US3801943A

US3801943A - Electoacoustic transducers and electromagnetic assembly therefor

Info

Publication number: US3801943A
Application number: US00259378A
Authority: US
Inventors: J Bertagni
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-06-16
Filing date: 1972-06-05
Publication date: 1974-04-02
Anticipated expiration: 1991-04-02
Also published as: FR2141983A1; NL7208047A; DE2229239A1; DK137820C; JPS5514600B1; FR2141983B3; DK137820B

Abstract

An electroacoustic transducer comprising a diaphragm and an electromagnetic assembly in driving relationship with the diaphragm, the assembly comprising a magnet, a core defining a gap in the magnet, a coil supported by a coil holder movably arranged in the gap with an end portion facing the diaphragm, a cap on said end portion having a top surface directed toward, and in driving relationship with, the diaphragm, the surface being preferably secured to the diaphragm by means of an adhesive. The adhesive may be soft or hard and the diaphragm is preferably flat. The cap seals the coil holder and a perforation is provided connecting the space formed between the cap, coil holder, core and gap with another surrounding whereby a rectilinear to and fro movement pump is defined by the cap, coil holder and core, the pumping action moving air to cool the coil. The lower end of the coil holder has a flange to prevent the coil from being shockremoved from the coil holder, and the upper end of the holder has a flange which is adhered to the cap.

Description

United States'Patent [1 1 Bertagni I541 ELEQTROACOUSTIC TRANSDUCERS AN ELECTROMAGNETIC ASSEMBLY THEREFOR [76] Inventor: Jose Juan Bertagni, 1027 [57] 22 Filed:

211 Appl. No.: 259,378

[30] Foreign Application Priority Data Hernandarias St., Buenos Aires, Argentina June 5, 1972 June 16, 1972 Argentina 236220 [52] U.S. Cl. 335/231, 179/115.5 [51] Int. Cl. 1101f 7/00 [58] Field of Search ..335/23l; 179/1155 R,

[56] References Cited UNITED STATES PATENTS 2,252,846 8/1941 Giannini et al. 179/1 15.5 R 2,501,032 3/1950 Harbaugh 179/1155 PC 2,848,561 8/1958 Gorike l79/ll5;5 R 3,329,777 7/1967 Kliewera. 179/1155 R 3,567,870 3/1971 Rivera 179/l15.5 R to the cap- 37 Claims, 8 Drawing Figures Primary Examiner-George Harris Attorney, Agent, or FirmMilton Osheroff ABSTRACT An electroacoustic transducer comprising a diaphragm and an electromagnetic assembly in driving relationship with the diaphragm, the assembly comprising a magnet, a core defining a gap in the magnet, a coil supported by a coil holder movably arranged in the gap with an end portion facing the diaphragm, a cap on said end portion having a'top surface directed toward, and in driving relationship with, the diaphragm, the surface being preferably secured to the diaphragm by means of an adhesive. The adhesive may be soft or hard and the diaphragm is preferably flat. The cap seals the coil holder and a perforation is I provided connecting the space formed between the 179]! BS cap, coil holder, core and gap with another surrounding whereby a rectilinear to and fro movement pump is defined by the cap, coil holder and core, the pumping action moving air to cool the coil. The lower end of the coil holder has a flange to prevent the coil from being shock-removed from the coil holder, and the upper end of the holder has a flange which 56 w 4 1 flw Apr. 2, 1974 ELECTOACOUSTIC TRANSDUCERS AND ELECTROMAGNETIC ASSEMBLY THEREFOR The present invention relates to an electromagnetic assembly for electroacoustic transducers and the like, and more particularly it refers to an electromagnetic assembly, as well as an electroacoustic transducer utilizing said assembly, which include, due to its structural and constitutive arrangement, a number of particular features, each of which may be used alone or in combination to achieve different effects and results. Thus, one of the features is to use the vibratory movement of the coil holder to act as a plunger of a to and fro moving pump in order to cool or refrigerate the core of the magnet, i.e. the pole piece of the magnet, the gap, the

coil holder and the coil or to replace the'elastic spider which resiliently supports the coil holder. The coil may also be mounted on the coil holder in a particular way in order to reduce the distributed capacity. The electromagnetic assembly of the present invention may also be used below the water, for example in connection with loudspeakers and microphones. As to the aspect of using the electromagnetic assembly under the water, said assembly may be combined with vibratory panels for other purposes, such asfor a washing machine. It is also possible to use the electromagnetic assembly in connection with vibratory panels to transmit such vibrations to granular materials arranged on said vibratory panel.

As may be appreciated from the foregoing, the electromagnetic assembly of the present invention is applicable to various types of electroacoustic transducers usable in a number of different fields within industry, the assembly being in driving relationship with the diaphragm of the transducer. In "-other words, the diaphragm can drive'the electromagnetic assembly or the electromagnetic assembly can drive the diaphragm. The acoustic vibrations may be those which can be or are to be heard (e.g., music, speech, etc.), or those whose purpose is mere vibration (e.g., washing machine).

Another feature of the invention is the use of a cap on the coil holder as the surface for transmitting the vibrations 'between'the coil holder and the diaphragm.

The known electromagnetic assemblies for electroacoustic transducers, and particularly for loud speakers have to face the well known problem of the generation of excessive heat within the zone ofthe gap and therefore, it is an important feature of the invention to be able to refrigerate said zone, whereby the usual life of the electromagnetic assembly is undoubtedly increased, or in other words, the life of the driving unit of the loudspeakers is increased. When the electromagnetic assembly is used for flat loudspeakers, it is an aim that the vibrations be more energetic and therefore the amount of heat which is generated is still larger. Thus, a good refrigeration within the air gap zone is even more important when the electromagnetic assembly is used in relationship with flat loudspeakers.

As a consequence of the requirement hereinabove stated, or in other words, to produce more energetic vibrations, it is necessary to provide suitable means in the coil holder which prevent the coil wound on the coil holder from being removed -by shocks therefrom. Another feature of the present invention, and which is also. an indirect consequence of the tendency to obtain larger or more energetic vibrations is that special means had to be conceived to link the ends of the coil with the connectingconductors which link the coil with the rest of the electronic circuit to avoid cracking or breakage at the linkage or welding points due to difference in masses existing between both types of conductors, upon being subject to variable rectilinear movements generated by the vibrations of the coil holder.

Still another feature which fonns part of the present invention is related to the output of the loudspeakers. It is well known that in the known loudspeakers the impedance is substantially constant in the zone of low frequencies Hz to 5,000 Hz) and thereafter a weak growing tendency'up to the region of 20 KHz (20,000 cycles) can be seen. In the known loudspeakers there is at least one low frequency peak. To oppose the impedance increase in the high frequency zone and to thus obtain a better output and a substantial elimination of the peaks, according to the tests carried out, it has been proved that it is possible to short-circuit the coil holder at the same time as the coil is wound in such a way as to obtain an additional capacity between the turns, for the zones which correspond to the residual capacity between the turns and thus obtain the already mentioned distributed capacity. There are several ways of short-circuiting the coil, as will be seen in the specific examples,

The present invention, according to one of the features thereof, utilizes an electromagnetic assembly for electroacoustic transducers and the like, comprising a magnet, a core defining a gap in said magnet, a coil holder, a coil wound on said coil holder movably arranged in said gap and an end portion of said coil holder projecting out of said magnet, wherein the coil holder is provided with a seal cap in the portion projecting out of said gap, 21 space being formed between said cap, coil holder, core and gap, at least one perforation connecting said space with another surrounding, thus a rectiliner to and fro moving pump being defined between said cap, coil holder and core.

The electromagnetic assembly may further comprise a resilient spider, mounted on the magnetic assembly and linked to said end portion of said coil holder. The resilient spider or support may be an impermeable and sealing diaphragm. v

The perforation mentioned in the above paragraphs which connects said space with said other surrounding, may be a through perforation in the core, so that one of the ends of said perforation faces said cap. The air mass which is conveyed by the coil holder with its cap through the perforation, may act as a cooling means for the core, the gap, the coil holder and the coil, but it may also act as a resilient cushion, specially when the electromagnetic assembly is used under water, where no refrigeration is necessary; in this event the other end of said perforation is sealed.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features will be explained with more details in relationship with the specific examples which will be now described by way of example in connection with the drawings, in which:

FIG. 1 is a perspective view of an electromagnetic assembly for electroacoustic transducers and the like, according to the present invention.

FIG. 2 shows in perspective view a detail of a portion of the flat diaphragm in the zone where it is to be linked to the electromagnetic assembly of FIG. 1.

FIG. 4 is a longitudinal section of the electromagnetic assembly applied to a flat diaphragm of a flat loudspeaker.

FIG. 5 is a detail in longitudinal section, similar to FIG. 4, but showing a possible alternative embodiment in relationship to the pumping system.

FIG. 6 is a detail in longitudinal section, similar to FIG. 4, but showing another possible alternative embodiment in relationship to the pumping system.

, FIG. 7 is a detail in longitudinal section of the coil holder, coil and resilient support.

FIG. 8 is a detail in longitudinal section, similar to FIG. 4, but showing another possible alternative embodiment in relationship to the pumping system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 to 4, it may be appreciated that the electromagnetic assembly 1 of the present invention comprises a magnet 2 which is a cylinder of small height and has a central cylindrical through hole 3.

A first disc 5, having a central projecting cylindrical core 6,, the diameter of which is smaller than that of ,.bore '3 and the height of which is larger than that of ration 10 the diameter of which is slightly larger than the diameter of the core 6, thus allowing between both the insertion of a coil holder 12, having a-coil 15. The height of the core.6 is such, when the three

members

2, 5 and 9 are assembled, that the upper base 6 of the core 6 is flush with the upper face 25 of the second disc 9 (FIG. 4). Thus, an air gap 11 is formed between the core 6 and the magnet 3. Gap 11 houses the coilholder '12. The coil holder 12 will perform a rectilinear alternative or to and fro movement, which is substantially coaxial with the longitudinal axis of the assembly.

The coilholder 12 is in one of the possible embodiments, a paper made cylinder 13, the inside face of which is sheathed with a metal sheet 14, for instance an aluminum foil. Although the coil holder 12 need not be an endless cylinder, but have a longitudinal cut so as to define a longitudinal slot, as is known in the art, it is preferred to make the coil holder as an endless cylinder. On the outside face of the coil holder 12 and more particularly on the paper 13, a coil 15 is wound which covers the lower half, leaving free the upper half, which defines the end .portion which projects out of the gap 11. Thus, the output within the high frequency range is increased. I

To achieve the features of the distributed capacity, already above explained, it is preferred to produce the coil for instance by means of two layers of turns, of which the inner layer of turns 16 (FIG. 7) is so wound as to progressively decrease the space between turns from bottom to top, while the outer layer of turns 17 is wound so as to progressively decrease the space between turns from top to bottom, whereby an optimum turn density is achieved to fulfil the purpose. Once the coil 15 is thus wound on the coil holder 12, both the lower end rim 18 as well as the upper end rim 19 are outwardly turned over thus forming each a flange, although for different purposes. The lower flange l8 prevents the coil 15 from being shock-removed from the coil holder 12, when the assembly is strongly vibrating. The upper flange 19 is provided, in order to increase the binding surface which links the upper end 19 with the resilient support which is a kind of a diaphragm 20. In the known electromagnetic assemblies this diaphragm or resilient support 20, also called spider, is generally made of an impregnated porous fabric to convey the necessary resiliency and yet rigidity and such known spider may be used in connection with the present invention; however, it is also possible to replace said fabric spider by an air impermeable member, such as of neoprene rubber.

In both cases the diaphragm 20 has an edge 21 defining the larger base portion, from which rises a frustoconical portion 22, the smaller. base portion 23 of which is provided with concentric ondulations ending in a connecting edge 24 which is connected by means of an adhesive (not shown) to the upper end or flange 19 of the coil holder 12 (FIG. 7). Bearing in mind that the flange 19 is turned over, the connecting surface between the resilient suspension 20 and the coil holder 12 is considerably increased. The edge 21 is connected to the upper face 25 (FIG. 1) of the second disc 9 of the magnet assembly comprised by disc 5, magnet 2, and disc 9. The smaller base portion 23 of diaphragm 20 provides the necessary resiliency to allow for the to and fro movement of the coil holder 12.

The upper flange 19 of the coil holder 12 satisfies also another purpose, namely to increase the supporting surface for the cap 26 made of an insulating material, which is preferably soft, and which, for example, may be made of polyethylene or polyurethane. This cap 26 has a lower cylindrical porjection 27 which enters the coil holder 12 and is sealed thereto at the same time as the main disc-shaped body 28, of larger diameter than the lower cylindrical projection 27, overlaps the upper flange l9'of the coil holder 12. The body 28 and flange 19 thus define additional sealing surfaces. Conveniently, cap 26 is connected to the coil holder 12 by an adhesive (not shown). It is apparent that the junction between lower cylindrical projection 27 and the main disc shaped body 28 defines a flange receiving portion which includes surfaces generally transverse and parallel to the axis of the coil holder, and this portion receives the flange 19 with the adhesive (not shown) being interposed between the opposed surfaces of the holder and the cap. The upper face of the upper I disc-shaped body 28 has two parallel channels 29, 29'.

The thus assembled electromagnetic assembly is connected, in case of being used with a sound transducer, with a'flat diaphragm 34 which may form part of a loudspeaker or a microphone. This flat diaphragm may for instance be of the type described in my US. Pat. No. 3,596,733, or any other type of known flat loudspeaker, as cited in the introductory portion of said patent. Diaphragm 34 corresponds to a flat loudspeaker having a front face 35 and a rear face 36. This rear face 36 has a circular recess 37, from which extends a pair of

parallel channels

38, 38. The circular recess 37 is filled with an adhesive (not shown) and the cap 26 is glued through its top face 26 thereto so that the portion of

theconductors

31, 31 and the insulated conductors 32, 32' projecting away from cap 26 become housed in the

channels

38, 38, thus building a homogeneous assembly and further reducing the possibility of breakage of the conductors. Obviously,

conductors

32, 32 are furthermore connected, as already stated, to a pertinent electronic circuit (not shown). In case the top face 26 is not perfectly parallel to the bottom wall of the recess 37, the adhesive which links the two members compensate any lack of parallelism and the homogeneous assembly is thus formed. As to the adhesive, the latter may be a hard adhesive, or in other words it produces a rigid connection between the recess 37 and the top 26'. It is also possible to use a soft adhesive, such as a resilient adhesive, whereby the intermodulation is improved.

Finally, if the assembly of coil 15, coil holder 12 and cap 26 operates as a percussion member with relationship to the diaphragm 34, no adhesive will be required. Whether or not the cap is secured to the diaphragm, the cap'is a member independent of the diaphragm and defines a large contact top surface directed towards the diaphragm and in driving relationship with the diaphragm, so that, when used as a microphone, for example, the diaphragm drives the cap through said surface, and when used as a speaker, for example, the cap drives the diaphragm through said surface.

If it is desired to connect the electromagnetic assembly 1 hereinabove described to an orthodox frustoconical loudspeaker, this is also possible and therefore the present invention has not to be interpreted in a limiting manner in this direction.

As to the operation of the structural embodiment described, it is obvious that in principle it is the same as the one of a known electromagnetic assembly as far as the generation of the electromotive force for vibrating the coil holder 12 is concerned. The air which is housed between the lower cylindrical projection 27 of the cap 26 and the core 6, is a mass of air which is pumped towards the inside and towards the outside through perforation 7, thereby achieving a refrigerating effect.

It would also be possible to replace perforation 7 of FIG. 4 by perforations 7'(FIG. 8) which are present in the first disc 5 surrounding the core 6, so that the movement of refrigerating or cooling air is performed through the gap 11. i

If the diaphragm is air permeable, the assembly can likewise operate in a satisfactory manner.

When the assembly is to be used below the water, perforation 7 is sealed (FIG. 5) by means of a plug rod 39. During the assembling of the electromagnetic assembly, the position of rod 39 is gauged with regard to perforation 7 in order to achieve good operation. In this event, no resilient spider suspension of the type of diaphragm 20 is necessary since the air cushion can carry out this role, but upon using the electromagnetic assembly under water, sealing means must be provided in order to avoid that the water enters the gap 11.

With regard to the embodiment shown in FIG. 6, here the plug rod 39 is replaced by a tubular member 40, having a valve 41 for gauging a crosssectional passage for the air, so that an optimum relationship can be achieved. Obviously, valve 41 may be replaced by a tubular member with a pre-established cross-sectional area for the air passage. In the same example which defines an alternative embodiment, a further modification is shown which as such is applicable to any of the other structural embodiments.

When the electromagnetic assembly of the present invention is used with a loudspeaker, for instance of the flat type as described in relationship to FIG. 2, it may be that special effects are intended to be achieved, such as an increased range of low frequencies. In other words, in a musical piece it may be desired that the human voice becomes submerged within the musical poem. To achieve this effect in the cap 26, and more particularly in the lower projection 27 a recess 42 may be made to house therein a weight member 43. Thus, the mass of the movable assembly and more particularly of the cap 26 and coil holder 12' with its coil 15 is increased, whereby the assembly will operate within a larger range of low frequencies.

The coil holder 12 described in relationship with FIG. 7 has a relatively low mechanical resistance and it may be convenient that such resistance is increased. In the embodiment shown in FIG. 6, the coil holder 12 may for instance be made of plastified (bakelized) cardboard, whereby a larger resistance against impacts is achieved upon the coil holder entering in impacting contact with the first disc 5. A coil holder so made, has i the drawback .that it is inefficiently cooled, but this drawback can be compensated by producing the weight member 43 of a good electricity and heat conducting material, such as copper, and since said copper disc or plate 43 faces the mass of air which is pumped through the perforation 7, the copper disc transfers the heat which it absorbs to the cooling air mass. If desired, a heat insulating layer 44 (FIG. 6) is arranged between the metal disc 43 and said cap 26.

If the cap 26 is made of expanded, bonded, cellular plastics, such as polystyrene, it may become advisable to arrange a heat insulating mass between the plate 43 and the recess 42, such as an asbestos disc-member (not shown) to avoid that the cap 26 becomes injured by heat.

Another possibility would be to manufacture the cap 26 from a rigid plastic, such as nylon.

It will be obvious that modifications may be introduced in this invention upon carrying it out into practice as far as certain structural details are concerned but always without departing from the fundamental principles which are clearly specified in the following claims.

I claim:

1. An electroacoustic transducer comprising a diaphragm and an electromagnetic assembly in driving relationship with said diaphragm, said electromagnetic assembly comprising a magnet, a core defining a gap in said magnet, a coil supported by a coil holder movably arranged in said gap with an end portion of said coil holder facing said diaphragm, an independent cap on 32. An electromagnetic assembly for electroacoustic transducers and the like, comprising a magnet, a core defining a gap in said magnet, a coil holder having a coil wound thereon, said coil holder being movably arranged in said gap and an end portion of said coil holder projecting out of said magnet, said coil holder being provided with a seal cap in the portion projecting out of said gap, a space being formed between said cap, coil holder, core and gap, at least one perforation connecting said space with another surrounding, whereby a rectilinear to and fro moving pump is defined by said cap, coil holder and core, said coil holder being an endless cylinder the outer face of which is of electricity insulating material and the inner face of which is sheathed with an electricity conducting layer.

33. An electromagnetic assembly for electroacoustic transducers and the like, comprising a magnet, a core defining a gap in said magnet, a coil holder having a coil wound thereon, said coil holder being movably arranged in said gap and an end portion of said coil holder projecting out of said magnet, said coil holder being provided with a seal cap in the portion projecting out of said gap, a space being formed between said cap, coil holder, core and gap, at least one perforation connecting said space with another surrounding, whereby a rectilinear to and fro moving pump is defined by said cap, coil holder and core, said cap having an upper face remote from said coil holder, said cap being provided on its upper face with a pair of channel members in which respective end portions of the conductor of the coil are housed and connected to respective other conductors. I I

34. An electromagnetic assembly according to claim 52, wherein said cap is of soft plastic.

35. An electromagnetic assembly for electroacoustic transducers and the like, comprising a magnet, a core defining a gap in said magnet, a coil holder having a coil wound thereon, said coil holder being movably arranged in said gap and an end portion of said coil holder projecting out of said magnet, said coil holder being provided with a seal cap in the portion projecting out of said gap, a space being formed between said cap, coil holder, core and gap, at least one perforation connecting said space with another surrounding, whereby a rectilinear to and fro moving pump is defined by said cap, coil holder and core, said core holder being made of plastified cardboard and said cap housing a good heat and electricity conducting metal disc facing said space.

. 36. An electromagnetic assembly according to claim 35, wherein a heat insulating layer is arranged between said metal disc and said cap.

37. An electromagnetic assembly according to claim 33, wherein said cap-is made of hard material.

Claims

1. An electroacoustic transducer comprising a diaphragm and an electromagnetic assembly in driving relationship with said diaphragm, said electromagnetic assembly comprising a magnet, a core defining a gap in said magnet, a coil supported by a coil holder movably arranged in said gap with an end portion of said coil holder facing said diaphragm, an independent cap on said end portion defining a large contact top surface directed towards, and in driving relationship with, said diaphragm.

2. An electroacoustic transducer according to claim 1, wherein said surface overlies said holder.

3. An electroacoustic transducer according to claim 2, wherein said surface is secured to said diaphragm.

4. An electroacoustic transducer according to claim 3, wherein substantially the entire top surface of said cap is secured to said diaphragm.

5. An electroacoustic transducer according to claim 4, wherein said surface is adhesively secured to said diaphragm.

6. An electroacoustic transducer according to claim 4, wherein said surface is adhesively secured to said diaphragm by means of an adhesive interposed between said surface and said diaphragm.

7. An electroacoustic transducer according to claim 6, wherein said adhesive is a hard adhesive to provide a rigid connection between said surface and said diaphragm.

8. An electroacoustic transducer according to claim 6, wherein said adhesive is a soft adhesive to provide a resilient connection between said surface and said diaphragm.

9. An electroacoustic transducer according to claim 1, wherein said diaphragm is a flat diaphragm.

10. An electroacoustic transducer according to claim 9, wherein said surface overlies said holder.

11. An electroacoustic transducer according to claim 10, wherein said surface is secured to said diaphragm.

12. An electroacoustic transducer according to claim 11, wherein substantially the entire top surface of said cap is secured to said diaphragm.

13. An electroacoustic transducer according to claim 12, wherein said surface is adhesively secured to said diaphragm.

14. An electroaCoustic transducer according to claim 12, wherein said surface is adhesively secured to said diaphragm by means of an adhesive interposed between said surface and said diaphragm.

15. An electroacoustic transducer according to claim 14, wherein said adhesive is a hard adhesive to provide a rigid connection between said surface and said diaphragm.

16. An electroacoustic transducer according to claim 14, wherein said adhesive is a soft adhesive to provide a resilient connection between said surface and said diaphragm.

17. An electroacoustic transducer according to claim 15, wherein said diaphragm has a recess which substantially houses said adhesive.

18. An electroacoustic transducer according to claim 16, wherein said diaphragm has a recess which substantially houses said adhesive.

19. An electroacoustic transducer according to claim 1, wherein said cap has a flange receiving portion including surfaces generally transverse and parallel to the axis of said coil holder, said end portion of said coil holder having laterally projecting flange means received in said flange receiving portion and wherein an adhesive is interposed between the opposed surfaces of said holder and said cap.

20. An electroacoustic transducer according to claim 1, wherein said coil holder has flange means extending laterally below said coil to prevent the coil from sliding off said coil holder.

21. An electroacoustic transducer according to claim 1, wherein said cap seals said end portion of said coil holder, a space being formed between said cap, coil holder, core and gap, at least one perforation connecting said space to another surrounding, whereby a rectilinear to and fro moving pump is defined by said cap, coil holder and core.

22. An electroacoustic transducer according to claim 21, wherein the upper end of the coil holder is sealed to a frustoconical air impermeable resilient support having a lower flange which is sealed to the electromagnetic assembly.

23. An electroacoustic transducer according to claim 21, wherein said perforation is a through perforation passing through said core, one end of said perforation facing said cap.

24. An electroacoustic transducer according to claim 23, wherein said through perforation is sealed at its lower end.

25. An electroacoustic transducer according to claim 23, wherein said perforation is provided with a valve member for changing the cross sectional area passage thereof.

26. An electroacoustic transducer according to claim 1, wherein said cap has an upper face remote from said coil holder, said cap is provided on its upper face with a pair of channel members in which respective end portions of the conductor of the coil are housed and connected to respective other conductors.

27. An electroacoustic transducer according to claim 1, wherein said cap is constituted by a soft plastic.

28. An electroacoustic transducer according to claim 1, wherein said cap is constituted by a hard plastic.

29. An electroacoustic transducer according to claim 14, wherein said cap has an upper face remote from said coil holder, said cap is provided on its upper face with a pair of channel members in which respective end portions of the conductor of the coil are housed and connected to respective other conductors, said adhesive substantially covering said channels and conductors and connections therein.

30. An electroacoustic transducer according to claim 21, wherein said core forms part of a disc abutting on said magnet, said disc having a plurality of perforations connecting the outside with said gap.

31. An electromagnetic assembly for electroacoustic transducers and the like, comprising a magnet, a core defining a gap in said magnet, a coil holder having a coil wound thereon, said coil holder being movably arranged in said gap and an end portion of said coil holder projecting out of said magnet, said coil holder being provided with a seal cap in the portion projecting out of said gap, a space being formed between said cap, coil holder, cOre and gap, at least one perforation connecting said space with another surrounding, whereby a rectilinear to and fro moving pump is defined by said cap, coil holder and core, said coil holder having an upper end and a lower end, said coil being wound on the lower half of said coil holder by means of two juxtaposed layers of turns, the spacing between adjacent turns of one of said layers progressively increasing toward the upper end and the spacing between adjacent turns of the other of said layers progressively increasing toward the lower end.

32. An electromagnetic assembly for electroacoustic transducers and the like, comprising a magnet, a core defining a gap in said magnet, a coil holder having a coil wound thereon, said coil holder being movably arranged in said gap and an end portion of said coil holder projecting out of said magnet, said coil holder being provided with a seal cap in the portion projecting out of said gap, a space being formed between said cap, coil holder, core and gap, at least one perforation connecting said space with another surrounding, whereby a rectilinear to and fro moving pump is defined by said cap, coil holder and core, said coil holder being an endless cylinder the outer face of which is of electricity insulating material and the inner face of which is sheathed with an electricity conducting layer.

33. An electromagnetic assembly for electroacoustic transducers and the like, comprising a magnet, a core defining a gap in said magnet, a coil holder having a coil wound thereon, said coil holder being movably arranged in said gap and an end portion of said coil holder projecting out of said magnet, said coil holder being provided with a seal cap in the portion projecting out of said gap, a space being formed between said cap, coil holder, core and gap, at least one perforation connecting said space with another surrounding, whereby a rectilinear to and fro moving pump is defined by said cap, coil holder and core, said cap having an upper face remote from said coil holder, said cap being provided on its upper face with a pair of channel members in which respective end portions of the conductor of the coil are housed and connected to respective other conductors.

36. An electromagnetic assembly according to claim 35, wherein a heat insulating layer is arranged between said metal disc and said cap.

37. An electromagnetic assembly according to claim 33, wherein said cap is made of hard material.