US 2400662 A
Description (OCR text may contain errors)
May 2 946- J. s. P. ROBERTON ET AL. 2,400,662
TELEPHONE TRAIISEJITTER AND RECEIVER Filed May 18, 1942 Patented May 21, 1946 UNITED: STATES PATENT oFFics 2,400,662 TELEruoNE TRANSMITTER AND RECEIVER James Samuel Paterson Roberton4 and Michael Lawrence Gayford, London W. C. 2, England. assignors to International Standardllectric Corporation, New York, N. Y.
Application May 1s, 1942, serial No. 443,424 in Gmt Britain May 27, 1941 5 claims; `(ci. 17e-115.5)
This invention relates to electro-acoustic transducers, that is to say to telephone transmitters and/or receivers, of the moving-coil type; and consists in improvements in the manu vfacture of such transducers whereby the poleterial, a permanent magnet A disposed concen' trically within the pot so that `the rim of the..
type with the required degree of accuracy inv the spacing of the inner and outer vpole-pieces, and
tape, which is iixed upon the rear or underside of a'diaphragm I6.
The central part of the diaphragm I6 is domed forwardly; and the space thus formed between the rearslde of the diaphragm and the upper surface of the inner pole-piece I3 is partly filled by a segment I1, preferably of synthetic-resin moulded material, which is held in place by the centralvnut and bolt I4, with its flat surface against the flat top of the inner pole-piece. v.For
acoustic reasons the segment has within it a hollow space communicating through slits with the space behind thel diaphragm. The hollow space may be provided by a recess formed in the otherwise flat underside of the moulded segment, and the slits by the introduction of a spacing shim at the position I8 surrounding the Vbolt I4 or by slight projections spaced around the underside of the periphery of the segment.
' the pot into the space around the magnet II.
more especially with such mechanical rigidity In order that the invention may be clearly understood, a description will now be given of two I of its embodiments as shown inlthe .accompanying drawing, inwhich:
Fig. l shows the rst embodiment in transverse section;
Fig. 2 the second embodiment in rear end view with the cover removed;
Fig. 3 a transverse section on the Fig. 2; and l Fig- 4 a part-section on the line 4-4 of Fig. 2.
As shown in Fig. 1, a short cylindrical magnet I I of high coercive force material carries an outer pole-piece in the form of a pot I2 iitted over the magnet from its lower end and an inner polepiece in the form of a anged disc I3 iitted onto the magnet at its upper end. The magnet and line 3-3 of The three parts of the magnet system are thus firmly embedded in the hard resinous material, and this has the valuable effect of fixing the parts rigidly with respect to one another and thus assuming the maintenance of a uniform air-gap, it being noted that, with .the powerful permanent magnets which modern materials make possible, any initial lack of concentricity of the pole-pieces at the air-gap is apt to become rapidly aggravated because the magnetic attraction across the air-gap is at its -greatest on that radius at which the gap is narrowest. This design also permits the acoustic properties to Ibe improved: the moulding material is allowed to Ilow into the pot up to a predetermined contour whereby the proper volume of air space inside the pot (behind the moving coil) is secured. The holes in thev and outside of the unit a small channel or holey may be provided in the front cover or mouthpiece to allow air to pass in or out from the chamits Dole-pieces are clamped' together by a central 55 bers between the membrane and the front of the nut and 'bolt I4. The inner and outer polepieces I3 and I2, which are of high permeability material, deiine between them an annular airgap; and in this air-gap is located a speech coil diaphragm. Pressure changes in the air behind the diaphragm will be prevented by air ow through the silk covered holes in the frame.
' This embodiment of the invention has been I5, in ther form of an edge-wound aluminium designed so as to be suitable for use on aircraft;
consequently one of the requirements is that lts magnetic field shall not influence a compass even at short range. and the efficiency of the magnetic screening' is one ofA the features of this design.
In the first place it has been found that the pot-with-internal-magnet design (parts I2 and .I I) is very satisfactory in reducing the external field to a small value. However, there is inevitably some flux leakage, especially around the air-gap and to a lesser extent from the back of the pot pole-piece, and to reduce this still further if not eliminate it completely the magnetic structure is completely enclosed by magnetic'screens. There is a front screen 22, which overlies the diaphragm I6, a back screen 23, which is in the form of a cup fitting over the insulating casing I9 and secured by screws, and a clamping ring 24 which overlaps both of the other two and which, being spun into position, serves mechanically to hold the assembly together. It has been found necessary to have the screening surface as nearly possible continuous all round the unit. The front screen 22 is of course in the form of a grid, i. e. it is apertured; but the holes in the mouthpiece or earpiece do not register with the holes in the front screen 22 so that it is not possible to injure the diaphragm by poking anything through the holes,
The leads from the speech-coil I are necessarily weak and fragile, In this design they are as short and as little exposed as possible, being taken to the tips of terminal screws 25 rigidly moulded in the frame; the winding end of the terminal is slightly below the level of the seating surface for the diaphragm. The back screen 23 forms a cover to protect the terminals.
Figs. 2 to 4 show a Second embodiment of the invention. This is a capsule-type microphone, with one terminal of the speech-coil connected to the back screen 23 and the other connected to an insulated central contact 26 in the back of said screen.
One of the interesting features of this design which does not appear in that shown in Fig. 1 is the presence of the disc 21 of high permeability material in the bottom of the pot I2. The purpose of this is to reduce the flux leakage which, as already mentioned, occurs from the back (base) of the pot. The flux passing from the central magnet to the rim of the pot is concentrated in the material of the pot, and thus is subjected to something of a bottle-neck at that part of the base of the pot which lies immediately below the periphery of the magnet. Further out the conditions become easier, although the thickness ofl metal is not increased, simply because the annular cross-sectional area increases with the radius. The disc 21 serves to open up this bottle neck, thus reducing the concentration of flux, and also reducing the leakage. It then becomes possible to make the pot I2 of thinner material than would otherwise be necessary to carry a certain flux as prescribed by the thickness of the inner-'pole-piece I3. A thinner pot is easier of manufacture by a drawing operation, is of less Weight, and gives more space behind for the terminals.
In this embodiment of the invention the diaphragm I6 seats upon a flange out-turned from the rim of the pot instead of upon a moulded part. Also, the acoustic resistance is provided by the lambswool lling 28 of slots in the moulding 29 which is secured to the pressed cup I2. This moulding carries the speech coil connections 30. In this construction there is only the internal filling of setting material and no external casing either continues with it or separate. The filling extends up on the outside of the flange on the disc I3, instead of inside it as in the previously described construction.
The capsule is normally fitted in a, moulded holder which has a short rubber horn. The design of this horn and the front holes and cavity of this holder has been considered inasfar as it affects the performance. The action of these in conjunction with the holes, cavities and protective oiled silk (or substitute) membrane 2| al1 combine to enable the desired response to be obtained, there being, in effect, a series of front resonators. The protective membrane 2I is supported at its centre by the perforated metal dome 20. As it is heid clear of this dome at the edges there is no trouble encountered through the membrane adhering to the dome when saturated with moisture and failing to transmit sound.
The internal lling 3| of the cup is preferably formed in position by injecting a suitable compound, such as plasticised polystyrene through small holes in the cup, a ring being held in the magnetic gap during this process. By the use of suitable pressures and temperatures, the cup can be filled without an undesirable flash being forced up the sides of the ring in the magnetic gap.
It will be understood that the invention may take other forms. In respect of the constructions already described for instance, the'moving coil I5 might be of copper, and the dome I1 might be omitted.
What is claimed is:
1. An electro-acoustic transducer comprising a pot of magnetizable material, al permanent magnet disposed concentrically within the pot so that the rim of the pot constitutes an outer pole, a disc of magnetizable material overlying the magnet in the mouth of the pot so that its rim constitutes an inner pole, a diaphragm, a speech coil attached to said diaphragm and located within the annular air-gap formed between said poles, and
a filling in said pot of setting material rigidly.
holding said poles in concentricity. Y
2. An electro-acoustic transducer according to claim 1, wherein said disc is formed with a downwardly projecting flange and said filling extends into the annular space between said flange and said magnet.
3. An electro-acoustic transducer according to claim 1, wherein apertures are provided in said pot which is embedded in a block of setting material which is continuous with said filling.
4. An electro-acoustic transducer according to claim 1, wherein said pot is embedded in a block of setting material .and an aperture is bored in said block to provide an air-passage through it extending through said filling into said air-gap.
5. An (ectro-acoustic transducer according to claim 1, wherein said pot is embedded in a block of setting material and an aperture is bored in said block from its outside surface to provide an air-passage through it extending through said filling into said air-gap, and wherein said airpassage is covered with a layer of /silk applied over the outer end of the bored passage.
JAMES SAMUEL PATERSON ROBERTON.
MICHAEL LAWRENCE GAYFORD.