Dynamic microphone and method of making the same

Abstract: A dynamic microphone has a diaphragm body formed of this synthetic resinous sheet material having a multilayer central portion (18) consisting of a separately formed patch (18'). The patch (18') comprises a layer of synthetic resinous material and this wire mesh. The centre of this diaphragm and the patch (18') are deformed together to integrate the patch. The magnet (34) of the microphone is preferably made of neodymium-iron-boron and is located within the voice coil (23).

Description: This invention is directed to microphones of the dynamic or moving-coil type and to the method of making them. It is traditional in such microphones to employ AlNiCo magnet structure and small area or diameter diaphragms. A small area diaphragm is beneficial because it means that both the diaphragm and voice coil attached to it present low mass so that the microphone is relatively insensitive to handling or to shock which could produce spurious noise. At the same time, a small diameter voice coil is compatible with AlNiCo magnets which require a high ratio of height-to-diameter (i.e., a "cylindrical" shape) in order to avoid the serious effects of self-demagnetization. Unfortunately, AlNiCo magnet designs also have a low level of flux density in the voice coil gap so that these traditional microphones have inferior acoustic sensitivity compared with modern microphones such as the "condenser" designs. Even if one were to increase the diameter of the AlNiCo magnet to improve the acoustic sensitivity, the penalties paid in increased magnet and consequent microphone sizes, not to mention the penalties paid in increased masses due to increased sizes of both the diaphragm and the voice coil, plus the penalty paid in increased mass due to the need to stiffen or compensate for the increased diameter of the diaphragm, render that approach impractical. Thus, it will be seen that features of both the diaphragm and of the magnetic circuitry play interrelated and mutually incompatible if not mutually exclusive parts in the efficacy of dynamic microphones insofar as improvements in their acoustic sensitivity and decrease in their handling or shock sensitivity are concerned. The prime function of a microphone diaphragm is to act as a receptor for acoustic pressure waves and to convert such waves into physical force or motion at the attached transducer, in this case the voice coil in its magnetic air gap. The diaphragm must have sufficient stiffness in the plane of its major face so that it will behave as a piston and, on the other hand, the means used to support the edge of the diaphragm in the direction normal to its major face must be compliant as possible to permit easy travel of the diaphragm in such normal direction. At the same time, the diaphragm and its edge mounting must be relatively rigid radially to prevent radial motion of the voice coil and to confine its motion in the axis of the air gap. The diaphragm and its edge mounting must also be resilient enough to return the coil axially to its mid-position at the frequency and amplitude of the acoustic waves being treated, due regard being had for the overall mass of the whole moving system. A conventional microphone construction is disclosed in US-A-3513270. This microphone has a diaphragm body of synthetic resinous material (in this case PVC) having a domed central portion and a surrounding attachment portion whereby the diaphragm is free to vibrate in a plane normal to the diaphragm, an annular voice coil being attached to and circumscribing the central portion on the concave side thereof and a fixed disc-shaped permanent magnet disposed within the coil. The coil has an internal diameter many times greater than its thickness and the thickness of the magnet is greater than that of the coil. It is an object of the present invention to make an improved dynamic microphone having an increased acoustic sensitivity and low level of shock or of handling sensitivity. JP-A-53-45227 discloses a vibrating plate for a speaker comprising a metal mesh whose pores are filled with a polymer film. The present invention is based on the discovery that it is possible to make an improved dynamic microphone having increased acoustic sensitivity and a low level of shock or of handling sensitivity if fabrication techniques involving the use of diaphragm lamination employing a layer of metal mesh are combined with the use of a Neodymium-Iron-Boron magnet having a "coin" shape (i.e., having a high ratio of diameter-to-height) and a voice coil having a diameter accepting the magnet. The present invention provides a method of making a dynamic microphone comprising forming a diaphragm body of thin synthetic resinous sheet material of high tensile and flexural strength and characterised by the steps of forming a multi-layer central patch of thin synthetic resinous material and thin-wire metal mesh contacting the central patch centrally of the diaphragm body with the thin-wire metal mesh in face-to-face contact with the diaphragm body and integrating while deforming both so that they are domed, with the diaphragm body on the concave side of the central patch. The microphone of the invention is characterised in that the magnet is of Neodymium-Iron-Boron composition and the domed central portion of the diaphragm includes a central patch comprising a thin film of said synthetic resinous material and a thin wire metal mesh laminated to the diaphragm body with the metal mesh in face-to-face contact with the diaphragm body. Preferably the diaphragm laminate is surrounded by a semitorroidal annulus. Preferably the ratio of diameter-to-height of the voice coil is at least about 10:1. Preferably the permanent magnet has a ratio of diameter-to-height which is at least about 7:1 in conjunction with a voice coil of a diameter slightly larger than that of the magnet. Preferably the dome shape of the microphone diaphragm encompasses about 40% of the total area of the diaphragm. An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a plan view of a diaphragm according to the invention, the central patch being omitted for the sake of clarity; Figure 2 is a view similar to Figure 1 but with the central patch; Figure 3 is a cross-section of the finally shaped diaphragm with the voice coil adhered in place; Figure 4 is a cross-section of the magnet assembly; Figure 5 is a cross-sectional view of the assembled microphone; Figure 6 is a plan view of the housing; and Figure 7 is an exploded detail of the central patch construction. Before describing the structural details of this invention by reference to the drawing Figures, it is well to discuss the method of making the invention. Such method involves the step of forming a preliminary laminate which is a thin film of synthetic resinous material having a plastic adhesive (adhesive such as SCOTCH-GRIP 1099-L nitrile rubber base adhesive available from 3M) sprayed onto one face thereof, with drying, and a sheet of fine mesh metal wire in face-to-face contact with the dry adhesive and initially tacked to it by hot ironing the thin film with applied adhesive onto the wire mesh. This preliminary laminate is cut, by stamping, into circular central patches and each central patch is plastically deformed, without heat, partially into final shape. A larger circular body is stamped from a separate thin film of the synthetic resinous material. The partially deformed patch serves to allow accurate positioning of the larger circular body centrally of it. The result is a multi-layer entity in which the mesh side of the partially deformed central patch engages centrally on the larger circular body. The multi-layer entity is then subjected to heat and pressure between a lower male die and an upper female die forming the final diaphragm shape by thermal and plastic deformation while penetrating the adhesive through the mesh and curing it to bond the synthetic resinous layer of the central patch to the synthetic resinous layer of the larger circular body together (while penetrating and capture-bonding the mesh). The voice coil is wound in multi-layer form and adhered in that form by the polyvinylbutyral coating on such wire. The diameter of a voice coil is slightly smaller than the diameter of a circular patch. The voice coil is wound with an even number of coil layers so that the two leads at the opposite ends of the coil wire are at the same end of the coil height and therefore may lie close to or against the face opposite the face contacted by the mesh and extend radially outward in free fashion for ultimate connection to the output circuitry in conventional fashion. However, the delicate leads may be locally adhered to a peripheral edge portion of the larger circular body so as to anchor them securely after the voice coil is adhered in place and before they are soldered in place. With reference to Figure 1, the final shape of the diaphragm will be seen, although it is to be understood that the drawing figure omits the overlying central patch for the sake of clarity. As shown, the finished diaphragm has an annular outer securing edge flange 10 by which the diaphragm assembly is mounted, an annular semitorroidal portion 12, the interrupted, depressed ring area 14 with circumscribing and interrupting upwardly struck flute portions 16 and, finally, the central dome portion 18. As noted above, the thin film from which the larger circular body is made is a synthetic resinous material, preferably processed from ULTEM 1000 (unmodified) available from General Electric Company, a polyetherimide resin having exceptional tensile and flexural strengths. The thin film is preferably about 0,00127cm (.0005 inch) thick and is drawn under heat and pressure to the final shape shown. The areas 10, 14, 16 and 18 bear the brunt of the pressure and consequently are drawn the most. Not shown in Figure 1 is the central patch and the voice coil, but the former overlies the central dome portion 18 and the latter is concentric with and directly beneath the interrupted depressed ring areas 14. The two leads from the voice coil are shown by the lines 20 which are dashed in those regions in which they underlie the diaphragm and are solid where they project beyond the diaphragm. The adhesive employed to adhere the voice coil to the interrupted undersurface 22 of the diaphragm assembly (see Figure 3) is available from LOCTITE and preferably is an instant adhesive known as PRISM 403. Referring to Figures 2 and 3, a plan view of the finished diaphragm and a section as indicated in Figure 2 are shown. As noted before, the larger circular body is first placed in registered position on the smaller, preliminarily deformed circular patch. For this registering, the smaller circular body is first plastically deformed (pressure only) so that it takes on the shape generally of the flutes 16 and the interrupted portions 14. The circular patch entity is then registered with the larger circular portion and the multi-layer entity is finally-deformed between the male and female dies so that it is deformed into final shape as indicated in Figure 1-3 before the voice coil 23 is adhered in position as indicated in Figure 3. That is to say, the peripheral edge 24 of the circular patch (purposely omitted from Figure 1 for clarity) as shown in Figure 2, is coaxial with but lies just outside the boundaries of the flutes 16 and of the interrupted portions 14. After forming into final shape, the central patch has interrupted depressed portions 14' and flutes 16' which are merged onto and formed simultaneously with the portions 14 and 16 previously described in conjunction with Figure 1 as is the integrated body 18' of the circular patch formed simultaneously and integrated with the domed body 18 of Figure 1. In Figure 3, the domed central patch 18' and the domed body portion 18 are shown as a single thickness because they are integral at this time. The diaphragm, minus the voice coil, can easily be handled at this time and the voice coil can be adhered in place on and concentric with the interrupted surface 22 and with its leads 20 adhered to the undersurface of the edge flange 10. The magnet assembly is shown in Figure 4. It comprises the high permeability steel cup 30 having the upper recess 32 which receives the Neodymium-Iron-Boron magnet 34 and the high permeability pole piece 36. The cup 30 and pole piece 36 may be made of 1215 steel. The annular air gap 38 receives the voice coil 23 with little radial clearance (typically about 0,114cm (.045")), the internal diameter of the cup recess 32 being about 1,943cm (.765") in a typical microphone. For this typical microphone, the magnet 34 is of 1,702cm (.670") diameter and of 0,254cm (.100") in thickness or height and it, as well as the cup 30 and the pole piece 36, may be provided with a central through bore or aperture as shown. Also, it should be noted that the voice coil would have about 350 turns of copper wire in four coil layers, the wire size being 50 AWG with polyvinylbutyral bond. Figure 5 shows the assembled microphone. The cup 30 of the magnet assembly is received in the bottom recess 40 of housing 42 and bottoms against the overhang 44. The cover 46 has an internal ledge 48 which clamps the edge 10 of the larger circular body peripherally against the housing face 50, the elevated wall 52 of the housing cover providing clearance for the motion of the diaphragm and being provided with a ring of apertures 54 to allow the pressure waves to impinge upon the diaphragm. In equilibrium position, the diaphragm assembly is engaged by the support structure only at its peripheral edge 10 so that the diaphragm is free to flex in both directions normal to its surface. The housing 42 is provided with a pair of vertical recesses, 90° apart, to receive the voice coil leads 20 for soldering. Other housing parts may be provided as deemed necessary or desirable. Figure 6 is a plan view of the housing 42. To complete the disclosure of the microphone, Figure 7 has been included to illustrate, in exploded fashion, the details of the preliminary laminate from which the central patches are stamped or cut. As shown, the preliminary laminate consists of the thin film 60 of synthetic resinous material ULTEM 1000 and the spray-applied adhesive SCOTCH-GRIP 1099-L (about 0,00254cm (.001 ") thick) is indicated at 62. Lastly, the metal wire mesh material is indicated at 64. This material is 50 mesh stainless steel wire having a diameter of 0,00305cm (.0012") and of a roll width of 101,6cm (40"), normally used for electrostatic shielding and available from the Swiss company TETKO INC. As noted before, the ULTEM 1000 material with the 1099-L adhesive applied thereto and air dried is hot-ironed onto the mesh material so that the preliminary laminate is tacked to the synthetic resinous material. Dimensionally, the ratio of diameter-to-height for the permanent magnet typically will be about 7:1 whereas this ratio for the voice coil is typically about 10:1. Taking 0,254cm (.100") for the magnet thickness or height as a practical lower limit for the Neodymium-Iron-Boron magnet before self-demagnetization becomes a controlling factor, the ratios specified for the magnet and voice coil lead to the condition wherein the thickness or height of the voice coil is about 70% that of the magnet, or 0,178cm (.070"), taking into account that the inside diameter of the voice coil must be slightly larger (0,114 cm (.045") typically) than the outside diameter of the magnet. Lastly, it should be noted that with the specific materials specified herein and in accord with the preferred configuration and dimensions detailed herein, it can be said that the wire mesh-reinforced central dome should be about 40% of the total area of the diaphragm. This obtains sufficient stiffening to meet the objectives of the invention while maintaining the overall mass of the diaphragm-plus-voice-coil to achieve a dynamic microphone competitive in performance with modern condenser-type microphone designs.

Claims: A method of making a dynamic microphone comprising forming a diaphragm body (10,12,14,16,18) of thin synthetic resinous sheet material of high tensile and flexural strength and characterised by the steps of forming a multi-layer central patch (16',18') of thin synthetic resinous material (60) and thin-wire metal mesh (64), contacting the central patch (16',18') centrally of the diaphragm body with the thin-wire metal mesh (64) in face-to-face contact with the diaphragm body and integrating while deforming both said patch and said diaphragm body so that they are domed, with the diaphragm body on the concave side of the central patch. A method as claimed in claim 1 in which the central patch has less than half the total area of the diaphragm body. A method of making a dynamic microphone as defined in claim 1 or 2 including the steps of forming a voice coil (23) having a diameter commensurate with that of the domed area of the integrated diaphragm body and affixing the voice coil (23) to the thin synthetic resinous material of the body in circumscribing relation to the domed area. A method of making a dynamic microphone as claimed in claim 3 in which the voice coil (23) is located on the concave side of the domed area. A method as claimed in claim 3 further comprising forming a permanent magnet (34) having a high ratio of diameter to height. A method as claimed in claim 5 further comprising fixing the periphery (16) of the diaphragm relative to the magnet. A method as claimed in claim 5 or 6 in which the voice coil (23) has a diameter and a diameter to height ratio greater than those of the magnet (34). A method as claimed in claim 5,6 or 7 wherein the diameter to height ratio of the magnet (34) is about 7:1 and the diameter to height ratio of the voice coil (23) is about 10:1. A method as claimed in any of claims 5 to 8 wherein the permanent magnet (34) is Neodymium-Iron-Boron. The method according to any of claims 5 to 9 wherein the magnet (34) presents North and South poles on its larger faces and including the step of enclosing the magnet partially with high permeability material (30,36) so as to leave an air gap within which the voice coil is partially received. A dynamic microphone comprising a diaphragm having a body of thin synthetic resinous material in the form of a domed central portion (18) and a surrounding attachment portion (16), an annular voice coil (23) attached to and circumscribing the central portion on the concave side thereof whereby the central portion is free to vibrate in a plane normal to the diaphragm and a fixed permanent magnet (34) of disc shape disposed within said voice coil (23), the voice coil having an internal diameter many times greater than its thickness and the thickness of the magnet being greater than the thickness of the voice coil; characterised in that the magnet is of Neodymium-Iron-Boron composition and the domed central portion of the diaphragm includes a central patch comprising a thin film of said synthetic resinous material (60) and a thin wire metal mesh (64) laminated to the diaphram body with the metal mesh in face-to-face contact with the diaphragm body. A microphone as claimed in claim 11 wherein the metal mesh (64) is positioned on the convex side of the domed portion. A microphone construction as defined in claim 11 or 12 wherein the area encompassed by the central portion is about 40% of the total area of the diaphragm. A microphone construction as defined in claim 13 wherein the ratio of diameter-to-height of the magnet is at least 7:1. A microphone construction as defined in claim 14 wherein the ratio of diameter-to-height of the voice coil is about 10:1. A microphone as claimed in any of claims 11 to 15 further comprising a magnet asssembly including a permeable cup-portion having a circular recess with a closed inner wall, an upstanding annular wall portion extending from the inner wall and forming a pole piece, the permanent magnet being disposed within the recess in abutment with the closed wall, and a disc like permeable pole piece disposed over the magnet, said magnet assembly having an annular recess for receiving said voice coil.

Inventors: TARDO TIMOTHY BRYAN, TARDO, TIMOTHY BRYAN
Applicants: PEAVEY ELECTRONICS CORP, PEAVEY ELECTRONICS CORP.
Application year: 1990
Main classification: H04R9
IPC codes: H04R7, H04R9, H04R31