US3358089A - Magnet assembly - Google Patents

Description

Dec. 12, 1967 R. J. PARKER MAGNET AS SEMBLY Filed June 10, 1964 Eai.

INVENTOR. ROLLIN J. PARKER ,4 r TOR-NE) 3,358,089 Patented Dec. 12, 1967 3,358,089 MAGNET ASSEMBLY RollinJ. Parker, Greenville, Mich., assignor to General Electric Company, a corporation of New York Filed June 10, 1964, Ser. No. 374,050 6 Claims. (Cl. 179-117) ABSTRACT OF THE DISCLGSURE A permanent magnet loudspeaker in which the permanent magnet assembly comprises a cup-shaped per- "manent'magnet and a cylindrical pole piece integrally "cast within the cup-shaped magnet. The crystal structure of themagnet is oriented in the direction of magnetic "orientation. Such directional grain magnets are produced by casting'the molten alloy against a cylindrical chill plate-Upon completion of solidification of the magnetic 'alloy, the permanent magnet and chill plate are an integral unit,with the chill plate acting as pole piece.

This invention relates to a permanent magnet assembly adapted'for use in a loudspeaker and to a process for producing such permanent magnet assembly.

Much progress has been made in the direction of miniaturizing-loudspeaker magnets and, in particular, in producing loudspeakers of reduced depth without sacrifice 'of' soundqualityf However, further miniaturization of permanent'magnet loudspeakers faces an obstacle inherent in the design of conventional loudspeakers themselvesr-To maintain a'given air gap density across the -'voice coil of a speaker requires a magnet of a corresponding predetermined strength. If the'length'of a magnet' is shortened to decrease the depth of the'speaker,

.of higher air gap densities than any other permanent -rnagnet assembly presently known.

' It is anadditional object of this invention to provide a permanent magnet assembly in which the diameter of a-themagnetmay be varied through a wide range, permit- .ting greatfiexibility. in the design of permanent magnet speakers, including a reduction in their depth.

It is stillan additional object of this invention to providea process for producing a permanent magnet assem- .bly by casting: a directional grain permanent magnet against a chill plate whereby, upon solidification, the chill plateand cast permanent magnet form a unitary structure, v

These an d other objects of the invention are achieved gin a'permanent magnet assemblyin which the position .-of .the permanent magnet and .the return path are, in effect, reversed. A cup-shaped permanent magnet, oriented ra- .dially, that is in a direction along the path of the magnetic circuit of the speaker magnet assembly, is adapted ,to fitexternally around the voice coil and a cylindrical or similarly shaped pole piece cast integrally within the cup-shaped permanent magnet is adapted to fit within the voice coil. The direction of orientation is referred to as "radialwbecause it is oriented generally along a path from the center to the outer extremity or lip of the cup- .shaped magnet. It is intended to distinguish such orientat i on from orientation in a single axial or single diametric United States Patent CH direction. The permanent magnet is cup-shaped and is radially oriented so that its preferred direction of magnetization lies along a path generally following the flow of magnetic flux to and from the air gap. One pole of the magnet is at the central portion of the cup-shaped magnet in contact with the integral pole piece. The second pole is at the opposite extremity or lip of the cupshaped magnet.

The invention will be more clearly understood from the following description taken in connection with the accompanying drawing in which:

FIG. 1 is'an elevational crosssectional view of a loudspeaker incorporating a permanent magnet assembly illustrating one embodiment of the invention;

FIG. 2 is an enlarged fragmentary view of the speaker of FIG. 1, showing with greater clarity the structure of the permanent magnet assembly;

FIG. 3 is a bottom elevational view of the permanent magnet assembly of FIGS. 1 and 2;

FIG. 4 is a perspective view of the permanent magnet assembly shown in FIGS. 1 and 2; and

FIGS. 5 and 6 are elevational cross-sectional views of other embodiments of speaker magnet assemblies of the invention.

Referring to FIGS. 1 and 2, the loudspeaker comprises a supporting frame or basket 1, at the outer edge of which a flange 2 supports frusto-couical diaphragm 3 by means of a securing ring 4. A flexible spider 5 is secured at its outer periphery to basket 1. A voice coil 6 is wound on a voice coil form 7 and is coupled to diaphragm 3. A permanent magnet assembly 8 is secured by rivets 9 to the supporting frame of the speaker.

As can be most clearly seen in FIGS. 2 through 4., the permanent magnet assembly 8 comprises a cup-shaped permanent magnet 10 and a soft steel cylindrical pole piece 11, located centrally within the cup-shaped permanent magnet. The cup-shaped magnet has a rim 12 at one end thereof and an aperture 13 in the base portion at the other end thereof. The magnetic circuit is completed by a soft steel washer 14 and disc 15. Washer 14 contains three recesses 16, each of which receives one of the rivets 9 which, in combination with frame 1 and recessed surfaces 17 and 18 in the side and bottom surfaces, respectively, of the permanent magnet holds the washer in place against the rim orlip 12 of the permanent magnet and secures the magnet assembly to the frame of the speaker. The disc 15 may be held on the pole'piece 11 either magnetically or by welding, brazing or other conventional means of attachment.

As magnetic flux moves across the air gap 19 to washer 14 (or in the reverse direction to disc 15), the flux traverses voice coil 6 which drives diaphragm 3 to which it is coupled in accordance with the varying electrical signals impressed on the coil. As illustrated by the arrows in FIG. 2, the magnetic flux .follows substantially the same path as the magnetic flux circuit of a permanent magnet speaker. It can thus be seen that the return path is, in effect, eliminated in the present invention by making the return path itself a permanent magnet.

The magnetic circuit of the speaker is completed by the simple addition of an annular washer at the rim of the permanent magnet and a disc on the pole piece. It is preferable that the depth of cup-shaped magnet 10 be substantially the same as the depth of pole peice 11 so that the upper surface of the pole piece and of rim 12 are in a single plane as illustrated in the drawings. It is possible to eliminate disc 15 by using a pole piece having In conventional speakers, to of the magnetic potential (H) is unavoidably lost in the steel return yoke. In the present invention, the yoke and joints are essentially eliminated so that substantially all magnetic potential is used in the air gap surrounding the voice coil. Moreover, the diameter of the voice coil does not limit the diameter of the permanent magnet so that great flexibility is possible in the design of the permanent magnet assembly. The result is that a speaker of very shallow depth is made possible by increasing the diameter of the permanent magnet to virtually any dimension desired to compensate for the decrease in its length resulting from a shallower speaker design. Miniaturization of speaker magnets with respect to depth is thereby made possible without loss of total magnetic energy or speaker performance.

An additional advantage of the invention is that it makes possible the useof varying diameter voice coils with a single permanent magnet assembly. This is accomplished by simply attaching a disc of smaller or greater diameter and a washer of correspondingly decreased or increased internal diameter to the permanent magnet assembly. This feature of the invention is illustrated in FIGS. 5 and 6. In FIG. 5, permanent magnet assembly 8 is made adaptable for use with a voice coil of smaller diameter than that of FIG. 2, by simply utilizing a disc 20 of reduced diameter together with a washer 21 of decreased internal diameter. Washer 21 is provided with a rim 22 of greater depth than the body of the washer so that clearance is provided at the air gap between pole piece 11 and washer 21. In FIG. 6, the permanent magnet assembly is made adaptable for use with a voice coil of larger diameter by use of a disc 23 of larger diameter and a washer 24 of increased internal diameter. Thus a single permanent magnet assembly may be used, for example, for both a highfrequency tweeter requiring a smaller diameter voice coil (FIG. 5) and a low-frequency woofer requiring a larger diameter voice coil (FIG. 6).

A still additional advantage of the present invention resides in the utilization of the chill plate, used in producing the radially oriented directional grain permanent magnet, as the flux collecting component or pole piece in the final speaker assembly. Highest energy products in permanent magnets are obtained by orienting the crystal structure of the magnets in the direction of their magnetic orientation. Such directional grain magnets are ideally suitable for use in loudspeakers where higher energy products result in higher speaker performance. Orientation of the crystal structure of such magnets takes place by casting the molten magnetic alloy against a chill plate which extracts heat from the molten alloy during solidification from one direction in the casting and results in the growth of a columnar grain structure in the preferred direction of magnetization. The production of such directional grain permanent magnets is more fully disclosed in U.S. Patent 2,578,407, the disclosure of which is hereby incorporated by reference.

In the present invention, the chill plate itself becomes the pole piece of the permanent magnet assembly. Pole piece 11 (FIGS. 1 and 2) is the chill plate used in preparing the directional grain permanent magnet. Upon completion of solidification of the magnetic alloy, the permanent magnet and chill plate form an integral unit forming the permanent magnet assembly of the invention, with the chill plate now acting as a pole piece.

In an illustrative process for practicing the present invention, a molten alloy of the type commonly known as Alnico 5 and consisting. essentially of about 8% aluminum, 14% nickel, 24% cobalt, 3% copper, balance substantially iron, is poured into a cavity mold corresponding in shape to the cup-shape of the permanent magnet. The mold cavities are arranged so that solidification of the cast material by loss of heat to or through the side walls of the cavity is held to a minimum. A soft steel insert plug, placed centrally in the cup-shaped mold, and in contact with the molten alloy at the central aperture thereof, acts as a metal chill and removes the heat liberated during solidification of the alloy. When solidified, the magnetic alloy and the centrally disposed metal chill form an integral assembly with the metal chill becoming the pole piece.

The directional or anisotropic properties of the permanent magnet are produced by subjecting the solid permanent magnet castings to a heat treatment in a magnetic field wherein the direction of the heat-treatingfield is made to coincide with the direction of final magnetization, Le, a radial direction along the path of the magnetic circuit. Such magnets exhibit magnetic properties which are appreciably higher in one direction than in a direction perpendicular to the preferred direction. The process for so directionalizing the magnets is described, for example, in Jonas U.S. Patent 2,295,082.

The directional grain permanent magnets to which the present invention is particularly directed are of the ironcobalt-nickel-aluminum alloy type commonly known as Alnico 5. While directional grain Alnico. 5 permanent magnets are the preferred magnetic materials useful in the practice of the present invention, other permanent magnetic materials may obviously be used so long as they are useful in the production of high energy permanent magnet loudspeakers.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A permanent magnet assembly adapted for use in a permanent magnet loudspeaker comprising acup-shaped directional grain permanent magnet having arim at a first end thereof and a hole at the opposite basel e'nd thereof and a pole piece cast integrally within the hole in the base of the cup-shaped magnet, the magnet being oriented crystallographically and magnetically in a direction along the contour of the cup-shaped magnet from the hole in the base to the rim thereof, said direction of orientation adapted to coincide with the path of the magnetic circuit in said loudspeaker.

2. The permanent magnet assembly of claim- 1 in which the permanent magnet is Alnico 5.

3. A permanent magnet assembly adapted for use in a permanent magnet loudspeaker comprising a cup+sha=ped directional grain permanent magnet having a rim at a first end thereof and a hole at the opposite base end thereof and a cylindrical pole piece cast integrally and cent-rally within the hole in the base of the cup-shaped magnet, the depth of said cylindrical pole piece being substantially the same as the depth of said cup-shaped permanent magnet, the magnet being oriented crystallographicallyandmagnetically in a direction along the contour of the cupshaped magnet from the hole in the base to the rim thereof, said direction of orientationadapted to coincide with the path of the magnetic circuit in said loudspeaker.

4. In a permanent magnet loudspeaker, a permanent magnetic assembly comprising a cup-shaped directional grain permanent magnet having a rim atone end thereof and a hole at the opposite base end thereof and a pole piece cast integrally within the hole in the base ofthe cupshaped magnet, the magnet being oriented crystallographically and magnetically in a direction along the contour of the cup-shaped magnet from the hole in the base to the rim thereof, said direction of orientation adapted to coincide with the path of the magnetic circuit in said loudspeaker, a washer mounted in contact with the rim of said cup-shaped magnet, and a pole piece element mounted on the pole piece of said permanent magnet assembly, said permanent magnet assembly, washer and pole piece element together forming the complete magnetic circuit for said loudspeaker.

5. In a permanent magnet loudspeaker, a permanent magnet assembly comprising a cup-shaped directional grain permanent magnet having a rim at one end thereof and a whole at the opposite base end thereof and a pole piece cast integrally and centrally within the hole. in the base of the cup-shaped magnet, the magnet being oriented in a direction'along the contour of the cup-shaped magnet from the hole in the base to the rim thereof, said direction of orientation adapted to coincide with the path of the magnetic circuit in said loudspeaker, an annular washer mounted in contact with the rim of said cup-shaped magnet, and a disc-shaped pole piece element mounted on the cylindrical pole piece of said permanent magnet assembly, said disc-shaped pole piece having a diameter less than the diameter of said cylindrical pole piece, said permanent magnet assembly, washer, and pole piece element together forming the complete magnetic circuit for said loudspeaker.

6. In a permanent magnet loudspeaker, the permanent shaped pole piece element has a diameter greater than the diameter of said cylindrical pole piece.

References Cited UNITED STATES PATENTS KATHLEEN H. CLAFFY, Primary Examiner.

magnet assembly set forth in claim 5 in which the disc- 15 MCGILL- Assistant Examiner-

Claims (1)

1. A PERMANENT MAGNET ASSEMBLY ADAPTED FOR USE IN A PERMANENT MAGNET LOUDSPEAKER COMPRISING A CUP-SHAPED DIRECTIONAL GRAIN PERMANENT MAGNET HAVING A RIM AT A FIRST END THEREOF AND A HOLE AT THE OPPOSITE BASE END THEREOF AND A POLE PIECE CAST INTEGRALLY WITHIN THE HOLE IN THE BASE OF THE CUP-SHAPED MAGNET, THE MAGNET BEING ORIENTED CRYSTALLOGRAPHICALLY AND MAGNETICALLY IN A DIRECTION ALONG THE CONTOUR OF THE CUP-SHAPED MAGNET FROM THE HOLE IN THE

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