US2553150A - Permanent magnet electrodynamic transducer - Google Patents

Description

May 15, 1951 A. l.. si-:IFRIED 2,553,150

PERMANENT MAGNET ELECTRODYNAMIC TRANSDUCER Filed omi. 28, 1949 7/IIIIIIIIIII/7 fais ATTORNEY Patented May I', 1951 UNITED STATES PATENT OFFICE PERMANENT MAGNET ELECTRODYNAMIC TRAN SDUCER Albert L. Seifried, Millington', NQJ., assigner to Thomas-ArEdison, Incorporated, West Orange,- N. J., a corporation of New Jersey Application `October 28, 1949, Serial No. 124,041

4 Claims. l

This invention relates to a small electroacoustical transducer for phonographs adapted for reproducing speech recordations, and more particularly thel invention relates to an improved transducer' which is adapted to feed into a small soundtube leading to anacoustic-type headset.

In'thereproduction of recorded speech, acoustic receivers are commonly used which are Worn on'the head and adapted to't over or into the ears, a pair of such receivers being herein referred to as a headset. Such acoustic headset ispreierred over a headset of the electroacous tic 'type-since it is ftypically smaller f and lighter. However, heretofore the acoustic-type headset has been cumbersome because of the need for a fairly' largeandstiff acoustic tube coupling thel headset to` the reproducer. The present transducer is-adapted to- Work' into a very small,

light?l coupling tube-Which Will not encumber to Another object is to provide such a transducer` which vhaseflicient high-frequencyV response 'to enable theV faithful reproduction abovementioned.

Other objects and features of the invention will be apparent from the following description and the appended claims.

In'the description of my. invention reference-` is had to the accompanying drawings, of which:

Figure 1 is aside elevational View of an electroacoustical transducer, with amounting bracketgin Which my invention is embodied;

Figure-2 isv a` front ,elevational view `of kthis transducer; A

Figure 3 isfa sectiontotenlar-ged scalel taken substantial-1y onv the line'B-'bi Figure 2;

Figure 4 is a backV ViewY of the-.transducer asv seen-from the llinea- 4 of Figure 3;

Figure 5 is a section taken` substantially on the line 5;-5 of Figure 3;

`Figure Gris a View to enlarged scale of the diaphragm assembly; and

Figure 'l is a fractional sectional view to enlarged scale taken through the axis of the transducerto show details'of the vibratory system of the transducer.

The present transducer includes a diaphragm' the essential parts of the device in concentric For instance, positionedwithin'the holder I is a thin'circular'diaphragm` i5 the peripheral edge of which contacts' the:

relation to each other.

inner surface vci this locatingl'flange. A singlelayer voice coil i6 is connected to the side-of the diaphragm which faces away from the holder I0. This coil is located in concentric relation tofthe peripheral edge of the diaphragm and is therefore located in concentric relation to the flange i2.. The-diaphragm is spaced from the Wall il by a thin annular gasket Il'. Another such gasket is positicnedagainst the outer side of the diaphragm and positioned-flat against this gasket is or similar elements, spaced at caudal-intervals` about the axis of the device. These rivets are positioned near the'rirnL flange through the disk I8, thegasket I'I, the diaphragm :I5 and the Wall I to clamp these parts tightly together.v An advantage of securing these parts intoa permanent `unitary subass'emblyis that the delicate-diaphragm is protectedlargely by the disk and the holder 'to permit stocking,

andhandling -of thesubassembly without likelihood of damasethereto.

In the inner side of the annular diskl I8 there are tvvo approximately parallel grooves 20 running from the inner to the outer edges of the disk (Figures 5 and 7). These grooves form clearanceopenings between the disk and the adjacent gasket II for lead-out rwires 2l of the voice coil ri, the portion of thelead-out Wires adjacent to thedisk beingscovered` byinsulatingv These lead-out f sleeves 22 (Figures 5 rardz).

I 2 and pass- Wires and sleeves pass through clearance slots 23 provided in the rim flange I2 and the wires are connected to a pair of terminals 24 anchored to an insulating plate 25. This plate is secured to the diaphragm holder as by an elongated one of the aforementioned rivets indicated in Figure 3 as I 9a.

The magnetic system of the transducer also comprises a member of magnetically soft material in the form of a cup 26. This cup has an outwardly-extending rim flange 2l which seats on the outer border of the annular disk I8 and the peripheral edge of which fits into the fiange I2 to locate the cup also in concentric relation with the voice coil. Mounted in this cup at the axis thereof is a cylindrical core 28 which is staked or soldered to the bottom wall of the cup at 29. The outer end of this core ts within the voice coil at a clearance spacing therefrom. Since the core is positioned'at the axis of the cup and the latter is positioned in concentric relation with the voice coil, the core is therefore also concentric with the coil. In the preferred construction, the core is made of permanent-magnet material as of one of the alloys of aluminum, nickel, cobalt and iron known as Alnico, in which case the outer end of the core is preferably provided with a soft iron pole piece 28a. However, as an alternative construction, it will be understood that the core may be made of magnetically soft iron and may be ener'l gized by a suitable eld coil not herein necessary to show.

The cup 26 is secured in assembled relation to the aforementioned subassembly by four rivets 3D, or like elements, which are interposed between the rivets I9. These rivets 30 pass through the flange 21 and the parts within the holder I in the manner of the rivets I9. In order that the rim flange 2l of the cup may lie flat against the disk I8 for close magnetic coupling therewith, it is provided with four notches 3! to clear the heads of the rivets ES (Figure l).

A resilient mounting for the present transducer is preferred and comprises a rubber ring 32 placed around the magnetic cup and held thereon by a circular plate 33 secured as by spot welding to the back side of the cup. In this ring there is a peripheral groove 34 received by an apertured leg 35 of a mounting bracket 36 which is held to a suitable support 31 as by screws 38 (Figure l). The plate 33 is welded to the cup before the latter is secured to the holder. To provide easy access to the rivets 3@ this plate is provided with four respective Clearance notches 33a.

The sensitivity of the present instrument is dependent, among other factors, upon the magnetic flux density in the air gap between the core 28 and the annular disk I8. In order that the gap density@ may be a maximum for a permanent magnet of given strength, the magnetic cup is made suioiently large in diameter to minimize leakage losses.

In order that good high-frequency response is obtained the Vibratory system-i. e., the diaphragm and voice coil-is designed to be resonant at the highest frequency to be reproduced. In this design, the diaphragm mass is kept low by choosing a thin diaphragm of light metal, say of .002 thick aluminum, and by cementing the coil directly to the diaphragm Without use of a coil frame; further, the stiffness of the diaphragm is made high and break-up of the diaphragm at high frequencies is prevented by 4 offsetting the central portion of the diaphragm at 39 and by using a diaphragm of effectively small diameter.

Novel features of the present invention whereby in a simple design leakage flux is minimized, the resonant frequency of the diaphragm is made high, and parts are accurately positioned relative to each other lie in the use 0f a large diaphragm and large magnetic cup equal in diameter to each other so that they can be centered by the common locating flange I3, and in the use of gaskets clamped against the sides `of the diaphragm which have a relatively small central opening to reduce suitably the effective diameter of the diaphragm.

A further feature of the present invention lies in the use of simple parts having essential locating surfaces in concentric relation to each other so that these surfaces can be established with a high degree of accuracy, and in the use of a construction wherein these parts are located accurately in relation to each other by one common locating surface, which is the inner peripheral surface of the locating flange I3.

The preferred embodiment of my invention herein shown and described is intended t0 be illustrative and not necessarily limitative of the invention, since this embodiment is subject to changes and modifications without departure from the scope of my invention, which I endeavor to express according t0 the following claims.

I claim:

1. An electroacoustioal transducer comprising a circular diaphragm, a circular diaphragm holder having a peripheral flange engaging the peripheral edge of the daiphragm and having a central opening for the emission of sound from the inner side of the diaphragm, a voice coil mounted on the outer side of the diaphragm in concentric relation with the peripheral edge thereof, an annular disk of soft magnetic material in juxtaposition with the outer side of said diaphragm and in peripheral engagement with said flange with the inner peripheral edge of said disk confronting the outer side of said coil in spaced relation thereto, an annular gasket between one side of the diaphragm and the inner wall of said holder and another annular gasket between the outer side of the diaphragm and said magnetic disk to space the diaphragm from the disk and holder for free vibration of the central portion thereof, clamping elements spaced at intervals about the axis of said holder near the rim thereof and traversing the holder, gaskets, diaphragm and disk to secure the same into a unitary subassembly, a cup-shaped member of soft magnetic material having an out wardly-extending peripheral flange seated on said disk and engaging the inner peripheral surface of said flange of said holder, said flange of said cup having portions cut away to clear the ange of said clamping elements, a central magnetic core mounted in said cup having the end portion thereof within said voice coil at a clearance spacing therefrom, and other clamping elements interposed between said rstmentioned clamping elements and securing said peripheral flange of said cup in mounted relation to said subassembly.

2. rIhe combination set forth in claim l Wherein said diaphragm holder has a at inner wall and wherein the diameters of the central openings of said gaskets are a minor fraction of the diameter of said diaphragm to provide a cylindrical air chamber between the diaphragm and holder having a diameter less than half the overall diameter of the diaphragm and having a length equal to the thickness of one of said gaskets.

3. An electroacoustical transducer comprising a circular diaphragm, a circular diaphragm holder having a rim flange fitting the peripheral edge of said diaphragm and having a central opening for the emission of sound from the inner side of the diaphragm, a voice coil mounted on the outer side of said diaphragm in concentric relation therewith, an annular magnetic disk mounted in said holder in spaced relation to the outer side of said diaphragm and in peripheral engagement with said rim flange with the inner periphery of said disk confronting the outer periphery of said coil at a clearance spacing therefrom, a magnetic cup-shaped member having an outwardly-extending rim ange seated on said magnetic disk and tting said first rim flange to locate the cup in concentric relation with said holder, a magnetic member mounted in said cup and having a circular end portion concentric with the cup and disposed within said voice coil at a clearance spacing therefrom, elements at the rim of said diaphragm holder securing said magnetic disk and diaphragm to said holder to form a permanent subassembly of said transducer, and other elements interposed between said rstmentioned elements and securing said magnetic cup in mounted relation to said subassembly.

4. An electroacoustical transducer comprising a circular diaphragm, a circular diaphragm holder having a rim flange fitting the peripheral edge of said diaphragm and having a central opening for the emission of sound from the inner side of the diaphragm, a Voice coil mounted on the outer side of said diaphragm in concentric relation therewith, an annular magnetic disk mounted in said holder in spaced relation to the outer side of said diaphragm and in peripheral engagement with said rim flange with the inner periphery of said disk confronting the outer periphery of said coil at a clearance spacing therefrom, a magnetic cup-shaped member having an outwardly-extending rim flange seated on said magnetic disk and fitting said iirst rim :dange to locate the cup in concentric relation with said holder, a magnetic member mounted in said cup and having a circular end portion concentric with the cup and disposed within said voice coil at a clearance spacing therefrom, clamping elements traversing said magnetic disk, diaphragm and holder at the rim of the latter to secure the same into one subassembly, and other clamping elements between said first-mentioned elements and traversing said holder, diaphragm, disk and flange of said cup to secure the cup to said subassernbly, wherein said flange of said cup has portions thereof cut away to clear the flange of said first-mentioned clamping elements.

ALBERT L. SEIFRIED.

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

UNITED STATES PATENTS Number Name Date 1,915,358 Giles June 2'7, 1933 2,025,127 Rowe 1 Dec. 24, 1935 2,440,439 Gilman Apr. 27, 1948 FOREIGN PATENTS Number Country Date 460,064 Great Britain Jan. 20, 1937

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