US3636278A

US3636278A - Acoustic transducer with a diaphragm forming a plurality of adjacent narrow air spaces open only at one side with the open sides of adjacent air spaces alternatingly facing in opposite directions

Info

Publication number: US3636278A
Application number: US800579A
Authority: US
Inventors: Oakar Heil
Original assignee: HEIL SCIENT LAB Inc
Current assignee: HEIL SCIENT LAB Inc; HEIL SCIENTIFIC LABORATORIES Inc
Priority date: 1969-02-19
Filing date: 1969-02-19
Publication date: 1972-01-18
Anticipated expiration: 1989-01-18
Also published as: JPS5547517B1; DE2003950B2; FR2031517A1; FR2031517B1; GB1305456A; CH524943A; DE2003950A1

Abstract

An acoustic transducer with a new kind of diaphragm geometry and a new kind of acoustical or vibratory excitation of the diaphragm. The diaphragm comprises a plurality of substantially equal spaced and substantially parallel diaphragm portions which define between themselves small air spaces, and means connecting the diaphragm portions to each other in such a manner so as to close each of the air spaces at three sides while the fourth side is left open and with the open sides of adjacent air spaces respectively facing in opposite directions. As a result of this arrangement, the narrow air spaces between adjacent diaphragm portions get alternatingly enlarged or reduced and alternatingly air is sucked in or expelled from the adjacent air spaces during vibration of the diaphragm portions. During each half-cycle of the vibration air is sucked in one direction into every second of the adjacent air spaces and expelled in the opposite direction from the other air spaces and during the next half-cycle the motion of air into and out of adjacent air spaces is reversed. Such diaphragm arrangement moves more air with less kinetic energy than conventional diaphragms. The vibratory diaphragm portions may be directly driven by applying an audio current, respectively an audio voltage to conductors attached to the diaphragm portions and located in a strong magnetic field, or the diaphragm portions may be indirectly driven by a pair of voice coils alternatingly attached to adjacent vibratory diaphragm portions to move the latter toward and away from each other. On the other hand, the vibratory diaphragm portions may be acoustically driven and audio currents, respectively audio voltages, may be produced in conductors attached to the diaphragm portions and moving in a strong magnetic field.

Description

United States Patent Heil [451 Jan. 18,1972

I ACOUSTIC TRANSDUCER WITH A DIAPHRAGM FORMING A PLURALITY ()F ADJACENT NARROW AIR SPACES OPEN ONLY AT ONE SIDE WITH THE OPEN SIDES OF ADJACENT AIR SPACES ALTERNATINGLY FACING IN OPPOSITE DIRECTIONS [72] Inventor: Osltar Heil, San Mateo, Calif.

[73] Assignee: Hell Scientific Laboratories, Inc., Belmont,

Calif.

[22] Filed: Feb. 19, 1969 [21] Appl.No.: 800,579

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas W. Brown Attorney-Michael S. Striker 1 ABSTRACT An acoustic transducer with a new kind of diaphragm geometry and a new kind of acoustical or vibratory excitation of the diaphragm. The diaphragm comprises a plurality of substantially equal spaced and substantially parallel diaphragm portions which define between themselves small air spaces, and means connecting the diaphragm portions to each other in such a manner so as to close each of the air spaces at three sides while the fourth side is left open and with the open sides of adjacent air spaces respectively facing in opposite directions. As a result of this arrangement, the narrow air spaces between adjacent diaphragm portions get altematingly enlarged or reduced and altematingly air is sucked in or expelled from the adjacent air spaces during vibration of the diaphragm portions. During each half-cycle of the vibration air is sucked in one direction into every second of the adjacent air spaces and expelled in the opposite direction from the other air spaces and during the next half-cycle the motion of air into and out of adjacent air spaces is reversed. Such diaphragm arrangement moves more air with less kinetic energy than conventional diaphragms. The vibratory diaphragm portions may be directly driven by applying an audio current, respectively an audio voltage to conductors attached to the diaphragm portions and located in a strong magnetic field, or the diaphragm portions may be indirectly driven by a pair of voice coils alternatingly attached to adjacent vibratory diaphragm portions to move the latter toward and away from each other. On the other hand, the vibratory diaphragm portions may be acoustically driven and audio currents, respectively audio voltages, may be produced in conductors attached to the diaphragm portions and moving in a strong magnetic field.

34 Claims, 21 Drawing Figures PATENTED JAN 1 8 i972 SHEET 2 BF 8 INVENTOR mm lie/0 'h/v/x #M-A/ ATTORNEY Mimi 1:

PATENTEUJANIBIHYZ 31636278 SHEET I; 0F 8 FIG. I20

W 38a 38 L F/G. up FIG. /3a

INVENTOR 05ml: flaw, 7)W/Q/f- 14 441 ATTORNEY PATENTEUJAnmm 3,636,278

sum 7 0F 8 INVENTOR 0:24,: lie/a ATTORNEY ACOUSTIC TRANSDUCER WITH A DIAPHRAGM FORMING A PLURALITY OF ADJACENT NARROW AIR SPACES OPEN ONLY AT ONE SIDE WITH THE OPEN SIDES OF ADJACENT AIR SPACES ALTERNATINGLY FACING IN OPPOSITE DIRECTIONS BACKGROUND OF THE INVENTION The present invention relates to acoustic transducers and more specifically to diaphragms and their use for sound windows, microphones and loudspeakers.

If sound has to pass between two separated rooms a sound window must be used and a lightweight, thin diaphragm serves best for this purpose.

Likewise, if sound is transduced into electric currents in a microphone or if sound is reproduced from electric currents in a loudspeaker, the lightest diaphragm assemblies will give the most faithful and efficient reproduction.

Air with a density of 1.2 mg. per cubic centimeter is 1,000 to 10,000 times lighter than solid bodies and this is the reason why diaphragms in the three mentioned devices can hardly be made thin enough so as to reduce the effective specific mass of the diaphragm.

It is an object of the present invention to provide for a diaphragm for use in sound windows, microphones or loudspeakers in which the efi'ective specific mass (grams per square centimeter) of the diaphragm can be reduced by a considerable factor for acoustic frequency without reducing the diaphragm thickness. A reduction factor of 10 is quite practical.

It is a further object of the present invention to provide in acoustic transducers such as sound windows, microphones or loudspeakers sound-transmitting diaphragrns which will give the most faithful reproduction of the sound.

SUMMARY OF THE INVENTION With these objects in view, the acoustic transducer according to the present invention comprises sound-transmitting diaphragm means including a plurality of at least partly vibratory diaphragm portions arranged to define between themselves adjacent narrow airspaces and means connecting said vibratory diaphragm portions to each other so that adjacent airspaces are alternatingly closed and left open at the opposite ends.

The acoustic transducer according to the present invention may also include means cooperating with the vibratory diaphragm portions for transforming vibrations imparted thereto by sound waves into electric energy and vice versa and in this case conductor means may be connected to the vibratory diaphragm portions while means for producing a strong magnetic field are arranged to cooperate with the conductor means for producing sound waves creating vibrations in the diaphragm portions during passing of an audio current through the conductor means, respectively during application of an audio voltage thereto and vice versa.

The airspaces or air pockets which alternatingly open to opposite sides may be formed by bending a thin diaphragm into meander shape and in this case the vibratory diaphragm portions will be formed by substantially parallel and spaced portions of the band, or on the other hand, the vibratory diaphragm portions may be formed by a plurality of separate thin sheets held in spaced, substantially parallel relationship by spacer means connected thereto and which have open ends alternatingly facing in opposite directions.

In electroacoustic transducers in which sound waves are transformed into electric impulses or vice versa, the vibratory portions of the diaphragm means may be directly or indirectly driven. In the first case, conductor means are, as mentioned before, carried by the vibratory portions of the diaphragm and these conductor means are arranged to move in a magnetic field, whereas in the second case separate electromagnetic means such as for instance voice coils are provided which are mechanically connected to the vibratory portions of the diaphragm means to cause the latter to move toward and away from each other during oscillation of the voice coils.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic explanatory view showing a diaphragm according to the prior art;

FIG. 2 is a schematic explanatory view showing a diaphragm according to the present invention;

FIG. 3 is a schematic perspective view illustrating one form of a diaphragm according to the present invention;

FIG. 4 is a schematic perspective view showing a modification of the diaphragm arrangement shown in FIG. 3;

FIG. 5 is a schematic perspective view showing a further modification of a diaphragm according to the present invention;

FIG. 6 is a schematic top view of a loudspeaker according to the present invention;

FIG. 7 is a fragmentary enlarged view similar to FIG. 6;

FIG. 8 is a cross section taken along line III--III of FIG. 6;

FIG. 9 is a section taken along the line IXIX of FIG. 8;

FIG. 10 is a cross section taken along the line X-X of FIG. 1 1 of another loudspeaker according to the present invention;

FIG. 11 is a cross section taken along the line XI-Xl of FIG. 10;

FIGS. 12a and 12b are perspective views illustrating the manner of forming the diaphragm means used in the loudspeaker as shown in FIGS. 10 and 11;

FIG. 13a is a partial plan view illustrating a thin band of plastic material with wires applied thereto before formation of this band into a meander shape as for instance shown in FIG. 1 I;

FIG. 13b is a partial view showing the band of FIG. 13a in folded condition;

FIG. 14 is a cross-sectional view of a further loudspeaker according to the present invention, the cross section being taken along the line XIV-XIV of FIG. 15;

FIg. 15 is a partial top view of the loudspeaker shown in FIG. 14;

FIG. 16 is a partial cross-sectional view showing a modification of the arrangement shown in FIG. 14;

FIG. 17 is a schematic cross-sectional view through an indirectly driven loudspeaker according to the present invention;

FIG. 18 is a simplified exploded view of the loudspeaker shown in FIG. 17; and

FIG. 19 is an exploded view of an earphone or microphone according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As mentioned above, the present invention relata to diaphragms in which the effective specific mass (grams per square centimeter) can be reduced by at least a factor of 10 for acoustic frequencies without reducing the diaphragm thickness, and the application of such a diaphragm for sound windows, microphones and loudspeakers.

The principle of the diaphragm according to the present invention with reduced effective mass will now be explained in connection with FIGS. 1 and 2.

FIG. 1 schematically illustrates a plane diaphragm D connected along peripheral edges thereof to wall means W defining an opening of a width 2na so that the diaphragm separates the airspace to the left side of the diaphragm from the airspace of the right side of the diaphragm. FIG. 2 schematically illustrates a diaphragm according to the present invention likewise extending through an opening of a width 2m in which the diaphragm D is however folded into a meander shape. In FIG. 2 the width of each fold is designated with a and its depth with b. The ratio of width to depth of the folded diaphragm represents the effective mass reduction factor f.

Upon application of sound waves in the direction of the arrow A, as shown in FIG. I, the diaphragm D will be displaced in direction of the arrow for a distance c, whereby it is assumed for simplification reasons that the diaphragm moves parallel to itself over its full width. In the folded diaphragm as shown in FIG. 2 and assuming that the diaphragm sections a remain rigid without any motion, the diaphragm sections 12 will be moved in direction transverse to the direction of the arrow A for a distance d and in this case it is also assumed for simplification reasons that the diaphragm sections b move over their whole width. These assumptions make the evaluation simpler without appreciably affecting the final result.

Even though the total mass of the folded diaphragm shown in FIG. 2 is by the factor l/f bigger than the total mass of the planar diaphragm shown in FIG. I, it can be shown that the total kinetic energy of the moving diaphragm mass necessary for transmitting a certain air displacement per time unit across the diaphragm is in the case of the folded diaphragm, as shown in FIG. 2, smaller by the reduction factor f than that for the planar diaphragm shown in FIG. I. It is assumed that the thickness of the diaphragms D and D'- is the same and the width of both diaphragms is Zna. The relative displaced volumes are represented in the two FIGS. by the two areas 2min and Znbd. For the same air motion, these areas will be equal, that is 2nac--2nbd, which means that a/b=dlcq, or c=d[f. With the air motions taking place in the same time, the displacement velocities v and v must have the same ratio, i.e., vJv j". The mass ratio of the moving parts of the two diaphragms is 2na/2nb=f=m /m assuming the same thickness of both diaphragms. The ratio of the kinetic energy of the two diaphragms is m v lm v =flf =l If.

For the same kinetic energy in the air the folded diaphragm of FIG. 2 has therefore by the factor f less kinetic energy than the planar diaphragm shown in FIG. 1 or, in other words, the folded diaphragm is equivalent to a planar diaphragm which is by the factor f reduced in thickness or mass. The effective mass is 2naf or 2n a lb. The actual mass of the folded diaphragm shown in FIG. 2 is 2n(a+b). The ratio between the actual mass and the effective mass is therefore ab+-b /a =b/a +b /a=l/f+l/fi A ratio of ten for the length to the width of the folds is quite practical and results in a real to effective mass ratio of 110:1. This makes a sound window physically quite strong. In electroacoustical transducers, increase in real mass makes the current carrying elements stronger, the ohmic losses relatively low, while still preserving a good matchto the mass of air. For a ratio of b/a=l0, the real mass of a 0.00l-inch aluminum diaphragm in a loudspeaker of this kind corresponds to an effective air equivalent to 0.225 inch (5.7 mm.) thickness, which is low enough for the highest audible frequencies. The recoil forces on the other hand exerted by the moved air onto the diaphragm are very ineffective in moving the diaphragm on account of the great effective to real mass ratio which is M I0. The diaphragm therefore needs no mechanical support for acoustical rigidity. In the case of microphones, air equivalents of 1 mm. are easily obtained since only low powers have to be converted where thinner diaphragms can be used.

In electroacoustic transducers which utilize abovedescribed diaphragms, good efficiency has been obtained which is due to two factors: (I) the reduced effective membrane mass (air equivalent of 1-5 mm.) guarantees small blind energies (acceleration and deceleration of mass) and the kinetic energy of the diaphragm is directly transformed into air vibrations, or vice versa; and (2) the actual total mass of the driving diaphragm can be relatively big. Since this mass corresponds to the mass of the conductive metal, ohmic losses will be low.

In electroacoustic transducers, the vibratory portions of the diaphragm, which may for instance be formed from an audiocurrent carrying metal foil or from an insulating foil to which parallel audiocurrent carrying metal strips or wires are attached, can be arranged in two different ways respectively shown in FIGS. 3 and 5. The two arrangements shown in these two Figures differ mainly from each other in the direction in which sound is ejected. In the arrangement shown in FIG. 3, the diaphragm according to the present invention is formed by a thin metal foil 2 bent into meander shape, as shown in FIG. 3, so as to form a plurality of narrow air spaces located ad- 5 jacent to each other and being alternately closed at the top and the bottom. Thin plastic foils 4 are connected in any known manner to the front and rear edges of the meandershaped foil 2 to close the aforementioned airspaces at the front and the rear. The diaphragm or metal foil is arranged in a strong magnetic field. For the sake of clarity, the magnetic poles which produce the magnetic field are now shown in FIG. 3 but only the direction of the magnetic field is indicated by the arrows 6. In this arrangement, the substantially parallel portions of the diaphragm 2 will be caused to vibrate toward and away from each other upon passing of an audio current through the diaphragm or metal foil 2 and sound will be emitted from the airspaces in the direction as indicated by the arrows 8, that is sound waves will be emitted substantially normal to the direction of the magnetic field.

In the arrangement shown in FIG. 5, the diaphragm or metal foil [0 is bent into a wavy configuration, as shown in FIG. 5, to form a honeycomblike structure and adjacent band portions are mechanically connected, for instance by cementing, but insulated from each other in the regions 10' where they abut against each other. Thus a plurality of rows of airspaces are formed which are altematingly closed at the top and at the bottom by thin plastic webs 12 connected in any convenient manner to the upper and lower edges of the foil 10. Only the upper webs 12 are shown in FIG. 5, but it is to be understood that the airspaces shown open at the top in FIG. 5 are closed at the bottom by corresponding webs I2. The direction of the magnetic field is indicated in FIG. 5 by the arrow I4, that is the magnetic field extends normal to the direction in which an audio current is passed through the foil strip 10. In this arrangement sound will be emitted during passing of an audio current through the foil 10 in the direction as indicated by the arrow 16, that is sound emission will be in the direction of the magnetic field. While the magnetic poles producing the magnetic field are not shown in FIG. 5, it is to be understood that these magnetic poles have to be formed of spaced iron laminates of perforated sheets so as to be sound transparent.

The depth of the airspaces formed by the diaphragm portions is related to the highest frequencies which are to be transduced effectively. A depth of 10-15 mm. will give perfect efficiency up to l0-l5 kHz. The diaphragm may be fonned by a thin metal ribbon or metal tape and tapes of aluminum, magnesium, copper or silver of a thickness of 0002-00002 inch give good efficiency up to the highest audio frequencies. Instead of metal ribbons also thin Mylar tapes coated with any of the aforementioned metals may be used.

One disadvantage of the arrangements shown in FIGS. 3 and 5, in which thin plastic sheets which respectively extend in a plane are connected to the edges of the metal tape, is that the portions of the metal tape adjacent to the edges thereof which are fixedly connected to the plastic sheets are prevented from vibration so that in these portions a low countervoltage is built up and a greater current which increases the ohmic loss is passed through the tape. Various arrangements are possible to at least partly overcome this disadvantage. For instance, if metal-plated plastic tape is used for forming the diaphragm, longitudinal portions of the plastic tape extending along its opposite longitudinal edges may be left unplated to thus provide a greater flexibility to the tape in the region of the opposite longitudinal edges thereof which are connected to the plastic sheets which close the airspaces along the tape edges. 0n the other hand, in the arrangement as shown in FIG. 5 substantially cup-shaped plastic members may be used instead of the parallel web portions 12 for closing the airspaces between the connected tape portions 10'.

An especially advantageous arrangement is shown in FIG. 4 which is a modification of the arrangement shown in FIG. 3. In the arrangement as shown in FIG. 4, the airspaces formed by the meander-shaped metal tape 2 are not closed by thin plastic sheets connected to the longitudinally extending opposite edges of the tape as shown in FIG. 3, but sleeves 18 formed from thin plastic foil, for instance Mylar, are respectively arranged in the airspaces formed by the metal ribbon 2 in the manner as shown in FIG. 4 in which the sleeves l8 substantially close the airspaces at opposite sides while extending slightly beyond the opposite end edges of the ribbon 2, and these sleeves may be connected at the outer edges 18 thereof to a carrying structure, not shown in FIG. 4, to thus resiliently support the metal ribbon 2 while permitting the latter to oscillate properly substantially over the whole width thereof when an audio current is passed therethrough. It is to be understood that the metal ribbon 2 shown in FIG. 4 is arranged in a magnetic field as indicated by the arrow 6 in FIG. 3.

A flat loudspeaker assembly using the diaphragm according to the present invention in an electromagnetic field is schematically illustrated in FIGS. 6-9. The assembly includes a plurality of flat strips of permanently magnetized ferrites 24 arranged parallel and spaced from each other and two sets of soft iron pole pieces 26, each comprising a plurality of spaced strips of a configuration as best shown in FIG. 8 which respectively engage the opposite poles of the permanent magnets 24 and extend partly into the space between the permanent magnets 24 to define with the permanent magnets 24 open spaces in which the diaphragms 2 are respectively arranged. The conductive diaphragm strips are arranged in the aforementioned spaces in the manner as shown in FIG. 2 to form rows of airspaces between the wavy strip portions extending in the longitudinal direction of the permanent magnets 24, and shown in a simplified manner in FIG. 6, which airspaces are alternatingly closed at the top respectively at the bottom by the 126 portions of the corrugated strip as described in connection with FIG. 3. The longitudinally extending airspaces are also closed at opposite ends by their plastic webs 4 connected to the transverse edges of the strips. The conductive diaphragm strips 10 respectively located in the spaces formed between adjacent permanent magnets 24 may be connected at the ends thereof in series or in parallel to each other. When an audio current is passed through the conductive strips 10 they will be vibrated and sound waves will be emitted through the spaces between the spaced, strip-shaped soft iron pole laminations 26, that is in the direction of the magnetic field produced. The assembly shown in FIGS. 6-9 may be mounted in a sound horn extending beyond the speaker and separating the sound emission of the front and rear side of the loudspeaker from each other.

A symmetric loudspeaker arrangement using a folded diaphragm according to the present invention is shown in FIGS. 10 and 11. The advantages of this arrangement are its simple construction, the possibility to use an extended diaphragm in a narrow space and the adaptability of the arrangement for the use of an acoustic horn to extend the frequency range to the lowest frequencies.

In the arrangement shown in FIGS. 10 and 11 a circular magnetic field is generated between two permanent magnetic ferrite rings 28 and 30 which are arranged spaced from each other along a common axis and each of which may be provided at its face facing the other ring with a pole shoe 28, respectively 30' of soft iron. The folded diaphragm arranged in the space between the pole shoes 28' and 30' is preferably in the form as discussed above in connection with FIG. 4 and the arrangement may include a meander-shaped conductive foil 2, the parallel portions of which extend in substantially radial direction as best shown in FIG. 11, to define between themselves a plurality of adjacent airspaces which alternatingly open in radial outward and radial inward direction. Mylar sleeves 18 are again located in the airspaces as shown and described in connection with FIG. 4, which Mylar sleeves extend in axial direction slightly beyond the conductive foil 2 and which substantially close the airspaces formed between the parallel foil portions toward the pole pieces 28 and 30'. This arrangement assures, as discussed above, a greater flexibility of the radially extending portions of the diaphragm. The

thus-formed bellowslike structure can easily be bent into ring form while maintaining equal spacing between the substantially parallel portions of the folded diaphragm. The diaphragm may be held between the pole pieces of the magnets by coaxial rings 34 formed from rubber or plastic material respectively connected to the

pole pieces

28 and 30 in any convenient manner and engaging with end faces thereof the radially innermost and outermost portions of the folded diaphragm 2 while simultaneously closing the small remaining openings of the folded diaphragm which extend beyond the Mylar sleeves 18. The magnetic circuit can be magnetically closed on the inside and/or outside of the ring structure by iron bridges, not shown in the drawing, to increase the field strength. This is however not necessary on account of the great coercivity of the ferrite material from which the magnets are formed. The ends of the tape 2 are connected to a device, not shown in the drawing, to produce an audio current to cause oscillation of the substantially parallel portions of the diaphragm and sound waves emanating in radially outward and inward direction. A sound reflecting cone 32 is preferably arranged in the opening of the magnet ring 30 coaxially arranged therewith to axially deflect the sound waves which emanate from the airspaces formed by the diaphragm in radially inward direction, and a corresponding outwardly projecting horn 36 may be connected to the ring 28.

Instead of using a diaphragm formed from a thin metal ribbon it is, as mentioned above, also possible to use a thin plastic tape to. which narrow metal tapes or wires are attached which are connected in series with each other. Such a series connection may for instance be obtained by winding a narrow metal tape or wire 40 in a close spiral about a tape ring 38, as shown in FIG. 12a, and by subsequently folding the ring 38 into meander shape 38', as shown in FIG. 12b. To facilitate the folding of the ring 38 as shown in FIG. into the meandershaped configuration as shown in FIG. 12b, cutouts 39 may be formed at longitudinally spaced portions along opposite longitudinal edges of the plastic tape in the region in which the plastic tape is to be bent into the meander shape, as shown in a developed portion of this tape in FIG. 13a, and the wires 40 may be flattened as shown as 40 in the regions between the opposite cutouts 39 to thus facilitate the bending of the wires in these regions. The wires 40 may be attached to the carrying plastic tape 38 in any manner known in the art, or the wires may be sandwiched between two thin plastic tapes which are heat-sealed to each other. After folding the tape 38 into meander shape, as shown in FIG. 12b, the flaps 38a between the cutouts 39 are bent over as shown in FIG. 13b to close the cells at opposite ends.

Instead of winding a wire in a continuous closely spaced spiral about the annular plastic tape in the manner as shown in FIG. 12a, it is also possible to provide a plurality of parallel wires or parallel narrow closely spaced metal tapes on a plastic-carrying tape before the latter is formed into a ring as shown in FIG. 12a, to bend then the end portions of the plastic tape out of the plane of the latter and to connect then the end portions slightly offset to each other so that one end of a wire on one of the connected tape ends is connected to the corresponding end portion of the wire adjacent thereto on the other of the tape ends so that the parallel wire portions are connected at the joined ends of the plastic tape into a continuous spiral. The thus-formed ring with the serially connected wire portions thereon is then further formed into the meander shape as shown in FIG. 12b. This arrangement is especially advantageous if the conductive portions of the diaphragm are formed by narrow parallel and spaced conductive portions applied to the carrier tape by vapor deposition.

A further loudspeaker arrangement is shown in FIGS. 14 and 15. In this arrangement, the meander-shaped diaphragm portions are held under tension in direction transverse to their elongation as will be described now in detail. The arrangement disclosed in FIGS. 14 and 15 comprises two permanent magnets 42 arranged spaced and parallel to each other, as best shown in FIG. 14, and a pair of

end plates

46 and 50 extending transverse to the elongation of the magnets 42 respectively en gage and are connected in any convenient manner respectively to opposite ends of the permanent magnets 42. These end plates are formed from soft iron, whereas plates 44 of nonmagnetic material are respectively arranged on the sides of the permanent magnets 42 which face each other and connected thereto in any convenient manner to define between the inner faces thereof and the inner faces of the end plates 46 and 50 a prismatic space. Each of the end plates is formed with a plurality of substantially rectangular openings 48, as best shown for the plate 46 in FIG. 15, through which the aforementioned prismatic space communicates with the outside of the loudspeaker assembly. Two rows of spaced pole piece laminates 54 and 54' respectively project from the inner faces of the

end plates

46 and 50 towards each other and each of the pole piece laminates 54 is formed by two outer flat iron strips 56 and an iron strip 58 which is thinner but wider than the outer strips 56 to project inwardly beyond the inner ends of the latter. The flat iron strips forming each laminated pole piece may be connected to each other in any convenient manner. The meander-shaped diaphragm is located in the space between the laminated pole pieces t and 54'. The diaphragm comprises a thin plastic carrier tape 60 bent into meander shape and connected along longitudinal edge portions, for instance by cementing to the outer strips 56 of the pole pieces and a central conductive portion 62 which may for instance consist of a thin metal foil applied to the plastic carrier tape 60 or of a metallic coating applied, for instance, by vapor deposition to the carrier tape. The inner ends of the thin pole piece laminates 58 extend up to the longitudinal edges of the conductive portion 62 of the diaphragm. The laminates 58 have to be thin enough so as not to come into contact with the conductive portions 68 during oscillation of the diaphragm por tion. As shown in FIG. 14, the pole piece laminates 54 are offset in transverse direction with respect to the pole piece laminates 54 so that laminates 54 and 54' respectively close successive airspaces formed between the parallel portions of the meander-shaped diaphragm at the upper and the lower ends thereof. As further shown in FIG. 14, the laminates 54 abut against and are connected in any convenient manner at their upper ends to the inner face of the end plate 46, whereas the laminates 54 are slightly spaced at their lower ends from the inner face of the end plate 50 so as to form between the lower ends of the laminates 54' and the inner face of the end pla e 50 an airgap S3. The magnetic force created in this airgap will attract the laminated pole pieces 54 toward the end plate 50 and thus create in the carrier tape 68 a continuous tension in direction transverse to its elongation so that slackening of the tape in transverse direction will be positively prevented. The end plate 50 is preferably provided with a plurality of spaced notches 52, as shown in FIG. 14, so as to concentrate the magnetic flux between the end plate and the pole pieces 54' in the region of the lower ends of the latter so as to hold the laminates 54 in perfectly spaced alignment with each other.

A modified arrangement is shown in FIG. 16 in which the pole piece laminates 54, each formed in the same manner as described in connection with FIGS. 14 and 15, are each connected at their upper respectively lower ends to the

end plates

46 and 50, respectively. The diaphragm is again formed by a plastic carrier tape 60' bent into meander shape as described before, but the parallel portions of the meander shaped carrier tape 60 are corrugated as shown in FIG. l6. Longitudinally extending metal strips 62 are connected to the substantially flat portions of the corrugations, and these strips may be connected in parallel or in series with each other.

The diaphragm portions in FIGS. 14 and are shown substantially parallel to each other. A deviation from parallelism in such a way that the diaphragm portions are wider spaced at their open ends and closer spaced at the closed ends facilitates an easier and smoother motion of the air in and out of the gaps, especially at high-sound energy levels.

In low energy level transducers with small diaphragm amplitude, such as microphones and earphones, very narrow parallel diaphragm spacings can be obtained by the use of thin ferromagnetic coatings on the diaphragms. The magnetic polarization induced in these coatings by the permanent magnetic field aligns the diaphragms parallel to each other in the direction of the lines of force and keeps them at equal distances from each other by the uniform repelling forces between the induced equal poles in adjacent coatings. If these ferromagnetic coatings are asymmetrically arranged between the permanent magnet poles, being closer to one pole than the other, a one-sided force tends to pull them towards the closer pole. If the diaphragm is held on the other side it will be stretched by a constant, well-defined force which will never diminish. This force is more reliable than the force in an elastically stretched diaphragm, which is subjected to fatigue. This one-sided magnetic force is a result of magnetic field inhomogeneity near the pole piece. The extra mass added to the diaphragm by the ferromagnetic material is small.

The loudspeakers having diaphragrns of the type described have at the sound emitting opening in the forward and backward direction an actual air cross section which is half the size of the diaphragm package cross section, because half of the cross section consists of open and half of closed air pockets. The gradual increase of this half cross section to its full value can be done by extending the closing elements of the pockets in form of wedges into the airspace. At the ends of these wedges the full air cross section is obtained. This transition is more practically done with a smaller number of bigger wedges, which are not coordinated with the air'pockets in distance and are preferably arranged perpendicularly to the pockets. A still more practical way to make this transition is to use curved sheets of uniform thickness which create wedgeshaped curved airspaces serving the same purpose of gradual increase in air cross section. The curved sheets are very rigid. The arrangement requires less volume and is shorter than the one with wedges.

The continuous transition described here is of importance. If omitted, the transducer will show an air resonance at the frequency, for which the depth of the airspace between diaphragms equals a quarter wavelength of the sound emitted.

The loudspeakers thus far described are directly driven, that is the conductors through which an audio current is passed and which are subjected to the influence of a magnetic are either formed by the diaphragm itself or by conductors extending along and fastened to the diaphragm. The diaphragm according to the present invention may however also be used in a sound transducer, in which the diaphragm portions are indirectly driven, that is in an arrangement in which conductors through which an audio current is passed to cause oscillation of the conductors in a magnetic field, are not mounted on the diaphragm portion themselves, but are mechanically connected to the diaphragm portions to cause oscillation of the latter.

Such a loudspeaker arrangement is shown in section and in a simplified exploded view in FIGS. 17 and 18. As shown in these two Figures, the indirectly driven loudspeaker comprises a pair of conventional permanent magnet voice coil driving units 64 arranged coaxially and spaced in axial direction from each other and each including a permanent magnet 66 and a pair of

pole pieces

68 and 70 defining between themselves an annular gap 72 in which a voice coil 74 of standard construction is adapted to oscillate during passage of an audio current therethrough. Nonmagnetic spacer members 76 are arranged abutting against and connected to the pole pieces 70 and, in the axial space between the spacer member 76, there are arranged a plurality of diaphragm discs 78, which have, preferably, a configuration as best shown in FIG. 18, as well as a plurality of substantially U-shaped spacer members 80. best shown in H6. 18, sandwiched between and connected to peripheral portions of the diaphragm members 78 in any convenient manner, for instance by cementing. The U-shaped spacer members 80 are arranged so that the open ends 81 thereof altematingly face in opposite direction so as to form with the diaphragm discs 78 connected thereto airspaces which altematingly open to opposite sides of the loudspeaker. The voice coils 74 are respectively and altematingly connected to successive diaphragm discs 78 by a plurality of driving rods 82, that is the upper voice coil, as shown in FIG. 17, may, for instance, be connected by a plurality of rods 82 to the uppermost diaphragm disc 78, and these rods pass then through openings 84 in the second diaphragm disc from the top and are connected again to the third diaphragm disc, and so on, whereas the lower voice coil 74 is connected by corresponding rods 82 to the diaphragm discs which are not driven by the upper voice coil. The voice coils are arranged in a known manner to oscillate during passing of an audio current therethrough toward and away from each other so that the diaphragm discs 78 respectively connected to the upper and the lower voice coils 74 will vibrate during passing of an audio current through the voice coils toward and away from each other to produce thereby sound waves passing out from the airspaces between adjacent diaphragm discs through the open ends 81 of the U-shaped spacers 80 as indicated by the arrows 86 in FIG. 18. For simplification reasons only two U- shaped spacer members 80 and two diaphragrns 78 are shown in FIG. 18. If desired, a horn may be arranged at one of the sound-emitting apertures of the transducer illustrated in FIG. 17.

It is also possible to provide only a single voice coil 74 connected by rods 82 to every second diaphragm disc, whereas the other diaphragm discs are not driven mechanically. These not driven set of discs may in this case be made heavier so as to remain substantially stationary. Instead of forming each of the diaphragms as a double cone, as shown in FIGS. 17 and 18, it is also possible to use diaphragm discs of different configuration and these diaphragm discs may have semispherically or otherwise curved surfaces.

H6. 19 schematically illustrates in an exploded view an earphone or microphone using a diaphragm according to the present invention. As shown in the exploded view of H6. H9, the microphone illustrated therein may include a pair of circular cover plates 88 formed from soft iron and provided at a central portion thereof with a plurality of parallel sound emitting slots 90 and a permanent magnet ring 92 which in the finished assembly is sandwiched between and connected in any manner known in the art at upper and lower faces thereof to the cover plates 88. A baffle 94 of nonmagnetic material having an outer cylindrical surface of a diameter substantially equal to the inner diameter of the magnet ring 92 is placed in the interior of the latter and the baffle 94 is formed with a central opening of square cross section to define in the interior thereof a prismatic space 94. Arranged in this space 94 are two sets of pole pieces 96 and 96' extending spaced from each other transversely through the space and being respectively in engagement with and connected to the upper one and the lower one of the cover plates 88 in any convenient manner. A meander-shaped diaphragm which may include a thin carrier tape 98 and wires 100 connected thereto in the manner as described before to define between the parallel portions of the meander-shaped tape a plurality of airspaces which are alternatingly closed at the top by the pole pieces 96 is arranged and at the bottom by the pole pieces 96' offset with respect to the pole pieces 96. The meander-shaped carrier tape 98 is connected, for instance by cementing, at the upper and lower longitudinal edges thereof to the respective pole pieces. Instead of using a meander-shaped carrier tape it is also possible to use a plurality of flat sleeves arranged parallel and spaced to each other and each provided with a closely wound wire spir l. The wire spirals on each sleeve are then connected in parallel or in series with each other while a pair of leads are connected in a known manner, not shown in FIG. 19, to a device producing an audio current. In such an arrangement the pole pieces 96 of the upper set may close the upper ends of the sleeves while the pole pieces 96 of the lower set may be arranged to close the spaces between adjacent sleeves at the lower ends of the latter. During passage of an audio current through the conductors carried by the diaphragm, the parallel portions of the meander-shaped diaphragrns or the parallel portions of the flat sleeves will oscillate toward and away from each other so that sound waves will be emitted through the slots in the upper and lower cover plate.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of loudspeakers or microphones differing from the types described above.

While the invention has been illustrated and described as embodied in sound transducers, such as loudspeakers or microphones, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In an electromagnetic transducer, diaphragm means comprising a plurality of closely spaced substantially parallelly arranged at least in part vibratable diaphragm portions defining between themselves narrow airspaces; and means connected to each of said vibratable diaphragm portions closing each of said airspaces all around with the exception of one side which remains acoustically open, said open sides of adjacent airspaces facing in opposite directions, whereby upon induction of vibration of said vibratable diaphragm portions, the vibratable diaphragm portions of adjacent airspaces will vibrate in opposite directions.

2. Transducer diaphragm means as defined in claim I, wherein said means for closing said airspaces comprise arcuate connecting portions integral with said diaphragm portions so as to form with the latter an elongated meander-shaped diaphragm having longitudinal extending corrugated edges.

3. Transducer diaphragm means as defined in claim I, wherein said means for closing said airspaces comprise a plurality of spacer members respectively located between said diaphragm portions and connected to edge portions of the latter in such a manner that adjacent airspaces defined between adjacent diaphragm portions and the spacer members connected thereto altematingly open to opposite sides of said diaphragm means.

4. Transducer diaphragm means as defined in claim 2, wherein said means for closing said airspaces further comprise a plurality of sleeves respectively located in said airspaces formed by said meander-shaped diaphragm, said sleeves having opposite arcuate portions respectively projecting beyond said longitudinal edges of said diaphragm and substantially closing said airspaces in the region of said edges.

5. Transducer diaphragm means as defined in claim 2, and including means connected to said diaphragm portions for vibrating the same to produce sound waves which then emanate from the open ends of said airspaces.

6. Transducer diaphragm means as defined in claim 2, wherein said arcuate connecting portions are located substantially in two parallel planes extending substantially normal to said diaphragm portions.

7. Transducer diaphragm means as defined in claim 2, wherein said arcuate connecting portions are located substantially in two concentric circles.

8. Transducer diaphragm means as defined in claim 2, and including means for closing said airspaces at said opposite longitudinal edges of said diaphragm.

9. Transducer diaphragm means as defined in claim 3, wherein said spacer members are substantially U-shaped with the open ends of adjacent U-shaped spacer members facing in opposite directions.

10. Transducer diaphragm means as defined in claim 2, including means cooperating with said diaphragm portions for transforming vibrations imparted to said diaphragm means by sound waves into electrical energy and vice versa.

11. Transducer diaphragm means as defined in claim 10, wherein said transforming means comprise conductor means connected to said diaphragm portions and flux-producing means cooperating with said conductor means for producing sound waves creating vibrations of said diaphragm portions during application of alternating electric impulses to said conductor means and vice versa.

12. Transducer diaphragm means as defined in claim 11, wherein said flux-producing means are constituted by means for producing a magnetic field so that during passing of an audio current through said conductor means sound wavecreating vibrations will be produced by said diaphragm portions and vice versa.

13. Transducer diaphragm means as defined in claim 11, wherein said conductor means comprise wires extending along and being carried by said diaphragm means.

l4. Transducer diaphragm means as defined in claim 13, wherein said wires are connected to each other to form a continuous coil.

15. Transducer diaphragm means as defined in claim 13, wherein said wires are connected together to form a plurality of coils connected to each other in series.

16. Transducer diaphragm means as defined in claim 11, wherein said conductor means comprise a thin ribbon of electrically conductive material connected at least in part to said portions of said diaphragm means.

17. Transducer diaphragm means as defined in claim 11, wherein said diaphragm means comprises a thin ribbon formed from electrically conductive material and constituting said conductor means.

18. Transducer diaphragm means as defined in claim 11, wherein said diaphragm means are formed from plastic material and wherein said conductor means comprise a coating of electrically conductive material applied at least to portions of said plastic material.

19. Transducer diaphragm means as defined in claim 2, wherein said vibrating means include electromagnet means and connecting means mechanically connecting said electromagnet means to said diaphragm portions.

20. Transducer diaphragm means as defined in claim 19, wherein said electromagnet means comprise a voice coil movable in an electromagnetic field.

21. An eleclroacoustical transducer comprising, in combination, permanent magnet means for producing a magnetic field, said permanent magnet means defining a space communicating with the outer atmosphere; diaphragm means located in said space and comprising a plurality of adjacent at least in part vibratable diaphragm portions arranged to define between themselves a plurality of narrow adjacent airspaces, and means connected to each of said vibratable diaphragm portions closing each of said airspaces all around with the exception of one side which remains acoustically open, said open sides of adjacent airspaces facing in opposite directions; and conductor means in said magnetic field and fixedly connected at least to said vibratable diaphragm portions to cause during passage of an audio current through said conductor means oscillation of said vibratable diaphragm portions to produce sound waves to be emitted through said open sides of said airspaces, or to cause during vibration of said vibratable diaphragm portions by sound waves impinging thereon audio currents in said conductor means.

22. An electroacoustical transducer as defined in claim 21, wherein said permanent magnet means comprise a pair of permanent magnets arranged spaced from each other aligned along a common axis and each forming an annular airgap in which a magnetic field is produced, wherein said diaphragm means comprise a plurality of spaced substantially parallel diaphragms forming said vibratable portions and arranged coaxial with said permanent magnets in the space between the latter, and U-shaped spacer means sandwiched between adjacent diaphragrns and connected to peripheral portions thereof so as to form said connecting means between said vibratable portions, each of said U-shaped spacer means having an open end and said spacer means being arranged so that the open ends of successive spacer means face in. opposite directions, and wherein said conductor means are in the form of voice coils respectively arranged in said annular airgaps formed by said pair of permanent magnet means, and including rigid means respectively connecting said voice coils alternatingly to successive diaphragms.

23. An electroacoustical transducer as defined in claim 21, wherein said conductor means extend along and are carried by said diaphragm means.

24. An electroacoustical transducer as defined in claim 23, wherein said diaphragm means form an elongated meandershaped ribbon.

25. An electroacoustical transducer as defined in claim 23, wherein said magnet means comprise at least two elongated permanent magnets arranged spaced from and substantially parallel to each other and two sets of pole shoe members extending spaced from and substantially parallel to each other transversely between opposite faces of said permanent magnets to define between the latter a space in which said diaphragm means is located, said space communicating with the outer atmosphere through the spaces between the pole shoe members of each set of pole shoe members.

26. An electroaooustical transducer as defined in claim 24, wherein said permanent magnet means comprise a pair of annular permanent magnets arranged spaced from each other along a common axis, said meander-shaped ribbon forming said diaphragm means being arranged in an annulus coaxial with said annular magnets in the space between the latter, and including means for resiliently supporting said diaphragm means on the faces of said pair of permanent magnets which are directed towards each other and for closing the airspaces between the vibratable portions of said diaphragm means toward said faces.

27. An electroacoustical transducer as defined in claim 26, and including a sound reflecting cone in the central space of one of said annular magnets and projecting with its apex toward the other of said annular magnets, and a horn projecting outwardly from said other magnet.

28. An electroacoustical transducer as defined in claim 24, wherein said permanent magnet means comprises a pair of permanent magnets arranged spaced from and substantially parallel to each other and each having a pair of opposite end faces and a side face facing the other of said pair of magnets, a pair of nonmagnetic plates respectively abutting against said side faces of said magnets, a pair of end plates of magnetizable material respectively abutting against said end faces of said pair of magnets and extending between the latter and each being formed with sound transmitting openings therethrough, and two sets of pole piece laminates projecting in the space between said nonmagnetic plates from said end plates towards each other, said meander-shaped ribbon forming said diaphragm means being connected at opposite longitudinal edge portions respectively to said sets of pole piece laminates and the pole piece laminates of one set being offset with respect to those of the other set so as to alternatingly close the airspaces defined between the vibratory portions of said diaphragm means at the edges of the latter facing said end plates.

29. An electroacoustical transducer as defined in claim 28, wherein said diaphragm means comprises a plastic carrier tape connected at edge portions thereof to said pole piece laminates and said conductor means comprise a thin metal ribbon connected to a central longitudinal portion of said carrier tape, and wherein each of said pole piece laminates comprises a pair of outer plate members having inner ends ending short of said metal ribbon and a centrai plate sandwiched between and being thinner than said outer plate and extending beyond the latter in the region of the respective edge of said ribbon.

30. An electroacoustical transducer as definedin claim 28, wherein said pole piece laminates of one set abut and are connected at the outer ends to one of said end plates, whereas the outer ends of said pole pieces laminates of the other set are separated by a small airgap from the other end plate so as to be magnetically attracted thereto, whereby said plastic carrier tape is tensioned in direction transverse to its elongation.

31. An electroacoustical transducer as defined in claim 30, wherein said other end plate is provided between the pole piece laminates of said other set with notches.

32. An electroacoustical transducer as defined in claim 22, wherein each of said diaphragms includes a central substantially cone-shaped portion.

33. An electroacoustical transducer as defined in claim 24, wherein said permanent magnet means comprises a single annular permanent magnet having opposite end faces, a pair of end plates of magnetizable material respectively abutting against said end faces and provided with a plurality of openings communicating with the space defined in the interior of said annular magnet, and two sets of pole pieces respectively projecting from said pair of end plates into said space, said meander-shaped ribbon forming said diaphragm means with said conductor means carried thereby being connected at opposite longitudinal edge portions to the inner ends of said pole pieces and the pole pieces in one of the sets being offset with respect to those of the other set so that the pole pieces of each set altematingly close the open ends of the airspaces formed by said meander-shaped ribbon and facing toward said end plates.

34. An electroacoustical transducer as defined in claim 33, wherein said annular permanent magnet has an inner cylindrical surface, and including a baffle of nonmagnetic material fitted into the interior of said annular magnet, said baffle being formed with a central opening of substantially square cross section and said two sets of pole pieces with said meandershaped ribbon therebetween being located in said opening of said baffle.

* Q t i

Claims

2. Transducer diaphragm means as defined in claim 1, wherein said means for closing said airspaces comprise arcuate connecting portions integral with said diaphragm portions so as to form with the latter an elongated meander-shaped diaphragm having longitudinal extending corrugated edges.

3. Transducer diaphragm means as defined in claim 1, wherein said means for closing said airspaces comprise a plurality of spacer members respectively located between said diaphragm portions and connected to edge portions of the latter in such a manner that adjacent airspaces defined between adjacent diaphragm portions and the spacer members connected thereto alternatingly open to opposite sides of said diaphragm means.

12. Transducer diaphragm means as defined in claim 11, wherein said flux-producing means are constituted by means for producing a magnetic field so that during passing of an audio current through said conductor means sound wave-creating vibrations will be produced by said diaphragm portions and vice versa.

14. Transducer diaphragm means as defined in claim 13, wherein said wires are connected to each other to form a continuous coil.

21. An electroacoustical transducer comprising, in combination, permanent magnet means for producing a magnetic field, said permanent magnet means defining a space communicating with the outer atmosphere; diaphragm means located in said space and comprising a plurality of adjacent at least in part vibratable diaphragm portions arranged to define between themselves a plurality of narrow adjacent airspaces, and means connected to each of said vibratable diaphragm portions closing each of said airspaces all around with the exception of one side wHich remains acoustically open, said open sides of adjacent airspaces facing in opposite directions; and conductor means in said magnetic field and fixedly connected at least to said vibratable diaphragm portions to cause during passage of an audio current through said conductor means oscillation of said vibratable diaphragm portions to produce sound waves to be emitted through said open sides of said airspaces, or to cause during vibration of said vibratable diaphragm portions by sound waves impinging thereon audio currents in said conductor means.

22. An electroacoustical transducer as defined in claim 21, wherein said permanent magnet means comprise a pair of permanent magnets arranged spaced from each other aligned along a common axis and each forming an annular airgap in which a magnetic field is produced, wherein said diaphragm means comprise a plurality of spaced substantially parallel diaphragms forming said vibratable portions and arranged coaxial with said permanent magnets in the space between the latter, and U-shaped spacer means sandwiched between adjacent diaphragms and connected to peripheral portions thereof so as to form said connecting means between said vibratable portions, each of said U-shaped spacer means having an open end and said spacer means being arranged so that the open ends of successive spacer means face in opposite directions, and wherein said conductor means are in the form of voice coils respectively arranged in said annular airgaps formed by said pair of permanent magnet means, and including rigid means respectively connecting said voice coils alternatingly to successive diaphragms.

24. An electroacoustical transducer as defined in claim 23, wherein said diaphragm means form an elongated meander-shaped ribbon.

26. An electroacoustical transducer as defined in claim 24, wherein said permanent magnet means comprise a pair of annular permanent magnets arranged spaced from each other along a common axis, said meander-shaped ribbon forming said diaphragm means being arranged in an annulus coaxial with said annular magnets in the space between the latter, and including means for resiliently supporting said diaphragm means on the faces of said pair of permanent magnets which are directed towards each other and for closing the airspaces between the vibratable portions of said diaphragm means toward said faces.

29. An electroacoustical transducer as defined in claim 28, wherein said diaphragm means comprises a plastic carrier tape connected at edge portions thereof to said pole piece laminates and said conductor means comprise a thin metal ribbon connected to a central longitudinal portion of said carrier tape, and wherein each of said pole piece laminates comprises a pair of outer plate members having inner ends ending short of said metal ribbon and a central plate sandwiched between and being thinner than said outer plate and extending beyond the latter in the region of the respective edge of said ribbon.

30. An electroacoustical transducer as defined in claim 28, wherein said pole piece laminates of one set abut and are connected at the outer ends to one of said end plates, whereas the outer ends of said pole pieces laminates of the other set are separated by a small airgap from the other end plate so as to be magnetically attracted thereto, whereby said plastic carrier tape is tensioned in direction transverse to its elongation.

33. An electroacoustical transducer as defined in claim 24, wherein said permanent magnet means comprises a single annular permanent magnet having opposite end faces, a pair of end plates of magnetizable material respectively abutting against said end faces and provided with a plurality of openings communicating with the space defined in the interior of said annular magnet, and two sets of pole pieces respectively projecting from said pair of end plates into said space, said meander-shaped ribbon forming said diaphragm means with said conductor means carried thereby being connected at opposite longitudinal edge portions to the inner ends of said pole pieces and the pole pieces in one of the sets being offset with respect to those of the other set so that the pole pieces of each set alternatingly close the open ends of the airspaces formed by said meander-shaped ribbon and facing toward said end plates.

34. An electroacoustical transducer as defined in claim 33, wherein said annular permanent magnet has an inner cylindrical surface, and including a baffle of nonmagnetic material fitted into the interior of said annular magnet, said baffle being formed with a central opening of substantially square cross section and said two sets of pole pieces with said meander-shaped ribbon therebetween being located in said opening of said baffle.