Movable-coil electrodynamic transducer with a diaphragm

Abstract: The present invention relates to a diaphgram for an electro-acoustical transducer, of the movable-­ coil electrodynamic type, for transforming electrical signals into acoustical signals and diffusing the electrical signals, characterized in that the diaph­ gram comprises a single-piece blade member, made of a metal material of very low density and very good heat conductivity and mechanical characteristics. The metal blade is resiliently suspended on the perimeter thereof, is metally coupled to the unit of which it forms a portion, and supports the acoustical current wire fixed at the center of mass thereof to the active metal surface. The acoustical current conductor wire communi­ cates to the blade the heat which is produced by the acoustical current, which is added to that generated in the blade mainly because of the deformation of the resilient suspension. This generated heat is quickly absorbed by the metal blade and efficiently dissipated by irradia­ ting it into the encompassing air and by conduction through the adjacent masses, thereby holding the tempe­ rature under a danger level.

Description: BACKGROUND OF THE INVENTION The present invention relates to a diaphragm electro-acoustical transducer, of the movable-coil electrodynamic type, for transforming electrical signals into acoustical signals and for diffusing the latter. As is known, in electro-acoustical transducers for transforming electrical signals into acoustical signals and diffusing the latter, the vibrating member, which is electrically driven by the signal, consists of a diaphragm to the active surface thereof, that is that surface which affects the transmission medium, usually consisting of air, there is rigidly affixed, at the center of mass thereof, a conductor of the acoustical or sound current, i.e. a movable coil which is driven because of the inter-action between the acoustical current and a constant magnetic flux through the gap therein the conductor is arranged. In general, a typical transducer for a middle-high range is provided with a diaphragm or membrane which essentially consists of an active substantially rigid curved surface, properly designed for the provided frequency spectrum, which is rigid with a supporting frame on which is rigidly affixed the movable coil. The diaphragm comprises moreover a resilient suspension system, having a suitable resiliency for the frequency range to be reproduced; in other words, this resiliency has a value which, in the parameter system of the diaphragm, for example the provided masses, determined the condition for a linear acoustical reproduction up to the maximum amplitude of the applied signals, with a substantially even response through the overall range; there being moreover provided a peripheral ring member which restrains the movable equipment to the rigid structure of the unit. More specifically, the rigid surface is provided by a light alloy blade, configured as a spherical cap, or the like, whereas the resilient suspension system comprises a ring member usually made of a thin plastic material, which material also forms the support member of the alluminium wire coil on which it is wound, and the restrining ring member. For designing such a system, it is necessary to consider several practical requirements imposed by the latest developments in the electric-acoustical field (such as, for example, the great spreading of the recording dynamic range), and the consequent increase of the risk due to the required performance severity imposed to the system from the reliability standpoint. The most important characteristic for an operating system is the capability of this system to support power, i.e. it capability to preserve, under extreme conditions, the qualitative and quantitative efficiency of less stringent requirements, so as to provide a continuous and even type of operation. For a maximum power level, among the several chain components, from the starting electrical input signal to the transduced acoustical signal, is the electrical signal to acoustical signal converter which is the most affected component. In order to bring this reasoning to a very elementary degree, (that is to reduce it, by arithmetically speaking, to the "least terms"), let us consider a system in which the full acoustic frequency range has been practically divided into several sections, each controlled by a suitable characteristic transducer, and let us consider exclusively the transducer of the top end limit of the range, for example from 3k to 16 k Hz , that is that for which the subject reasoning will be clearly evident (naturally under the hypothesis that the considered range is not further subdivided). For a faithfull reproduction or playback, as stated, apart other requirements which are herein neglected, there is necessary to provide a perfect linearity, that is an even ratio of the value of a parameter, for example the power of the input electrical signal, and that of the corresponding acoustical signal at the output of the transducer; and this for all of the frequencies: which means a substantially even response through the overall frequency range, that is with an offset less than ±2 dB. If the values of the characteristic parameters of the transducer, in particular of those of the movable assembly, have been properly designed in order to provide the desired characteristics, it occurs that the conductor wire used for forming the movable coil will have an insufficient cross-section for supporting the current intensity corresponding to the maximum signals of a recursive average duration, with a consequent superheating; moreover, the diaphgram construction will have an insufficient thickness to resist against the stresses exerted thereon by the movable coil subjected to its maximum displacements; on the other hand, its mass can not be increased to allow, considering the mass of the movable coil, a playback of the top limit of the sound frequency range practically devoid of any loss. By concluding, if the transducer being considered is operated in extreme conditions, as it actually occurs, then the most deleterious drawbacks will be due to: a) the over-heating of the movable coil which is susceptible to deleteriously damage the movable assembly or equipment, because of the high temperature involved; b) the premature wear of the parts of the diaphragm structure, which are subjected to very high mechanical stresses, with a consequent heat generation, such as great stretching and binding deformations, characteristic of the resilient suspension system and mainly because of a very great tension stress to which the diaphragm is subjected along its coupling line to the diaphragm supporting unit, because of instantaneous pulses, even of moderate value, all which will contribute to quickly put the transducer out of service. Since it is not possible to increase, in a constant characteristic transducer, the mass of the diaphragm and movable coil, without negatively affecting the result, the single approach to be used is that of efficiently and quickly dissipating, as far as possible, the generated heat so as to hold the temperature of the diaphragm, for the provided operation type, under a risk level. The document US-A-3 991 286 discloses a heat dissipating device for loudspeaker voice coil wherein a diaphragm member may be fabricated of thermally conductive metallic material, is fixedly attached to o integrally formed with a coil form and is resiliently supported on a frame by means of a spider element which also may be fabricated of thermally conduttive material. But, the diaphragm structure in such a device comprises at least two pieces, namely the diaphragm member and spider element, which must be attached to one another, thus reducing the effective conductivity of the assembly. DE-A-2 357 425 discloses a process for manufacturing a loudspeaker membrane in beryllium or alloy thereof, that is to say a membrane of light and low density material, but does not teach or suggest any use of such a material for dissipating heat in an electric-acustic transducer. US-A-2 392 143 discloses a loudspeaker incorporating an acoustic metal diaphragm which comprises a central, vibratile portion and a peripheral suspension therefor portion radially from said vibratile portion, means providing a channel between said vibratile portion and said suspension and constituting a connection therebetween and a voice coil secured to said diaphragm within said channel. This type of diaphragm may be formed from a single or two pieces. In a patent abstract of JP-A-58 30293 a dome type speaker is disclosed as incorporating a diaphragm composed only of a dome part and having a rim part which joins to a supporting frame through a soft elastic material. However, even if diaphragm may be a single piece, it has no corrugated portion and necessitates to be completed with at least a coil form portion for voice coil to be fixed to the diaphragm. In addition, the disclosed assembly does not provide any heat dissipation. DE-A-21 10 466 discloses a membrane for a loudspeaker but this membrane is lacking of corrugations and coil form; the voice coil is applied under the membrane. It is, instead, one object of the present invention to solve the above mentioned problems, by providing an electric-acoustic transducer, of the movable-coil electrodynamic type, having a diaphragm or membrane for transforming electric signals into acoustical signals and diffusing the latter, which affords the possibility of quickly dissipating the generated heat, both by irradiation and conduction, in order to hold the temperature under a risk level. Another object of the invention is to provide a diaphragm or membrane for an acoustic transducer, which can be easily produced and which is adapted to resist against severe use conditions while providing a higher performance than the conventional diaphragms or membranes. Another object of the present invention is to provide an electric-acoustic transducer with a diaphragm which is very reliable and flexible in operation and, moreover, is very competitive from a mere economic standpoint. In accordance with the present invention, the above and other objects are achieved by a diaphragm electric-acoustic transducer which comprises: a pole portion or pole tip; a support unit formed of metal, surrounding said pole portion and defining a gap therewith; a diaphragm surrounding said pole portion and having a peripheral ring portion at the outer periphery attached to said support unit; an electrically conductive voice coil attached to said diaphragm, and wherein said diaphragm is formed in a single piece from a high heat conductivity metal and has a corrugated portion adjacent said peripheral ring portion, a cylindrical portion on a opposite side of said corrugated portion from said peripheral ring, and a central cap portion extending across an end of said cylindrical portion, the conductive voice coil being attached to, and closely in contact with, said cylindrical portion of said diaphragm; characterized in that said corrugated portion of said diaphragm is composed of a number of spiral-like undulations providing a resilient suspension system, and in that said resilient suspension system is provided with a damper, made of rubber or other elastomeric material, and symmetrically arranged with respect to the suspension system. Thus, the acoustical current conductor will transmit to the diaphragm the heat generated by the acoustical current, which heat will be added to that generated in the diaphragm mainly because of the deformation of the resilient system during the operation; the heat being quickly absorbed by the metal diaphragm and efficiently dissipated by irradiating it into the air and by conduction through the adjoining masses, so as to hold the temperature under a set dangerous level. Further characteristics and advantages of the present invention will become more apparent hereinafter from the following detailed disclosure of the subject diaphragm, or membrane, which is illustraded, by way of an indicative but not limitative example, in the figures of the accompanying drawings, where: Figure 1 is a schematic cross-sectional view illustrating a first embodiment of the transducer according to the present invention; Figure 2 shows a different embodiment of the transducer according to the invention; Figure 3 is a top plan view of the diaphragm shown in Figure 1; Figure 4 is a bottom view of the diaphragm shown in Figure 1; in the embodiment being disclosed the diaphragm is anchored to its supporting unit through the interposition of a resilient gasket; Figure 5 illustrates yet another embodiment of the transducer according to the invention; Figure 6 shows a graph of the diaphragm response, in which on the abscissa axis there are shown the frequencies, while on the ordinate axis there are shown the pressures in dB's. DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the figures of the accompanying drawings, the diaphragm or membrane used in the transducer according to the invention is characterized in that each portion of said diaphragm is made of a very good heat conductive material, having a very low density and suitable mechanical characteristics. From preliminary tests on the diaphragm it has been found as preferable an aluminium alloy with a high magnesium contents; however, the processing of such an alloy, in order to obtain a diaphgram suitable for a mass production, has required a great designing effort and a particular processing method in order to obtain, from a blade member as that mentioned, having a thickness of few 1/100 of millimeter, susceptible to be highly work-hardened, a tridimensional object with different and partially opposite properties, distributed according to the function of the several parts. As shown in figure 1, the diaphragm, indicated overally at the reference number 1, is applied to a diaphragm supporting unit, overally indicated at the reference number 2. The diaphgram has a central cap 10, or diaphragm active surface, of substantially rigid configuration for the provided frequency spectrum; from the zone 10 a cylindrical portion 11 axtends thereon there is assembled a movable coil 12 which closely contacts the metal material of the diaphragm so that the heat generated by said coil will be quickly and efficiently absorbed and dispersed both by irradiation and by conduction through the metal mass to which the diaphragm is anchored. The diaphragm is moreover provided with a waved portion, which provides a resilient suspension system, indicated at 15 and including a peripheral ring element 16 coupled under the top flat portion 3 of the unit 2, which includes a central permanent magnet 4 arranged in the magnetic circuit and made of a high permeability iron, consisting of the portions 2, 3, 5 and 6. The thus designed diaphragm has been subjected to severe operation tests and extreme conditions, and it has been found that this diaphragm has very high performance characteristics, with respect to conventional diaphragms. The aluminium alloy which has been used for making the subject diaphragm is AlMg7, with a thickness of 0.05 mm; the diaphgram has been constructed according to three embodiments for 500-8000, 800-10,000 and 2,000-16,000 Hz, with an active surface of convex and concave cap configuration having respectively a diameter of 55, 50 and 34 mm. In this connection it should be apparent that, for making the diaphragm, another alloy can be used provided that it has the above defined characteristics. As shown in figure 2, since the diaphragm is made in a single piece, with the three components or parts thereof disposed in the precise order: active surface, gap movable coil, peripheral resilient ring member, the active surface will be arranged on the top and the resilient suspension zone will be arranged under the flat portion closing the magnetic circuit. Then, during the assembling operation, the diaphragm will be arranged in its position before the flat portion, which will prevent the diaphragm from being removed and replaced without demagnetizing the unit, which operation can not be easily performed remotely from the shop and which requires, in the unit, the provision of a field coil: however, from this undesirable characteristic no drawback derives for the reliability of the device. As shown in figure 2, the diaphragm, indicated overally at the reference number 20, according to a more complex and delicate embodiment thereof, carries into practice the inventive idea. In fact, the blade member, indicated at 20, is provided, between the resilient zone and cap, which are always indicated at the same reference numbers used in figure 1, with a loop 21 for housing the movable coil. From the construction standpoint, the blade member must be looped on itself so as to reach and cover, by a second layer, the already formed cylindrical portion, the base of the cap, and must then be bent through 90°, in order to form the resilient suspension system which, in the case being disclosed, advantageously comprises eleven crimping legs, of spiral shape, which are incised in the metal material in a symmetrical way in the overlaying and underlaying portions so as to simultaneously hold the static planarity of the ring member unchanged. The cylindrical portion projecting from the cap base, with a very thin gap between the two portions forming it, is adapted to closely receive the movable coil: on the outside or on the inside of it or, preferably, in the gap between the two metal layers. Figure 5 shows a different diaphragm, indicated at 30, which is conceptually similar to that shown in figure 1, with the single modification related to the shape of the central pole shoe of the magnetic circuit since, above said pole shoe, under the diaphgram cap, there is arranged a solid member made of a non magnetic material which defines the profile at a small distance so as to provide a small volume compression chamber adapted to increase the stiffness of the movable assembly (without affecting the mechanical strength thereof), so as to provide a more even response through the overall range. According to a preferred embodiment, for a wave range from 1,000 to 20,000 Hz, the active surface of the diaphragm has a spherical cap shape having a base diameter of 34 mm, the weight of the movable portion of the diaphragm being 510 mgr. The diaphragm is made of an AlMg7 alloy, with a rubber damper on the peripheral ring member of the undulated portions; the movable coil has 30 turns, on two layers of copper wire of 0.12 mm diameter and, at 800 Hz, has an impedance of 8 ohm. It has been found that, the values of the other parameters being the same, when the bending radius of the cap was changed from 35 to 20 mm, the main resonance toward the top limit of the acoustical range passes from 9,500 to 35,000 Hz with a negligible loss of the acoustical frequencies before the resonance region. Moreover, the shown recording, which relates to the transducer of figure 1, must be considered with great attention since the diagrams of substantially uneven configuration represent the responses obtained from the transducer of figure 2, which has not been modified, with diaphragms having a cap bending radius of respectively 23 and 29 mm. Under the above mentioned diagrams there are shown the response diagrams, of very high evenness which have been obtained from the same transducers by suitably increasing, in both cases, the stiffness of the movable assembly system, without increasing the mechanical strength and accordingly the impedence thereof: this has been obtained by providing, in the rear hollow of the diaphragm cap, a compression chamber having size and shape characteristics suitably designed for the two cases. More speficially, the greater resulting stiffness is such as to selectively compensate, in several points of the spectrum, the different performance of the diaphgram at this point, so as to provide a substantially even acoustical output pressure. From the above disclosure it should be apparent that the present invention fully achieves the intended aim and objects. In particular, the fact is to be pointed out that the subject diaphragm, which is made of a single piece very thin blade member, provides great advantages with respect to conventional diaphragms, mainly with respect to the evenness of the characteristics of the single diaphragm portions. While the invention has been disclosed and illustrated with reference to preferred embodiments thereof, it should be apparent that the disclosed embodiments are susceptible to several modifications and variations within the scope of the appended claims.

Claims: A diaphragm electric-acoustic transducer of the movable coil electrodynamic type, comprising: a pole portion or pole tip (4); a support unit (2) formed of metal, surrounding said pole portion (4) and defining a gap therewith; a diaphragm (1) surrounding said pole portion and having a peripheral ring portion (16) at the outer periphery attached to said support unit (2); an electrically conductive voice coil (12) attached to said diaphragm (1), and wherein said diaphragm (1) is formed in a single piece from a high heat conductivity metal (14) and has a corrugated portion (15) adjacent said peripheral ring portion (16), a cylindrical portion (11,21) on an opposite side of said corrugated portion from said peripheral ring, and a central cap portion (10) extending across an end of said cylindrical portion, the conductive voice coil (12) being attached to, and closely in contact with, said cylindrical portion (11,21) of said diaphragm; characterized in that said corrugated portion (15) of said diaphragm (1) is composed of a number of spiral-like undulations providing a resilient suspension system, and in that said resilient suspension system is provided with a damper, made of rubber or other elastomeric material and symmetrically arranged with respect to the suspension system. A diaphragm electric-acoustic tranducer in accordance with claim 1, wherein said peripheral ring portion (16) of said diaphragm is attached inside a top plate (3) of said support unit (2). A diaphragm electric-acoustic transducer in accordance with claim 1, wherein said peripheral ring portion (16) of said diaphragm is attached to outside a top plate (3) of said support unit (2). A diaphragm electric-acoustic transducer in accordance with claim 3, wherein said cylindrical portion (15) of said diaphragm (1) contains a loop structure (21) formed by the diaphragm being folded back on itself, and wherein said conductive voice coil (12) is positioned inside said loop structure (21). A diaphragm electric-acoustic transducer in accordance with claim 1, further comprising a solid member made of a non-magnetic material and positioned between said cap portion (10) of said diaphragm (1) and said pole portion (4), said solid member defining a profile at a small distance to provide a small volume compression chamber for increasing a stiffness of said diaphragm.

Inventors: CESATI MARIO, CESATI, MARIO
Applicants: OUTLINE SNC, OUTLINE S.N.C. DI NOSELLI G.& C.
Application year: 1990
Main classification: H04R7
IPC codes: H04R7, H04R9, H04R31