US3440364A - Speaker magnet having curved preferred direction of magnetization - Google Patents

Description

April 22, 1969 J. PARKER SPEAKER MAGNET HAVING CURVED PREFERRED DIRECTION OF MAGNETIZATION Filed D90. 21. 1965 PIC-3.2

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2 INVENTOR ROLLIN J. PARKER T RN'Y United States Patent US. Cl. 179-117 4 Claims ABSTRACT OF THE DISCLOSURE A permanent magnet has a preferred direction of magnetization along a curved path, the open ends of which intersect the side surfaces of the magnet. The magnet is particularly useful in permanent magnet speakers as it simplifies the magnetic circuit in such speakers and concentrates the magnetic flux at the air gap.

This invention relates to permanent magnets and permanent magnet assemblies for use in speakers and to a process and apparatus for the production of such magnets and magnet assemblies.

Permanent magnets ordinarily used in permanent magnet speakers are anisotropic, i.e., they are oriented along a preferred direction of magnetization. This is accomplished by subjecting the permanent magnets, during cool ing of the magnet in its magnetic hardening process, to a magnetic field coinciding with the preferred direction of magnetization. Such method of orientation is disclosed, for example, in Jonas US. Patent 2,295,082.

Typical permanent magnet speakers contain a permanent magnet, a pole piece and one or more return path elements, all arranged in magnetic circuit relationship. The magnetic flux in such circuits flows axially through the magnet, turns 90, and flows radially across an air gap to interact with a voice coil. Thus the path of the magnetic circuit is generally rectangular. To enhance the performance of such speakers, the magnets are oriented along a preferred direction of magnetization which runs axially through the magnet to coincide with the path of the magnetic circuit. The structure of permanent magnet assemblies in loudspeakers is, to a large extent, dependent upon and determined by the path of the magnetic circuit in such permanent magnet assemblies, which in turn is closely interrelated with the preferred direction of magnetization or orientation of the speaker magnet.

It is a principal object of the present invention to provide a speaker magnet which is oriented along a path which permits substantial improvements in the design of permanent magnet circuits used in loudspeakers.

It is an additional object of this invention to provide a permanent magnet assembly for use in loudspeakers which is simpler, has fewer parts and nevertheless posesses greater magnet efficiency than presently known magnet assemblies.

The objects of this invention are generally achieved by the provision of a permanent magnet having a principal or preferred direction of magnetization lying along an axially disposed curved path, the open ends of which intersect the side surface of the magnet. In effect, the magnetic flux flows out the sides of the permanent magnet rather than the ends. The result is a higher efiiciencv speaker magnet because the fiux is concentrated where it should bein the air gap. A second result is that the magnetic circuit in a speaker comprising the permanent magnet and the return path elements is considerably simplified. No bottom return path element or top pole piece is necessary and it is possible, as set out more fully below, to simplify and make more compact even those elements of the return path which remain. Yet another aspect of the invention involves a unique method for orienting the permanent magnets in the manner described.

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

FIGURE 1 is an elevational sectional view of a cylindrical speaker magnet of the invention showing the manner of its orientation;

FIGURE 2 is a sectional view of a permanent magnet assembly of the invention comprising a return path element and a permanent magnet;

FIGURE 3 is a sectional view of a speaker embodying the permanent magnet assembly of the invention; and

FIGURE 4 is a sectional view of an apparatus suitable for orienting permanent magnets in accordance with the invention.

As can :be seen in FIG. 1, a cylindrical speaker magnet 1 having end surfaces 2 and 3 at opposite axial extremities and aside surface 4 is oriented so that its preferred direction of magnetization is along a curved or arcuate path 5 disposed symmetrically around the central axis AA of the permanent magnet. It will be noted from FIG. 1 that the curved path 5 intersects with side surface 4 of the magnet at 6 along the top portion of the magnet and at 7 along the bottom portion of the magnet. Thus, a first pole will be created along one extremity of a side surface of the magnet at 6 and a second pole will be created on the opposite extremity of the side surface of the magnet at 7. The path of preferred magnetization shown in section in FIG. 1 extends symmetrically completely about the axis of the permanent magnet. The path of orientation is therefore described by rotating curve 5 through a com plete 360 arc about the central axis AA of the magnet. Any plane passing through the central axis of the magnet would produce the orientation path shown in section in FIG. 1.

The utilization of a permanent magnet in a speaker magnet circuit, oriented as shown in FIG. 1, is shown in FIG. 2. As there illustrated, all that is necessary to complete the magnetic circuit is an open cylindrically shaped return path element 8. The permanent magnet 1 fits within and is coaxial with return path element 8. A flange 9 on the inner surface of return path element 8 contacts the permanent magnet 1 along its lower extremity 7. The opposite extremity 6 of the side surface of the permanent magnet is spaced from return path element 8 to form the air gap 10.

The permanent magnet-return path structure shown in FIG. 2 permits a number of significant advantages in the construction of permanent magnet speakers. Measurements indicate that of the total magnetomotive force is made available at air gap 10 as opposed to 65% to 75% in conventional magnet structures. Additionally, virtually of the total magnetic flux may be made to enter the general region of the voice coil versus a loss of as much as 20% of such flux to regions outside the voice coil region in conventional magnet structures. In substance, use of a magnet oriented as illustrated in FIG. 1, with the return path assembly illustrated in FIG. 2, makes possible the utilization in speaker structures of a much higher percentage of the energy of conventional permanent magnets.

The invention also makes possible the utilization of return path elements of thin-gauge metal. This is possible because the greater efliciency of the magnetic circuit permits the use of lower flux densities to achieve an equivalent air gap density and thus equivalent speaker performance. A conventional return path or yoke structure in a speaker must have sufiicient cross-sectional area so that the flux density does not exceed the saturation point of the metal. Because the flux density per unit area of return path is smaller in the speaker structure of this invention, less cross-sectionalarea in the return path element is necessary. This enables efiicient speaker magnet structures to be built utilizing thin-gauge, soft-steel return paths. An additional reason why the return path elements of the invention need be of less cross-sectional area is based upon the fact that the magnetic flux is distributed over a Wider area at the point of contact with the return path element.

The foregoing is illustrated more clearly in the speaker shown in FIG. 3 of the drawing. Speaker basket 11 has depended from its central portion a tubular return path element 12 integral with and formed from the speaker basket. It is possible to use metal from the speaker basket, previously discarded in the stamping of such baskets, to make the return path elements of the speaker both because of the simplified design of the return path and because of the ability to use lighter gauge metal. This aspect of the invention is more fully disclosed in my copending application Ser. No. 515,322, filed of even date herewith and assigned to the same assignee as the present application. The remaining components of the speaker are a frusto-conical diaphragm 13 and a flexible spider 14. A voice coil 15 is wound on a voice coil form 17 and is coupled to diaphragm 13.

Another advantage of the invention is also illustrated by FIG. 2. The width of the air gap 10 is the same as the width of the throat 17 formed between the remainder of the return path element and the lateral sides of the permanent magnet. While this structure is not unknown in the speaker art, it has almost always been avoided because greatest efficiency in speaker structures is obtained by maintaining a large degree of separation between the axial flow of magnetic fiux in the magnet and the return axial flow in the lateral portion of the return path. This discourages radiation of flux from the sides of the magnet to the return path element at areas other than the air gap. In conventional speaker assemblies the flux flows in an axial direction through the permanent magnet-any radial flow of flux other than at the extremities of the magnet must be discouraged. In the speaker magnet assembly construction shown in FIG. 2, there is already a radial component (toward the sides of the magnet) in the flow of flux in the magnet 1 so that radial flow is not discouraged except for a very small area at the axial center of the magnet. It is thus possible to reduce considerably the dimensions of the throat area of the' permanent magnetreturn path system and in turn to miniaturize the construction of the speaker. While the invention does not require in all cases that the air gap and throat width be the same dimension (as in FIGS. 2 and 3), the invention nevertheless does make it possible for the throat section to be narrower tlfin in conventional speaker structures.

The present invention also encompasses a unique method for orienting magnets along the direction set forth in connection with the description of FIG. 1 above. An apparatus suitable for so orienting the permanent magnets of the invention is shown in FIG. 4. Such apparatus includes a ceramic tube 19 containing the permanent magnets 20, 21, 22 and 23. Soft iron washers 24, 25, 26 and 27 at the' juncture of each two permanent magnets act as pole pieces. An outer cylindrical tube 28 of any magnetically permeable material, such as soft iron, surrounds the entire structure and acts as a return path element. The outer tube 28 is wound in periodic fashion by windings 29, 30, 31 and 32 to create a series of poles of opposite polarity at the extremities of the magnets. Thus, it is wound in a first direction around magnet 20, an opposite direction around magnet 21, the first direction around magnet 22, and so forth, to create alternating north and south poles surrounding abutting end portions of the permanent magnets. As can be seen in FIG. 4, this arrangement permits magnetic flux to flow from washers 24, 25, 26 and 27 through an arcuate or curved path in each of the cylindrical magnets to an opposite pole at the next successive washer, through the return path element to repeat the circuit. This circuit is illustrated in the drawings for three of the magnets but would of course be repeated throughout the orientation device.

By arranging any number of such periodic fields around a thermally insulating, temperature-resistant tube such as tube 19 containing the speaker magnets, a number of such speaker magnets may be oriented in accordance with the teachings of this invention. The permanent magnets are first heated to a temperature above their Curie temperature, and preferably about 300 C. above their Curie temperature, and then cooled through the Curie temperature to room temperature in accordance with a prescribed cooling cycle, while the magnets are subjected to a magnetic field in the foregoing apparatus. The cooled magnet will be oriented along the curved path described above.

A specific example of the process of the invention is as follows:

Ceramic tube 19 (FIG. 4) is loaded with a plurality (as for example, a hundred) sand-cast cylindrical alnico 5 permanent magnets ground to proper dimensions and having the following composition in percentages by weight: 8 aluminum, 14 nickel, 24 cobalt, 3 copper, balance iron. The tube is placed in an induction heater and the tube and magnets are heated to 1100 C. The Curie temperature of the alnico 5 composition is about 800-850" C. The tube and magnets are then removed from the induction heater and placed in the magnetic field orientation device illustrated in FIG. 4. Each magnet is aligned such that the line of contact between two adjacent magnets falls centrally within one of the washers 24, g

25, 26 and 27. The permanent magnets are then allowed to remain in the orientation field while the periodically arranged windings are energized until the magnets have cooled through their Curie temperature to about 600- 700 C., which ordinarily takes about five to ten minutes at ambient temperatures.

The permanent magnets to which the invention is particularly directed are those prdouced from iron-cobaltnickel-aluminum alloys commonly known as alnico 5. While alnico 5 is the most widely used ferromagnetic material in speaker magnets, 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 cylindrical permanent magnet having end surfaces and a side surface therebetween, said permanent magnet having a first pole around a first portion of the side surface of the permanent magnet at an extremity thereof and a second pole around an axially spaced second portion of the side surface of the permanent magnet at the opposite extremity thereof, the preferred direction of magnetization of the permanent magnet lying along a curve disposed symmetrically about the axis of the magnet and connecting the first and second poles.

2. The permanent magnet of claim 1 composed of alnico 5 magnetic material.

3. A permanent magnet having end surfaces and a side surface therebetween extending along the axis of the magnet, said permanent magnet having a first pole around a first portion of the side surface of the permanent magnet and a second pole around an axially spaced second portion of the side surface of the permanent magnet, the preferred direction of magnetization lying along a curve disposed about the axis of the magnet and connecting the first and second poles.

4. A permanent magnet assembly for a speaker comprising a permanent magnet and a return path element together adapted to provide a magnetic circuit, the permanent magnet being coaxial with and spaced within the return path element, the return path element contacting the permanent magnet along a side surface of the permanent magnet at an extremity thereof, the opposite extremity of the side surface of the permanent magnet being spaced from the return path element to form an air gap, the preferred direction of magnetization of the permanent magnet being along an open curve, one end of which intersects the side surface of the permanent magnet along that portion contacting the return path element,

the other end of which intersects the side surface of the 6 magnet at that portion adjacent the air gap.

References Cited UNITED STATES PATENTS 2,188,091 1/1940 Baermann 148-103 10 KATHLEEN H. CLAFFY, Primary Examiner. ARTHUR A. MCGILL, Assistant Examiner.

US. Cl. X.R. 148-103

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