US2566850A - Electroacoustic transducer - Google Patents

Description

Sept. 4, 1951 E E M01-T 2,566,850

ELECTROACOUSTIC TRANSDUCER Filed OC. 19. 1946 /NVEA/TOR E. E. MOTT ATTORNEY Patented Sept. 4, 1951 ELECTROACOUSTIC TRANSDUCER Edward E. Mott, Upper Montclair, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 19, 1946, Serial No. 704,485

1 Claim. 1

This invention relates to electroacoustic transducers and more particularly to ring armature transducers, such as disclosed in Patent 2,249,160, granted July 15, 1941, to Edward E. Mott, especially suitable for use as and in loudspeakers.

In electroacoustic transducers, the power which can be translated is dependent upon the amplitude of diaphragm vibration. Translation of high power with a device having a diaphragm of convenient size entails large amplitude diaphragm displacement. In transducers of the ring armature type and of presently known constructions, considerations of efiiciency, particularly from the standpoint of the magnetic circuit, dictate the use of a small air-gap between the free or unsupported marginal portion of the armature. Thus, in such constructions, because of the ccnsiderations noted, the maximum permissible amplitude of armature vibration may be limited and, accordingly, the amplitude of diaphragm displacement likewise may be limited.

One object of this invention is to increase the maximum permissible amplitude of armature vibration in ring armature type electroacoustic transducers without degradation of the magnetic efiiciency, whereby an increase in the maximum power translated is realized.

Another object of this invention is to decrease the static force upon the diaphragm in a ring armature transducer, whereby the mechanical stiffness of the vibratory system is decreased.

In accordance with one feature of this invention, in an electroacoustic transducer of the type disclosed in the patent above-identified, the ring armature and inner pole-piece are so constructed and arranged that the armature may execute large amplitude vibrations without danger of contact thereof with the pole-piece.

More specifically, in accordance with one feature of this invention, in such a transducer, the armature and inner pole-piece are provided with juxtaposed portions inclined to the direction of armature displacement. These portions may be positioned in close proximity, thereby to provide a low reluctance air-gap, but because of the disposition thereof relative to the direction of vibration of the armature, large amplitude vibrations are attainable.

The invention and the above-noted and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is a view in cross-section of an electroacoustic transducer illustrative of one embodiment of this invention; and

Figs. 2 and 3 are fragmentary sectional views of transducers illustrative of other embodiments of this invention.

Referring now to the drawing, the transducer illustrated in Fig. 1 comprises a plate I0, for example of insulating material, having a pair of contacts or terminals Il and I2 affixed thereto. Mounted upon the plate I0 is a magnetic structure including a pole-piece having'a cylindrical portion I3 and an annular ange I4 which is seated upon the plate. The magnetic structure includes also a cylindrical, axially magnetized magnet I5 coaxial with the cylindrical pole-piece portion I3, seated upon the flange I4 and secured thereto, as by soldering.

Seated upon the magnet I5 is an annular, magnetic spacer I6 upon which the peripheral portion of an annular magnetic armature Il rests. The armature carries a dished non-magnetic diaphragm member I8, the periphery of which is secured to the inner marginal part of the armature, as by a suitable cement.

An auxiliary magnet I9 overlies the armature Il and extends over the magnet I5 and pole-piece portion I3. As indicated in Fig. l, the auxiliary magnet is magnetized radially so that its poles are of opposite polarity with respect to that of the magnet I5 and pole-piece tip in juxtaposition thereto. Advantageously, the auxiliary magnet is of less strength than the main magnet I5. The auxiliary magnet rests against a magnetic annulus 20, which provides a low reluctance path between the two magnets, and an insulating spacer 2| having an annular ilange portion 22 overlying the armature.

The plate I0, magnet structure, spacers and auxiliary magnet are held securely in assembled relation by a metallic band or sleeve 23, the edges of which are crimped over the plate and auxiliary magnet.

An annular signal coil 24 is mounted between the magnet I5 and pole-piece I3, is insulated therefrom by an insulating member 25 and has its ends connected to the terminals II and I2.

The pole extremity of the cylindrical pole-piece I3 is tapered to provide a truste-conical pole face as indicated at 26. The inner marginal portion 21 of the armature is bent or inclined to conform to the inclined pole face 26 to provide an airgap of uniform length inclined relative tothe direction of armature and diaphragm vibration, i. e., vertical in Fig. 1. Because of the construction and arrangement of the pole face 26 and armature portion 21, it will be appreciated that the air-gap therebetween can be made quite short, yet the armature may execute large ampli- 3 tude vibrations without contact between the armature and the pole-piece i3. Also, it will be appreciated that because of the inclinationl of the portion 21 and pole face 26, the static magnetic force eilective upon the armature is relatively small whereby the stiness of the vibrating system, i. e. the armature and diaphragm unit, also is relatively small. Thus, emcient translation may be attained. Additionally, because of the low stiffness of the vibratory system, a low resonance frequency for this system may be obtained.

The transducer illustrated in Fig. 2 is of construction similar to that shown in Fig. l and described hereinabove. I1 is provided with an upwardly extending, in clined annular or frusto-conical portion 30 conforming to and uniformly spaced from the in"J clined pole face 3i of the pole-piece i3. Also, the auxiliary magnet le is provided with an annular inclined face 32 conforming to and uniformly spaced from the armature portion 33, whereby a small air-gap ytherebetween'may be f employed and, thus, high magnetic emciency ob tained.

The embodiment of this invention illustrated in Fig. 3 is similar to that shown in Figs. l and 2. However, in this embodiment, the annular or frusto-conical armature portion 33, which conforms to and is spaced uniformly from the pole face 3i, is inclined at an angle less than a right angle with respect to the body of the armature.

Reference is made of the application Serial No. '104,483 flied October 19, 1946, of Edward E. Mott wherein a related invention is disclosed and claimed.

Although specific embodiments of this invention have been shown and described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claim.y

However, the armature,

An electroacoustic transducer comprising a magnetic structure including cylindrical inner and outer pole members, the inner member having an outer frusto-conieal pole face at one end thereof, an annular magnetic armature of width small in comparison to its diameter, said armature having a fiat body portion extending from said outermember toward said inner member and having a misto-conical inner marginal portion extending upwardly from said body portion, said frusto-conical pole face and said frustoconical marginal portion being in juxtaposition and conforming to each other, means on said outer pole member defining an annular Iulcrum supprt for said armature adJacent its periphery and coaxial therewith, and an annular magnetic member /overlying said armature and extending from said outer pole member toward said inner member, said magnetic member having a irustoconical face in juxtaposition and conforming` to said frusto-conical inner portion o! said armature.

EDW E. MOTT..

REFERENCES CITED The following references are of record in the le of this patent:

OTHER REFERENCES Magnets, by C. R. Underhill, 'pages 67 and 68, McGraw-Hill Book Co., Inc., 370 Seventh Ave., N. Y.. N. Y., 1934.

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