US2974204A - Transducer - Google Patents

Description

March 7, 1961 M. c. sUPlTILov 2,974,204

TRANSDUCER Filed July 6, 1954 2 Sheets-Sheet 1 IN V EN TOR.

March 7, 1961 M. c. sUPmLov 2,974,204

TRANSDUCER Filed July 6, 1954 2 Sheets-Sheet 2 6A. ffy@ .f 6 c. l 6G.

INVZ'OR. Z'CBYM/'z/V )PM am United Svtates Patent "ice TRANSDUCER are Filed `luly 6, 1954, Ser. No. 441,335

-19 Claims. (Cl. 179-1155) `This invention relates to a transducer and particularly to an electrodynamic type of microphone and/or speaker. While the invention is adaptable for a wide variety of size s Iof microphone and/ or speaker, the invention to be disclosed is particularly useful for combination microphone-speaker constructions of smal-'l size used in connection with dictating machines, portable sound recording and reproducing machines and other devices where a microphone having the ability to function effectively as a small speaker is to be used. l

While there are many types of transducers which may be used in conversion of energy between sound and electricity, the electrodynamic type of transducer has many outstanding advantages. Such a transducer has a relatively high output, is rugged, has a long useful life, may be designed to have any desired impedance within a substantial range, and may have good frequency response.

As is well known, such a type of transducer hasl an annul-ar air gap in which a voice coil operates. From an acoustic angle, the combination of a moving voice coil within a narrow annular air gap is complicated as far as the desired range of voice frequencies is concerned. To have uniform sensitivity, it is essential that a moving coil and diaphragm should generate in the voice coil substantially equal values of potential for unit pressures of sound waves on the diaphragm throughout the entire operating range of the transducer. This involves complicated compensating acoustic elements such as resonant cavities and acoustic shunts for the purpose of correcting undesired frequency response characteristics. Such acoustic compensating elements frequently require the housing forthe transducer assembly as an acoustic element. This means, therefore, that an acoustic design for a particular transducer construction fixes the housing dimensions and materials and makes it diicult to use the same transducer assembly in a different housing. l In transducer constructions where the diaphragm has a diameter up to about 3", desirable frequency response presents. serious design problems. In a speaker, two import-ant considerations present themselves. One is a frequency response which, of course, must be desirable and in some respects may have to be the converse of vthe frequency response of the microphone. The other consideration'involves efficiency of energy conversion and relates to the fact that the speaker operates at a relatively high power level as compared to a microphone. Thusy a small transducer may be overloaded when used as a speaker for suicient volume to function as such and results in rattles and undesirable speaker performance.

As far as the microphone alone is concerned, it has been widespread practice to provide ducts for air passages from the interior of the casing to atmosphere and also air shunts across the diaphragm of the microphone in order to improve the frequencyA response of the microphone. Such a recourse makes it impossible to transfer the microphone construction from one housing to a different type of housing without a redesign of the passageways, these passageways functioning as acoustic shunts;

2,974,204 nPatented Mar. 7, 1961` In accordance with this invention, a construction is provided wherein the acoustic shunt action is concentrated in the transducer assembly proper, independently of a transducer housing. A transducer embodying the present invention may be readily used with various kinds of housings and the acoustic performance thereof accurately controlled. A construction embodying the present invention is characterized by ruggedness and sim'- plicity making for quick assembly and ready adaptability to various kinds and sizes of housings.

To a substantial degree, the frequency response cha-racteristics of a transducer embodying the present invention will remain constant in spite of changes of housing. By controlling the diaphragm of the transducer, a definite change in frequency response may be effected in the unit. This control of frequency response is particularly desirable in devices such as dictating machines. In such machines, generally sharp lfrequency cut-offs below about cycles per second and above about 4,000 cycles per second, or up to as high as 6,000 cycles per second are desired, depending upon acoustic considerations. The low frequency cut-off is desired so that 60-cycle and 1Z0-cycle hum will not be readily picked up in the dictating system with-which the transducer is used. The top frequency range'is dictated by various considerations, some preferring a cut-off at about 4,000 cycles per second while others prefer a higher frequency cut-oh to 6,000 cycles per second or even higher. Within this frequency range, however, control over the response characteristics is necessary without at the same time tying a transducer construction to a particular type of housing. Thus in some instancesv a flat response may be desired andin other instances a responserhaving a dip or peak may be required, depending upon the overall characteristics 'of the entire dictating system.

Another feature of the present invention is a novel diaphragm useful for transducers of the general type and size referred to previously and a method of making the same. While a diaphragm made in accordance with this invention and embodying the present invention may be used in any size desired, its greatest field of application resides in connection with Itransducers of Ithe hand type which are ordinarily used as combined microphones and speakers.

Y In order `that the invention may be fully understood, reference will now be made to the drawings wherein an exemplary embodiment of a construction embodying the invention is disclosed and wherein an exemplary method of making the diaphragm is also disclosed, it being understood that variations may be made both in the construction and in the method without departing from the scope of the invention except as defined in the appended claims.

Figure 1 is a sectional elevation of a transducer embodying the present invention.

Figure 2 is an exploded view of the various parts making up the transducer of. Figure 1.

Figure 3 is a bottom view on a reduced scale along line 3-3 of Figure 1.

Figure 4 is an elevation on an enlarged scale of the diaphragm and voice coil assembly, the view being taken along broken 1ine-4--4 of the diaphragm of Figure 2.

Figure 5 is `a sectional elevation of a die arrangement with a diaphragm disposed therein Afor processing in accordance with the invention. Figures 6A to' 6H inclusive show some transducer characteristic curves with various kinds of diaphragms.

The transducer assembly includes support cup 10 of ferromagnetic material. Cup 10 has body portion 11 and annular ange supporting portion 12. Disposed within body 11 is annular positioning member 13 of nonmagnetic material and either of metal or non-metal. For example, positioning member 13.1may be of brass. Po-

sitioning member 13 has a smoothA cylindrical inner surface and has two externally extending shoulders 14 and 15 and external annular bands 16, 17 and 18. Positioning member 13 has top edge 19. Band 18: haallllter diameter large enough to provide a pressftitpwithintbodyf 11. Slidingly disposed within the bore-oty the,V nommag-y netie` positioning memberl is cylindlic'al-v permmnt; mag.

net having magnet pole. faces 21 and; 2,2` atthe two.` ends thereof. Permanent magnet 2t)v isvof;` suitable, terial, such as Alnico V` or other suitable material.r

Permanent magnet- 20 hasy an axial length which is greater than the axial length of positioning member 1,3., When permanent magnet20 is disposed withintthebore; of positioning member 1,3 and; the, p0, ning; men 'er pressed intoy body 11, one pole ofy the, magnet may,l conf. tact the bottom of the cuptbody Whilepthef other polo or the` magnetI can extend above the level'v of free; edge, 19 of the non-magnetic positioning membep. It` is;l understood, of course, that the two end faces ofi' the permanent magnet are ground to a smooth nish.

Ferromagnetic pole piece is provided forA covering and contacting the free pole face yof thepermanentymgnet. Pole piece 25 has annular portion 26 extending around band portion 16, of the nonanagnetic positioning member. The outside` `diameter of polepiece,` 25kV is` smaller than the inside diameter of body portionyll of the support cup to form annular air gap 27. Pole, piece 25l has its outer surface 28 curved conyexlyj asv shown, central portion 29 being concave. Pole; piece 25 makes, intimate contact with the free face of the` permanent magnet.

At this point, it might. be observed that. pole piece25 preferably has a tight press fit over bandz; 176;` of positioning member 13;` Magnet 20, however, canzcasily:lit-,with in member 13` and thus .variations in magnet sizewillbe accommodated; When pole piece 25 is fitted over p0- sitioning member 15,` the. partsl may be,` assembledV by forcing member 13 into body portion `11. Positioning member 13 will be forced into body portion 11 deep enough so that the permanent magnet is caught between the cup bottom and the inside surface of thetpole piece. The assembly is thus tightly retained in such relation.

Resting upon annular flange portion12is a diaphragm generally indicated by. 33. Thewdiaphragm consists of annular support portion 34, roll 34a, voice.. coil bobbin support part 34b, and domel 36. The fabricated diaphragm has cemented thereto at 341) voice coil, bobbin 35. Voice coil bobbin 35 carries. voice coil 37 of conventional construction, insulated leads 38 of the voice coil being attached to annular support portion 34 andextending out throughr suitable apertures 39E*Y in the cup ange for connection vto the outside.V Voice coilbobbin 35and voice coil 37. operate in annular. air gap 27; The diaphragm may be made of suitable-material, such 'as cotton, silk, nylon, plastic, or any.' other material as, paper. When a porous` material is used for the diaphragm, it maybe impregnated with a` suitable thermosetting plastic compound, such as forexample Bakelite resin. The resin functions to stiffen the diaphragm, waterproof the same, close the pores and strengthen thematerial. The diaphragm treatment `will be givenv later. The voice co-il bobbin may be of porous or non-porous material. The voice coil and `cemented region at 34h render the bobbinnon-porous.

The diaphragm is mounted so that annular support portion 34 lies on flangel part 12 of the cup. Gasket 40 is disposed at the rim of the, diaphragm and within ferrule 41 of the ange. Thereafter, conventional fine and coarse screens. 42` and 43 are disposed in position for completing the construction. Coarse screen 43 has its edge curled under the flange. Coarse screen 43 is strong enough so that the microphone is protected against damage While line screen 42 keeps `dirt and dust out.

In accordance with one aspect of this invention, the diaphragmisdivided into two.separate.areasvwhioh` are respectively porous or open tor permit air to pass through., and non-porous or closed to air. Where a diaphragm is made of a material such as, for example, Woven material which is normally porous, it is possible to utilize one feature of the present invention and fabricate the diaphragm to obtain the desired porous and non-porous diaphragm parts. Thus the woven material may be dipped -into or have a. thin solution of a thermosetting plastic resin applied thereto. The fabric may have a thickness of several thousandths of anv inch, say about .004. The coated fabric is then subjected to hea-tand pressure between two complementarily shaped dies to provide the desired shape of diaphragm. This procedure so far -is old and widely used. Thus loudspeaker spiders are manufactured in this fashion. However, and in accordance with this invention, `the complementarily shaped dies have the clearances between the opposed parts abnormally large where the diaphragm material is to be porous. Thus with normal die clearances, the diaphragmA material may have its thickness .reduced to less than onehalf of the original thickness of the woven material atV .those regions Where the. diaphragm is to. be nonfporous;

Under the heat and pressure atthese regions, of normal die clearance, the plastic resin is forced into the pores of the woven material. Where the die clearance is abnormally large, the pressure will be below the value for compressing the woven material and squeezing the resin into the pores. Itis understood that the transition from the parts of the dies having normal clearance to the,` parts of the dies having abnormally large clearances may Ibe gradual if desired. This will result in a gradualChange4 of porosity of the diaphragm material from porous to non-porous.

It is clear that, there are various degrees of porosity. The advantage ofthe new diaphragm is that the nonporous part which may be considered, as`V the variable factor is restricted to an area which isy substantially lest than the area of the entire diaphragm. Hence variations in porosity of the porous part in a diaphragm embodying4 the present invention will have muchless elfect than the same variation in a diaphragm having itsentire area of the same unit porosity.

Inasmuch as part 34 of thediaphragm is clamped and` rests upon an imperforate metal surface, it will be Clear that whether part 34 is porous or non-porous is immaterial. For the purposes at hand, it is easier to fabricate the diaphragm so that roll 34a and annular part 34 are both porous. y

Referring for example to Figure 5 -anvil 45 and die 46 have suitable complemental shaped surfaces 47 and 48 for hot pressing diaphragm 33 to the required con-l tour. Voice coil support35 has not been cemented to the diaphragm as yet.` Anvil part 45 may have electric; heater 50 within the body, threaded inserty 51 permitting introduction of the heater within the anvil body.y Any other disposition ofv heaters. is4 possible, theL die construe tion being conventional except for the clearances betweenv the die, and anvil. I t is understood that die 46 is carried by a ram so that the diaphragm blank may be subjected to a hot press operation, i

In this instance it is desiredV to have dome 3 6 no-nporous and roll 34a porous. As will be evident, the line of demarcation between the porous and nonfporous parts of the diaphragm as a Whole may be placed at any de Sired part of the diaphragm. Asv indicated in Figure 5',

the, simple sectioning 52 indicates norm-a1 die clearance, while sectioning 53 indicates larger than normalA die clearance.

By normal die clearance isk meant a die, clearance` which willl force the impregnating resin. into the ports of the diaphragm material under operating conditions of pressure and temperature. Nov amount of die clearajIlCe can bey stated as normal since that; dependson die'pressure and diaphragm material. However, by havingl certain partsI of the, dielclearance. larger than non mal, then the porous diaphragm part'wll result. `As-a' rule, the larger than normal die clearance may vary. Thus the die clearance may be enlarged from about 25% to over 100%, depending upon die pressure used, nature-of the resin and temperature. The die clearance change need not necessarily be symmetrical around the diaphragm, although this will be the simplest and easiest construction of the dies.

Depending upon the nature of diaphragm stock, the nature of resin and desired die pressure andtemperature, aA large variety of diaphragm patterns of porous and non-pourous Iregions is possible. r In the diaphragm illustrated in the drawing, thev por-v ous part, for practical purposes, is confined to roll 34a. As-has been previously pointed out, ypart 34b, to which the voice coil bobbin is cemented, is rendered non-porous by virtue of the presence of the inner layer of material of which the bobbin is made. As a rule, voice coil bob bins' are o f kraft paper vor 'may be of plasticor the like. With dome 3 6 of the diaphragm non-porous and roll 34a porous, a path for air around the diaphragm will be establishedwhen the transducer is assembled as il lustrated in Figure 1. Thus referring to this figure, air from outside of dome 36 pass through the porous portion 34a into annular air gap 27, around the bottom edge of the voice coil bobbin and thence inside the voice coil bobbinto the inside surface of the diaphragm. Such anv air path contains resistance determinedby the dimensions Yo f the clearances on the outside and inside of the voice'coil and bobbin within the air gap. As a rule, etliciencyrequirements make it advisableto reduce the air gap to a minimum size. In practice, therefore, the shunt air path described above is effective principally for low frequencies. If desired, however, one or more fine apertures may be provided in the voice coil bobbin and support part 34b to provide additional Vshuntair paths. Thus for example, it is possible to. have diaphragm domev 36, now-shown as non-porous, porous over part or allof its area and roll 34a and annular part 34;; now shown as porous, non-porous in part or in whole. .It will thus be evident that a very large range of transducer operating characteristics may be obtained by having diaphragm dome 36 porous or non-porous, in whole or lin part, annular part 34a porous or non-porous, in whole or in part, and/or providing air relief ports through diaphragm part 34b and the underlying area of voice coil bobbin. In practice, however, a large range of transducer characteristics may be obtained by having the, diaphragm constructed with regard to porous or nonporous portions as illustrated in Ithe drawing and simply making the diaphragm material and the closeness of the weave or porosity thereof as the variable.

Where the diaphragml is of naturally non-porous ma- 'teriaLasplastie or metal, then ports as described above Will-be desirable.;l n l Y 'gThe curves shown in the drawing illustrate the variousitypes of responses which may be obtained by controlling the porosity of the diaphragm.. The various curves are plotted on semi-logarithmic paper wherein the Y axis shows response in decibels (db) and the X axisshows frequency arranged on a logarithmic scale. Inasmuch as decibel values involve a logarithmic ratio, it will be clear that fundamentally both the X and the Y values are logarithmically related.

Thus referring to curves 6A to 6H inclusive, some characteristics of dynamic transducers are illustrated. The curves show transducer output against frequency. The transducerv under test was placed in anY anechoic chamber `(having substantially zero sound reflection) "several `'feet away from a loudspeaker. Theloudspeaker waal supplied with currents at *frequencies covering'v a range.;fro'm"below about 100 cycle'sperfsecond to frequencie'sI beyond;l0,000 cyelesper second. The entire sound generating system;` for energizing .the .speaker :had

: 1-substantially smooth flat characteristic of acoustical energy generated over the frequency range.

In curve A the response may effectively start at around cycles per second and effectively eut off at about 10,000 cycles per second. In between these two extremes, it will be noted that there are a number of sharp, peaks and dips where the response drops to low values. By having the porosity of the diaphragm reduced, such as by making the weave of the diaphragm material liner, but still having the porosity uniform over the en tire diaphragm, the response may be improved as'shown in curve B. In this curve it will be noted that the dips are not quite as high as in curve A and there are fewer resonance peaks. i: Curve C shows a response where the diaphragm ma. terial has a closer weave but with the pores still open. I n this curve, the resonance peaks and dips have been `still further reduced but the peaks arestilll sharp enough so that, in a speaker, rattles would be heard.' f Referring now to curve D, the porosity may be uni form but even less than in curve C. The responses shown by curves C to H inclusive may be obtained by using conventional diaphragms having uniform porosity over their area. However, characteristics illustratedby curves C to G inclusive would be obtainable from a uniform diaphragm only by laboratory type of fabrica-V tion requiringpreeise control of porosity yand would' not be practical for production. By utilizing'the invention and having the diaphragm porosity non-uniform, i.e. one part porous and another part non-porous, precise char# acteristics may be easily obtained in quantity produc-- tion. Thus again, referring to curve D, -this character#v istie may be obtained by having the diaphragm dome non-porous and having roll 34a porous. Curve E shows a characteristic easily obtained with the new diaphragm by having a non-porous dome and having roll 34d sor'newhat less porous than in D.

In curve F, the porosity of roll 34a has been reduced. It will be noted that the lower frequency cut-off has been affected and that the lower part of the entirefrequency response curve has been depressed, leaving a small peak near the high frequency end.

In curve G, the porosity of roll 34a has been reduced even more. The tendency to form a generally round peak near the high frequency are of the range is 'now quite pronounced. Curve H shows the entire diaphragm non-porous. This curve shows a generally sharp high frequency resonant peak for the transducer. Referring to curves D, E and F, it will be noted that by controlling the closeness of the weave of the diaphragm'a substantial control may be exercised over the frequency response. Thus in curve D, a dip in the central part of the curve may be desirable in certain record-v ing systems where the recording system as a wholev might have a peak in that same region. Thus the dip maybe usedto compensate for the peak in the system. Curve F may be a desirable response for a transducer where the lrecording and reproducing system as a whole has a tendency to -droop at the high frequency end.A

It will thus be seen tlhat a transducer -unit substantially independent of the housing has been provided. Such a transducer unit embodying the present invention may have its frequency response characteristics predetermined and the characteristic be substantially unchanged irrespective of the casing or housing within which the transducer unit as a whole is disposed. Thus whether the transducer unit embodying the present invention is dis?. posed in a handle for use in an office type of dictating machine or is disposed in a small cabinet for use on a desk or is disposed in the dictating machine cabinet itself, is immaterial and the unit will still provide substantially the same response. In order to control the area of porosity in the diaphragm without changing the molding dies, a control of the moldngpressure. as well4 asontrolling thegtlrikuess aar-1,204.-

ofthe diaphragm material and the nature of the plastic used for` moldingwill all be useful. Thus the closeness of the weave of the diaphragm material, the nature of the diaphragm material such as, for example, whether the material is linen, nylon, or, other textile fibers, the size of the fibers used in making the diaphragm and the nature of the molding material used mayall be readily controlled andvariedto obtain desiredV results with the same set of dies; The die pressure may also be varied. Thus one set of dies may be used for obtaining awide variety of diaphragms inorder to obtain a wide varietyv ofk frequencyv response characteristics.

Variations in construction ofthe pole piece andpositioning member are possible. The positioning member may haye one annular shoulder. The maximum outside diameter of the positioning member should ge great enough so that the positioning member may be press ttedto a desireddepth in the cup. T he annular shoulder will form a bottomV annular wall (as seen in Figure 1') for the annular air gap. 'lf desired, the positioning member may have apertures 15a axially thereof at various regions along the annular shoulder or the shoulder may be scalloped' for communicating with the air chamber in the bottom ofthe ferromagnetic cup.

The free top edge portion of the positioning member may be accurately finished and used as a guide to cooperate with thepole piece for locating the pole piece so that annular air gap 27 is uniform. However, by providing shoulder 16, accurate press fit positioning of the pole piece on the positioning member is rendered simple.

In the drawing the diaphragm dome isy shown as provided with a concave center portion. The pole, piece is also concave at the center to clear the diaphragm. These may be ommitted if desired.

WhatA is claimedv is:

1. An electrodynamic type of transducer comprising aferromagnetic member provided with a cylindrical portion having an annular externally extending supporting yflange at the edge, a non-magnetic positioning sleeve member having stepped outer surfaces providing at least one annular shoulder, the maximum outer diameter being large enough so that the positioning member may be Vpress fitted into the cylindrical portion to a desired depth,

a straightpermanent magnet having ground pole faces at the free ends thereof and having a length greater` than the length of the positioning member, the magnet being small enough to slide into the` positioning member, a shallow cup-shaped ferromagnetic pole piece, means for maintaining saidvpole piece on said"y positioning member, said p ole piece having an outer annular surface cooperating withv the inner cylindrical surface near the liange to provide an annular yair gap, said annular shoulder on the positioning member being at the inner end of the -annularair gap,A anda diaphragm and' voice coil assembly carried by said' flange, the voice coil extending into the annular air gap 2. rlhe construction according to claim l` wherein said pole piece has a part overhanging the end of` the positioning member andA is. ladapted to be press fitted over the positioning member whereby the permanent magnet determines the limit to which the positioning member and; the pole piece are presstted into position.

3. An electrodynamic type oftransducer comprising a ferromagnetic member having a cylindrical portion terminating in an annular externally extending supporting flange at one end and having a magnetic circuit completing portion atthe other end, a `non-magnetic positioning sleeve member having stepped outer surfaces providing two. annular shoulders, the` maximum outer diameter of said' positioning member being large enough so that the positioning member must be forced into the cylindrical portion to a desired depth, a straight permanent magnet having ground pole faces atthe, ends thereof andy having ber, the magnet being-small enough toslide `into the-posi-Y tioning member; said positioning member having-a free endportion near the mounting-flange, the free end p0rtion having an outer diameter less-than the outer-diameters, at theshoulders, a shallow cup-shapedferromagnetic pole piecedimensionedto bev-press-,fittedf over the free end' portion of' thepositioningmember, saidpolepiece having an outer annular surface-cooperatingwith the opposed innencylindrical s urfaceof the-cylindrical portion near theange to form an annular airgap, the shoulder nearest the freeend offthe positioning membercooperating with0 the edgeY portionofthe pole piece to position the saine, the permanenty magnet being tightly retainedbetween the inside surfaces ofthe pole piece4 and magnetic circuit Ycompleting portion when` thevarious parts'are pressfitted together, and a diaphragm and voice coil assemblyl carried by said flange, the- -voicecoil extending into the annular air gap.`

4'. An electrodynarnicI type of transducer comprising al ferromagnetic member` having a cylindrical portion terminating in an annular externally extending supporting tlangeatone endv and havinga magnetic circuit completing portion at the otherend, a non-magnetic` posi-V tioning sleeve member having stepped outer4 surfaces providing at least one annular shoulder,I the maximum outer diameter being large enough so that the positioningv member mayA be` pressfitted into the cylindrical portion adesired depth, -astraight permanent magnet having ground pole facesf-at'the free-ends thereof yand having-a length greater than the length of the-positioning member, the magnet being-small-enough-to` slide-into the positioning member, one pole face of the magnet normally being in intimate contact with-the magnetic circuit completing portion, a` shallow cup-shaped' ferromagnetic-pole piece forI the other pole face, means-for maintaining said-pole piece on said positioning member, said pole piece having an outer`A annularsurface cooperating with the inner cylinder surface near the flange to provide an annular air gap, said annular shoulder on the positioning member being at the inner end of the annular air gap, and a diaphragm and voice coil assembly carried by said flange, the voice coil extending into said annular air gap, said diaphragm having a.` roll portion with the outer edge of the roll portion functioning as asupport, portion and se. cured to said flange-for supporting-the entirediaphragm and voice coil assembly, said diaphragm dome and roll cooperating with the pole-piece and ange to provide air spaces which together with the annular air gap form an interconnected air chamber, said ange extending continuously to-form a sealing-wall, pa-rt of said entire-diaphragm being porous and' the remainder being non.- porous so that all airpassing between atmosphere and the interior ofL said transducer can only' pass through the porous part of the diaphragm.

5. An electrodynamic type of `transducercomprising af ferromagnetic cup having an annular externally extending supporting flange at the cup edge, a non-magnetic positioning sleeve member having stepped outer surfacesA providing at least two annular shoulders, the-maximum outer diameter of said positioning memberbeing large enough so that thepositioning member must be forced into the cup to a desired depth, a straight permanentmagnet having ground' polefaces at the endsthereof and having a length greater than the length of the position-Y ing member, the magnet being small enough to slide into the positioning member, said positioning member having` a` free end portion near the mounting ange, the f ree end portion having an outer diameter less thanthe outer diam.` eters at the sil'ioulders,` a, shallow. cup-shaped ferromag-` netic poley piece dimensioned; toz be press fitted over the free end portion o f; the positignius member, said. mic piece having an outer annu-lar surface. CQOPel'ating--With theopposed inner surface ofv thecup near the flange-.to form auV annular-l air gap, thev s ht'iuldelt,-,nearestthe-151%` aaneen-1.

.9v4 end of lthe positioning membercooperating with the edge portion "of the pole piece to position the "same, thepermanentmagnet being tightly retained between the inside surfaces of the pole piece and cup when the various parts are press fitted together, a diaphragm and voice coil assembly carried by said flange, the voice coil extending into the annular air gap, said diaphragm havingan annular support. portion which is porous, means for mounting said diaphragm at the support portion' so that part is clear of the cup flange, the diaphragm part within the voice coil being non-porous. l

6. The construction according to claim 4 wherein said diaphragm has the diaphragm dome of non-porous material and wherein the roll is porous. 7. For use in a dynamic type of transducer having an annular air gap across which extend lines of magnetic force, a diaphragm and voice coil assembly, said voice coil including a cylindrical bobbin for operating in said air gap, said bobbin being attached to said diaphragm at a circular intermediate region of said diaphragm, said diaphragm having an annular porous roll portion beyond the intermediate region, said annular roll portion being porous lin a continuous annular band and said diaphragm within the roll portion being non-porous, said diaphragm being of porous fabric with the non-porous part having a plastic filling the fabric pores.

8. An electro-dynamic type of transducer comprising a ferromagnetic cup having a flange, a cylindrical ferromagnetic member disposed in said cup, said member having a diameter smaller than the inside diameter of the cup and being disposed symmetrically within said cup with the end functioning `as a pole piece, permanent magnet means within the cup between said ferromagnetic member and cup bottom, vsaid member and cup wall together providing an annular air gap, a voice coil assembly in said annular air gap, a diaphragm attached to said voice coil assembly, said diaphragm having a portion covering said pole piece with an air chamber therebetween and having an annular roll portion extending away from the air gap in a direction generally perpendicular to the axis of the cup, means for rigidly supporting the outer edge of said roll portion on said flange with the roll having clearance from the flange so that said diaphragm and voice coil assembly yinay vibrate, one of said diaphragm portions being non-porous and the remaining portion being porous, said cup, cup flange and member cooperating with the entire diaphragm to form a closed air space within said transducer with all air coupling to atmosphere being effected only through the porous portion of the diaphragm.

9. The construction according to claim 8 wherein that diaphragm portion which covers the pole piece is nonporous and the annular roll portion of the diaphragm is porous.

10. An electro-dynamic type of transducer comprising a ferromagnetic cup having la flange, a cylindrical permanent magnet having ground pole faces at the ends thereof `disposed in said cup, said permanent magnet having a diameter smaller than the inside diameter of the cup and being disposed symmetrically within said cup, a nonmagnetic sleeve press fitted into said cup and surrounding said magnet, said sleeve defining an annular air chamber around the permanent magnet at the bottom of the cup, a generally cup-shaped pole piece for the permanent magnet near the cup edge, said pole piece and cup wall together providing an annular air gap, at least one air passage through lsaid. sleeve between the chamber at the bottom of the cup and said annular air gap, a voice coil assembly in said annular air gap, a diaphragm attached to said voice coil assembly, said diaphragm having a portion covering said pole piece with an air chamber therebetween and having an annular roll portion extending away from the air gap in a direction generally perpendicular to the" axis of the magnet, meansffor rigidly supporting the outer edge of said roll portion on said flange with the roll having clearance from the flange so thatsaid diaphragm. and voice coil assembly may vibrate, one of said diaphragm portions being non-porous and the remaining portion being porous, said cup, cup flange and' pole piece cooperating with the entire diaphragm to form a closed air space within said transducer with all air coupling to atmosphere being effected only through the porous portion of the diaphragm, whereby the. acoustic properties of the transducer may be controlledby the position of said sleeve, the coupling yair passage through saidsleeve, the porosity of said one portion and the shape and area of the non-porous diaphragm portion.

1l. rllhe construction according to claim 10 wherein that diaphragm portion which covers the pole piece is non-porous and the annular roll portion of the diaphragm is porous.

12. The construction according to claim 10 wherein said sleeve has a pair of stepped shoulders at the air gap end of said sleeve, one stepped shoulder cooperating with the pole piece for positioning the same, the other stepped shoulder providing an annular region to form an extension of the air gap so that said voice coil may vibrate at satisfactory amplitude.

13. The construction according to claim 10 wherein said sleeve has a pair of stepped shoulders at the air gap end of said sleeve, said pole piece being press fitted over one stepped shoulder, the other stepped shoulder cooperating with the inside surface of the cup to yfolrn an extension of the annular air gap so that said voice coil may move over a satisfactory range of amplitude.

14. An acoustic diaphragm comprising an integral body 0f normally porous fibrous material in the form of a fabric, said body having an impervious central rigid vibratile portion coated with an infusible thermosetting resin to lill the normally open pores of said fabric and a porous peripheral flexible portion in which the individual fibers are thinly coated with a quantity of an infusible thermosetting resin insufficient to completely fill the normally open pores of said fabric.

l5. An acoustic diaphragm as claimed in claim 14 wherein the thermosetting resin is a phenol-formaldehyde resin.

16. An acoustic diaphragm as claimed in claim 14 wherein the central portion is non-planiform.

17. An acoustic diaphragm as claimed in claim 14 wherein the central portion in non-planiform and the peripheral portion is corrugated concentrically to said central portion.

18. An acoustic diaphragm comprising an integral body of normally porous flexible fibrous material in .the form of a fabric, said body having an impervious central rigid non-planiform vibratile portion coated with an infusible thermosetting resin which has been impregnated in its fusible state in the individual fibers of said fabric and cured to its infusible state under conditions of temperature and pressure sufficient to cause the resin to flow and fill the normally open pores of said fabric, and said body having `a porous peripheral undulated flexible portion in which the individual fibers are thinly coated with the same infusible thermosetting resin which has been impregnated in its fusible state in the individual fibers of said fabric and cured to its infusible state under conditions of temperature and pressure insufficient to cause said resin to ow and fill the normally open pores of said fabric.

19. An acoustic diaphragm as claimed in claim 18 wherein the thermosetting resin is a phenol-formaldehyde resin.

(References on following page) maman;

References lCited'iin the le of. this .l patent 2,020,212 A UNITEDJSTATESPATENTS" 1,815,987V Peterson July 28; 19:3'1`l` 2,26l;1 11. 11,886,816, Jensen Nov 8,1, 1932` 2,=302;1f78 1,911,802 Burgman May30, 19,33 2,469,773

1,941,073 Blattner Dec. 26, 1933 2,517,727` 1,941,476 Jensen -..Y Jan. 2f.' 19.3.4 2,641,329.` 1,941,477. Jensenv Jan 2,1934 2,714,134, 1,954,966, Thuras; Apr.- 17,1934 10 22,873,313

1,997,051 Engl'1olnf1` .A Apr. 9, 1935 2,001,081l .,'Ihuras May 141935 2,020,211- mam-,, 1 v Nov. 5., 1.935 ,70118.33`

12 Quam 'Nov.-5, 1935- Jensen July 7, 1936 BriggsV Apr... 9, 1940 Enghlm Nv. 4, 1941 Brennan@ y Nov. 17; 1942 Knowlesm..T May 10,1949 Smith Aug. 8', 1950 Levy June 9, 1,953.r Touger et a1 July 26, 1955' Haerther et al Feb. 1.7, 1959 FOREIGN1 PATENTSY

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