US2369676A - Electromechanical-conversion device - Google Patents

Description

Feb. 20,1945.- y H T 12,369,676

ELECTROMECHANICAL- CONVERS ION DEVICE Filed Jul-y 10 1940 4 Sheets-Sheet 1 Feb. 20,1945; F, v HUNT'ETAL 2,369,676

v ELEECTROMEGHANICAL-CONVERS ION DEVICE Filed Ju-ly'lO, 1940' 4 Sheets-Sheet 2 J'aH/v ALw/v P/ERcE Feb. 20, 1945.

F. v. HUNT ETAL 2,369,676

ELECTROMECHANICAL-CONVERSION DEVICE Filed July 10, 1940 4 Sheets-Sheet 3 f'msasmcw l/. Hu/vr JbH/v ALVIN PIE/7C6 Patented Feb. 20, 1945 aseasrs Frederick Vinton Hunt, Belmont, and John Alvin Pierce, Cambridge, Mass.

ELECTROMECHANIC AL-CON VERSION VICE Application July 10, 1940, Serial No, 344,750

7 35 Claims.

The present invention relates to electromechanical-conversion devices, and more particularly to vibration-translating devices, such,

for example, as are used for reproducing or recording sound with wax, shellac, lacquer, metal, and similar phonograph-record materials.

In Letters Patent 2,239,717, granted April 29,

1941, there is disclosed a new and improved phonograph reproducer thatreproduces phonograph records with complete fidelity, encompassing the entire range of audible frequencies, with very low distortion, and with very small wear or the material of which the records are constituted.

An object of the present invention isto im prove upon reproducers of the type described in the said Letters Patent. v

A further object of the present invention is to provide a new and improved phonograph reproducer that shall be easy to fabricate and that shall be sufficiently simple in mechanical behaviour to allow a quantitative mathematical analysis of the behaviour and a quantitative engineering design in advance of actual construction.

The reproducer disclosed in the said Letters Patent employs viscous damping material that, however, may become subject to aging and deterioration. A- further object of the present invention, therefore, is to provide a phonograph reproducer which shall require no viscous damping material.

Another object is to provide an electromechanical conversion device embodying a novel jack, or clamp, whereby the action of a screw in pressing together two pairs of conductors also provides a stron'g'mechanical clamp, thereby relieving the conductors themselves of mechanical strain.

Other and further objects will be explained hereinafter and will be particularly pointed out in the appended claims.

The invention will now be described in connection with the accompanying drawings, in which Fig. 1 is a plan of a phonograph reproducer embodying theinvention, shown disposed in contact with the surface of a phonograph record; Fig. 2 is a section, upon a larger scale, taken upon theline 22 of Fig. 1, looking in the direction of the arrows; Figs. 3 and 4 are similarly enlarged sections, taken upon the lines 3 -3 and 4-4, respectively, of Fig. 1, looking in the direction of the arrows; Fig. 5 is a fragmentary I described hereinafter.

of Fig, '1; Fig. '12 is a plan, similar to the plan of Fig. 5, illustrating a preferred embodiment of the invention; Fig. 13 is a developed plan of the unitary ribbon inductor and mechanical supportin member used in the embodiments of the invention illustrated in Figs. 12 to 18, inclusive;

Fig. 14 is a section of the reproducer head, taken upon the line l4-'-l4 of Fig. 12, looking in the directions of the arrows; Fig. 15 is a section of the jack for-mounting the reproducer head of .Figs. 12 and 14 on the tone arm, the section being taken upon the line l5l5 of Fig. 14; Fig. 16 is an underside plan of the reproducer head illustrated in Fig. 14; Figs. 17 and 18 are sections of the reproducer head, taken upon the lines I'l-l'l of Fig. 16 and lB-l8 of Fig. 14, respectively, looking in the directions of the respective arrows; Fig. 19 is a schematic plan representing the structures shown in detail in previous views; Fig. 20 is a correspondin schematic elevation; Fig, 21 is an electric-circuit diagram equivalent to the mechanical system of Figs. 19 and 20; and Figs. 22, 23, 24 and 25 are symbolic graphs representing performance characteristics more fully A stylus needle 3, the lower'end of which is adapted to be disposed in contact with the undulations in the soundtrack of a phonograph record I, is mounted at the front end of a movable tone arm. As explained in the said Letters prise the tone arm and arepivoted, as described in the said Letters Patent, to permit the tone arm to swing freely back and forth, and also up and down.

' As illustrated in Figs. 1 to 4, the front open ends "of the tubes 5 may be held together by a yoke '35, carried by an insulating bottom plate 18. The yoke 35 and plate l8'may be constituted of Bakelite. The plate 18 may support a highintensity of U-shaped permanent magnet 54 havplan of a modification, the magnet and otherv parts being omitted, for clearnes's; Fig. 6 is a corresponding elevation, partly in section, upon the line 6-6 of Fig. 5, looking in thedirection of the arrows; Fig. 'Lis an underside plan of the forward portion of the embodiment illustrated in Figs. 5' and 6-; Figs. 8, 9, 10 and 11 are sections taken upon the similarly numbered'section lines ing a north-pole piece 68, providing a north pole N, and a south-pole piece 10, providing a south pole S. A small stationary soft-iron pole piece 11 is held in place on the insulating plate 18 by a pin or screw 9. A permanent horse-shoe ma g-. net 1 ill of somewhat different shape, having softiron pole pieces H2, and a centrally disposed pole piece H4, is shown in Figs. 12 to 18.

between the pole pieces N2 of Fig. 18. It may be constituted of a pivotless-inductor dynamic-type device in the form of a substantially horizontally disposed single-turn or multiple-turn inductor, shown as the elongated loop l3 of Figs. 1 and 2 and at 38 in Figs. 5, 6, 16 and 18. The loop constitutes a combined electric circuit and mechanical support. The sides of the loop are shown disposed transversely, in the magnetic field of the permanent magnet 54 or I I0.

The inductor loop 38 shown in Figs. to 18 is in the form of an interrupted cylindrical shell, provided with rear-end supporting arms 66, one at each end of the interrupted cylindrical shell.

In the modification shown in Figs. 1 to 4, the rear ends 66 of the loo-p I3 are secured to mounting blocks 80 that are held to the insulating bottom plate 18 by screws 64, and they may be cone nected to a transformer (not shown) by wires 58 and 60 that may be insulated from each other in any well known way. The inductor element is provided with an intermediately widened portion, bent into the form of a stiff conical shell 36 at the extreme forward or outer closed free end of the loop l3. Ribbon-like arms 82 and 84, of any desired length, connectthe shell 36 to the ends 66 of the loop. The active portion of the loop thus extends throughout the length of the pole pieces 68 and 10. a

As is further explained in the said Letters Patent and embodied in the devices here disclosed, the stylus 3 is rigidly mounted at the bottom apex tip of the cone 36, substantially normal to the substantially horizontal plane of the inductor loop, and the upper base of the cone is substantially in the plane of the inductor loop. The height .of the cone provides the necessary leverage, and the cone shape provides a substantially maximum-stiffness lever for the mass of material employed for the purpose, inasmuch as a conical shell of given mass provides a structure of the maximum stiffness to connect the stylus point and a loop of given size. The conical shell 36 thus connects the stylus-3 and the loop driven thereby, to provide a very high ratio of stiffness to inertia, leading to small phase shift between the impressed motion and the electrical output, and correspondingly excellent transient response.

Whether constituted of a single loop as here illustrated or, where greater voltages are needed, a plurality of loops as described in the said Letters Patent, the inductor coil is easy to fabricate, since it may be made from a thin ribbon blank, as shown in Fig. 13, constituted of some electrically conducting metal, such as duraluminum or, preferably, coin silver, that may be punched from sheet stock. When bent into shape, the ribbon is substantially vertically disposed to provide great stiffness and rigidity.

In the modification of Figs. 1 to 4, the rear arms 66 of the loop are twisted out of the vertical plane toward the horizontalby (90-A) degrees, as illustrated in the sectional view of Fig. 3. As described in the said Letters Patent,but not illustrated here, the side arms 82 and 84 of the loop I3 and the upper or base portion of the conical shell 35 may be bent into the form of a U-shaped channel. The function of this channel form is to provide great rigidity of theforward portion of the inductor loop. In the modifications illustrated in Figs. 5 to 18, even greater rigidity is provided by securing the interrupted cylindrical inductor portion 38 of the loop about the cylindrical upper base portion 31, which is integral with and partakes of the rigidity of the conical shell stylus support 36. This rigidity provides that the forward portion, or the inductor loop, which lies in the magnetic field, shall always move as a single whole, "without relative vibration of the electromagnetically active portions of the loop. On account of the disposition of the loop ends 66 in substantially the same plane as the remainder of the inductor loop, the active portion of the loop is very stiff for lateral forces applied in the plane of the loop. However, because the loop ends 66 have the form of ribbons whose width is large in comparison with their thickness, the inductor loop has great flexibility for motion occurring about a longitudinally disposed axis of the loop. When a lateral force or displacement is applied to the stylus 3 positioned at the apex of the comcal shell 36, the only motion which the electromagnetically active portion of the loop can execute is a rotation about the longitudinally disposed axis, and because the inductor-loop structure has great flexibility for thistorsional motion, the force required on the stylus 3 to produce this motion of the loop is extremely small.

The mass of the inductor-loop element, together with the conical shell and the sapphire stylus, may be made less than approximately 30 milligrams. Most of this mass, owing to the shape and disposition of the inductor loop, is concentrated close to the longitudinally disposed axis of vibration, so that the equivalent mass of the en-. tire vibratory system, as viewed from the stylus apex as a driving point, may be less than about 5 milligrams. The mass reactance of the dynamical moving system, even at the highest frequencies, is thus so small that very small forces are required to hold the stylus positive contact with the record groove. In consequence of the small mass 'reactance of the vibratory system, the wearing of even very soft record materials is minimized and the transient momentary deformation of the record material is so slight that very high frequencies are reproduced without appreciable translation loss. v

A very light, unrestricted vibration-translating device is thus provided, adapted to vibrate about an axis which is quite rigid and stifi for lateral displacement in the plane of the loop, but which is very flexible, so as to move substantially freely, in torsion; a lateral vibrational displacement of the stylus 3 below the plane of the loop is converted into a rotary motion of the stiffened front portion of the loop. No pivots are required to obtain the desired motion of the inductor loop since the longitudinal axis of vibration is established by the inherent differential fiexibilities obtained by means of the conformation of the mechanicalsupporting-and-inductor loop itself. As the stylus 3 moves up and down, and to the right and the left, the stylus yields and,moves freely in all directions, communicating the appropriate vertical or rotational motion to the inductor loop by virtue of the inherent flexibilities obtained by the conformation of the loop.

The loop I3 or 38 constitutes both the mechanical system to which the stylus 3 delivers its energy and the electric system for conveying the energy. A voltage is induced in the loop when the stylus needle 3 is caused to vibrate to drive the stylus support and, by conventional transformation, the electrical energy may be delivered from the lowimpedance loop to ordinary Communication circuits. The conical stylus mount and the inductor loop are so light and have such great rigidity on account of their geometrical configuration that the electromagnetically active system vibrates I ascacrze' dition to providing the requireddampingxof the free. vibrations of, the inductor loop induced-(by freely as; a "rigid .unit. response .to the force exerted on the stylusbythe'soundttrack. 3:1:

" ,Sudden-Ijars. of the turntable, ortone-arm mounting might be thought by some .to be liable to cause the reproducer to bounce out of the record groove.v As is explained andclaimed inzthe said Letters Patent; the .mass distribution and pivoting of.the'tone arm maybe arrangedin .suchaway as to avoid this difllculty. As is further explained-in the said Letters Patent, the stylus-supportingarm may be made suf-; ficiently. long so as to enable. it to swingsomewhat beyond the center ofthe record, thus reducing the error introduced by change. in the track-.v

ingangle: Theloopl3 is shown disposed at,;an

angle tothe direction of extension of the suppo-rting arm;to; reduce the trackingsangle to: amini-;

mum, or low, value.

1 the stylusmotion, provide'a means for transmite ting a;portion'of the force pressingqth'e stylus 3 into {contact with the record groove; the said force, 1 being produced by i the i net unbalanced weight of the reproducerhead and tone-arm. Also described as a modification in the said Let-J tersPatent, there was provided an. auxiliary spring (not shown here) for transmittingthis force pressing the stylus into contact with the record groove. It is undesirable torely uponthe L'As explained in the said Letters Patent, -.ane

described more fully in. our published, paper in .the Journal of the Acoustical Societygof America for July, 1938 (vol. 10., page 14) it is necessary for a phonograph reproducer stylus to be fiexiblein the vertical direction in order to;avoid deforming therecord-material in the pinched sections ofthe groove. According to the, present invention, the pressure exerted by the stylus needle .3fon the.

reoord-andthe mechanical impedanceof the dy,

namical system for vertical 'motion are so small that, ,instead of tearing through the material of the record in the pinched sections of the groove, the stylus rises up slightly andpassesoverthem withoutcau'sing any damage; and-this without failing to follow substantiallythe centerline. of the recordgroove. This results in thetotal elimination of all even harmonics in the lateral motion position of the magnetic field, as illustrated, the otherwise distortion-producingvoltageisthus inmechanical strength of the said damping'mate I rial to transmitthis force on account of the possibilityof cold-flow of the damping material, with consequent derangement ofthe, relative positions of the-reproducer parts. I

According tothe present invention, ,,therear portionsof the vibratory loop are inthe form of two flat ribbon members 66, each ,inclinedto-the horizontal by an angle A, as indicated'inthe sectional view of Fig. 3, toform between them a dihedral angle. ('180.- -2A). A sufficient elastic trengthin the vertical direction may thus be secured -to carry the said unbalanced ,weight. Atthejsame time, and by virtueof the ribbonlike form of the members 66,-,there is'no appreci able stiffening, of the structurefor rotationalmotion. By, virtue ofthe inclination of the meme;

vb'ersv 66 to the horizontal by theindicated angle, the same desirable features of rigidity forlateral displacement in the plane of the vibratory loop; and great flexibility in torsion about an axis line parallel tothis plane, are secured, with the addi-- tional provision ofa suitable vertical-stiifness.

I This vertical stiffness can be altered by altering the said dihedral angle, and rotation takes place about a horizontal axis lying at the intersection of the two planes of the inclinedribbon members 66, The unbalanced weight of the reproducer head may .thus be supported by the vertical stiffnessdevelopedby this inclination. The damp ing material has no function,to *perform-except auxiliary springs to transmit a verticalcompo;

troduced into the twosides of the loop inopposite directions, and issimultaneously accurately cancelled out, .so that onlythe voltage caused by thev lateral motion of the stylus needle. remains to produce the desired electrical output, correspond-' ing in facsimile to the recorded groove modula: tion... I .1 ;.According to the invention disclosedin the said Letters Patent and embodied in the modification illustrated, injFigs. l dto l, damping; is provided; partlyby a damping membrane [9 connecting the cone 36 at theflclosed, outer end'of the .looptothe pole piece 1'! and lying within the loop 5|3, and

' partly bytwo similar membranes 2l and123.-The

membrane l9v is disposedat the extreme forward to providedamping and thereis, therefore, no difiiculty arising from cold-flow of the damping material, nor any necessity to provide additional nent of force; a a v In Figs. 5 to 11, the conical shell 36 is shown as separate from its inductor-andelastic supporting ribbon, and-as embodying the upper cy- 1 lindrical section 31. It is, however, combined into a unitary. vibratory structure with its supportend of the loop, at the points where. thecone 36 is joined'thereto, almost on the axisofrotation, where the amplitude of motion is least,. and anyefiect cinch-uniform elasticity of the material is minimized The membranes 2| and Bare disposed at the extremeen'ds of the loonand connect-the ends 66 ofthe loop I3 with the-rear poritions ofthe adjacent magnetic structure or the base 18, bridgingthe spaces'between them; .lThe. membranes ,2l, and 23 absorb'and prevent-refiece tion ofs'uch dampedivibrationsas may reach td-these points; ,1--; .v dam n membranes, '2! an ih ing-ribbon by wrapping an intermediate narrow portion 38 .of the latter around the major portion of the, periphery of the cylindrical section. The supporting ribbon is .bent' at its junction lines 45 with the cylindrical shell 31 to form the two substantially flat supporting and conducting members 66, hereinafter calledthe'Fbackeflats. The cylindricalshell 3'!v is customarily insulated from the inductor-.38 by the cement used t'ojoin them into. a unitary structure, but this insulation is not important. at this junction because the inductor 38 may be of high electrical conductivitywhile the cylindrical shell 31 is very thin and oflow conductivity, and very little loss of output is brought about by imperfect insulation between them. The developed form of the unitary inductor and-mechanical supporting member may be similarv to that shown in Fig. 13. The preferred embodiment, illustrated in Figs. 12, 14 and-16, isthe same in essence. as thatillustrated by'Figs. 5, ,6

and 7, except that the back flats have been -ta-" p'e're'd to secure added stiffness-ofthe transverse compliance on therequivalent pivot. A magnetic field: late 1 be provided in the structures of Figs; fto'11; the sameas illustrated in Figspl2Lto 18,

and-hereinafter:described. Of the unitary in-l ductorand mechanical-supporting member, the narrow interruptede'cylindrical-shel1 portion 1 I 38,

wrapped aroundthe cylindricalshell 31 consti i tutes the inductor portion;- while i the "widened supporting-arm portions 56, extending beyond thetwo junction lines 45,'one=at each end of the interrupted-cylindrical shell, constitute the. elastic-conducting andesupporting member.

in Fig. 13,- extending beyond lines 43, provide portions-forsclamping, welding, or soldering to fixed conducting supporting elements 65.

The conductors 58 and 60may be respectively connected to the supporting elements 65. The conducting leads '58 'andaMTmay be held to an insulating supporting block- 1| by a screw "13.

A supporting-tube'1 5 is held on the tone arm'by a screw 11, ina mating tube-83 The tube 15 carries the insulating supporting "block- 1|. The

Tabs 1 "j shownrectangular in- FigL' but triangular tubes and'83 may be clamped together by the screw'11." The tube 15,the supporting block1|, and the vibrator-y I elements and the mounting supports carried thereby, may be removed from the tonearm byunscrewing the screw'11i The tube 15 contains theleads 'SBand fifljinsulatedby the. supporting block 1|, andthetube- 83'two corresponding spaced mating leads 85' and '81," insula'tedtrom the tube 83. The conductors 58,

80, 85" and 81 are-thus disposedhetween' the clamping members 15 and 88. The screw 11 extends into *the'tone armthrough a-slot 19*"in the tube 15'.-

The-end of thetone arm on which thereproducer head ismounted comprises an outer respectivemating'lead conductors and-6O from the reproducer head; at the same time, the force pressing-these leads together acts to spread the split: inner tube 83 into close mechanical contact= withthe outer: shell. 15 of the reproducer head; The singleoperation of tightening the screw .11 i affords a good xelectri'cah contact between the leads and a strong-mechanical attach ment 'of. the-reproducer'head on the' tone arm without reliance on" the electrical: conductors tormechanicalstrengthn The operation 1 ot. .the repro ducer: may be "better understood'in connectionwith the schematic rep": resentations of:Figs. 19 andzzo, wherethe rela tively short and'wide back-flat supporting mambe'rsiG are" presented as disp'osed-instwo planes intersecting along. a line'perpendicularrtotthe planeof Fig; 20',-as indicated at. P; Theback flatmembersfifi, as viewed'in the horizontal plane of Fig. 19, may be disposed approximately at right angles to each: other. This 90 angleissapproxi mately. the optimum angle. for providing. maximum transverse stiffness rot the eflective :pivot' with a: low. stiiin'ess for torsional rotation of .the. inductor.

A-sstructure is. thusprcvided thatislvery rigid? ammo 101:. .lateral displacement-int the plane of Fig..19, but veryl-iiexible'in torsion about the said axial line'through Pi ins.=Fig. 20. As heretofore de scribed inconnection: with Fig. 3, so'also in Fig. 9 andrEiga20g: the angle/A of inclination of the back-ifla'timembersxfio to the .horizontal will providesthe necessary verticalhstiifness to. transmit the 'forceipressingzthe istylus 3 into contact with the record groove.

In connectionawith the featuresof the present invention'ithat relate to provision f or positive tracking," we make particular-reference to Fig. 2 ot our saidpaper'in theJournalof. the Acoustical Societyiof: America .Ior July, l938 (vol. 10, p. 14). There it is. shown that the included angle of a record groove which is laterally'modulated must.

change as the recording. stylus. executes .a motion perpendicular to the record groove. 1 In order forthe motion of the stylus 3 to be entirely controlled .by the modulationof the record groove,

it-will be cbvi'ous that at least a portion of the recordigroove oneach side ofthe midline of the stylus '3 must be in continuous contact with the stylusat all ti'm'es. Such'a condition of positive guidance-of the stylus is sometimes called positive tracking and it will be obvious that when this condition is satisfied the stylus needle 3, in executing-the lateral motion corresponding to the record groove. must'also execute a vertical motion ass, result of the change in the included angle of the record groove.

The structures disclosedherein make provision for this vertical motion and the magnetic field, represented'symbolically by the arrow H+H in Fig. 19, is symmetrically disposed, so that no net voltage is induced in the inductor by vertical motion'of the stylus In order to hold'the stylus in positive contact with the record groove at all times, and for any modulation impressed upon the-record groove; it is necessary that the vertical" compliance afforded by the elastic supportingzmembers 86 fall'within certain limits. These limits'are'determined by the consideration that the force necessary to deflect the stylus vertically from itsequilib'rium position by about 0.01 inch should be capable of imparting to the efiective mass of the vibratory element for vertical motion an accelerationvof about five hundred times that due togravity; In a-similar way, the desirable value of compliance for transverse motion of the stylus: must be-so 'chosen that the maximum displacement from' equilibrium produced by lowfrequency modulations of the record groove will not require transverse forces greater than that pressing thestylusin'contact with the said record groove. indicates that the transverse mechanical complianceLat.the styluslshould lie between one and two times the mechanical compliance at the stylus for verticalmotion; i

' Thewdimensional proportions and the disposition. ot -ithe-.:back-'flatmembers 86 necessary to I secure :thisidesirable. proportion of transverse and For typical applications, the foregoing aseaeve v f "the supD rting block .1 (Fig. 6)}: -.'The face: 9 3 of the supportingblocki'll imaybe butisnot'nec essarily in a planerperpendicularztothe axis of the cylinder'31." The back-flats fitindicated in Fig; 19 may be twisted out or a 'plan'e through the use of an angle on -(Fig.8 )i-att.the point of attachment of the back-flats to the cylindrical section, different from the angleez (Figkll) v formed at the rear of'the back-flats, where the back-fiat rii'ember is attachedi to the mounting blocks.

In actual practiceit' is notnecessary that the axis of vibration be strictly parallel to the. surface of the recordl. In the mathematical treatment based on Figs, 19 and '20, however, such l parallelism has been assumed for convenience. An important advantage is gained by inclining the axis of vibration vvith respect to the record surface inasmuch as such' inclination maybring the point'of attachment of the back-flats 66 closer to the recordsurface. This has the advantage of providing that when, thestylus. is defiecte'd upward thecompon'en't'of the resulting relative-motion along the record groove isj'reduced. Moreover, there 'is-inevitably some fric- -tional drag induced by the passageof the record groove under the stylus tending to deform the elastic support for the conical shell. vIf this deformation is'present to any considerable extent, a to-and-fromotion of the stylusalong the direction of motion of the record groove may be excited in a manner similar to that in which a 7 from entering theprotective -housing;:vvhere it might enter the narrow'air gap in the magnetic circuit andinterfere with the satisfactory operation of the reproducer; We have found the following described disposition of the back-flats to provide suflicient stiffness for the longitudinal.

--motion of the stylus tdm'ake these auxiliary-provisions unnecessaryin most cases. However,for

' reproducers or'vibration-translating devices constructed according to the present invention but with dimensions-different from those-we have used in our experimentalwork, or in devices designed for other frequency'ranges, these auxiliary restraints maybe desirablefand our experiments and our analysis 'have'in'dicated the adequacy of the mentioned methods for avoiding'such diflic'ulties. i v

violin string is setiinto vibration by the gliding bow.

I This to-and-fro motion of the stylus apex in;

and along the direction of, the record groove and z the resulting motion'of the inductor portion of the vibratory elementproduce no electrical output by virtue of theisymmetrical disposition of the magnetic 'field. .HoWever, this motion has the effect of superimposing a fluctuation upon the uniformlinear velocityof the recordjgroove and impressing upon the electrical output of the reproducer aflutter or frequency ;modulation. 5

- In the present invention, .We disclose anddescribe" a methodof-disposingthe back-flat members 66 through choice, of th angles n and a2, aswillbe described below, and of; shaping these members so that a very low .mechanical compliancefor I longitudinal motion ofthe stylusis secured. It

is possible to restrain thislongitudinalmotion of the stylus along the, record groove in several'other' Ways such as, foreXample'byproviding a-flexible wire or ribbon member. We have found that the longitudinal motion of the stylus'may be damped effectively by using a restraining member com- The term back-flat, throughout this description, has been-used to referto theelastic-supi porting-and 'conducting membersBG-{Whether or *notthese are in a single plane. In the si'mplified case,inwhich "11. p r

the. banana has is 1181mm time. .the I mounting face 93. Underthese conditions, arotation of the conical. shell 36 about the axis through the. line P' (see 20.). formed by the intersection of the )planes of the back-flats (and,

therefore, inthe plane of the back-flats, since they are in this case horizontal) involves bending ofthe back-flat'members 66-of thefixed-fixed type. .By fixed-fixed type is. meant that. form of elastic deformation oftheribbomhke beams,

in which the vend portions are constrained to re- ,main approximately parallelv to each other.q ;On

the other hand, fora vertical displacement ,ofthe stylus apex 3, the bending. of the back-fiats fifi is of the fixed-free type,-' as in an-ordinary'canti- 1 lever beam loaded at one end. If the; effective :len'gths'of theback-flats are-the same; for these two types .of deformation, @the compliance Ct v for vertical motion .would be approximately eight times the compliance Ct for transversemotion of the tylus. The above-described considerations, however,: have. indicated that this would provide anundesirable relation between the verti cal and transverse compliances, and that itis desirable in a phonograph reproducer, that the compliancefor transverse motion of the stylus tip should'be between one ,andstwoi timesithat for vertical motion of the stylus tip.

Now, if the angle at the front or theback- I fiat members 66 is made'difierent from-zero, and

prising a thin filament or'block of a viscous A dampingmaterial 22,-such'as1'Visca1o-id, rubber,

Vistinex or other. plastic, attached to theconical shell near thestylusapex and ,extending'therefrom to the fixed point of attachment 20. supported from the insulating block;'l-l;, as illustrated in Figs. 6-and 7. Another andpreferred modification of the restraint forthelongitudinal vibration of the stylus may taketheform-of a thin film of elastic material '25 surrounding the apex of the conical shell above its point-of contact with the record groove and. cemented to a but.-

= ton-likev projection 21. on the bottom of the mountingplate ilflshown in Fig; '2, or similarly at-.

tached to the bottom-platecof a protective housing (not illustrated), surrounding*the reproducer head pictured in Figs. 12/60 18. Such an elastic film may also serve the purpose. of preventing dust and small particles of extraneous material the vertical and the transverse compliances at the stylus tip are measured and plotted as a function of the angle 002 at the rear of the back-flat membars .66, experimental curves are obtained similar to' those shown'by the 'solid linesof Fig. 22..,,v In

general," the verti'ealfcompliance CV decreases rapidly as the angle on: isincreased from zero toward ninety, degrees. 1.0171 the other hand, the

torsional compliance Ct increases slowly as. a2

' is first increased from Zero, anddisplays abroad maximum. at some angle in approximately; equal to in, this equality occurring-when the-back-fiat members 56' become substantial-ly plane. This indicates that-fora range of values of az, the

this largerwaluef of :front angie 31x1, the torsional "compliance Ct "again displaysa broad maximum ata-higher valueofmz, againapproximately equal -to thew-increased value of. the-angle: ad; n the other. hand; ifv -theyangle-mh chosen too small, fit'may 'be that the maximum of the curve 0f transverse torsional complianceoccurs at soasmall vava'l-ue of :2 that the verticalcompliance Cv-is greater thanithezcompliance Ct .for. all values of "an, as indicated in Fig. 23. :Itwillbeseen from theseflgures that it .is possible to select values of a1. and (:2 such that an-y desirable .ratio between,- transverse and vertical compliance may be secured in the resulting structure. .Havingse- *cureda desirable ratio betweenth'ese compliances,

their absolutema-gnitudes may be made to have desired values by an appropriate selectionlof the Width and'wthe thickness of'zthe .bacbfiats-fifi.

ltmaybe pointed out also that an rincreasein :the frontanglewmaat thepoint of attachment of A the' back-fiat i5 tothe conical shell-provides a desirable stiffening of this junction for the type of deformation'described above as a consequence of the longitudinal frictional force along the record groove. 'If on is increaseditoo"fartoward 90?; the transverse stiffness" of the efiective pivot 'would be undesirablyflreduced. ,Our measurements haveindicated "that the angle armaybe chosen intermediate-1y, 'suchas'about so that high transverse stiffness 'o f'the effective-pivot .is secured 'while at 'the'same" time a high degree of stiffness; or very low complianceCL; isprovided for-longitudinal forces atthestylus apex. The

measurementsindicatedbythe curves arms. 22

"further indicate that the reanangle 122 at. the .t

point or attachment-of the back-flatson their mounting blocksjmay' be chosen "independently to'yield suitable ratios of transverse andvertical compliance at the stylusapex. "Wehave found a suitable angle :12 to be about 6, and'a suitable thickness of 'the'ba'ck fiats'to'be about .0015

v inch. It follows that an ofthe elastic complig is the distance from the apexlof the-stylus to the center-of gravityof the vibratory system: J; is the'distance fromsthe stylus apex toe line through perpendicular to. the planeof Fig. 20 .and corresponding, if 411:0:2, to the intersect on of the planes containing. the back-flats or, if 1116 112, to the line representing. the axis of rotat on determined by the elasticity of the 'backefiats alone m isthe total mass" of the vibratorysystem; Its the moment of inertia or thevibratorysystem about-anaxis perpendicular-to the'plane of Fig. 20, and-passing through the stylus apex; C is the transverse compliance in centimeters per dyne or the equivalent-pivot established by the differential rigidity of the back-flats 66; C, is the 'rotationalzco'mpliance, in radians per dyne-centimeter, providing the restoring torqu for angular displacements. of the'vibratorv about the equivalent pivot;

' finiteaand zero values.

is :the transverse displacement; or the stylus 'apex'ifrom its-equilibriumtpositi'onzinduced by .thepdisplacementof the-center'line of the rec- I ord groove' (y .isnotzillustrated in Fig.20,. be-

cause the stylus is there shown as occupyin its equilibrium position) 'inis the equivalent displacement vfrom equilibrium of. the inductor portion transverse to the .mag-

netic'field :-(a: is notillustrated'in Fig. 20 be- -:cause Fig..20 represents the equilibrium condition) and grand :1: represent the-time derivatives of y and r, respectively, that is, the velocities o'fthe corresponding displacements. The solution ofv the differential. equations describing the motion of the: system may be representedby'the electricalcircuitbf Fi .21. In this circuit, 11 asconstant current.generator, repre- "-where o cv I e ghgcla 11, I 1090,

{unis .theangular .frequencyotth impressed transverse displacementy.

.Theperformance zindicated by this expression is represented by' the''symboliccurves of 'Figs. 24 and 25, inwhich the dashed 'lines'at the'critical frequencies 02' and wrepresent;respectively, in-

In practice, some viscous damping 'is inevitable in a realizable structure so that these infinite or zero values never occur.

Thevirnportant conclusion tobe drawn from thismathematical analysis is that the structure may be-so: designed that. the critical frequencies .10: and wo may be :made to'coincide. In such a case,' the corresponding'terms'involving oz and wo in. Equation 1 canceL'and the frequency response :ot the reproducer is perfectly uniform up to frequencies so high that "the assumptions upon which the equivalent electrical and mechanical systems are based no longer hold. This frequency, in practical realizable structures, lies well beyond'theaudible range of frequencies.

'Further analysis of the conditions necessary to make wa m yield *the physical interpretation that the axis of percussion pf'the vibratory system' assa whole should be made to coincide with the elastically determined axis passing through P atthe distance 'hfrom'the stylus apex. Thus, if the line through the-stylus apex perpendicular to Fig. 20 and 'the axis of rotation through P defined by'the' elasticity of the back-flats 55 are conjugate centers of oscillation for impulsive forces applied either at the stylus apex or at this axisthrough P,1 the response of the reproducei' willwbel perfectly uniformw'ith respect to fre- I This desirablev relationship may be quencywithout any necessity for the provision of damping materials. 1 further explainedby calling attentionto the fact that in .most textbooks on Mechanics (see references below) the terms center of percussiorrfland center of oscillationv are discussed in connection with the motion f"a rigid body having a fixed and immovable pivoting axis The center of percussion is. then analyzed as that pointqat which the bodymay be struck an impulsive blow without causing :anyforpe reaction on the pivot transverse. to the line joining the pivot and the point of application of the impulsive force. .It is further shown in these textbooks that if the impulsive force is applied at the previously immovable pivoting point, then the previous point of application is an instantaneous center of {oscillation for the resulting motion; thus the previouspivoting point and the previous pointxof application are conjugately related and each is an instantaneouscenter of oscillation for impulses.

applied at the other. 7

Now, in the presentinstance, as in all prior; art vibration-translating devices, the pivot is not perfectly immovable but has, for transverse forces, an elastic compliance whose mechanical impedance is not negligible in comparison .with the mass reactance of the vibratory systemat the higher frequencies of exitationq. If, therefore, the mass distribution of thevibratory system were such that'the stylus apex or point of-application of the impulsive forces were not 'a" cenquency-terms inthe response Equation 1 cancel out as above mentioned.

ter of percussion with respect to the elasticipivot,

then transverse force reactions would occur at the pivot uponjthe application of such'impulsive vibromotive forces at the stylus apex. "Since the.

vibrator has inertia, these force reactions :at the pivot would "be ia-ble-to excite resonance oscillations whosefrequency is determined by the-mass and dimensions of the vibrator and'theelastic pivot compliance; The said resonance oscillations comprise a transverse motion of the"' effective pivot and hence, by cyclically-alterin the relative transverse displacements of the stylus and the pivot, produce cyclic motion of the' inductor portion transverse to the magnetic; field. The effect of these motions on the electrical out-. put is exhibited by the frequency-dependent terms'inthe response Equation 1. I In the .previous Letters Patent referred to above,'th'ese spurious resonanceoscillations are controlled and damped by the provision of damping material,

shown in Figs. 1 and 2 herewith at 19, Hand 23, positioned so as to have the maximum damping effect on the undesired spurious oscillations and a minimum effect on the desired vibrations caused by the modulation of the record groove.- Some prior-art devices have sought to avoid the difliculties introduced by these oscillations by using such'rigid and, therefore, massive construction that the oscillation frequencies are above the useful range; but it is an object of this: 'disclo-' sure to show how, even with extremely light constructions of the vibratory system and even though the resonant oscillation frequencies fall within the useful range of reproduction the effect of these oscillations may be avoidedso that there is no necessity of providing any damping.

material to control them. For if the mass distribution of the vibratory system is chosen so that the elastic pivot is a center of oscillation conjugate to the'stylus apex as a driving point,

no force reactions occur at the pivot to excite the resonance oscillations and the corresponding fre By equating the above defining expressions for we and we, we may solve for the momentof inertia of the corresponding uniform-response vibrator, as follows:

The transverse pivot compliance C is usually very much smaller than the torisional compliance Cs multiplied by h. This expression therefore may be writtenin the following approximate form: Y

It follows that the product. gh is equivalent to the square of the radius of gyration of the vibratory element about the stylus apex or point of application of the impressed force, and this is, therefore, a necessary condition for the de sign of a uniform response vibratory system.

I tz- Since the geometric mean of two quantities is defined as the square root of their product, this necessary condition may also be described. by stating'that the radius of gyration of the vibratory element, about the stylus apex, must be velocity having any desired frequency and impressed at some particular point, then this point and the equivalent elastic pivot should form con; jugate centers of oscillation. 'The'term ,conjugate centers of oscillation is well understood in text books on "Mechanics and refers to the phenomenon whereby an impulsive force applied at one of the centers ,andperpendicularto the linejoining the two centers of oscillation produces about the conjugatecenter a purely rotational motion. (See, for example, Introduction to Theoretical Physics, by L. Page, pp. 115-116, published by Van Nostrand, 1938, or Handbook of Engineering Fundamentalsf'by O.-W. Esh

' bach, pp. 4-41 to 4-42, published by. Wiley,'l936.)

These terms will be Iunderstoodto describe the motion of the vibratory system insofar as it is controlled alone by. the effects of inertia and it may. further be understood that only these inertial effects are significant in determiningthe motion for impressed velocities at very high frequencies. Forthis reason, the conjugatecenter of oscillation may be referred to as an inertial axis or'an axis of percussion, as distinguished in the above description from the axis of vibration for excitation at low frequencieswhich is determined by the elasticity of the supportin means.

The preceding analysis may therefore be summarized by stating the requirement that the elastic axis must coincide with the, inertial axis for uniform response at all exciting frequencies.

In the preferred embodiment ofthe invention shown in Figs. 12 to lathe vibratory element difiersfromthat of Figs: to 11 by a tapering of the backfiats 66, as before stated, from a narrow portion-at the junction line to a wider portion at the junction'line 43. This tapering 'provides a reduction in the transverse-pivotcompliance Cp, which serves to increase the values of the critical frequencies m2 and zoo, and thereby renders the reproducer uniformly responsive over a wider range of impressed frequencies. In this modification, the leads 58 and 60 from the reproducer head are continued over the lower face 93 of the supporting block H and form the mounting elements corresponding to the blocks of Figs. 5 to 11, to which the mounting tabs 40 are attached. In these figures, a magnetic circuit .is also illustrated comprising the horse-shoe N0 of hard, permanent magnetic material, seated upon soft-iron pole shoes I I2. The pole-shoes I I2 and the central pole'piece H4, which may be of softiron or preferably of a hard magnetic material, are attached to a brass block H5, which serves toatta'ch the magnetic structure to the insulating block H by the screwsll8, and to a stylus, inductor means, means whereby the stylus is connected to the inductor means, and supporting means for the inductor means comprising metallic ribbons disposed at an angle to each other and the planes of which intersect substantially along a line that constitutes a longitudinal axis of vibration of the inductor means.

5. Vibration-translation apparatus comprising an inductor loop of interrupted cylindrical shell form, a cone-shaped stylus support along the base of which the-interrupted-cylindrical loop is secured to provide rigidity, and supporting arms, one at each end of the interrupted-cylindrical loop.

6. Vibration-translation apparatus comprising an inductor loop of interrupted cylindrical-shell form, a cone-shaped stylus support along the base'of which the interrupted-cylindrical loop is secured to provide rigidity, and supporting arms, one at each'end of the interrupted cylindrical loop, the arms being disposed substantially in hold themagnetic circuit in appropriate relation to. the inductor portion of the vibratory system.

Further modifications and changes will occur to persons skilled in the art, and all such are considered to fall within the spirit and scope of the invention, as defined in the appended claims.

What is claimed is:

1. Vibration-translating apparatus comprising a vibratory element having conjugate centers of oscillation, means for applying vibratory impulses at one of the centers, movable inductor-and-elastic supporting means for deriving inductively an i electric signal from the vibration of the vibratory element and for supporting the element to vibrate about substantially the conjugate center in response to vibratory impulse applied at the said one center, and means for establishing a magnetic field in which the movable inductor means moves.

2., Vibration-translating apparatus comprising a vibratory element, movable inductor-and-elastic-supporting means for deriving inductively an electric signal from the vibration of the vibratory element and for providing an effective elastic pivot for low-frequency vibrations and a distribution of mass for the element such that the radius of gyration of the element about the point of application of the exciting force is substantially the geometric mean of the distance from the said point of application to the center of gravity of the element and the distance from th said point of application to the axis through the said efiective pivot, and means for establishing a magnetic field in which the movable inductor means moves.

3. Vibration-translationapparatus comprising a vibratory element having a mass m and provided with an apex at a distance 9 from the center. of gravity thereof, the element having elasticsupportin'g members adapted to provide an axis of rotation at a distance h-from the apex and havinga moment of inertia I about an axis through the apex parallel to the first-named axis substan tiallyequal to l mgh(l l,

where Cp is thetransverse compliance of the I said elastic-supporting means.

' 4. Vibration-translation apparatus comprising plane's inclined to one another.

'7. Vibration-translation apparatu comprising an inductor loop'of interrupted cylindrical-shell form, a cone-shaped stylus support along the base of which the interrupted-cylindrical loop is secured to provide rigidity, and supporting arms, one at each end of the interrupted cylindrical loop, the arms being disposed substantially in planes inclined to each other, and the line of intersection of the planes constituting a longitudinal axis of vibration of the inductor loop.

8. Vibration-translation apparatus comprising an inductor loop of interrupted cylindrical-shell form, a cone-shaped stylus support along the base of which the interrupted-cylindrical loop is secured to provide rigidity, and supporting arms. one at each end of the interrupted cylindrical loop, the supporting arms comprising sections of warped surfaces providing for a transverse compliance of the stylus apex one to two time larger than the vertical compliance of the stylus apex.

9E Vibration-translation apparatus comprising a stylus, inductor means, means whereby the stylus is connected to the inductor means, and supporting ribbons for the inductor means com prising sections of warped surfaces providing for a ratio of transverse to vertical compliance of between one and two.

10. Vibration-translation apparatus comprising a stylus, inductor means, means whereby the stylu is connected to the inductor means, and

supporting ribbons for the inductor means comprising sections of warped surfaces providing for a ratio of transverse to vertical compliance of between one and two, the supporting means comprising arms disposed substantially in planes inclined to each other. 11. Vibration-translation apparatus comprising a stylus, inductor means, means whereby the stylus is connected to the inductor means, and supporting means for the inductor means comprisin sections of warped surfaces providing for a ratio of transverse to vertical compliance of between one and two, the supporting means comprising arms disposed substantially in planes inclined to each other, and the line of intersection of the planes constituting a longitudinal axis of vibration of theinductor means.

v12. Vibration-translation apparatus comprising a stylus, a support for the stylus and a combined mechanical-electrical connector-jack for supportis larger than the compliance for longitudinal sulated from the mating portions, and means for simultaneously clamping together the mating electrical leads and the mating portions.v

l3. Phonographic apparatus comprising 1 a stylus adapted to be disposed in contact with a record groove, combined inductor-and-elasticsupporting means, the said supporting means comprising-two ribbon members providing at the stylus simultaneously a large compliance for transverse-motion, a compliance for vertical motion lower than thefirst-named compliancefand a compliance for longitudinal motion lower than the first two named compliances.

14.,Phonographic apparatus comprising; a stylus adapted to be disposedin contact with a record groove, combined inductor-and-elasticsupporting means, the said supporting means comprising two ribbon members providing-at the stylus simultaneously a large compliance for transverse motion, a compliance for vertical motion lower than the first-named compliance, and a compliance for longitudinal motion lower than the first two named compliances, and having the vibrational axis determined by the inertia of the parts coincident with the axis determined by the elastic supporting means.

15. Vibration-translating apparatus comprising a stylus, a stylus support comprising a conical shell having an integral cylindrical section atits base, movable inductor-and-elastic-supporting means attached to the cylindrical section constituting the mechanical system to which the stylus delivers energy and the electrical system for conveying' the energy, and means for establishing a magnetic field transverse to the inductor-andlastic-supporting means, the inductor-and-elastic-supporting means being unitary to constitute the sole support for the conical shell and being rigid to move in the magnetic field as a single whole without relative vibration of the electromagnetically active portions of the inductor means.- e

l6. Vibration-translating apparatus comprising a stylus, a stylus support comprising aconical shell having anintegral cylindrical section at its base,inductor means attached to the cylindrical section,-.;means ,for establishing a magnetic. field transverse to the inductor means, and elastic supporting means comprising two metallic ribbons,

attached to the cylindrical section at junction lines forming equal angles with a;plane perpendicular to the axis of the cylinder and attached at their'other ends to supporting means on junction lines forming equal, but smaller-than the firstnamed, angles with the said-plane. i

' 17. Vibration-translating apparatus comprising a stylus, a stylus support comprisinga conical shell having an integral cylindrical section at its base, inductor means attached to the cylindrical section, means for establishinga magnetic field transverse to the inductor means, and elastic-v supporting means comprising two metallic ribbons attached to the cylindrical-section at junction-lines forming angles of, about 25 with a plane perpendicular to the axis of the'cylinder and attached at their other ends to supporting of about motion.

19. Phonograph apparatus comprising a stylus adapted to'be disposed in contact with the sound track of a record, a support for supporting the stylus in contact with the sound track, the support havinga rear portion fixed at one end and a movable portion carried at the other end, and means for mounting the stylus at the free end of the movable portion onlyto, constitute the movable portion the sole support for the stylus, the

vibratory axis established by the support being disposed substantially at the center of oscillation conjugate to the point of contact between the stylus and the sound track.

20. Phonograph apparatus comprising a stylus adapted to be disposed in contact with'the sound track of a record, asupport for supporting the stylus in contact with the sound track, the support having a rear portion fixed atone end and a movableportion carried atthe other end, and means for mounting the stylus at the free end oi the movable portion only to constitute the movable portion the sole support for the stylus, the

movable portion being shaped to prevent substantial mechanical deformation thereof in response 1 to movement of the stylus along the sound track, and the rear portion being flexible in order that it may flex mechanically in response to movement of the stylus along the sound track, thereby to cause the movable portion to move substantially as a wholein response to-movement of the stylus along the sound track, the vibratory axis established by the upport being disposed substantially at the center of oscillation conjugate to the point of'contact between the styl ,s and the sound track.

21 Phonograph apparatus comprising a stylus adapted to be disposed in contact with the sound track of a record, means for creating a magnetic field, a support for supporting the stylus incontact with the sound track, the support having a rear portionfixed at one end and a movable inductor portion carried at the other end, and

means for supporting the stylus at the free end of the inductor portiononly to constitute, the inductor portion the sole support for the stylus, the, inductor portion being disposed in the magnetic field in order that electric currents may flow therein in response to movement of the stylus along the sound track, the inductor portion being shaped to prevent substantial mechanical deformation thereof in responseto -movement of the stylus along the sound track,v and the rear portion being flexible in order that it may flex mechanically in response to movement of the stylus along the sound track, thereby to cause the inductor portion to move substantially as a whole in response to movement of the stylus along the sound track, the vibratory axis established by the support being disposed substantially at the center of' oscillation conjugate'to the point of contact between the stylus and the sound track.

22. Phonograph-reproducing' apparatus comprising a stylus, vibratory supporting means for the stylus, and electrical means for deriving an electrical voltage from the vibrations of the supporting means, the supporting means being disposed to provide great flexibility for lateral motion of thestylus, whereby the recorded sounds may be faithfully; reproduced in the electrical output, and the supporting means being disposed so as to provide, great flexibility for vertical motion of the stylus without producing a corresponding electrical output, thereby removing the prin.

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