an effective magnetic shield for my bucking coil 8, I have made the outer portion of it of preferably the best grade of magnetic material, such as pure soft iron,-but in order to still utilizel the useful function of the disc centering the core 2, I have made the portion of the opening therein which makes contact with the 'core 2 of non-magnetic material, as brass, having found that a suitable radial depth of Anon-magnetic material can be satisfactorily pressed into a larger opening of themagnetic portion ofthe disc. As an eXample'I employ a depth of nonmag netic-material of 1/8 inch, having found that with the external diameter of the non-magnetic portion of the disc substantially equal to, and preferably less than, the internal diameter of the'bucking coil 8 entirely satisfactory results are had.

While I have referred to the shield 10-as magnetically secured to the disc4, this is merely a'preferred arrangement. ,The shield 10 can be moved anywhere along the core 2 between the coil 5 and discl 4, and the coil 8l moved-to follow, and still obtain the beneficial results of my invention, though not to the same degree. Also a construction in which the distance between field coil 5 and bucking coil 8 is increased, as -by deepening the cylindrical magnetic element will result in obtaining the beneficial effects of my invention.

I havey further found that with a given structure the leffectiveness of hum neutralizat1on can be affected by some suitable means for variably extending orlessening the effective magnetic projection of the core 2 in order to modify the magnetic fieldl components, as by a screw 12 ofmagnetic material o erating in a threaded hole 12 in the' end of, For example, having a coil 8 and shield 10 that does not completely neutralize for coil 6, improvement can be had by screwing element 12 in or out as found by trial lto obtain a further reduction in hum.- The effectiveness to the gap, so that a screw device 12 may core 2. I

prove of value as a final adjustment in quantity production of reproducers to adjust for production differences in positioning coil 6.

An'adjusting device, such as the screw 12,

may be used to advantage in other ways. In

those uses of the reproducer with a. vacuum' tube amplifier system energized from the same source as the coil 5, it may be found uneconomical to try to eliminate all of the hum current arising from such energizing, so that there is some residual 'hum current accompanying the sound representing currents ntroduced from the output of the am lifier -iuto coil 6, which may be neutralized y the adjustment of coil 8.

In Fig. 2 the connections for the coils are shown, the arrangement including another way of controlling the neutralizing as be tween the dynamic coil and the bucking coil. The core 2 of Fig. 1 is again shown with field coil l5 connected to be ener ized by way of terminals 13 and 14, as from a source 26 of periodically fluctuating unidirectional current. Dynamic coil 6 is shown connected to be supplied with sound representing currents through a transformer 23 connected to, for

example, theoutput terminals 21 and 22 of an amphfer system. The coil 6 is connected in-series with coil 8 through a potentiometer 24 across the terminals 17 and 18, or other suitable means, permitting adjusting the magnitudes of the hum bucking current collected in coil 8 applied to coil 6, a control equivalent to that provided by the screw 12 in Figs. 1 and 2, or which may be used in conjunction with an adjustment had by use of screw 12.

As a practical procedure the coil 8 maybe constructed with more turns than needed for .neutralizing for a normal position of coil 6,

and the potentiometer used for securing an adjustment for taking care of production discrepancies in location of coil 6. )r the potentiometer adjustment may be utilized for correcting for residual hum current coming from a preceding amplifier. v

In the fragmentary Fig. 3 I show another manner of locating the bucking coil 8 with respect to dynamic coil 6. In this arrangement the core 2 is stopped short of the opening in disc 4 so that a screw attachment 12,

with enlarged head as shown, may form` the annular gap with the opening in disc 4, with coil 6 loca ed in this gap as before. The screw attachment 12 has its shank threaded for adj ustably screwing into a 4threaded hole` 12 in core 2. The coil 8 is inset under an overhang at the rear of the head of screw attachment 12 as shown. This arrangement vtherefore provides for locatingv the 'bucking coil with respect to the dynamic coil so as air gap longitudinally as well as laterally.

In some dynam-ic speaker .construction provision is made for positioning the coil 6 laterally in the gap by inserting a fiexible 'diaphragm 27 at the truncated portion of the conical element 7 which diaphragm has a central hole which permits of securing the center of the diaphragm to a fixed point in the end ofcore 2, as by a bolt or screw, thus assuring the cylindricalcoil support 7 always taking correct position in the gap withI respect to core 2. The fiexible construction of the diaphragm 27 is relied upon to permit coil 6 to move cone 7 in spite of the central pinning of the diaphragm.

As an additional feature I employ a washer 28 of' other suitable arrangement'of material, such as rubber or cork, between the core 2 and diaphragm 29, and which can thus be compressed within limits by bolt 29 to permit of longitudinally setting the position of coil 6 in the gap for final hum elimination adjustment in quantity production, and like conditions making such adjustment desirable.

While I do" not wish to be understood as offering any specific theory in support of the extraordinary results in hum elimination obtained -by me from relocating the bucking coil and\shielding it, yet it is thought that a fuller mnderstanding of the principles involved will be had by those skilled in the art by reference to practical observations made by me..

one-third of thehum had without the use of any coil. Upon moving the bucking coil to the position at the back of disc I found that I had to increase the number of turns to secure the minimum hum, but reduced the hum to about one-twelfth of that had without any coil. Then with the shield 10 covering a bucking coil of turns I secured a reduction of the hum to about the 1/500th part of the hum without any coil, and substantially down to the limit of human audition.

The feature of having to increase the number of turns to secure minimum hum as the Abucking coil was moved away from the field lexisting along that part of the core 2 between the field coil and the gap,-that is, thatl portion of the total flux of the system not passing through the dynamic coil 6,.-and since the elimination of linkage of the bucking coil with this stray field improved the neutralizing had it appears reasonable that the poor neutralizing had with the bucking coil located close to the field coil and within the stray field is in some way associated with the stray field component of the total flux which stray field, because of encountering a different' magnetic reluctance, differs in phase from the field passing through the dynamic coil 6. Poor neutralizing can well be accounted for by a distortion difference between the induction in the buckingcoil and the dynamic coil making exact opposing impossible, so that alnplitude adjustment is not altogether effective.

It seems sound to reason that the stray field component, due to different'reluctance by reason of a longer air gap, is out of time harmony or phase with the main field component continuing in the iron path 0f core 2 to and through the principal but short gap,

so that the bucking coil having an induction A represented by the vector sum of both'components is not in continuous opposed step to the induction in 'the dynamic coil of the main field component only.

The additional and marked effectiveness of the shield 10 'lends weight to ,suspecting 'the trouble to rest with the different conduct of the stray field component. It is seen that without the shield it is still possible for stronglstray field from core 2 to the rear of the disc 4 to link with the coil 8 without linking the dynamic coil, ,butwhich field is largely short circuited from coil 8 when the shield disc is in place.

In other words the fundamental require-` ment of reall effective hum buckingA is to have the buc g coil linked onl by flux undergoing the same hysteretic in uences as 5 the iiux linking the dynamic coil. Because of the relation of the 100 turns in the dynamic coil to the 50 turns in the bucking coil I have used, it is apparent that magnitude or volt ,turns oer no diliiculty, being susceptibld to the usual principles as long as like flux characteristics, vectorially speaking, are had -for both coils. l

Theoretically, therefore, a perfect hum `bucking condition would be obtained if the two coils could be made equal and magneti- 1 cally coincident, but in view of the fact that both coils carry the sound representing currents any approach to' this theoretically perfeet association for humc bucking must be avoided in order to avoid substantial sound current neutralization. By separating the coils as shown in Figs. 1 and 3 the mutual electrical couplin with respect to soundl currents is reasonab y smalland of no practical consequence. j

It is obvious that the passage of sound current through the bucking coil amounts to a loss in eiliciency in proportion to the relative electrical and motional impedance of the bucking coil and electrical impedance of the dynamic lcoil, so that my ability to secure complete neutralization of hum with a bucking coil having but one-half the turns of the dynamic coil is of substantial value in maintaining e'ciency; l

. While I have disclosed my invention with particularity with respect to specific embodiments thereof, no limitations by reason of such choice for explanatory purposes are intended, the`invention being one of broad scope capable of numerous adaptations readily apparent to those skilled in the art.

Havingv fully. described my invention, I claim: Y

1. In an electromagnetic sound reproducer the combination of a magnet energized by direct current, an air gap in the magnetic circuit of said magnet, a dynamic coil adapted to be energized with sound representing currents operatively located in said air gap, and means -for eliminating disturbances in vsaid dynamic coil due to periodic fluctuations of said direct current comprising a. second coil connected in opposing series ,with saidv dy# namic coil, said second coil being so closely associated with said air gap that it is substantially independent of the magnetic leak- .age of said system not eiective on said dynamic coil.

' '2. In an electromagnetic sound reproducer the combination 'of a magnet energized by direct current, an air gap in the magnetic circuit of said magneta dynamic coil adapted Y to be energized with sound representing cur- 05 rent operatively locatedlin sald air gap, and

dynamic coil due to periodic pulsations of said direct current comprising a second coil connected in opposing series with said dynamic coil, said second coil being magnetically shielded from at least a portion of the stray fields of said magnetic system extraneouis1 to said gap not effective on said dynamic co 3. In an electromagnetic sound reproducer the combination of a magnet including means for the energizing thereof by direct current, an air gap in the magnetic circuit of said magnet, a dynamic coil adapted to be energized with sound representing current operatively located in said air gap, a second coil associated with said magnetic system con- 'means for eliminating disturbances in said 'lliA nected in opposings'eries with said dynamic coil, and a magnetic shield substantially iso- .lating said -second coil from stray fields of said magnetic system extraneous to said gap not eective on said dynamic coil.

4. In an electrodynamic sound reproducer thecombination of a magnet including means for the energizing thereof by/drect current lhaving an alternating component, an airgap in the magnetic circuit of said magnet, a dynamic coil adapted to be energized with sound representing current operatively located in said air gap, a second coil associated with said magnetic system in close relation to said air gap, said coil being connected in opposing series with said dynamic coil, and magnetically eifectivemeans for altering the stray field conditions lwith respect to one or both of said coils in the neighborhood of said gap so as to produce in said two coils harmonious flux inter-linkages as induced from said alternating component of thelield.I

5. In an electrodynamic sound reproducer the combination of a magnet including means for the energizing thereof by direct current,

`1 an annular airgap in the magnetic circuit of said magnet' formed by a circular magnetic core projecting into a circular openingin a magnetic disc, a dynamic coil operatively positioned in said airgap, a second coil encircling said circular opening on one side of said disc, said second, coil being connected in opposing series with said dynamic coil, and a shield of magnetic material magnetically associated with said disc covering said second coil and through which said core extends in projecting into said circular opening, the opening in said shield through which s'ald core ex.ends being lined with non-magnetic material, whereby said shield serves the additional function of xedly positioning said core in the circular opening of said disc without unduly short-circuiting magnetic flux from said annular airgap.

6. In an electromagnetic sound reproducer the combination of a magnet including means for the energizing thereof by direct current, an annular airgap in the magnetic circuit of said magnet formed by a circular magnetic core projecting into a circular opening in a magnetic disc, a dynamic coil operatively positioned in said airgap, a second coil encircling said circular opening on one side of saiddisc, said second coil being connected in opposing series with said dynamic coil, and a shield of magnetic material substantially covering said coil having an opening through which said core extends in projecting through said circular opening without said core making magnetic contact therewith.

7. In an electrodynamic sound reproducer the combination of a magnet including a disc of magnetic material having a circular opening therein, a circular core projecting through said opening of such diameter as to form an annular airgap with said disc, a direct current coil onsaid core for the energizing of said magnet, a dynamic coil operatively positioned in said airgap, and means for eliminating disturbances in said dynamic coil due to periodic fluctuations of said direct current comprising a second coil encircling said core between said direct current coil and said airgap, said second coil being connected in opposing series with said dynamic coil, and a shield of magnetic material interposed between said second 4coil and said direct current coil.

' 8. In an electromagnetic sound reproducer n the combination of a magnet energized by direct current, an airgap in the magnetic circuit of said magnet5 a dynamic coil adapted 'i 35 to be energized with sound representing current operatively positioned 1n said airgap, and means for eliminating' disturbances in said dynamic coil due to periodic uctuations of said direct current comprising a second 4u coil of less .turns than said dynamlc coil connected in opposing series therewith,V said second` coil being associated with said magnetic circuit so as to have a' greater flux linkage therefrom than is had by said dynamic coil,

and furtherso associated with said magnetic circuit as to be substantially independent of stray flux extraneous to saidy aix-gap not linkin said dynamic coil.

n witness whereof, I have hereunto subscribed my name this 5th day of March, 1929.

` BENJAMIN F. MIESSNER. v

DISCLAIMER 1,830;v401.-Benjamin F. MiessnergfShort Hills, N. J. ELEc'rBoMAGNE'rIc SOUND REPRODUCER. Patent dated November 3, 1931. Disclaimer filed November 25, 1932, by the assignee, Me'essner Inventions, Inc. l

Hereby enters this disclaimer to 4that part of the claim in said specication which is in the following words, to wit:

* 3. In an electromagnetic sound reproducer'the combination of. a magnet including means for the energizing tli'ereof ,by direct current, an air gap in the magnetic circuit ofA said magnet,y a dynamicv colll adapted to be energize with sound rpresenting current operatively located said gap, a second coil associated with sai magnetic `system connected in opposing series with said dynamlc coll, and a magnetic shield substantially isolating said second coil from stray fields of said magnetic system vextraneous to said gap not effective on said dyanmic coil.

[Oficial Gazette December A20, 1982.]

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