CA2016478A1

CA2016478A1 - Movable-coil electrodynamic transducer diaphragm

Info

Publication number: CA2016478A1
Application number: CA002016478A
Authority: CA
Inventors: Mario Cesati
Original assignee: Individual
Current assignee: Outline SNC di Noselli G and C
Priority date: 1989-05-11
Filing date: 1990-05-10
Publication date: 1990-11-11
Also published as: EP0397621B1; IT8920460A0; DE69022474T2; IT1229706B; DE69022474D1; DK0397621T3; ES2077665T3; EP0397621A3; JPH0349500A; EP0397621A2

Abstract

" MOVABLE-COIL ELECTRODYNAMIC TRANSDUCER DIAPHGRAM "

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

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BACKGROUND OF THE INVENTION
The present invention relates to a diaphgram for an electro-acoustical transducer, of the movable-coil electrodynamic type, for transforming electrical signals into acoust1cal signals and for diffusing the latter.
As is k~own, in electro-acoustical transducers for transforming electrical signals into acoustical signals and dif~using the latter, the vibrating mem-ber, which is electrically driven by the signal, con-sists of a diaphgram to the active surface thereof, that is that surface which affec~ the transmission medium, usually consisting of air, there i5 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 ~ter-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 dia~hgram or membrane which essentially consists of an active substantially ,~
:
. rigid curved surface, properly designed ~or the pro-. vided frequency spectrum, which is rigid with a sup-porting frame on which is rigidly affixed the movable co il .
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The diaphgram comprlses moreover a resilien~
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 diaphgram, for example the provided masses, determines the condition for a linear acoustic-al reproduction up to the maximum amplitude of the applied signals, with a subs-tantially even respon~e through the overall range; there being moreover pro-vided a peripheral ring member which restrains the movable equipment to the rigid structure of the unit.
i~ore specifically, the rigid surface is pro-vided by a light alloy blade, configur~ed as a spherical cap, or the like ~! whereas the resilient r 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 in-crease of the risk due to the required per~ormance severity imposed to the system from the reliability ~ . .1 .

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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 quanti-tative efficiency of less stringent requirements, so as to provide a continuous and even type o~ opera-tion.
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 arith-metically speaking, to the "least termsl'), let us consider a system in which the ~ull acoustic ~requency range has been practically divided into several sec-tions, each controlled by a suitable characteristic , ., transducer, and let us consider exclusively the trans-. ducer of the top ~ld limit of the range, for example from 3k to 16 k Hz, tnat is that for which the sub-ject reasoning will be clearly evident (naturally ., under the hypothesis that the considered range is not further subdivided), ..... .. ..
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For a faithfull reproduction or playback, as stated, apart other requirements which are here-in neglected, there is necessary to provide a per-fect l~nearity, that is an even ratio of the value of a parameter, for example the power of the input electrical signal, and that of the corles~onding acoustical signal at the output of the transducer;
and this for all of the frequencies: which means a substantially even response through the overall fre-~uency range, that is with an offset less than + 2 dB.
; If the values of the characteristc 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 ha~-e an insufficient cross-section for supporting the current intensity corres- -ponding to the maximum signals of a recursive average duration, with a consequent superheating; moreover, the diaphgram construction will have an insuf~icient thickness to resist agains-t 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 solmd frequ~ncy ran~ ~ ctically devoid of any loss.
By concluding, if the transducer being con-sidered 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 o~ the parts of the diaphgram 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 diaphgram is subjected along its coupling line to the diaphgram supporting unit, because of instantaneous pulses, even of moderate value, all which will contribute to quickly put the transducer out of service.
Since it i5 not possible to increase, in a constant characteristic transducer, the mass of the diaphgram and movable coil, without negatively affect-ing 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 2 ~

temperature of the diaphgram, for the provided opera-tion type, under a risk level.
SUMMARY OF THE INVENTION
Accordingly, the aim of the present invent$on is to solve the above mentioned problems, by provid-ing a diaphgram or membrane ~or an electro-acoustic- .;?
al transducer, of the movable-coil electro~ynamic type, for transforming electrical signals into acous-tical signals and diffusing the latter, which a~fords the possibility of quickly dissipating the generated heat, both by irradiation and by conduction, in order to hold the temperature under a risk level, Within the scope of the above mentioned aim, a main object of the invention is to provide such a diaphgram or membrane which can be easily produced and which is adapted to resist against severe use conditionswhile providing a hi~her performance than the conventional diaphgramSor membranes.
Another object o~ the present invention is to provide a diaphgram for electrical-acoustical transducers which is very reliable and flexible in operation and, moreover, is very competitive from a mere economic standpoint.
~ ccording to one aspect of the present in-vention, the above mentioned aim and objects, as , ~ ..

~l~11i478 well as yet other objects, which will become more apparent hereinafter, are achieved by a diaphgram comprislng a single-piece blade member, made o~ a very low density good heat conduct~ and high mecha-nical characteristic metal materials, said blade member being resiliently suspended on the perimeter there-of, and being metally coupled to the unit of which said blade member forms a portion, and supporting :~
an acoustical current conductor affixed,at the center of mass thereo~, to the active metal sur~ace.
.The acoustical current conductor will trans-mit to the blade member the heat generated by the acoustical current, which heat will be added to that generated in the blade member mainly because of the deformation of the resilient system during the opera-tion; the heat being quickly absorbed by the metal ; blade member and efficiently dissipated by irradia-ting it into the air and by conduction through the adjoining masses, so as to hold the temperature under a set dangerous level.
BRIEF DESCRIPTION OF THE DRA~JINGS
Further characteristics and advan~ages of the present invention will become more apparent here-inafter from the following detailed disclosure of the subject diaphgram, or membrane, which is illus-2~$~

traded, by way of an indicative but not limitativeexample, in the figures of the accompanying drawings, ; where:
Figure 1 is a schematic cross-sectional view illustrating a first embodiment of the diaphgram or membrane according to the present invention;
Figure 2 shows a different embodiment of the diaphgram according to the invention;
Figure 3 is a top plan view of the diaphgram shown in Figure 1;
Fi~ure 4 is a bottom vieu o~ the diaphgram shown in Figure 1; in the embodiment bein~ disclosed the diaphgram is anchored to its supporting unit - through the interposition of a resilient gasket;
; Figure 5 illustrates yet another embodiment of the diaphgram according to the invelltion;
Figure 6 shows a graph of the diaphgram re-sponse, 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 ~IBOD~lE~TS
With reference to the figures of the accompa-hying drawings, the diaphgram or membrane according to the invention is characterized in that each por-tion of said diaphgram is made of a very goo~ heat , ~ ., 2 ~

conductive material, having a very low density and suitable mechanical characteristics, From preliminary tests on the diaphgram 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 fe~ 1/100 of milli- ;
meter, 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 diaphgram, indicated overally at the reference number 1, is applied to a diaphgram supporting unit, overally lndicated at the reference n~ber 2.
The diaphgram has a central cap 10, or diaph~
gram ac~ive surface, of subs-tantially rigid cor~igura-tion for t~le 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 diaphgram so that the heat generated by said coil will be quickly and efficiently absorbed and dispersed both by irradia-tion and by conduction through the metal mass to which the diaphgram is anchored.
The diaphgram 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 por-tions 2, 3, 5 and 6.
The thus designed diaphgram has been subjec-ted to severe operation tests and extreme condition and it has been found that this diaphgram has very high performance characteristics, ~ith respect to conventional diaphgrams.
The aluminium alloy which has been used for making the subject diaphgram is Al~g7, with a thick-ness of 0.05 mm; the diaphgram has been constructed ac-cording tothree embodiments for 500-8000, 800-10,000 and-2,000-16,000 Hz, with an active surface of con-vex and concave cap configuration having respective-ly a diameter o~ 55, 50 and 34 mm In this connec-tion it should be apparent that for making the diaph-gram,another alloy can be used provided that it has .. ..
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the above defined characteristic~.
A~ shown in figuIe 2, ~ince the diaph~ram is made in a eingle pisce, with the three component~
or parts thereof disposed in the preci~e order:
active surface, gap movable coil, peripheral re-~ilient ring member, the active surface will be ar-ranged on the top and the resilient suspen~ion æone will be arranged under the flat portion clo~ing the magnetic circuit, ~ hen, duri~g the assembling operation, the diaphgram will be arranged in its position before the flat portion, which will prevent the diaph~ram from being removed and replaced without demagnetizing the unit, which operation can not be easily perform-ed remotely from the ~hop and which require~, in the unit, the provision of a field coil: howeYer, from thi~ undesirable characteri9tic no dra~bac~ derives fo~ the reliability of the device, As shown in figure ~, the diaphgram, in-dicated overally at the re-ference number 20, accord-ing to a more complex and delicate embodiment there-of,carries into practice the inventive idea, In fact, the blade member, indicated at 20, i~ provided, between the re~ilien-t zone and cap, which are always indicated at the same reference 1 ' r 7 ~

numbersused in figure l, with a loop 21 for hou~ing the movable coil. From the construction ~tandpoint, the blade member must be looped on it~èlf so a~ to ~each and cover, by a second layer, the already form, ed cylindrical portion, the base of the cap, and must then be bent through 90, in order to form the resilient ~uspension sy~tem which, in the case being di~closed, advantageously comprises ele~en crimping legs, of spiral shape, which are incised in the metal material in a symmetxical way in the overlaying and underlaying portions ~o ao to simultaneou~ly hold the static planarity of the ring member unchange~.
The cylindrical portion projecting from the cap base, with a very thin gap between the two portions forming it, is adapted to clo~ely receive the movable coil: on the outside or on the inside of it or, preferably, in the gap between the two metal layers.
~ igure 5 show~ a different embodiment of the diaphgram, indicated at 30, which is conceptual-ly similar to that shown in figure 1, with the ~ingle modification related to the shape of the central pole shoe of the magnetic circuit since, above ~aid pole shoe, under the diaphgram cap, there is arranged a solid ~ember made of a non magnetic material which ..
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defin~ the profile at a small distance ~o as to pro-vide a small ~olume compression chamber adapted to increase the stiffness of the movable assembly (with-out affecting the mechanical ~trength thereof), ~o as to provide a mcre even re~ponse through the over-all range.
According to a preferred embodiment, for a wave range from l,000 to 20,000 ~z, the active surface of the diaphgram has a spherical cap shape having a base diameter of 34 mm, the weight of the movable portion of the diaphgram being 510 mgr.
The diaphgram is made of an N Mg7 alloy, with a rubber damper on the peripheral ring member of the ondulated portions; the ~ovable coil has 30 turns, on two layers of copper wire of 0.12 mm dia-meter and, at 800 Hz, has an impedance of 8 ohm.
It has been found that, the values of bhe other paramete s 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 acou~tic-al range passes from 9,500 to 35,000 Hæ with a negli-gible loss of the acoustical f~equencies before the resonance region.
~ oreover, the shown recording, which re-late~ to the transducer of figure l,must be consider-.. . ..

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ed with great attention since the diagrams o~~ubstantially uneven configuration represent the re~ponses obtained from the transducer of figure 2, which ha~ not been modified, with diaphgrams having a cap bending radius of respectively ~3 and 29 mm.
Under the above mentioned diRgrams the-re are shown the ~esponse aiagrams, of very high evenness,which have been obtained from the same trans-ducexs by ~uitably increa~ing, in both cases, the stiffnes~ of the vable a~sembly sy~tem, without increasing the mechanical strength and accordingly the impedence thereof: this ha~ been obtained by providing, in the rear hollow of the diaphgram cap, a compression chamber having size and shape charac-teristics suitably designed for the two cases. More speficially, the greater resulting stiffness is such as to ~electively compensate, in several points of the spectrum, the different performance of the diaphgram at this point, so a~ to provide a subs-tantially even acoustical output pressure.
From the above disclosure it should be apparent that the present invention fully achieves the intended aim and objects.
In particula~, the fact is to be pointed ...., 20~7~

out that the subject diaphgram,which is made of a single piece very thin blade member,provides great advantage`s with respect to conventional diaphgrams, mainly with respect to the e~enhes~ of the character-istics of the single diaphgram portions.
While the invention ha~ been di~closed and illustrated with reference to preferred embodi-ments thereof, it should be apparent that the dis-closed embodiment~ are susceptible to several m~di-fications and variations all coming ~ithin the spirit and scope of the appended claim~.

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Claims

1. A diaphgram for an electrical-acoustio-al transducer, of the movable-coil electrodynamic type, for transforming electrical signals into acoustic-al signals and diffusing the latter, characterized in that said diaphgram comprises a single-piece blade member, made of a good heat conductiviti and mechanic-al characteristic very low density metal material, said blade member being perimetrically resiliently suspended and metally coupled to a supporting unit therefor, said blade member supporting an acoustical current conductor affixed at a center of mass there-of to an active metal surface, said acoustical cur-rent conductor transmitting to said blade member heat produced by said acoustical current, which heat is added to heat generated in said blade member be-cause of deformation of the resilient suspension system in operation, said heat being quickly adsorbed by said metal blade member and efficiently dissipated by irradiation into air and conduction through adia-cent masses so as to hold the temperature under a set level.

2. A diaphgram according to claim 1, cha-racterized in that said blade member is made of light AlMg alloys, dural, titanium with a thickness for alluminium alloys from 0.04 and 0.07 mm depending on a provided frequency range.

3. A diaphgram according to claim 1, cha-racterized in that it is completely made of a very low density metal alloy, including metals of very good conductivity and mechanical strength, with a thickness from 0.04 to 0.07mm, without any interrup-tions or joints, and so designed that the heat pro-duced by an acoustical current through the acoustic-al conductor affixed at the center of mass thereof to the active metal surface, resiliently anchored metally to the unit, is quickly diffused through the diaphgram, with the heat therein present because of the friction between the molecules of the resilient suspension system, subjected to deformations, stret-ching and the like, due to the movement so as to be dispersed by conduction in the adjoining metal masses and by irradiation into the air so as to hold the temperature under a set level.

4. A diaphgram, according to claim 1, cha-racterized in that said blade is made by a single making process differentiated exclusively with re-spect to the ways in which it is applied to the zones of the blade member corresponding to the single com-ponents that is: active rigid ow face rigid with the movable coil support, resilient suspension sys-tem, peripheral ring for anchoring to the unit,

5. A diaphgram, according to claim 1, cha-racterized in that it can be applied and replaced in an already magnetized unit, which is obtained during the making process without interrupting the blade member during the forming of the diaphgram, by simply bending on itself said blade member so as to arrange the resilient suspending portion thereof in front of the perforated disc closing the magnetic circuit as the diaphgram cap.

6. A diaphgram, according to claim 1, cha-racterized in that the acoustical current conductor is housed in a gap formed by the walls of the blade member bent on itself.

7. A diaphgram, according to claim 1, cha-racterized in that the acoustical conductor is affix-ed either in the inside or on the outside of the resulting support.

8. A diaphgram, according to claim 1, cha-racterized in that the resilient suspension, obtain-ed by symmetrical ondulations on the blade member, is completed by a damper, also symmetrical with re-spect to the suspension system, made of a rubber or other elastomeric material, and adapted to change the resonance characteristics and provide a further peripheral resilient region.

9. A movable coil electrocynamic trans-ducer, characterized in that said transducer com-prises a diaphgram according to the preceding claim, provided for any frequency range, the geometrical and physical characteristic being suitable to dis-place the top natural resonance frequency beyond the acoustical range, by simply reducing the bending radius of its cap shape active surface, without sub-stantially affecting the resonance response, where-as by increasing the stiffness of the system of the movable assembly, which is obtained by reducing the rear hollow of the diaphgram to a minimum volume compression chamber, by introducing a geometrical-ly solid body coherent with the cap profile, it is possible to pass from a starting response variable by more than + 4 dB to a response restrained within + 1 dB.