US3171904A - Electrodynamic loud-speakers and microphones - Google Patents

Description

March 2, 1965 G. R. PouToT 3,171,904

ELECTRODYNAMIC LOUD-SPEAKERS AND MmRoPHoNEs /oal zd E E- EMM/4 5 F 1E.. E 1d ad 9d /Od 2d dw ad 1 EY INVENTOR GEORGES @055x27- Pol/TOT United States Patent O 3,171,904 ELECTRODYNAMEC LUD-SPEAKERS AND MICRPHONES Georges Robert Pontet, 5 Rue Maillard, Paris Xl, France Filed June 19, 1961, Ser. No. 117,987 Ciaims priority, application France, June 23, 1960, 830,847 1 Claim. (Cl. 179-1155) My invention relates to improvements in electrodynamic loud-speakers and microphones, It is a well known fact that, as concerns its fundamental components, the structure of a loud-speaker is the same as that of a microphone, the two types of apparatus differing chiefly by the direction of operation: the loudspeaker produces vibrations of the atmosphere under the action of the electric oscillations to which it is subjected whereas the microphone operates in a reverse manner and produces electric oscillations under the action of vibrations received from the outer atmosphere. Apart from certain details in structure which are well known per se for any one skilled in the art, it is possible to consider the microphone as the reverse of the loud-speaker and a large number of technical arrangements which are applicable to one type of apparatus are also applicable to the other for reversed operation. For this reason and with a view to greater clarity, I will no longer mention microphones as such in the present specification while the expression loud-speakers is intended to convey also the meaning microphones so that the referencel to microphones should be implicitly understood after each expression loudspeakerf It is also a well-known fact that a loud-speaker includes two chief components of which one is transducer, that is an arrangement with transforms electric oscilla* tions into mechanical vibrations, while the other component is constituted by a diaphragm which transmits and diifuses the mechanical oscillations passing out of the transducer to the outer atmosphere.

It is a well-known fact that various types of loudspeakers are known in the art, and electrodynamic loudspeakers are those wherein the transducer is electrodynamic, that is, it resorts to the effects of a magnetic field on a current, with a view to transforming oscillatory electric energy into vibratory mechanical energy or reversely in the case of microphones, in accordance with the well known law of Laplace.

The transducer forms furthermore the driving means for the loud-speaker and its effects are transmitted to the atmosphere through the agency of the diaphragm. The problems to be solved by those skilled in the art for such a transducer or driving means have, generally speaking, been already solved in a comparatively satisfactory manner. The case is not the same for the problems relating to the diaphragm and chiefly to the manner of shifting the driving means with a view to obtaining through the agency of the diaphragm, a shifting of the largest amount of air possible, with a maximum delity. Generally speaking, the diaphragm has been hitherto constituted by a cone of paper or the like material, the area of which adjacent the apex is made rigid with an electric winding forming part of the transducer, while the surface of said cone has a remaining area which is generally equal to one or more square decimeters, so as to provide for the shifting of a sufficient amount of ambient air.

The operation of such a system would be theoretically accurate if all the points of the cone were to move in unison with 'the movable section of the driving means. But, in practice, the case is different, chiefly for the high frequencies of the acoustic spectrum; this is ascrib- ICC able to numerous reasons which it is not necessary to disclose in the present specification, since anyone skilled in the art is well acquainted therewith.

My invention has chietiy for its object, to provide loud-speakers or microphones which satisfy better than those employed heretofore the various requirements appearing in practice, and chiefly, they include diaphragms which are more faithful and/or more efficient, or are more advantageous than the known diaphragms.

My invention has for its object to provide a loudspeaker or microphone of an electrodynamic type, which includes at least one magnet generating a magnetic field acting on at least one conductive part such as a wire or ribbon which is rigidly secured to the actual diaphragm ensuring a coupling between said wire or ribbon on the one hand, and the surrounding air on the other hand. Said diaphragm is made of a non-conductive material the thickness of which is sufficient for eliminating practically any elasticity within the range of accelerations and/ or stresses appearing in the loud-speaker or microphone, said material having furthermore a specific weight which is under 0.1 and even, preferably under 0.02.

According to my invention, such a loud-speaker is advantageously characterized by the fact that said conductive wire or ribbon is laid on the so-called rear surface of the diaphragm which surface faces the magnet or magnets, while at least 50%, and even preferably at least of the length of said ribbon, wire or the like conductive part, is inserted in the actual gap of Said magnet or magnets.

The material used for the diaphragm and referred to hereinabove is advantageously a synthetic resin, preferably in an expanded condition which bestows it with an extremely low specific weight and more particularly the material preferably is expanded polystyrene.

ln a preferred embodiment, the diaphragm of such a loud-speaker or microphone has a large front surface with reference to its thickness, while the major part of its rear surface is rigid with the movable elements of the transducer or driving means of the loud-speaker or the like, said movable elements being Substantially linear. The thickness of the diaphragm is preferably between 3 and l2 millimeters.

On the other hand, the material forming the diaphragm and the thickness of the latter (excluding any possible projecting section carry-ing the conductive wire, ribbon or the like part), are advantageously such that any force applied to the rear surface of said diaphragm excluding such possibly projecting sections may, within the range of accelerations and/or stresses appearing normally in loud-speakers and microphones, be transmitted without any substantial deformation to any point of the front surface of the diaphragm which extends within an angle of less than 50 to either side of the perpendicular to said rear surface, passing through the point of application of the force to be considered.

According to a particular embodiment of the invention which seems to be of special advantage, the coupling is performed, by means of a conductive ribbon or wire, which is given longitudinally the shape of a curve rep resenting rectangular relaxation oscillations, which curve is termed hereinafter a Greek pattern. Said ribbon is rigidly secured to the diaphragm. The front surface of the latter may assume any desired outline, but preferably it has a rectangular outline, the length and breadth of which are several times the thickness of said diaphragm. The various sections of the Greek pattern are however energized all simultaneously by a number of magnets, the poles of which extend over a line the length of which is greater than their spacing. Said conductive ribbon or Wire is carried on the rear surface of the diaphragm, as

Hof FIG.V l1,

1- My invention will beoreadily understoodupona reading of the following disclosure relating to V,embodiments illustrated in theaccompanying drawing,.which,includes diagrammatic views of said embodiments and wherein FIGS. 1 and 2 are sectional Views, respectively through line IV-I ofFIG. 2 and .lijneII-ll ofFIG. 1 of a loudspeaker in accordance with a preferred embodiment of my invention,.

^ Y FIG. 3 .is a partial detail View on enlarged scale' of the loudspeaker-illustrated Yin FIGS.' 1 and `2, Y

FIGS. 4 and 5 are respectively a transverse sectional View and a lateraly elevational View of the chief components of a loud-speaker wherein the coupling is performed by means of a rectilinear ribbon,

FIGS. 6 and 7 aredetail views atrightangleswith each Aother and showing. means for connecting the conductive ribbon :of FIGS. 4 and 5,

i FIG..S is a view. similar toFIG. 4 for the-.casefwhere the coupling is obtained through two movable parallel ribbons,

FIGS.. 9.and 10 .are two partial'cross-sectional views at right angles with refe-rence to each other, respectively taken. along lines IX-IX of FIG..8 and lines X--X ofi FIG. 8, y FIG. .l1 is a plany view from underneath of a coupling system having a. large area which is started vibrating by a conductive ribbon in the shape of afGreek pattern,

FIG. 12 is a corresponding view along line XII- XII FIGS. 13 and 14 arepartial views respectively in transverse cross-section` and viewed through the rear surface of the diaphragm -of the main components of a loudspeaker wherein the coupling is obtained through a plurality of windings.

.It should be .understood that these figures andthe cor- .responding parts of the disclosure are Agiven solely by way of example, and by no means in a limiting sense.

In the various gures, the following elements o ,f the embodiments illustrated, which play the. same .part,. have received `the same reference numbers followed by an alphabetic. index-which differs from one figure to another. Thus,

The reference number 1 designateslmy improved dia- Phragm, v

The ,reference numberv Z'Mdeslignates Vthe ,movable con- 4ductive section of the drivingmeans or transducer, which movablesection is rigidlysecured to said diaphagml, Thereference numbers?? and designate the poles or fractionsl of a pole of the transducer-energizing magnet or magnets which have the'same polarity,

f VThe reference number 4 designates the poleof such ,a magnet, the polarity of which is opposed to that of thepoles 3, and 5,

. The reference numbers 6, '7, 8 and 9 designate the sides of the transverse cross-section ofthe diaphragm,

6 designating more particularly the so-called front side, .7 the rear side which may be omitted, 8 and 9 the lateral surfaces extending between the front surface and themovable section of the driving means,

The reference numbers andll designate the parts (such as a ribbon, wire or winding, and the like said embodiment is constituted Aby a plate or sheetiof expanded polystyrene, the specific weight of which is 0.013

4 to 0.015 gram per cubic centimetenandthesize of which is 81 mm. along its breadth l, while its length L is 120 mm. and its thickness e1 is equal to 6 mm. Inside said sheet or plate la, there .are provided (for instance by a set of cutters) sixteen parallel'grooves or slots 14, the depth of which is equal to 4 mm, and the breadth o -f which l1, is equal to 4 mm. Said` grooves Iarethus separated by projections or webs 15,thebreadth I3 of said Webs being equal to l The front surface of the coupling means thus formed is shown at 6u while its rear surface 7a includes the webs 15. Over the outer'free edges of said webs 15, there is positioned and secured a conductive wire 2a, the cross-sectionof which may be round or flat. Said vwire which is advantageously of aluminium is inthe case considered, a round wire of a diameter of about 0.5 mm. Preferably, the Wire 2a is previously subjected to the action of a template which gives itthe shape of a Greek pattern (as shown at 2g, 2h, 2z' etc. in FIG. 11) while one of the surfaces of said, Greek pattern, and possibly also the free edge of the webs are coated witha suitable glue, so that the positioning and the securing of the continuous wire 2a over thesuccessivewebslS, may be obtained accurately and rationally. This latter gluing operation is performed `advantageously by means of the abovementioned template. Whenuthe glue has set, the template is removed, and the uends of the wire 2a are connected with yielding conductive braids 16 or the like adapted to provide the desired electric connections vof the wire.A A A At the. two ends of the diaphragm 1a, (FIG. 1) considered longitudinally, thereare assembled, for instance A through gluing, two tongues 12a, 13a. which provide for the suspension of the diaphragm 1a with reference to the frame 18a-18b-2j5af25c of thearrangement. This enlsures an accurate positioning ofthe diaphragm, whereby its teeth or webs may engage freely the magnetic gaps lformed in tl 1e transducer; said gaps are provided in the mannerl disclosed hereinafter, in a manner such that said webs4 may move inside said gaps and execute a rectilinear translational movement (in the vertical direction appearving in,FIG. 2), whereby they carry along with them the as more particularly favorable, the use of polyvinyl chlorideor Polyurethane, preferably in `an expanded condition, and the sizes of which are in the example considered, alengthof 81 mm., a breadth l2 of 8 mm. and a thickness of 2 mm. These tongues connect, in the present case,

' the diaphragm 1 z withtwo metal members 19a, 1917 having a square shaped cross-section. Said members 19 may be disconnected and adjusted with reference to the Aframe 18a 18 tg, 25a, 25b of the arrangement. Insulated 1 conductive terminals 20a, 20b, 'pass through saidmembers Y, 19 and ensure the feed of current to the conductive braids The magnetic circuit of the transducer or driving means includes in the embodiment described magnetic slots 21,

vtherlumber of whichis equal to that of the webs 15, and

consequently, in-the present caseth 'ei r number'is equal to seventeen. Each of said slots is provided with a useful gap of which the sizes are: breadth I4 (FIG. 2) 2 mm.-

. height h (FIG. 2) 4.5mm., length v(FIG. 1) 1l8 mm. The field in each of said gaps is equal to about 2,000 gauss,

and is obtained as follows:

v Between two blades 22 and 23 of a highly magnetic i metal sheet,.of a thickness of 0.5 mm., of a length of 118 plate projects by 1 mm'. beyon'd'the small blades'ZZ fand 23 .along three of the sides of"the"l at'ter, whereas "on the fourth side, that is, at the upper side in FIG. 2, there is provided between the successive blades 22 and 23 a useful gap forming a sort of slot 21. Finally, the blades 22 and 23 form through their upper edges the poles 3 and 5 of the energizing magnets.

As far as the details of execution of the magnetic section of the arrangement are concerned, it seems of advantage to proceed as follows: firstly seventeen members which are identical with that which has just been described are executed, each including two small blades 22 and 23 and a small ferrite plate 24. These seventeen members are then assembled with a spacing between them of 2 mm. which allows the free flowing of air behind the diaphragm that is, at the lower ends of FIGS. 1 and 2 by means of two comb-shaped structures 25a and 25b advantageously of aluminum including the desired corrugations or utings, for engagement over the outwardly projecting ends of the different ferrite plates.

These two combs 25a and 25h serve as stays for the longitudinal angle bars 18a, 18h (FIG. 2) which form with said combs 25a and 2512 the above-referred to frame of the loud-speaker. The whole arrangement is designed furthermore in a manner such that the webs of the diaphragm are inserted each substantially at mid-height into the corresponding gap or slot 21 and thereby the wire 2a is adapted to move in a magnetic field which is unvarying during the movements of the diaphragm.

As already mentioned, the above described embodiment illustrated in FIGS. l to 3, may be subjected within the scope of the accompanying claims to various modifications, of which a number are disclosed hereinafter.

According to a rst modication which is very advantageous in many cases, instead of the webs 15 carrying a conductive element constituted by a wire, there is provided a conductive element constituted by a metal structure, also in the shape of a Greek pattern cut in a thin plate, preferably of aluminium and glued atwise over the free edges of said webs, as clearly shown in FIG.3. This may lead to an easier or more convenient machining, taking into account the desired accuracy to be obtained. It may furthermore be preferable to give the breadth I3 of the corresponding webs of the diaphragm a value generally larger than 1 mm., say about 2 mm.

According to a further modification, instead of producing a winding which in practice extends in an unvarying direction between the two braids 16, there are formed, for instance by means of a template, windings including a plurality of forwardly and rearwardly directed sections of a conductive wire which is then generally of a smaller diameter than the wire 2a. This allows obtaining driving or transducing members of which the impedance may be higher than in the embodiment illustrated in FIGS. 1 and 2.

According to a still further modification, there are produced in a similar manner, two or more windings which are mechanically associated but are electrically insulated from each other, for instance with a View to tapping oit a counter-reaction voltage adapted to control the transducer and the associated amplifier. Such a counter-reaction system would be of no interest in the case of conventional loud-speakers because the movements of the cone provided in the latter have no accurate relationship with the movements of the driving windings. The case is quite diiferent in the case of a loud-speaker according to my invention, wherein the movements of the diaphragm are exactly those of the conductive section of the driving means and wherein consequently the interest of such a counter-reaction system may be considerable.

With the various embodiments of my improved loudspeaker or microphone, the following further features are advantageously incorporated:

The front surface 6a of the diaphragm 1a is smooth in the manner of a plate, and its surface sizes, that is its length and its breadth when said surface is rectangular,

range between eight times and fifty times the thickness e1 of said diaphragm including its webs such as 15.

The actual breadth I3 of each web 15 ranges between 20% and 100% of said thickness e1 and the depth of each web measured in the same direction as e1 ranges between 30% and 70% of e1.

'The thickness of the diaphragm 1a without the webs, is such that the forces applied to its rear surface by the webs 15 starting from the conductive wire or ribbon may be transmitted without any substantial deformation to the correspondent point of the front surface 6a of said diaphragm, any point of said front surface being located through the actual structure of the diaphragm and its webs, within a virtual angle of less than 50 to either side of a line perpendicular to the rear surface, and passing through the point of application of the force to be considered. This arrangement is disclosed with further detail hereinafter, reference being made to FIG. 8.

The point of application of the force is to be understood as the point at the root of a web, more particularly along its outer surface as shown at A or B in FIG. 3. It will be appreciated from a consideration of the embodiment of FIGS. 1-3 that since the thickness of the diaphragm from the front to the back surface is 2 mm., and the clear spacing between the web is 4 mm. (these dimensions will be documented more particularly hereinafter) that an angle drawn from A or B to a point on the front surface of the diaphragm midway between the webs will be inclined at an angle of 45 whereby every point on the front surface of the diaphragm will lie on the base of an isoceles triangle having base angles of 45 and a vertex located at points A or B.

The thickness of the diaphragm in FIGS. 1-3 is equal to e1 minus the depth of the webs which as shown on page 5 is equal to 6 mm.-4 mm. or 2 mm. The spacing between the webs is specified at page 5 as being l1 which is 4 mm.

FIGS. 4 and 5 relate to a `further embodiment of my improved loud-speaker. The coupling is obtained by means of a rectilinear conductive ribbon 2d of which the plane registers with the medial plane of the gap between the magnet poles 3d and 4d inside which gap the ribbon is engaged. Said ribbon may be given any allowable desired length, for instance l0 to 30 cm. or more. It is therefore inserted through one of its edges inside the gap of a corresponding magnet 3d, 4d. Near its other edge, it engages in a suitable manner the apex of a diaphragm 1d according to my invention, having a cross-section, the shape of which approximates that of a triangle with a front surface shown at 6d, while its sides 8d and 9d converge towards the above-mentioned apex, thus forming advantageously an angle t of about The feed connections for the ribbon 2d thus held in the diaphragm are shown at 11d and the tongues for the suspension of the coupling 1d are shown at 12d. FIGS. 6 and 7 show in full detail an advantageous embodiment of the connections lttd, which play the same part as 11d. Said connections 10d are formed by an undulating section 2d of the loud-speaker driving means in a plane perpendicular to that of FIG. 7, together with the feed of current into the conductive ribbon 2d.

As to the suspension tongues 12d, they are advantageously made of a yielding and light material, as in the case of the tongues 12a and 13a illustrated in FIGS. l and 2. Said tongues connect the diaphragm with the frame of the loud-speaker, or else with the case or baiiie board of the latter if any, which is however no longer necessary in the case of loud-speakers according to my invention.

In all cases, the suspension tongues for the diaphragm of a loud-speaker or a microphone according to my invention should have desired yieldingness and/ or elasticity so that the movements of said diaphragm may constitute translational movements parallel with the corresponding planes of symmetry of the gaps in the magnets.

. sponding thereto.

electro dynamiccoupling is performed by means of'a Wire or ribbon'whichv is wound or formed into a'rectangular or substantially rectangular shape, of which the longer sides 2e andaZf are several times longer than the'breadth of the rectangle or substantially rectangular iigure corre- These two larger sides 2e and 2f are inserted respectively in' one of the two gaps extending between the poles, 4e on the one hand, and 3e,'5e of a magnet on the other hand. The remainder of the embodiment is substantially the same as in the case of FIGS. 4 and 5, except as concerns the transverse cross-'section'of the diaphragm le; said cross-section is in the present case substantially that of an isosceles trapezium of which the smaller base forms a section of the rear surface of the diaphragm. Near the ends of said smaller base, are fitted the sections 2e and '2f of the operative conductive ribbon, which is fed by two connections e made of a further pleated ribbon, of which the mean plane is perpendicular to the plane of the rectangular structure e and 2f. Considering a predetermined length of the diaphragm 1d or 1e (measured perpendicularly to the plane of FIGURES 4 and 8), the front surface of the diaphragm 1e lof FIG. 8 may have an area which is substantially double that of the diaphragm 1d of FIG. 4, said diaphragm remaining furthermore perfectly rigid and faithful, taking into account the fact that the geometrical conditions'as to the angles are observed in this case infa manner similar-to that which has been disclosed for asimilar purpose, in the case of the embodiment of FIGS. 1 to '3. VThis may be explained in a clearer manner as follows (FIG. 8): the

vforces being applied to lthe points'A and B of the rear surface '7e of the diaphragm, by the conductive ribbons'ze and 2f, any point P whatever on Vthe front surface 6e of the diaphragm lies within a vertical'geometrical-angle (which therefore does not correspond to a material' stru'cture inside the diaphragm), whichangle' is'equal at a maximum to "50 to either side of the perpendiculars AB or DEto said rear surface passing throughsaidpoints A and D. In the case'of a point P (FIG. 8),'it`is'apparent that the latter lies bothin the angle `BAC=aVY andsinside the angle EDF =d, these two^angles 'being equal to less lthan 50".r Experiencehas shown that Ysuchl ageometrical condition relating to all the points of the -frontsurface of the diaphragm with reference to Athel points ofapplication of forces on the rear surface of the' latter corresponds to an absence of any substantial deformation of thediaphragm which 'appears then as quite rigid and incapable of moving without any deformation or phasefshift-ing withl reference to its rear surface. This cooperates consequently inthe obtention of high'A grade results bymy improved' loud-speaker or microphone, Ysaid high grade being modifications proposed for the latter figures. Eachof the long sides of this Greek patternis :inserted insidefthe corresponding gap of a magnet whichis' not illustrated, in a manner similar to what has been disclosedv with-referenice to FIG. 8.

Lastly, FIGS. 13 and 14v relate to a loudspeaker-according to the invention, wherein the coupling is-obtained by means of a number of circular windings. In FIG. 13, are shown in particular two of said windings,2m and 2n,

positioned in the circular gaps oftwo Acorresponding mag- G0' The control of said Adiaphragm 1b`is obofqsaid windings-2m, 2n, 2p, 2q are shown.

u nets 3m,\4m, 5m, and Sn, 411, and Sn. In FIG. 14, four Preferably the diameters of said windings (which are equal tosay 2 cm.) are substantially equal to one half of the spacing of their axes. The diaphragm is shown atlm. It may have a large area as inthe case of FIGS. 1 and 2; the

linear sizies of said area depends merely on the number of windings suchA as 2m, Zn to be positioned on the rear surface 7m of vsaid diaphragm, in a manner such that the latter may move rigidly asa unit and without any, parasitic or distorting elfects as already mentioned hereinabove.

ThusLwhatever may be the embodiment adopted, my invention provides electrodynamic loud-speakers and microphones, executed in a novel manner and provided with substantial advantages among which I wish to mention more particularly the very large useful areas provided by the front surface of the diaphragm 1 which I obtain, in association with a faithfulness in the reproduction and/ or in the coupling, which is far superior to what was usually obtainable hitherto; this invention also provides a very simple and sturdy structure of a comparativelyv low cost price, considering the high grade of operation and advantageous features reached.

As already mentioned, my invention is by no means limited to those'applications or embodiments of its different parts, which have been more particularly disclosed and it covers, in contradistinction, all the modifications thereof, falling within the scope of the accompanying claim, and for-instance, in case ofan embodiment similar to that of FIG. 11, the openy rectangular sections of the ribbon 2g, 2h, 2i need not be all interconnected, the successive longer'sides of said rectangles beingV in fact, if

desired, all independent of one another, and also it is possible to'resort, as required, to sizes or numerical'data which are different from those mentioned with reference to the examples disclosed chiefly in -the case of the examples illustrated in FIGS. 1 to 3.

In Vthe accompanying claim, the expression"electrodynamic apparatusfshould Ybe construed as including elec- "trodynamic loud-speakers yand electrodynamic microphones.

What I claim-is: i Sound reproducing'apparatus ofthe electrodynamic .type comprising a diaphragm of expanded synthetic -resin having a specic weight less than 0.1 gram per cubic centimeter, said diaphragm having a front and rear surface which are substantially parallel to one another, said front surface being aflat planar surface, said diaphragm including lintegral equally spaced rectangular webs extending rearwardly` from the rear surface of the diaphragm and'dening a series of equal spaces between adjacent webs,sa1d webs beingspaced apart a distance such that the spaces Vtherebetween are substantially equal tov the distance which each web projects from the rear surface of the diaphragm'whereby said spaces between adjacent webs have a square profile, a continuous conductive element secured to said webs at a location remote from the rear surface of`the"diaphragm, means elastically supportingthe diaphragm topermit'rectilinear translational `movement thereof and"fixedly supported magnetic means 'including a plurality of magnetic elements equal in number to theiwebs'and having'magnetic gaps in which areV accommodatedrespective'webs, said lmagnetic means and conductive yelement constituting a magnetic circuit in `rwhich variations in the magneticfield of the magnetic elements produces cor-responding 'displacement -of' the con- Vductive'element aswell as ythe webs and diaphragm secured thereto,-said diaphragm having a distance between the front and-rearsurface thereof which isrelated to the size of fthe spaces between'adjacent webs such that every point onthe front ysurface ofthe diaphragm lies on the base of at least one isoceles` triangle having a Vertex at the intersection of the rear surface of the diaphragm with Vthe edges of the webs and including a vertex angleof less ythan whereby the diaphragm will undergo sub- 9 stantially rectilinear translational movement as a result of variations in the magnetic lield.

References Cited by the Examiner UNITED STATES PATENTS 1,604,532 10/26 Riegger 179-1155 1,749,635 3/30 Gerlach 179-181 1,987,412 1/35 Neumann 179-1155 2,297,218 9/42 Henrich et al. 179-181 2,978,669 4/61 Harris 179-1155 kt\\ 2,978,671 4/61 Harris 179-1155 3,111,187 11/63 Barlow 179-181 FOREIGN PATENTS Germany. Germany. Germany. Great Britain. Great Britain.

OTHER REFERENCES Rigidity of Loudspeaker Diaphragms, Barlow, Decem- 10 ber 1958, Wireless World, vol. 64, No. 12, pages 564-569.

ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner.

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