US2951190A - Electro-mechanical transformer - Google Patents

Description

Filed April 22, 1955 M. BAERMANN ELECTRO-MECHANICAL TRANSFORMER 2 Sheets-Sheet 1 2| I3 33 N s 5 s N FIG. 4

INVENTOR.

MAX BAERMANN ATTORNEYS Aug. 30, 1960 Filed April 22, 1955 FIG. 2

FIG. 3

M. BAERMANN ELECTRO-MECHANICAL. TRANSFORMER 2 Sheets-Sheet 2 27 so A N s 5 5 N N S 33 3| I2 l4 u INVENTOR. MAX BAERMANN ATTORNEY United States Patent ELECTRO-MECHANICAL TRANSFORMER Max Baermann, Bensberg Wulfshof, Koln, Germany Filed Apr. 22, 1955, Scr. No. 503,236

Claims priority, application Germany Oct. 28, 1954 14 Claims. (Cl. 317173) This invention pertains to the art of electro-mechanical transformers, and more particularly to apparatus for transforming variable electric currents into mechanical vibrations or vice versa, such mechanical vibrations either being converted into or the result of acoustic vibrations.

Numerous devices are known for the purpose of converting variable electric currents into mechanical vibrations or vice versa. They are generally known as loud speakers or microphones or pickups. As the invention is particularly adaptable to loud speakers, the invention will be described with particular reference thereto, although it will be appreciated that it has broader applications.

In the =art of loud speakers as now known, a current carrying conductor is normally mounted for movement in a fixed magnetic field. Varying electric currents flowing in this conductor create a magnetic field which reacts with the fixed magnetic field to cause the conductor to move proportional to the amount of current flowing. This movement is normally then transmitted to a diaphragm wherein the movements of the conductor are converted to acoustical energy.

With microphones or pickups a similar arrangement is provided. A conductor is movably mounted in a fixed magnetic field. Movement of this conductor due to external forces generates electric voltages proportional to the amount of movement. These electric voltages may then be amplified, as is known.

In either the case of the loud speaker or microphone, magnetic field must be employed which have a considerable strength. The production of such strong fields involves a very considerable expenditure for high alloy permanent magnet material. This considerable expenditure results from the fact that a considerable weight of material is required, and the cost of the material per unit of weight is relatively high.

T reduce the weight of the permanent magnet material required, it has been proposed in the past to employ a fixed electrical conductor having a ferro-magnetic circuit with an air gap, and a permanent magnet movably supported in the air gap, and connected to a mechanically vibrating element. With such an arrangement, the problem has existed of an initial mechanical stress on the movable magnet supports and the diaphragm due to the attraction of the movable magnet for the ferro-magnetic pole pieces. Such initial stress is particularly detrimental to a high fidelity transformer.

Also, in either of the above described arrangements, special electrical coupling means must be employed to either match the high impedances of the electronic amplifying tubes to the necessarily low impedance of the electrical conductors, or to isolate, in the case of a loud speaker, the plate current of such tubes from the conductor, or both. If this plate current were not isolated from the electrical conductor, then its magnetic field about the conductor would react with the fixed magnetic field of the permanent magnet, whether fixed or movable,

resulting in a displacement of the movable member from its normal resting position, and an initial stress on the diaphragm.

The present invention contemplates an electro-mecham'cal-vibration transformer including a ferro-magnetic circuit consisting of a conductor and core having an air gap, and a permanent magnet movably mounted in the air gap, and connected to a mechanically-vibrating element which overcomes all of the above referred to objections and others, is simple in construction, economical to manufacture, and is capable of transforming mechanical vibrations into electrical current or vice versa with high fidelity.

In accordance with the invention, the permanent magnet connected with the mechanically-vibrating element is placed in the air gap of the ferro-magnetic circuit in such a manner that it can vibrate or move parallel to the pole surfaces forming the air gap, such magnet being mag netized perpendicularly to its direction of movement. The permanent magnet in its normal resting position in the air gap is so arranged that when viewed in a crosssectional plane parallel to the line of movement, the pole surfaces of the magnet and the limiting surfaces of the air gap only partly overlap.

With such an arrangement an electric current proportional to the size of a sound vibration flovw'ng through the electrical conductor, produces poles on each side of the air gap proportional in their strength to the current flowing through the coil. Under theinfluence of this electromatic magnetically formed pole, the permanent magnet will vibrate in the air gap and move the diaphragm of the loud speaker in accordance with its own movements.

Further in accordance with the invention, and in some respects preferably in accordance with the invention, two permanent magnets oppositely magnetized may be provided in the air gap of the ferro-magnetic circuit, such magnets being spaced from each other in the line of movement. Alternatively, two ferro-magnetic circuits with opposite polarization can work simultaneously on one permanent magnet. Both such arrangements have the result that the magnetic forces tending to bias or pull the magnet in any direction can be balanced out and the supporting elements for the magnet can be freed from initial stress in the undeflected position.

In one embodiment of the invention the permanent magnet may be rod-shaped and magnetized transversely to its lengthwise axis. This shape is especially adapted to large diameter loud speakers when several driving systems must work on the same diaphragm simultaneously. It can also be used advantageously for microphones.

Further in accordance with the invention, the perma nent magnets can also be ring-shaped and radially magnetized. With this form of construction it is possible to have the greatest area of opposed pole surfaces and magnet surfaces working on each other in the smallest volume or space. Also, this construction offers many advantages; for example, the fastening of the elastic supporting parts for the magnet; the connection of the magnet with a cone-shaped diaphragm such as is now customary in building dynamic loud speakers, and the fastening the loud speaker to housings or other surrounding apparatus are made simple and easy.

The magnet material employed may be as desired, but preferably the magnet material should have a coercive force of at least one thousand oersteds, although greater or less force may be employed. Basically, a high coercive force insures that the permanent magnet when subjected to an external magnetic field of a specified size, will not become de-magnetized. Also, the greater the coercive force the higher will be the efficiency of converting acoustic energy to electrical energy or vice versa. Permanent magnet material, such as manganesebismuth, iron-barium-oxide, and the like are especially suitable. Materials having a low Weight-to-coercive-force ratio should also be employed.

Further in accordance with the invention, the ferromagnetic circuit can be adjustably rare-magnetized by either an electro or preferably by a permanent magnet, the purpose of such pre-magnetization being to balance out any magnetizing effects of the ferro-magnetic circuit brought on by direct currents flowing in the coils of this ferromagnetic circuit, such as would result if the coils were placed directly in the anode circuit of the final amplifier tube or tubes. The need for an outlet transformer between this final amplifier tube and a loud speaker is thus avoided. if this pre-magnetization is obtained by electro-coils, the amount of pre-magnetization may be controlled by either varying the current in the coils or the number of turns, or both. If permanent magnets, as preferred, are employed, the amount of pre-magnetization may be determined by adjusting the amount of magnetic material in the ferro-magnetic circuit, or by providing an adjustable shunt for portions of the magnetic field of the pre-magnetizing magnet. Normally, the pre-magnetizing would be adjusted to such a value, that it equals and opposes the magnetizing function of the direct current flowing in the anode circuit of the final amplifier tube and in the coils of the loud speaker.

Further in accordance with the invention, the ferromagnetic circuit may take the form of one or a pair of three-legged cores with the center leg broken to define the air gap and the windings wound on this center leg on both sides of the air gap.

The principal object of the invention is the provision of a new and improved electro mechanical-vibrating transformer which is simple in construction, economical to manufacture, and which has improved fidelity characteristics.

Another object of the invention is. the provision of a new and improved electro-acoustic transformer employing a movable permanent magnet connected to a mechanically-vibrating element and a ferro-magnetic cirouit having an air gap in which the magnet is positioned wherein the supporting elements for the magnet when it is in a position of rest are not under any mechanical stress.

Still another object of the invention is the provision of an electro mechanical-vibratingtransformer having permanent magnet connected to and movable with a mechanical movable element on a pre-determined line of movement wherein the permanent magnet is polarized transversely to such line of movement.

Still another object of the invention is the provision of a new and improved loud speaker which does not require the use of current isolating or impedance matching devices between the final electronic amplifying tube and the loud speaker.

Still another object of the invention is the provision of a new and improved electro-acoustic transformer such as a loud speaker, so arranged that the plate current of the electronic tube energizing the loud speaker may flow through the coils thereof without imposing an initial stress on the diaphragm or movable elements.

Still another object of the invention is the provision of a new and improved electro-acoustic transformer such as a loud speaker, having means for adjustably compensating for and neutralizing the effects of the electronic amplifier tube energizing it whereby the movable members will not have any initial stress imposed thereon in theresting position. 7

Still another object of the invention, having a movable permanent magnet or magnets connected to a mechanically-vibrating element and disposed within the air gap formed by spaced surfaces of a term-magnetic circuit, the poles of the permanent magnet or magnets and the surfaces of the air gap being so arranged that magnet variations induced in the ferro-magnet-ic circuit cause a simultaneous pushing and pulling effect on the magnet or magnets, whereby to obtain a maximum energy transformation.

Still a further object of the invention is the provision of a new and improved electro mechanical-vibrating transformer including a ferro-magnetic circuit having an air gap and permanent magnet arranged in the air gap in such a manner as to present a maximum area of permanent magnet surface to the ferro-magnetic circuit air gap surface.

Still another object of the invention is the provision of a new and improved electro-acoustic transformer employing a movable permanent magnet so shaped that several driving systems are working on the same diaphragm simultaneously.

The invention may take physical form in ceitain parts and arrangement of parts, preferred embodiments of which will be described in detail in this specification, and illustrated in the accompanying drawing, which are a part hereof, and wherein:

Figure 1 is a cross-sectional view of a loud speaker system embodying the present invention;

Figure 2 is a view similar to Figure 1 of an alternative embodiment of the invention, and in particular showing a pre-magnetizat-ion arrangement with an adjust-able shunt for controlling the degree of pre-magnetization;

Figure 3 is a view similar to Figure 1 illustrating a still further embodiment of the invention.

Figure 4 shows a still further alternative embodiment, the view being taken approximately on the line 4-4 of Figure 5; and

Figure 5 is a cross-sectional view of Figure 4 taken approximately on the line 5-5 thereof.

Referring now to the drawings wherein the showings are for the purposes of illustrating preferred cmbodiments of the invention, and not for the purposes of limiting same, Figures 1 and 2 show a loud speaker including a mechanically-vibrating element in the form of a cone 10, a ferro-magnetic circuit including a core .11 and an electrical winding 12 and a permanent magnet indicated generally at A connected to the cone 10 and positioned in an air gap 13, defined by spaced pole surfaces 14, 15 in the ferromagnetic circuit.

. The core 11 may in accordance with the invention take a variety of different forms and shapes, but in the em Ibodiment shown is generally cup-shaped including base -17, a side 18 and an upper edge 19 which turns inwardly to form the pole surface 14. A central post or stem 20 extends generally centrally upwardly through the cup and terminates in an outwardly extending flange 11 the outer edge of which is formed by the inner pole surface 15, and which as shown is directly opposite from and of the same width as pole surface 14.

The core 11 may be of any desired magnetic material having high permeability and low magnetic losses. It may either be formed from powdered material or laminated as is known in the art.

The winding 12 is wound about the central stem 29 and insulated therefrom by a layer of electrical insulation 23. Terminal leads, not shown, may extend outwardly through the core.

The cone 10 may be of any desired material and is known in the loud speaker art. it is resiliently mounted on the upper end of the stem 20 by means of a resilient disc or washer 25 fastened at its outer ends to the cone 10, and supported on the upper surface cf the stem 2% by means of a screw 26 and spacing sleeve 27. The washer 25 is made resilient by a plurality of arcuate cuts 29 forming a labyrinth connection from the outer edges of the washer 25 to the central portion thereof. Obviously, any other form of resilient mounting for the cone 10 may becmployed.

invention shown is comprised of two annular rings 30, 31 spaced in the line of movement by a washer 33, and connected to each other and to the inner end of the cone in any suitable manner. The washer 33 may be constructed of any light weight non-magnetic material, polystyrol preferred.

The magnets 30, 31 are supported in accordance with the invention for movement in a direction parallel to the pole surfaces 14, 15, and are magnetized transversely to this line of movement which is indicated generally by the double-headed arrow 35. Thus, the magnets 30, 31 are in the shape of annular rings with the inner and outer edges of these rings being oppositely polarized. The magnet 30 in the embodiment shown has the outer edge magnetized with a north polarity, while the inner edge has a south polarity. The ring 31 is oppositely magnetized from the ring 30. Thus, both a north and south pole are presented to the pole surface 14; likewise, both a north and south polarization are presented to the pole surface 15.

It will be further noted from Figure 1 that the poles of the magnets 30, 31 overlap or extend beyond the limits of the surfaces defining the pole surfaces 14, 15.

While two magnets 30, 31 are provided, it will be appreciated that a single magnet having poles substantially equivalent to the four poles shown could be employed.

For the purposes of describing the operation of the loud speaker of Figure 1, let it first be assumed that no current is flowing in the winding 12. With such an arrangement the north and south poles of the magnets 30, 31 respectively are attracted outwardly to the outer pole surface 14. As they are symmetrically arranged, however, to the edges or limits of the surface 14, there is no longitudinal force exerted. In a similar manner the south and north poles of the magnets 30, 31 respectively are attracted toward the pole surface with a force equal and opposite to the attraction of the outer poles to the outer pole surface 14. Thus, these forces are automatically balanced. As the inner pole surfaces are also symmetrical to the inner pole surface 15, it will be seen that the magnets are under no stress when no current is flowing in the coil 12.

If it be assumed that a current flows in the coil 12 a magnetic field is immediately developed, which for the purposes of illustration may be assumed to form the outer pole surface 14 with a north polarization, and the inner pole surface 15 with a south polarization. Under such an electro-magnetization the north pole will tend to repel the north polarization of the magnet 30, and attract the south polarization of the magnet 31. In a like manner the south polarization of the inner pole surface 15 will repel the south polarization of the magnet 31. The effect is to cause the magnets to move upwardly in an amount proportional to the strength of the poles 14, 15. It will be noted that there is in effect a pushing and a pulling effect on the permanent magnet.

If the current in the coil 12 is then reversed, the polarization of the surfaces 14, 15 is reversed and the magnets are then caused to move in the opposite or downward direction in a similar manner to that described above.

If desired, adjusting means can be provided to accurately position the magnet 30 along the line of movement relative to the pole surfaces 14, 15. Also, the spacing between the pole surfaces and the surfaces of the magnet may be varied as desired.

In describing the operation of the embodiment of the invention shown in Figure 1 it was assumed that there was no D.C. magnetizing current flowing through the winding 12. If such a current had been flowing, then the magnets 30, 31 would have been moved in a direction depending upon the polarization caused by such current and in an amount proportional thereto. The cone 10 and the spring washer would have thus been pres tressed. Any variations in the current in the coil 12 would thus move the magnets from this prestressed resting position, which is considered undesirable, and with the embodiment of the invention shown in Figure 1 it is preferred that some means be provided for isolating the winding 12 from the plate current of the electronic tube energizing the loud speaker.

Figure 2 shows an embodiment of the invention wherein means are provided for neutralizing or counteracting the eifect of this DC current flow in the coil 12. In the embodiment of the invention shown in Figure 3 like parts will be referred to with like numbers. Similar parts will be referred to with like numbers with a prime mark added.

Thus, in accordance with the embodiment of the invention shown in Figure 2, the stem 20' is formed separately from the base 17 of the cup and has an outwardly extending flange 35 at the lower end thereof. An annular magnet 36, polarized in an axial direction substantially as shown, is disposed between the lower surface of the flange 35 and the upper surface of the base 17. An adjusting slug 38 of ferro-magnetic material extends through an opening 39 in the base 17', and is supported on an adjusting screw 40 threaded into the lower end of the stem 20. The slug 38 is arranged to fit co-axially into the opening of the magnet 36. By adjustably positioning the slug 38 in the opening 39, a variable magnetic shunt may be provided for the flux lines as indicated by the dashed line 41.

The effect of the magnet 36 is to cause a pro-polarization or pie-magnetization of the pole surfaces 14, 15 in an amount such that when the coil 12 has the plate current of an electronic tube flowing therethrough, the two magnetizing forces will be equal and opposite, and the sum total of the two magnetizing forces at the pole faces 14, 15 is zero. Thus, the magnets 30, 31 will have in the no signal position, a position without stress or strain on the cone. The adjustment of the plug 38 enables the loud speaker to be adjusted for varying plate currents as might be encountered in practice.

Otherwise the operation of the speaker shown in Figure 2 is identical to that of Figure 1.

It will be noted that in all instances the magnetic forces on the pole faces 14, 15 are always equal and opposite and thus do not tend to pull the magnets 30, 31 to one side or the other, but only to cause them to move in a direction parallel to the pole surfaces 14, 15.

The magnets 30, 31 may be formed of any desired material, although the permanent magnet material such as manganese-bismuth,iron-bariurn-oxide are preferred, lecause of their high magnetic force and relatively light weight. Obviously, any desired magnetic material can be employed, however.

Referring now to Figures 4 and 5, wherein a still further alternative embodiment of the invention is shown, the ferro-magnetic circuit is comprised of a three-legged core arrangement including outer legs 50 and an inner central leg 51 split at the middle to define an air gap 52, defined by oppositely facing pole surfaces 53, 54. Two of such ferro-magnetic circuits are employed, the pole surfaces of the lower being indicated by the numerals 53' and 54', with the air gap 52' defined there by being aligned with the air gap 52. These two ferromagnetic circuits are supported in spaced relation with the air gaps 52, 52 aligned as above described by means of bolts 56 extending through the corners of the core, and with spacing washers 57 accurately locating the two ferro-magnetic .circuits.

The speaker cone 10" is mounted on an elastic washer 25, essentially similar to the one described with reference to Figure l for movement in a line of movement parallel'to the pole surfaces 53, 54. Two such washers 2.5 are provided, one at each end of the two ferromagnetic circuits. A rod 60 of non-magnetic material extends between the two'washers 25 and supports for movement with the cone 10 a permanent magnet 62,

, 62 as viewed in a plane through the line of movement.

The permanent magnet in the embodiment shown in Figure 4 generally has a rectangular cross section with its polarized sides being presented tothe air gap'52, 52'. These surfaces of the magnet 62 are provided with 'a shoe of non-permanent magnetic material 64 reduced in thickness at its midsection as shown-at 65.

The purpose of this shoe 64 is to provide a "path of high permeability for the surfaces of the magnet 62. Ordinarily the magnetic material employed, such as manganese-bismuth or iron-barium oxide, has a low permeability, and the shoe 64 assists in directing the flux lines from the pole surfaces 53, 54 and 53', 54 into the magnetic material of the magnet 62. As shown the shoe 64 is cut away at 65 so as to reduce the weight of the shoe'd i to the minimum.

Each leg 51 and 51 of the ferro-magnetic circuit are provided with electrical windings 70, all Of which are preferably identical in size and number of turns.

These windings are so interconnected that when a current fiows therethrough, the polarization of the pole surface 53 will be equal and opposite to the polarization of the pole surfaces 53 and 54, and the :same as the pole surface '54. In effect diagonally opposite pole surfaces are the same, While adjacent pole surfaces are opposite.

It will be noted that the magnet 62 is symmetrically disposed relative to the pole surfaces 53, 53', 54, 54'. When no current is flowing in the coils 70, the magnet 62 and the cone will be unstressed and in the position shown. If a current flows through the coils 70- to produce the polarization shown in the drawings, that is with the pole surface 53 a north pole, and likewise for the other poles, the effect will be that the north polarization of the pole surface 53 and the south polarization of the pole surface 54 will repel the nolth and south poles respectively of the magnet 62 causing the magnet to move downwardly. In a like manner the south polarization of the pole face 53' and the north polarization of the-pole face 54' will attract the north and south polarization of the magnet 62, and further draw the magnet 62 downwardly. When the current flows in the coils in the opposite direction, a direct opposite effect will result, and the magnet 62, the rod 60 and the cone 10 will be forced in the opposite direction in an amount directly proportional to the current flowing in the coils 70 With the arrangement shown in Figure 4 it is preferred that the DC. magnetizing current of the final electronic amplifying tube be excluded from the coils 7G, particularly when only a single amplifying tube is employed.

However, where the final electronic amplifying tubes are arranged in a push-pull arrangement, the connections of the coils 70' may be so connected that the steady D.C. plate currents produce equal and opposite and therefore bucking magnetic fields, resulting in no polarization of the pole surfaces when there is no signal.

Alternatively, the fer-ro-magnetic circuit may be premagnetized in 'a manner somewhat similar to that described with reference to Figure '2. Thus, Figure 3 shows an-embodiment of the invention in which the leg 50 has a split in which a permanent magnet 72 is positioned, such permanent magnet being polarized in the longitudinal direction of the leg 50' as shown. The split may be perpendicular to this direction or at an angleas shown. .The magnet 72 is adjustable in a direction parallel to the surfaces of the split. -By moving the magnet 72 into and out of the split, the amount of prernagnetization introduced 'into the leg 50 can be adjusted from a maximum toa minimumsothat any magnetizing effects of the DC. currents in the coils 76 can be adjustably balanced out. Obviously, mag

netic shunts, similar to the shunt member 38 in Figure 2 could be employed to effect such'adjust'ment.

Obviously, one magnet'will be employed initheu'pper leg 5i?" and one in the lower leg 50, and in both ferromagnetic cores. I V

The invention has been described with particular reference to loud speakers. Obviously, the cone 10 or it) could be the diaphragm of amicrophoneor other elements capable of picking up =or'of being mechanically vibrated. In such case the forces vibrating the cone 10 or '10 would move the magnet in a direction parallel to the pole surfaces and cause a variable magnetic flux to thread through the coil 12. Such variable magnetic flux will, as is known in the art, generate a voltage in the coil 7d or the coil 12, which can then be amplified in accordance with known means.

Thus, it will be seen that embodiments ofthe invention have been described which accomplish all of the objects of the invention, and others which provide an electromechanical vibration transformer which is simple in operation and capable of high fidelity sound reproduction.

The invention has been described in detail with reference to preferred embodiments thereof. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification, and it is my intention to include allsuch modifications and alterations insofar as they come within the scope of the appended claims.

Having thus describedby invention, I claim:

1. An electro-acoustic transformer comprising in combination a ferro-m'agnetic circuit having spaced parallel pole surfaces defining an air gap, a mechanically vibrating element, a permanent magnet positioned in said 'air gap and connected to and movable with said element, winding means inductively associated with said ferromagnetic circuit and operable to induce a magnetizing force in said ferromagnetic circuit in response to magnetizing direct current applied'to said winding means, and pre-magnetizing'means associated with said ferromagnetic circuit for producing therein a magnetic force equal and opposite to said magnetizing force induced in response to the'direct current applied to said winding means, whereby the pole surfaceshave substantially no magnetic polarityin the absence of a'signal current applied to said windingmeans.

2. An 'electro acoustic transformer, comprising in combination a ferro-magnetic circuit, including electric conductor means, and having parallel pole surfaces defining an air gap, a mechanically vibrating element, a permanent magnet disposed within said air gap and'connected to and movable with said element in a direction parallel to said pole surfaces, and pre-magnetizing means for said ferro-maghetic circuit.

3. A combination of claim 2 wherein said pre-magnetizing means comprise a permanent magnet in series with the ferromagnetic circuit.

4. The combination of claim 3 wherein the amount of permanent magnet in series with'the ferro-magnetic cir' cuit is adjustable.

5. An electro-acoustic transformer, comprising incombination a pair of ferro-magnetic circuits, each including electric conductor means and each having spaced parallel pole surfacesdefining air gaps, a mechanically vibrating element, and a permanent magnet disposed in both of said air gaps, and connected to and movable with said element parallel to said pole surfaces, said 'ferro-magnetic circuits having opposite magnetic polarization.

6. An electro-acoustic transformer, comprising in com bination apa'irof ferro-magneticcircuits, each comprised of a three-legged core with the center leg of 'eaclt' core having-asplit forming pole surfaces defining aligned air gapsyacoil on said center leg on each side of said split, a permanent magnet 'disposed in said air gaps and mov- 9 able in a direction parallel to said surfaces, said magnet having an axis of polarization in a direction transverse to the line of movement, and a mechanical vibrating element connected to and movable With said magnet.

7. The combination of claim 6 wherein permanent magnet means are in series with the legs of said core and premagnetize said pole faces.

8. An electro-mechanical transducer comprising a ferro-magnetic unit having magnetic core means with a gap therein and winding means on said core means, said magnetic core means presenting confronting pole faces at opposite sides of the gap, a permanent magnet unit disposed loosely between said pole faces for movement in the gap substantially parallel to said pole faces, said permanent magnet unit comprising permanent magnet means polarized substantially perpendicular to said pole faces and presenting opposite polarity poles which face toward said pole faces at the opposite sides of the gap and are spaced from said pole faces, and a vibratory member coupled to said permanent magnet unit to move therewith, one of said units at one end of the gap presenting a first pair of opposite polarity magnetic poles at the opposite sides of the gap and at the other end of the gap presenting a second pair of opposite polarity magnetic poles at the opposite sides of the gap, the poles of the second pair having a polarity at each side of the gap opposite to the polarity of the correspondingly disposed poles of the first pair.

9. The transducer of claim 8 wherein said magnetic core means presents said first and second pairs of opposite polarity magnetic poles at the opposite sides of the gap.

10. The transducer of claim 8 wherein said permanent magnet unit presents said first and second pairs of opposite polarity magnetic poles at the opposite sides of the gap.

11. The transducer of claim 10 wherein said gap in the magnetic core means is annular, and said permanent magnet unit is ring-shaped and is radially polarized.

12. The transducer of claim 8 wherein there is provided permanent magnet means in series with the magnetic loop through said magnetic core means and across said gap.

13. The transducer of claim 12 wherein there is provided means for adjusting the magnetic force of said permanent magnet means in said loop.

14. The transducer of claim 13 wherein said adjusting means comprises magnetically permeable shunt means inductively associated with said magnetic core means and adjustable with respect thereto.

References Cited in the file of this patent UNITED STATES PATENTS 1,752,909 Hoxie Apr. 1, 1930 2,073,561 McCracken Mar. 9, 1937 2,141,820 Paredes Dec. 27, 1938 2,435,587 Harry Feb. 10, 1948 2,492,255 Angehrn Dec. 27, 1949 2,507,708 Greener May 16, 1950

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