US 3919498 A
An electroacoustic transducer having a pair of spaced confronting permanent magnet plates and a vibratable diaphragm disposed between the magnet plates and carrying a conductor. The magnet plates have circular concentric poles on their confronting faces with poles of like polarity registering with each other across the diaphragm. The conductor on the diaphragm follows a spiral path and each zone of the diaphragm confronting an interpolar transitional surface region of the magnetized plate faces is provided with a plurality of spiral convolutions of the conductor. The spiral convolutions of adjacent zones have opposite directions.
Description (OCR text may contain errors)
United States Patent 11 1 Beer 1 1 ELECTROACOUSTIC TRANSDUCER  Filed: June 14. 1973 1211 Appl. No.1 369,825
 Foreign Application Priority Data June 20. 1972 Sweden 8109/72  [1.5. CI 179/1155 PV: 179/1155 H  int. Cl. H04R 9/02  Field of Search 179/1155 PV. 115.5 R.
 References Cited UNITED STATES PATENTS 3.013.905 12/1961 Gamzon et 179/1155 R 3.139.491) 6/1964 L \ons 179/1155 PV 3.2091184 9/1965 Gamzon et a1. 179/1155 P\" 3.283.086 11/1966 Evans 262.911) 12/1926 United Kingdom 1 1 Nov. 11, 1975 1.329.295 7/1962 France 179/115 5 484.339 111/1929 German} 179/1155 P\' 1.430.700 4/1965 France 179/1155 H Primary E.\'aminerl(athleen H. Claffy Assistant Erumz'ner-George G. Stellar Attorney. Agent. or Firm-Hill. Gross. Simpson. Van Santen. Steadman. Chiara & Simpson  ABSTRACT An electroacoustic transducer having a pair of spaced confronting permanent magnet plates and a vibratable diaphragm disposed between the magnet plates and carrying a conductor. The magnet plates have circular concentric poles on their confronting faces with poles of like polarity registering with each other across the diaphragm. The conductor on the diaphragm follows a spiral path and each zone of the diaphragm confront ing an interpolar transitional surface region of the magnetized plate faces is provided with a plurality of spiral convolutions of the conductor. The spiral convolutions of adjacent zones have opposite directions.
12 Claims, 7 Drawing Figures U.S. Patent Nov. 11, 1975 FIG. 3
B /4 ZEN Hm w I v N ELECTROACOUSTIC TRANSDUCER BACKGROUND OF THE INVENTION This invention relates to electroacoustic transducers of the electrodynamic type for use in loudspeakers. earphones or microphones. More particularly. the invention relates to electroacoustic electrodynamic transducers of the type having a vibratable diaphragm which carries a conductor for electrical current and is disposed in a permanent magnetic field. Transducers of this type are illustrated and described in US. Pat. Nos. 3,013,905 and 3,l64,686.
An advantage of transducers of this type is that they can easily be constructed such that the diaphragm vibrates substantially uniformly in phase over its entire movable surface. However. known transducers of this type have a construction such that they are relatively expensive to manufacture and, moreover, the conductor cannot easily be given the high impedance that is desirable especially in the case of microphones and earphones. The present invention provides an electrodynamic electroacoustic transducer which avoids these and other drawbacks.
BRIEF SUMMARY OF THE INVENTION In accordance with the invention there is provided an electrodynamic electroacoustic transducer comprising a permanent magnet having a magnetized face. a vibratable diaphragm confronting this face in spaced parallel relation therewith, and a continuous flexible conductor which is carried by the diaphragm. The magnetized face of the permanent magnet provides a north pole and a south pole and one of the poles forms a closed loop around the other. A transitional surface region of the magnetized face separates the two poles, and the magnetic field produced between the poles thus traverses this surface region. The diaphragm is disposed in this magnetic field and the conductor on the diaphragm is arranged such that the lines of the mag netic field intersect it substantially at right angles in the zone of the diaphragm confronting the transitional surface region. Moreover, in that zone of the diaphragm the conductor comprises a plurality of spiral convolution so that each line of force of the magnetic field bridging the transitional surface region intersects the conductor a plurality of times.
In a preferred embodiment the magnet is a plate of a magnetically anisotropic material which has been magnetically oriented with the direction of preferred magnetization generally parallel to the face confronting the diaphragm, so that the magnetic field exists predomi nantly at that face and only minimally at the opposite face. In this way a strong magnetic field can be obtained.
A stronger and more uniform magnetic field surrounding the conductor may be obtained if two similar permanent magnets are used which are disposed on op posite sides ofthe diaphragm with poles oflike polarity in register with each other across the diaphragm.
The above and other features and advantages will be clear from the following detailed description of exemplary preferred embodiments illustrated in the appended drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. I is a view in axial section of a transducer ac' cording to the invention intended for use in an ear phone;
FIG. 2 is a view on line II-ll of FIG. I;
FIG. 3 is a view in axial section of the magnets of the transducer shown in FIG. 1:
FIG. 4 is a view on line IVIV of FIG. 3;
FIG. 5 is a view in axial section ofa modified embodiment;
FIG. 6 is a plan view, generally corresponding to FIG. 2, of the diaphragm of the embodiment of FIG. 5;
FIG. 7 is a plan view. generally corresponding to FIG. 4, of the magnetized face of one of the magnets of the embodiment of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION In the drawing the dimensions and spacings of the elements are somewhat disproportionate as compared to the actual dimensions and spacings. Examples of actual dimensions and spacings will be given hereinafter in order that the invention may be better understood.
Referring to FIGS. 1 to 4, these figures show an electroacoustic transducer 10 intended to be used in an earphone. The sound generating element is a generally flat, circular diaphragm 11 which is clamped along its peripheral portion between two rings I2 held in place in a circular housing 13 by a retaining clip 14. The diaphragm 11 is made from a polyester film ofa thickness of about 0.0l mm and has circular corrugations I5 concentric with the peripheral portion and thus is very flexible. To one side of this film there is laminated an aluminum foil of a thickness of about 0.0l mm. From this foil a continuous conductor 16 is formed which forms a multiplicity of spiral convolutions defining a current path between peripheral and center portions 17 and 18 of the foil as described in more detail hereinafter.
A pair of flat, circular permanent magnet plates 20 are mounted in the housing [3 on opposite sides of. and equally spaced from, the diaphragm 11. The confronting faces 21 of the plates are concentric with and parallel to each other and to the diaphragm and define between them an air gap 22 in which the diaphragm can vibrate. The center portion of the diaphragm and the center portion of the plates are clamped together by means of a bolt 23 and nuts 24. Spacers 2S and 26 on the bolt and flanges on the rings 12 maintain the diaphragm and the plates in the proper spaced relation.
Each plate 20 is provided with a multiplicity of perforations 27. The walls of each perforation form a exponential horn directed away from the face 2] and having its axis parallel to the axis of the plate.
The plates 20 are magnetized is such manner that the confronting faces 21 each have three poles, namely a circular north pole at the center. a circular south pole between the periphery and the center and a circular north pole along the periphery. Thus, each plate produces two annular concentric permanent magnetic fields having a common south pole. The magnetization is shown diagrammatically in FIGS. 3 and 4 where the perforations 27 have been omitted for clarity. From these figures it is seen that the direction of polarization is radial and that the magnetic fields exist predominantly at the face 21 and only minimally at the opposite face of each plate. This will be explained in more detail as the description proceeds.
In FIG. 4 the magnetic poles are represented by concentric annular fields designated N and S and separated by annular fields designated 28, hereinafter referred to as transitional surface regions. It should be noted that this representation is for illustrative purposes only, and that in actual fact the poles and the transitional surface regions are not as sharply defined as FIG. 4 indicates but merge gradually. Therefore, the annular fields N and S actually show only the centers of the poles, and the annular fields 28 show regions in which the magnetic fields are strong. Since both plates are magnetized in exactly the same manner, the poles and the transitional surface regions of both plates are congruent.
The plates may be formed from any suitable permanent magnet material but preferably are made from a highly coercive magnetically anisotropic material, such as plastic-bonded barium ferrite or a cobolt rare earth magnet material (one material of the last-mentioned class is known as GECOR and sold by General Electric Company). In the illustrated embodiment, the plates are made from the first-mentioned material (known as Ferroxdure), and before the magnetization of the plates the material has been oriented such that the plates have a preferred direction of magnetization that is radial and generally parallel to the confronting faces 21. As a consequence. the magnetic fields exist predominantly at the confronting plate faces 21 and only minimally at the opposite faces. and in addition they can be made very strong.
As shown in FIG. 2 the conductor 16 forms a continuous current path between the peripheral and central portions 17 and 18. More particularly. the conductor forms a multiplicity of spiral convolutions 30 about the central portion 18. In the interest of clarity only relatively few convolutions are shown while in the actual embodiment the number of convolutions may be several hundred; thus, the width of the conductor may be of the order of0.l mm and the spacing of the convolutions may be of the order of 0.01 mm.
The direction of the convolutions is reversed about halfway between the peripheral and the central portion, forming respective conductor sections, so that a current flowing in the conductor reverses direction at a point that is opposite the south poles of the magnet plates, that is. opposite the place where the magnetic field in the air gap 22 changes direction. Thus, the spiral convolutions 30 cover two concentric annular zones of the diaphragm 11, one outer zone confronting the outer transitional surface regions 28 of the magnet plates 20 and one inner zone confronting the inner transitional surface regions.
The conductor 16 is electrically connected to two terminal members, one formed by a lug 31 secured to the peripheral foil portion 17 and the other formed by the bolt 23 and the spacer 25 which are in electrical contact with the central foil portion I8. Through these terminals the transducer can be connected in the output circuit of an audio amplifier. for example.
The general mode of operation of the transducer shown in FIGS. I to 4 is similar to that of the known transducers of the same type. It should be noted. however, that by virtue of the described arrangement of the conductor in a multiplicity of spiral convolutions, the conductor can be made very long to have the high impedance that is often desired. Moreover. as a consequence of the threepole magnetization of the magnet plates and the reversal of the direction of the spiral convolutions, the influence of disturbing exterior mag- 4 netic fields is eliminated; the conductor acts as a bifilar winding.
In FIGS. 5 to 7 a transducer IA is shown in which the diaphragm 11A has the conductor 16A on both sides and is held stationary only at its peripheral portion [7A. Although no corrugations on the diaphragm are shown, the diaphragm may be provided with circular corrugations. Moreover, the two permanent magnet plates 20A are magnetized such that they have four circular and annular poles of alternating polarity. see FIG. 7. Although no perforations are shown. it will be appreciated that the magnet plates preferably are acoustically transparent.
The conductor configuration on one side of the diaphragm is the same at both sides of the diaphragm 11A and shown in FIG. 6 in which in the interest of clarity the conductor is shown as a single spiral line between the peripheral and central foil portions 17A and 18A. Since there are three transitional surface regions 28A on the confronting faces of the magnet plates, the sprial convolutions 30A reverse direction twice. The two portions of the conductor on either side of the diaphragm are electrically connected with each other at the center of the diaphragm as shown at 32 in FIG. 5. The peripheral foil portions 17A are electrically connected with a pair of terminal lugs 31A.
Although in the illustrated embodiments the diaphragm and the permanent magnet plates are circular, these elements may be square, rectangular, or oval or have any other suitable shape. Naturally, the shape of the spiral convolutions must correspond to the shape of the poles and the transitional surface regions. Moreover, in some applications it may be sufficient to have a magnet only on one side of the diaphragm.
While in most cases the best results are obtained if the permanent magnet plates have a relatively large number of poles, two poles may sometimes give satisfactory performance. In that case that is only one transitional surface region and all spiral convolutions run in one and the same direction.
What is claimed is:
1. An electrodynamic electroacoustic transducer comprising:
a. a unitary permanent magnet plate having a plurality of magnetic poles on one face thereof at least two of the poles extending along respective closed loops around a central pole. with said poles alternating in polarity. and forming respective transitional surface regions separating adjacent poles of opposite polarity. the opposite face of said plate being directed to the atmosphere;
b. a flexible diaphragm, of nonmagnetic and electrically nonconducting material, having a peripheral portion held stationary with respect to the magnet plate and a vibratable portion, disposed in the magnetic fields defined by said poles. bridging the tran' sitional surface regions between adjacent poles, and in spaced confronting relation with said one face of the magnet plate; and
c. a continuous flexible conductor for carrying electrical current. said conductor being affixed to the vibratable diaphragm portion confronting the tram sitional surface regions between said pairs of adjacent poles, said conductor comprising a plurality of concentric sections, each section having a plurality of mutually adjacent unidirectional spiral convolutions, with the convolutions of each section corifronting a respective transitional surface region S and reversed in direction with respect to the convolutions of any section adjacent thereto.
2. An electrodynamic electroacoustic transducer according to claim 1, wherein said flexible diaphragm also has its central portion held stationary with respect to the magnet plate; and a pair of fixed terminal members disposed respectively at the central portion and the peripheral portion of said one face of said magnet plate and electrically connected with respective ends of said conductor.
3. An electrodynamic electroacoustic transducer according to claim 1, wherein said magnetic plate has a central magnetic pole and two annular magnetic poles concentrically disposed with respect thereto, and said conductor comprises two sections.
4. An electrodynamic electroacoustic transducer according to claim 1, wherein said magnetic plate has a central magnetic pole and three annular magnetic poles concentrically disposed with respect thereto, and said conductor comprises three sections, the innermost and outermost of which have the convolutions thereof running in the same direction, with the intermediate section having its convolutions reversed with respect to the other two sections.
5. An electroacoustic transducer as set forth in claim I, including a second unitary permanent magnet plate having a plurality of magnetic poles on one face thereof with the pattern of such magnetic poles and intrapolar transitional surface regions thereof being congruent with the pattern of magnetic poles and intrapolar transitional surface regions of the first-mentioned magnet plate, the two magnet plates being disposed on opposite sides of the diaphragm with the poles and the transitional surface regions of one magnet plate facing and registering respectively with the poles and the transitional surface regions of the other magnet plate, the oppositc face of said second magnet plate being directed to the atmosphere.
6. An electroacoustic transducer as set forth in claim 1 in which the magnet plate is formed with a multiplicity of perforations. the walls of which form exponential horns having their axes extending transversely of said one face. each perforation in the magnet plate connecting the air space between said diaphragm and said magnet plate with the atmosphere.
7. An electroacoustic transducer as set forth in claim 1 in which said permanent magnet plate is magnetized predominantly at said one face and only minimally at the opposite face.
8. An electroacoustic transducer as set forth in claim 7 in which the magnet plate is made from a magneti cally anisotropic material and thus having a direction of preferred magnetization. its actual magnetism being aligned with said direction of preferred magnetization, said actual magnetism being generally parallel to said one face at the transitional surface region.
9. An electroacoustic transducer as set forth in claim 1 in which said one face of the magnet plate is circular and flat and the poles and the transitional surface region are annular and concentric with said one flat face.
10. An electroacoustic transducer as set forth in claim 9 in which the vibratable diaphragm portion has circular corrugations concentric with said one flat face.
11. An electroacoustic transducer as set forth in claim 9 which includes means secured to said magnet plate and holding the center portion of the diaphragm in stationary spaced relation to said magnet plate.
12. An electroacoustic transducer as set forth in claim 11 in which said holding means also is a fixed terminal member connected to one end of the conductor.