IMPROVEMENTS IN ELECTROACOUSTIC TRANSDUCER DIAPHRAGMS

Abstract: Improvements in electroacoustic diaphragms, principally of the moving coil type "cones" and domes. The improvements sought, particularly the minimisation of "diaphragm break-up" within the transducers bandwidth, are achieved by using anisotropic materials for the diaphragm which preferably have a higher azimuthal tensile modulus than meridional tensile modulus. This is preferably implemented by the inclusion of selectively oriented high specific modulus linear materials such as ceramic, metal or polymer tape, fibre or whiskers in a lower modulus matrix. The preferred directions of alignment of these high modulus inclusions being azimuthal and meridional. Filament winding is recommended as an effective method of incorporation of the azimuthally oriented inclusions.

Description: IMPROVEMENTSINELECTROACOUSTTCTRANDUCER DIAPHRAGMS This invention relates to diaphragms for εlectrcacc stic transducers and in pcrticular to transducers of the type in which th≤ diaphragm iε, coupled to rπ electromechanical transducer for example moving coil type loudspeaker and is formed into a three dimensional shape to achieve rigidity for example 'cones' of conical or flcrcd form and so called tv/eetcr domes. The principal problem to be overcome is the occurrence of the phenomenon of 'diaphragm break-up' at frequencies within or near the frequency bandwidth that the transducer is required to reproduce. 'Diaphragm break-up' is principally caused by the appearance of bending waves on the diaphragm occurring εs the major diaphragm/energy transportation mechanism on at least some region of the said diaphragm. This phenomenon is responsible far degradation of transient response, production of nonlinear distortion and noπlinearity of the transducers axial frequency response transfer function. Two types of bending waves can and do occur. At low frequencies, asymmetric modes with nodal diameters appear and these are usually minimised by curvature of the diaphragm, for example flared * cones' and by the use of azimuthεlly oriented corrugations or rigid stiffening rings. Axisymmetric bending waves occur at high frecuenoies. For a. cone they first appear at the outer boundary at a frequency f, , called the lower limit of the cone transition region and progressively move inward with increasing frecueπcy until a.t f, , the upper limit of the cone transition region, the whole diaphragm is subjected to axisymmetric bending wavws. For an isαtropic ' cone' type diaphragm, the onset of ' iaphragm break—up' can be predicted approximately by analysis of a conical membrane. Thi gives the equation for the lower limit of the cone transition region. f.. = c cos S. / 2TTR. tb c b where c is the velocity of longitudinal waves in the diaphragm, θ. iε the semi apex angle of a cone which is a surface tangent to the ' cone' at the outer boundary and ?•__ is the radius of the outer boundary. BAD ORIGIN L ft OMPI Below f , the whole diaphragm moves in phase and the diaphragm radiation can be reasonably described by the radiation characteristics of a rigid ' one' . With constant acceleration/ frecuency drive, a rigid cone exhibits a flat exial pressure rmplitude/frecuency response function until it approaches the characteristic cone frequency f = c tcπS, / TTFι, (= c / TTH) c o b J b v α' ■ * vhere H iε the cone depth end c the velocity of sound in sir. t'eycπd f the axial radiation attenuation approaches δdb/αctave aεymtotically. Consequently, the usefulness of decreasing the cone εεmi apex angle θ, to postpone the onset frequency αf ' iaphragm break-up* is limited by its effect on the characteristic cone frequency. Tt may be noted that, in this simplified analysis, a flared ' oπε' suf ers in comparison to a cons since it wculd normally have a greater effective cone depth H and/or a larger 5.. Generally the preferred desig ' π procedure would be the selection cf a material with as high a value cf specific tensile modulus ( -=c~ } as possible and then selection of cone profile to meet overall design requirements. Lf the suitable isotropic^materials, metals such as Aluminium, Eεryllium end titanium exhibit some of the highest values of specific tensile modulus available and diaphragms αf these materials have been made but they exhibit the undesirable characteristic of very low internal loss factors ! v igh mechanical ~ ) ϊechanical damping of these diaphragms by application of skins of high loss factor polymers can be used tα reduce the overall mechanical Q but brings with it a significant reduction in the effective specific, tensile strength and modulus. The relatively recent introduction of very high specific tensile-, modulus linear materials ( generally ceramic fibres or whiskers such as C, G, Si C, Al C„ but also p ' olymers such as aromatic pclyamides) suggested that improved diaphragms could be produced using these materials. Attempts to produce significantly improved diaphragms using randomly oriented high tensile modulus fibres appears to have failed for reasons such as ; effective tensile modulus of a randomly oriented fibre composite being BAD ORIGINAL [ } significantly less than the individual fibres axial tεnεile modulus, difficulty of actually forming the diaphragm shape of • such a composite, difficulties -with achieving sufficient fibre to matrix interface bond strengths and certain prooerties of the fibre materials (for example the self lubricating properties of graphite) reducing composite modulii and strength. The objects of this invention are to provide materials with optimally adjusted properties suited to particular diaphragm requirements, principally the minimisation of 'diaphragm break-up' effects within the desired transducer bandwidth, and methods qf manufacture of said diaphragms. Examination of the conical membrane equations for aπisotropic materials with meridional variation of the azimuthal tensile modulus shows that the parameter c in the equation for the lower limit of the cone transition region f tb = c cos 9 b / 2TTR b largely represents the circumferential or azimuthal longitudinal wave velocity. The mainly azimuthal orientation αf high specific tensile modulus linear material in a lower specific tensile modulus matrix produces an anisotropic material with an azimuthal specific tensile modulus greater then the meridional specific tensile modulus and hence a diaphragm with an f.. higher than that for a similar diaphragm of comparable composition but random oriented ■ linear material. While f... is relatively independent of the meridional tensile modulus, the overall diaphragm behaviour, particularly at frequencies below f.. , iε not. Consequently some control over the meridional modulus is desirable. The ratio αf azimuthal to meridional tensile modulii can be controlled to some extent by the inclination of the included material to the meridional direction and the ratio of the modulii of the linear to matrix materials. A more effective method of reduction αf this ratio is the inclusion αf meridionεlly aligned high modulus linear material into the diaphragm. .πe object is therefore to gain maximum advantage from the properties of the diaphragm material by alignment of the material and/or its components to achieve maximum strength and modulii in the preferred azimuthal and meridional direction. The effective mechanical ~ of the diaphragm will depend upon the type and orientation of the strain but, for a composite, will generally be dependant upon the type of linear material and matrix material and the nature αf the linear materiel to matrix material interface bond. Modification of the Q can be achieved by any of the usual methods such as coating the diaphragm with a high loss f ctor material ( for example polyvinyl acetate). Thus the first aspect of this invention is directed to the selection αf an isσtropic material for the transducer diaphragm with the preferred directions of material properties being azimuthal and meridional and preferably so chosen th t the azimuthal specific tensile modulus iε made greater than the meridional specific tensile modulus. This being generally achieved by the inclusion αf selectively oriented high specific tensile modulus linear material such as ceramic, metal or polymer fibre, tape or whisker, in a lower modulus material for example metal or polymers such as epαxies, polyurethsπes,'polyesters, polyolεfines, nylon, polyvinyls. The inclusion may be semi- continuously distributed throughout the diaphragm section or discretely incorporated dividing the diaphragm into sectors and/or annular regions or farming spirals αf either or both rotational sense which may if required be joined to farm s rigid framework. The εεcαnc aspect of this invention iε directed to rretho s of manufacturing aπiεotropic diaphragms as described above. The largely azimuthal orientation of an included high specific tensile modulus linear material can be readily achieved by the methods αf filament winding (wet or dry) with single or multiple feeds onto a suitably shaped mandrel. This method provides a means of control of many important diaphragm parameters (.such as meridional variations of azimuthal tensile modulus and diaphragm thickness) by variation of ; the matrix materials formulation, the inclusion to matrix content ratio and the meridional feed angle during the winding process, '.".'here continuous high specific tensile modulus fibres such as the ceramic fibres Boron, Carbon/Graphite or O?:JI Silicon Carbide or polymers such as of the aromatic polyεmide type constitute the included material, the continuity of fibre achieved overcomes many of the. problems associated with randomly oriented composites. Meridional tensile modulus and strength can be increased by increasing the meridional winding angle although significant increases in meridional tensile modulus are better achieved when a layer or layers of meridionally oriented high specific modulus linear material are laminated with a layer or layers of azimuthally oriented layers. Alternatively, meridionally oriented and largely azimuthally oriented material may be interlaced or woven to increase interlaminar shear strength, The process of joining the wound material (and/or layers) to form a continuous diaphragm could be concurrent with or subsequent to the said forming processes and cculd involve the use of any suitable process ( for example polymerisation, ultrasonic welding, pressure, heat, solvent, adhesive or vacuum deposition techniques). Likewise prestresεing of the high modulus material, if found necessary, could be carried out concurrent with or subsequent to the forming process. In certain applications it may be desirable to laminate a selectively anisαtropic diaphragm as described above with other layers of polymers, metals or ceramics. In any diaphreg containing such a selectivεly eπiεctrσpic diaphragm, azimuthally oriented corrugations or rigid rings may be employed to minimise the asymmetric bending effects. ■ '

Claims: The claims defining the invention are as follows: 1. diaphragm for an electroa.cαustic transducer of the type in which the diaphragm is. of three dimensional shape to achieve rigidity (for example tweeter domes end conical or flared cones). Wherein the selected diaphragm material is aπiεαtropic with the preferred directions of material properties being azimuthal and meridional and preferably with an azimuthal specific tensile modulus greater than the meridional specific tensile modulus. 2. A diaphragm as claimed in claim 1, wherein the diaphragm rraterial is made anisotrαpic by the inclusion of selectively oriented, linear material(ε) of higher specific tensile modulus than the matrix material and where the preferred directions of orientation of the inclusions are azimuthal and/or meridional. 5 3. A diaphragm as claimed in any one of claims 1 or 2 wherein the included linear material(ε) are of a high specific modulus such as ceramic (.far example B, C, Si C, Λl D„), metal or polymer fibre, tape or whiskers. 4. A diaphragm as claimed in any one of * the claims 1 to 3 3 wherein the included linear material is itself αf a composite nature such as metal or polymer with ceramic, metal or polymer fibre, tape or whisker reinforcement. 5. A diaphragm as claimed in any one of claims 1 to 4 wherein the matrix material is of a metal or a polymer such as 5 epoxies, polyesters, polyurethaπes, polyolεfiπes, nylons, pclyvinyls. δ. A diaphragm as claimed in any one αf the claims 1 to which employs the process of single or multiple feed filament winding in its manufacture. 3 7. A diaphragm as claimed in any of the claims 1 tα S formed of laminations of azimuthally end meridionally reinforced layers. 8. A diaphragm as claimed in any one of the claims 3 to C wherein the high modulus linear materials in the meridional = and azimuthal directions are interlaced or woven. rτι t θ!.:?ι 9. A diaphragm as claimed in any one of claims 6 to 8 in which the material is joined tα forπrr a continuous diaphragm during or after the winding and/or weaving and/or laminating process by any suitable process such as polymerisation, ultrasonic welding, heat, pressure, solvent, adhesive or vacuum deposition. 10. A diaphragm in any one of the claims 1 tα 9, in which the included linear material is prεstrεssed during or after the initial process of manufacture. 11. A diaphragm as claimed in any one αf claims 1 to 10, which has interlaminatiαns or coatings or skins on either or both sides αf high loss factor polymers (such as polyvinyl acetate). 12. A diaphragm as claimed in any one αf claims 1 to 11, which has interlaminations or skins of any ceramic, metal, polymer or composite material. 13. A diaphragm as claimed in any one of claims 1 to 12, which has azimuthally oriented corrugations or rigid stiffening rings affixed.

Inventors: SYLVESTRE P
Applicants: SYLVESTRE P
Application year: 1981
Main classification: H04R7
IPC codes: H04R7