US2490227A

US2490227A - Electrodynamic loud-speaker

Info

Publication number: US2490227A
Application number: US755699A
Authority: US
Inventors: Murkham Leonard Walter
Original assignee: RESLOSOUND Ltd
Current assignee: RESLOSOUND Ltd
Priority date: 1941-12-04
Filing date: 1947-06-19
Publication date: 1949-12-06
Anticipated expiration: 1966-12-06
Also published as: FR940616A; GB552328A

Description

Dec; 6, 1949 L. W. MURKHAM ELECTRODYNAMI C. LOUDSPEAKER Fiied June 19, :|947

Patented Dec. 6, 1949 UNITED STATES P OFFICE ELECTRGDYNAMIC LOUD-SPEAKER land, a British company Application June, 19, 1947,- Seria-l No.` 155,699 In Great Britain December 4, -1-941 Section 1, Public Law 690, August 8,1946 Paten-t expires DecemberI 4, 1961 4 claims. (ci. 17e-115.5)

This invention relates to vloud speakers or the like of the moving-coil type in which the diaphragm is vibrated by means of a coil which carries the speech current and is located in the magnetic iield set up by a magnet. In loud speakers of this type the moving-coil is usually connected to the diaphragm by being wound on a former in the shape of a short cylindrical shell one edge of which is attached concentrically 'to the diaphragm, and the magnetic iield is usually the eld set up across the small annular air gap between the o-uter cylindrical surface of an inner pole piece and the inner cylindrical surface oi a surrounding outer pole piece, the said 'former with the coil in it projecting into this air gap. Again in such loud speakers, it is usual for the outer pole piece to take the form of a plate with a circular concentric hole in it Vforming the inner cylindrical surface aforesaid, and for the inner pole piece to be extended rearwardly in the direction away from the diaphragm, the magnetic circuit being completed by means of a more-or-less pot-shaped structure whose bottom' joins the rear end of the inner pole piece and whose rim or edge joins the back surface of the plate forming the outer pole piece.

In loud speakers of this kind, in order to obtain maximum acoustic efficiency, it is necessary lto work with the smallest possible clearances between the coil and former on the one hand and the surfaces forming the annular air .gap on the other, and diiculty has been heretofore experienced in centering the-coil correctly in the gap and rendering the centering immune from disturbance by mechanical shock.

The invention has among its objects the provision ci improved means whereby this centering may be effected and securely maintained, the assembly of the parts in correct centre being entirely automatic. Another object is that of simplifying the magnet construction. Another object is the provision of improved means for preventing the deleterious effects of vair cushioning at the back of the diaphragm. Y

The invention comprises the arrangement that both the diaphragm unit and' the plate forming the outer pole .piece are centred by tting them accurately in a circular recess formed say in the back surface of the front plate of the outer housing which houses said diaphragm and the magnet.

The invention also comprises in constructions in which the inner pole piece is centred relative to the plate forming 'the outer' p'cile piece by means of a non'ferrous 'bush mounted on 'the back o'f said plate, in which bush the rearwardly extending part of said inner pole vpiece Very accurately fits, eiecting the attachment of the nonferrous bush to the rear surface of said plate by heat springing, a counter-sunk' recess in the rear surface oi said plate being shrunk, by cooling, on to said bush.

The invention also comprises, in constructions in which the inner pole .piece is centred relative to the plate formingl 'the outer polepiece by means of a nonierrous bush mounted 0n the back of said plate, in which bush the rearwardly extending part of said inner pole Vpiece Very accurately nts, the arrangement that holes or cut away portions are formed in 'the bush .for the purpose of reducing cushioning behind the diaphragm.

Other features of the invention will appear hereinafter.

In order that the invention may be the more clearly understood a loud speaker in accordance therewith will now be described, reference being made to the accompanying drawings wherein:

Figure 1 is a sectional side elevation of ythe vwhole assembly;

Figure 2 is a plan of one of the parts of Figure 1;

Figure 3 shows a construction which may be employed in the bottom of the outer container I3 of Figure 1. A

Referring to Figure 1 the diaphragm I and the magnet are housed in a cylindrical container of nonmagnetic material, whose lfront end 2, which is just in front of the diaphragm, has a central opening 3 therein through which the sound waves from the diaphragm are adapted to pass. This iront end 4has formed in its rear face a shallow circular recess 4, and both the diaphragm AI and therefore the movingl coil la and the circular plate 5 forming the outer pole piece of vthe magnet, are centred by fitting closely within this shallow recess. Thus the diaphragm, which is of the usual .paper-.like material or of the material generally used 4for this purpose, -is framed in a flat ring framing t of some stiffer material which ts very accurately in this shallow circular recess and the said circular plate 5 forming the outer pole Lpiece of the magnet ts accurately behind the diaphragm 'in the same circular recess thereby clampi-ng the `frame 6 in place and ensuring that the said outer pole piece and the moving coil la shall both be accurately centred. The clamping action of the said circular plate is against the ring 'framing of the diaphragm which is slightly thicker `than the diaphragm proper. 'This fact, added to 'the fact that the 3 front face of said outer pole piece is chamfered slightly towards its central opening as shown, ensures plenty of room for the diaphragm to vibrate in. In like manner the shallow recess 4 in the front end 2 of the housing is formed to enable the diaphragm to vibrate freely.

The inner pole piece 1 is centred, relative to"Y this circular plate 5 forming the outer pole piece by means of a non ferrous bush (see also Figure 2) mounted on the rear surface 'of said circular plate, in which bush the rearwardly ex-.

than the outside diameter of the disc forming the bush 8. Said circular plate 5 is heated until its expansion permits the bush 8 to be tted into this cylindrical recess and, upon cooling, the bush in said recess.

tending part of said inner pole piece very accurately fits. Thus, the moving coil I and the outer pole piece 5 both being accurately centred by the shallow circular recess 4 in the front plate 2 of the container, and the inner pole piece I being accurately centred by the nonferrous bush 8 on the rear of the outer pole piece 5, the moving coil Ia will be accurately centred with respect to the annular clearance as a whole, and this centering cannot become displaced.

It will be seen that the machining of the shallow circular recess 4, the inner and outer ma- `vrearwardly from said cylindrical recess.

will be gripped tightly and secured permanently The accurate machining, both of the inner surface of the bush and of the inner and outer surfaces of the circular magnet plate 5 may be effected after this heat springing operation. It will be seen that the bush 8 projects The machining of the inner surface of said bush is chining of the plate 5 forming the outer pole piece, Y,

tively to said outer periphery m'ust all be very accurate. Once these parts have their surfaces yaccurately formed their assembly automatically becomes correct and cannot thereafter become incorrect.

The nonferrous bush 8 also serves to locate the inner pole piece I axially relative to the outer pole piece. Thus the inner pole piece is formed with a small shoulder 9 which engages with the bush to limit the rearward axial movement thereof relative to said outer pole piece, and, in construction, the rear end of the inner pole piece is secured, by means of a single central screw I, to the bottom II of the pot portion of the mag net, the dimensions being such that, in screwing up the screw, this shoulder, engaging the bush,

strains the bush slightly, thereby ensuring that Athe circular plate 5 forming the outer pole piece is pulled hard down on to the edge or rim of the wall I2 of the pot, which wall constitutes the permanent magnet. The said bottom II of the pot is made quite separate from the cylindrical 4wall I2 thereof, and all four parts viz. the outer pole piece 5 and bush 8, the inner pole piece l, the pot bottom II and the cylindrical pot wall I2 are secured together by means of the one screw I0 aforesaid passing through the pot bottom II into the rear of the inner pole piece '1. The actual pole surface of the inner pole piece 'I opposite to the corresponding surface of the outer pole piece 5 is of greater diameter than the remainder of said inner pole piece. The said shoulder 9 which abuts the nonferrous bush 8 is very small and is some distance behind the said pole surface of increased diameter.

It will be appreciated that the parts of the -magnet must be machined accurately to ensure vthat the rear end of the inner pole piece I abuts firmly against the rear plate II of the magnet, land at the same time, the cylindrical wall I2 of the magnet is clamped tightly between said rear plate and the front plate 5 forming the outer pole piece. The intensely concentrated field of the lmagnet adds to the mechanical strength.

The attachment of the nonferrous bush S to the rear surface of the circular magnet plate 5 `constituting the outer'pole piece is preferably such that the forward portion thereof is of the same diameter as the inner or pole surface of the circular plate, the rear portion being of smaller diameter and forming the bearing surface for the irmer pole piece 7. In this way also a shoulder for engaging the aforesaid small shoulder 9 on the inner pole piece is formed.

It will thus be seen that the nonferrous bush 8 is solidly attached to the magnet plate 5 without the use of any screws and n such a way that concentricity between the pole surface of the magnet plate on the one hand and the bearing surface of the bush, and therefore the pole surface of the inner pole piece 7, on the other hand. can never be disturbed.

When the one central screw I0 is screwed up, the bush 8`may be withdrawn say .015 of an inch from the cylindrical recess owing to the fact that the error in the length of the inner pole piece I up to the small shoulder 9 may be as much as that amount. This will cause no damage and will not in any way weaken the magnet structure in respect of its lateral or centralising strength. Once the magnetising is effected the magnetic Vforce pulling the two magnetic plates 5 and II towards one another is so strong as to aiord the bulk of the longitudinal strength of the magnet structure.

The cylindrical housing of nonmagnetic material consists of the aforesaid front end 2 and an integral pot-shaped portion I3. When the magnet as a whole has been assembled, the assembly of the remainder is effected by locating rst the diaphragm I and then the outer pole piece 5 of the magnet, in the shallow circular recess 4 in the rear face of the front end 2 of the outer container, then placing the pot portion I3 of the outer container in position against the said front end and then screwing said front end to said pot portion by means of a number of screws I6 screwed into thickened portions of the wall of said pot portion. The dimensions are such that this presses the outer pole piece 5 of the magnet against the ring framing 6 of the diaphragm and 'completes the clamping action as described.

To obviate the disadvantages of air cushioning at the rear of the diaphragm I, which tends to reduce the bass output and give articial accentuation elsewhere, a number of, say four, holes I5 are drilled through the circular plate 5 form- Ving the outer pole piece, say in the chamfered area as close as possible to the gap without interfering with the magnetic path. (These holes must also pass through the nonferrous bush 8.) This prevents air cushioning behind the part of the diaphragm outside of the coil Ia. Additional holes or cut away portions I6 are also formed in the bush 8 in line with the annular clearance. i. e.

in line with the coil Ia, thus preventing serious cushioning behind the part of the diaphragm inside of the coil as air is permitted to escape through the annular gap, though it is true that this narrow gap does offer some resistance. Also, to allow air to escape from within the magnet pot a number of holes I1 are formed in the rear plate II of the magnet structure. Alternatively the air could be arranged to escape past the rear plate of the magnet structure, the latter being spaced by spacers from the cylindrical Wall I2, but this is not usually so satisfactory. The outer` container is very considerably larger than the magnet and thus the trapping of air therein is not deleterious. To enable the container to be thus larger than the magnet the latter abuts against pedestals I8 extending from the bottom of the container pot.

Rubber pads or spring washers or plastic rings I9 or other form of compressible packing may be interposed between the plate I I and the pedestals I8 or rear of the housing. Similar packing 20 may also form a water tight joint between the front and the pot-portion of the outer housing.

In cases where the loud speaker will not have to be installed out of doors the outer container could be formed with openings to the atmosphere. Where the loud speaker may have to be installed out of doors and openings to atmosphere are required, the arrangement illustrated in Fig-ure 3 may be employed. In this arrangement the bottom of the pot portion I3 has a central hole 2| formed in it and this hole is covered by means of a cap 22 which is spaced away from the bottom of the pot portion I3 by means of lugs 23 integral with said pot portion I3. Air can flow freely out of the hole 2I and then between the lugs 23 and over the rim of the cap 22 but no matter at what angle the loud speaker is mounted rain water cannot have access to the housing I3.

In the above described apparatus the magnet is preferably made of one or other of the alloys known to be used for making such magnets from such alloys as they permit of a large reduction of weight without efliciency loss.

It will be appreciated that the invention may be applied not only to pressure type loud speakers but also to all moving coil types and to dynamic microphones.

I claim:

1. A loud speaker or the like of the moving coil type, comprising a diaphragm, an adjacent speech-current-carrying coil for vibrating said diaphragm, radially spaced and concentric inner and outer pole pieces providing an annular air gap space therebetween, the outer pole piece comprising a circular plate having a hole into which the inner pole piece projects, an outer housing having a front opening, a front closure plate detachably secured to said housing for closing said front opening, said front closure plate having a circular recess in its rear surface, said diaphragm and outer pole piece being secured in said recess in said front closure plate.

2. A loud speaker or the like according to claim 1, wherein said diaphragm unit comprises the diaphragm proper and a flat ring framing therefor of some stiffer material which ts very accurately in said circular recess and said circular plate ts accurately in said recess behind said diaphragm and thereby clamps said framing in place.

3. A loud speaker or the like according to claim l, wherein said diaphragm unit comprises the diaphragm proper and a fiat ring framing therefor of some stiier material which ts very accurately in said circular recess and said circular plate ts accurately in said recess behind said diaphragm and thereby clamps said framing in place, and wherein said framing is slightly thicker than said diaphragm proper.

4. A loud speaker or the like according to claim 1 and wherein the inner pole piece is centred relative to said circular plate by means of a nonferrous bush mounted on the back of said circular plate, in which bush the rearwardly extending part of said inner pole piece very accurately nts, said bush having a plurality of holes in is rim and a plurality of cut-away portions along its inner edge.

LEONARD WALTER MURKHAM.

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

UNITED STATES PATENTS Number Name Date 1,766,473 Wente June 24, 1930 1,847,702 Thuras Mar. 1, 1932 2,104,433 Marshall Jan. 4, 1938 2,126,566 Marshall Aug. 9, 1933 2,295,483 Knowles Sept. 8, 1942 2,403,535 Kremer July 9, 1946 FOREIGN PATENTS Number Country Date 490,750 Great Britain Aug. 19, 1938