Patent Publication Number: US-8989429-B2

Title: Electrodynamic transducer having a dome and a buoyant hanging part

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/FR2011/000025 filed Jan. 14, 2011, claiming priority based on French Patent Application No. 1000156 filed Jan. 15, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The invention generally relates to the field of sound reproduction by means of loudspeakers, also named electro-dynamic or electro-acoustic transducers, which convert an electrical energy generally delivered by an amplifier into acoustical energy. 
     2. Description of the Related Art 
     Acoustical energy is radiated through a diaphragm the displacements of which induce variations of pressure of the ambient air, which propagate in space under the form of an acoustic wave. 
     In the Rice-Kellog type electro-dynamic transducer, which is the most common, the diaphragm is driven by a movable coil including a solenoid in which passes an electric current (from the amplifier) and which dives into an air gap filled with a magnetic field produced by a permanent magnet. Interaction between the electric current and the magnetic field induces a force known as the Laplace force driving the movable coil, which in turn drives the diaphragm, the vibrations of which produce an acoustic radiation. 
     Numerous designs were imagined for diaphragms; most common shapes are a cone (the generatrix of which may be straight or curved) and a dome, or a combination thereof. 
     In the case of the cone, the movable coil is generally fixed onto the edge of an opening formed in the center of the diaphragm. The size and mass of the moving part are somewhat important, reason for which such architecture is especially adapted to the manufacture of transducers designed for the reproduction of low range and mid range frequencies, requiring diaphragm vibrations of low frequency and great amplitude. 
     In the case of the dome, the movable coil is generally fixed to a peripheral edge of the diaphragm. The size and mass of the moving part may be minimize, reason for which such architecture is especially adapted to the manufacture of transducer designed to reproduce of high range, due to diaphragm vibrations of high frequency and low amplitude. 
     Whichever its shape, the diaphragm is generally fixed to a chassis of the transducer trough a peripheral suspension which, in addition to its primary function of holding the diaphragm, has three general functions:
         return effect to the diaphragm toward a rest position,   producing a secondary acoustic radiation which adds to the radiation of the diaphragm.   centering and axial guiding of the moving part (including the diaphragm and the movable coil) with respect of the air gap,       

     In cone diaphragm transducers, due to great displacements, the suspension is generally not sufficient to guide the diaphragm with respect of the air gap. This is which complementary centering devices are generally provided, like of the spider type (Cf. French patent application FR 2 667 212 in the name of the applicant). 
     In the case of dome shaped diaphragms, the displacements of which are far smaller, a sole peripheral suspension is generally provided to ensure all three functions discussed hereinbefore. Such a topology has been known for a long time, Cf. U.S. Pat. No. 2,242,791 (Edward C. Wente/Bell Laboratories) of June 1948. A more recent example is exposed in US patent application No. US 2008/0166010 (Stiles et al). 
     It is known that axial guiding and centering of the diaphragm with respect of the air gap are an essential function of the suspension. Indeed, it is necessary to avoid (or at least minimize) the transversal movements (swinging, pitch) of the diaphragm, considered as defects which generate distortions within the emitted sound signal. In particular, the coil may rub on the walls of the air gap. Such a rubbing induces strong distortions and parasite noises which prevent use of the transducer. 
     This is why the centering of the moving part with respect of the air gap is a tricky assembly operation, which requires taking into account all manufacturing clearances (in particular of the magnetic circuit) and also requires a very precise fixation of the suspension on the transducer chassis. Such an operation is difficult to automate. Despite all precautions, rubbing of the moving coil on a side wall of the air gap may arise and it is usual, in order to minimize such occurrence, to provide between the moving coil and the air gap important internal and external operational clearances, of several tenths of millimeter. 
     However, enlarging the air gap has harmful consequences:
         it decreases, in a same magnetic circuit, the density of magnetic flow within the air gap, which decreases in proportion the driving force provided to the moving coil and thereby decreases the efficiency of the transducer,   it decreases the capability of dissipating heat produced by Joule effect within the coil, due to the thickness of air layers which surround the coil and act as thermal insulators.       

     Part of the efforts made by the loudspeaker manufacturers is directed toward the research of the best compromise between centering clearances of the moving part with respect of the air gap (and hence suspension dimensioning and/or fixing clearances), and the acoustic performances of the transducer. As already stated, increasing the clearances decreases the performances. Of course, in an industrial manufacturing environment, the choice is generally directed to an increase of clearances, and a decrease of acoustical performances. 
     In order to address this problem, the applicant has made the opposite choice, in order not to scarify performances and to search for pertinent and rational solutions in the transducer architecture. 
     The invention therefore aims at proposing a solution to the problems disclosed hereinbefore, in particular for high range transducers, and at providing improvements to dome diaphragms in order to facilitate the assembly thereof without sacrificing the acoustical performances. 
     SUMMARY 
     The invention therefore provides, in a first aspect, an electro-dynamic transducer including:
         a main magnetic circuit defining an air gap,   a moving part comprising a dome shaped diaphragm and a movable coil fixed thereto and diving into the air gap;   a support to which the moving part is suspended;   a suspension linking the moving part and the support;       

     wherein the suspension is floating with respect of the support, allowing a radial degree of freedom. 
     Accordingly, the suspension no longer provides a centering function. This function is provided directly at the level at the air gap, when the moving coil is powered by a modulation electrical current. Such architecture allows for a decrease of the operation clearance around the moving coil, thereby enhancing the sensitivity of the transducer. 
     The decrease of clearances reduces the thickness of air layers around the solenoid, and hence the thermal resistance between the solenoid and the magnetic circuit. This enhances thermal dissipation and, consequently, allows for an increase of the permissible power of the transducer. 
     In one embodiment, the support comprises a peripheral groove and the suspension is under the form of a ring an inner edge of which is cast into the grove. A clearance of more than 0.1 mm is preferably provided between the suspension and a bottom of the groove. 
     The support may comprise a plate in which the groove is formed, and a rod fixed to the plate and through which the support is fixed to the magnetic circuit. 
     In one embodiment, the groove is limited by two facing flanges and the suspension is slightly constrained between the flanges. 
     The suspension is preferably made of a polymer reticulated foam, such as melamine foam. 
     In one embodiment, at least one wall of the air gap is coated with a low friction material, such as PTFE. 
     In addition, the air gap and the moving coil are preferably such dimensioned that the occupation rate of the moving coil within the air gap is equal or greater than 50%. 
     In one embodiment, the magnetic circuit comprises a pole piece around which the moving coil is positioned, and a clearance of less than a tenth of a millimeter is provided between them. 
     A lubricant (preferably pasty) may be provided between the suspension and the support. 
     The invention provides, in a second aspect, a coaxial two-way or more loudspeaker system comprising a low range electro-dynamic transducer for the reproduction of low range and/or mid range frequencies, and an electro-dynamic transducer as disclosed previously, for the reproduction of high range frequencies. 
     In this system the high range transducer may be mounted in a coaxial and frontal position with respect of the low range transducer. 
     In a third aspect, the invention provides a loudspeaker enclosure including a transducer or a coaxial loudspeaker system as disclosed hereinbefore. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the invention will become apparent from the detailed description of preferred embodiments, considered in conjunction with the accompanying drawings in which: 
         FIG. 1  is a sectional view showing a high range dome transducer in one embodiment of the invention. 
         FIG. 2  is a view of a detail of  FIG. 1 . 
         FIG. 3  is a sectional view, with enlarged scale, of a detail of the transducer of  FIG. 1 , in a different point of view. 
         FIG. 4  is a sectional view showing a coaxial loudspeaker system comprising a low range transducer, and the high range transducer of  FIG. 1  mounted therein in a coaxial and frontal position. 
         FIG. 5  is a view similar to  FIG. 4 , showing a coaxial loudspeaker system comprising a low range transducer, and a high range transducer in an alternate embodiment. 
         FIG. 6  is a perspective view showing a loudspeaker enclosure including a coaxial loudspeaker system as illustrated on  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1-5 , more precisely in  FIG. 1  and  FIG. 3  is illustrated an electro-dynamic transducer  1  adapted for reproducing high range frequencies, i.e. of about 1 kHz to 20 kHz. 
     The transducer  1  comprises a magnetic circuit  2  which includes a permanent central annular magnet  3 , sandwiched between two pole pieces which form field plates, i.e. a back pole piece  4  and a front pole piece  5 , glued on opposite face of the magnet  3 . 
     The magnet  3  and the pole pieces  3 ,  4  have rotational symmetry around a common axis A 2  forming the general axis of the transducer  1 . 
     The magnet  3  is preferably made of a rare earth element neodymium iron boron alloy, which has the advantages of offering a high density of energy (up to twelve times higher than a permanent magnet of barium ferrite). 
     As depicted on  FIG. 1 , the back pole piece  4 , called yoke, is of one piece and made of soft steel. It has a form of a cup with a U-shape transversal section, and has a bottom  6  fixed to a back face  7  of the magnet  3 , and a peripheral side wall  8  extending axially from the bottom  6 . The side wall  8  ends, at a front end opposite to the bottom  6 , by an annular front face  9 . The bottom  6  has a back face  10 . 
     The front pole piece  5 , called core, is also made of soft steel. It is of annular form and has a back face  12 , by which it is fixed to a front face  13  of the magnet  3 , and an opposite front face  14  which extends in the same plane as the front face  9  of the side wall  8  of the yoke  4 . 
     As depicted on  FIG. 1 , the magnetic circuit  2  is extra-thin, i.e. its thickness is small with respect of its overall diameter. In addition, the magnetic circuit  2  extends up to the outer diameter of the transducer  1 . In other words, the size of the magnetic circuit  2  is maximum with respect of the overall diameter of the transducer  1 , which increases its power handling together with the value of the magnetic field, and hence the sensitivity of the transducer  1 . 
     The core  5  has an overall diameter smaller than the inner diameter of the side wall  8  of the yoke  4 , so that between the core  5  and the side wall  8  is defined a secondary air gap  15  in which is concentrated most part of the magnetic field generated by the magnet  3 . 
     In the air gap  15 , the edges of the core  5  and of the yoke  4  may be chamfered, or preferably (and as depicted on  FIG. 1 ), rounded so as to avoid harmful burrs. 
     The transducer  1  also comprises a moving part  16  including a dome shaped diaphragm  17  and a movable coil  18  fixed to the diaphragm  17 . 
     The diaphragm  17  is made of a light and rigid material, a thermoplastic polymer or an aluminum-based alloy, magnesium or titanium. The diaphragm  17  is such positioned as to cover the magnetic circuit  2  on the side of the core  5 , and such that its axis of rotational symmetry be merged with the axis A 2 . 
     Hence, the apex of the diaphragm  17 , located on the axis A 2 , may be regarded as the acoustical center C 2  thereof, i.e. the equivalent punctual source from which the transducer  1  acoustically radiates. 
     The diaphragm  17  has a circular peripheral edge  19  which is slightly turned up, in order to facilitate the fixing of the movable coil  18 . 
     The movable coil  18  comprises a conductive metal (e.g. copper or aluminum) wire solenoid, having a preferred width of 0.3 mm, spiral winded to form a cylinder, an upper end of which is glued to the turned-up peripheral edge  19  of the diaphragm  17 . Here, the coil  18  has no support (but could have one). 
     The movable coil  18  dives in the air gap  15 . The inner diameter of the moving coil  18  is slightly higher than the outer diameter of the core  5 , so that the inner operation clearance between the moving coil  18  and the core  5  is small with respect of the width of the air gap  15 . However, in an alternate embodiment, the operation clearance may be dimensioned in a conventional way. 
     In a preferred embodiment, the edge at least of the core  5  (and possibly the inner surface of the lateral wall  8 ), is coated with a low friction polymer layer, such as polytetrafluoroethylene (PTFE, also known as TEFLON™), having a thickness of about or smaller than a hundredth of a millimeter, and preferably of several tens of pm (such as about 20 μm). 
     In consequence, despite the small clearance between the core  5  and the moving coil  18 , on the one hand, the positioning of the moving coil  18  within the air gap  15  is somewhat easy and, on the other hand, in operation the axial movement of the moving coil  18  is not impeded by the proximity of the core  5 , even in the event both elements would accidentally and temporarily contact. 
     Practically, the moving coil  18  and the air gap  15  are preferably such dimension that:
         clearance between the moving coil  18  and the core  5  including its coating be smaller than a tenth of a millimeter, and for example comprised between 0.05 and 0.1 mm. In a preferred embodiment, the inner clearance is of 0.08 mm (it might however be classically dimensioned);   the outer clearance between the moving coil  18  and the side wall  8  of the yoke  4  is smaller than 0.2 mm, and for example comprised between 0.1 and 0.2 mm. In one embodiment, the outer clearance is of 0.17 mm.       

     Accordingly, the maximal width of the air gap  15 , for a moving coil  18  having a width of 0.3 mm, is of 0.6 mm (with an inner clearance of 0.1 mm and an outer clearance of 0.2 mm). In such a configuration, the occupation rate of the moving coil  18  within the air gap  15  which is equal to the ratio of the sections of the moving coil  18  and air gap  15 , is of about 50% (considered as a minimum). In the preferred configuration, for an air gap width of 0.55 mm, an inner clearance of 0.08 mm and an outer clearance of 0.17 mm, the occupation rate of the moving coil  18  in the air gap  15  is of about 55%. 
     Those values, which are equal to or greater than 50%, are to be compared to occupation rates of known transducers, generally smaller than about 35%. 
     As a result, the density of magnetic flow within the air gap  15  is increased, and the sensitivity of the transducer  1  is subsequently increased, whereby the sensitivity is proportional to the square of the augmentation of the density of magnetic flow within the air gap  15 . 
     It is advantageous to fill the air gap  15  with a mineral oil loaded with magnetic particles, such as of the type sold by FERROTEC under trade name Ferrofluid™. Such a filling has the following advantages:
         it contributes to the centering of the movable coil  18  within the air gap  15 ;   it functions as a dynamic lubricant, and therefore contributes to the silent operation of the transducer  1 ;   its thermal conductivity, which is far higher than the thermal conductivity of air, contributes to the evacuation, toward the magnetic circuit  2  (and more specifically toward the yoke  4 ), of the heat produced by Joule effect within the movable coil  18 .       

     The transducer  1  further comprises a support  20  fixed to the magnetic circuit  2  and to which the moving part  16  is suspended. The support  20 , which is made of a diamagnetic and electrically insulating material, for example a thermoplastic material such as polyamide or polyoxymethylen (charged with glass or not), has a general shape of rotational symmetry around an axis merged with the axis A 2 , and has a T-shaped section. 
     The one-piece support  20  forms an endoskeleton for the transducer  3  and includes an annular plate  21  contacting the front face  14  of the core  5 , and a cylindrical rod  22  which protrudes backwards from the center of the plate  21 , and which is located in a complementary cylindrical recess  23  formed within the magnetic circuit and formed by a succession of coaxial drillings made in the yoke  4 , the magnet  3  and the core  5 . 
     As depicted on  FIG. 1 , the endoskeleton  20  is rigidly fixed to the magnetic circuit  2  by means of a nut  24  screwed onto a threaded section of the rod  22  and tightened against the yoke  4 , within a counterbore  25  formed in the back face  10 , at its center. Thereby, the plate  21  is tightly urged against the front face  14  of the core  5 , without rotational possibility. This fixing may be completed by a glue film between the plate  21  and the core  5 . 
     Given its frontal situation with respect of the magnetic circuit  2 , the plate  21  extends within the lenticular inner volume limited by the diaphragm  17 . The plate  21  comprises a peripheral annular rim  26  and a central disc  27  to which the rod  22  is connected. The disc  27  may be provided with holes  28  a function of which is to maximize the volume of air underneath the diaphragm  17 , in order to decrease the resonance frequency of the moving part  16 . 
     The rim  26  has substantially the shape of a pulley and comprises a peripheral annular groove  29  which radially opens outwardly, facing an annular peripheral portion  30  of the inner surface of the diaphragm  17 , in the vicinity of the edge  19 . 
     The groove  29  splits the rim  26  in two facing flanges forming side walls of the groove  29 , namely a back flange  31 , contacting the front face  14  of the core  5 , and a front flange  32 . The flanges  31 ,  32  are connected through a cylindrical web  33  forming the bottom of the groove  29 . 
     The moving part  16  is mounted onto the endoskeleton  20  by means of an inner suspension  34  which connects the diaphragm  17  and the plate  21 . This suspension  34  has the shape of a ring and is made of a light, elastic, acoustically non emissive material (the material may be porous). This material is preferably resistant to heat within the transducer, and its elasticity is chosen so that the resonance frequency of the moving part  16  be smaller than the lowest frequency reproduced by the transducer  1  (i.e. 500 Hz to 2 kHz). Polymer foams (such as polyester or melamine foams) are well adapted, due to their high porosity. 
     In an alternate embodiment, the suspension  34  is made in a fabric of natural fibers (such as cotton) or synthetic fibers (such as polyester, polyacrylic, Nylon™, and more specifically aramides such as Kevlar™), or in a mixture of natural and synthetic fibers (such as cotton-polyester), wherein the fibers are impregnated with a thermosetting or thermoplastic resin and are thermoformed to provide spider type corrugations. 
     In the absence of acoustical emissivity of the suspension  34 , only the dome diaphragm  17  emits an acoustical radiation, whereby fundamental modes, resonances, and more generally parasite acoustical radiation of suspension  34 , which would interfere with radiation of the diaphragm  17  and would therefore decrease the performance of the transducer  1 , are avoided. 
     The suspension  34  has a section in a substantially polygonal shape and comprises a straight inner edge  35 , i.e. with rotational symmetry around the secondary axis A 2 , and a peripheral outer edge  36  of substantially frusto-conical shape. 
     Through its outer edge  36 , the suspension  34  is glued to the peripheral portion  30  of the inner surface of the diaphragm  17 . Alternately, in case the movable coil  18  includes a cylindrical support fixed to the diaphragm  17  and onto which the solenoid is mounted, the suspension  34  may be fixed, through its peripheral edge (which would then be cylindrical), onto the inner surface of such support. 
     As depicted in  FIG. 1 , the thickness of suspension  34  (measured along the axis A 2 ), although smaller than its free length (measured radially between the flanges  31 ,  32  and the inner surface  30  of the diaphragm  17 ), is not immaterial but of the same order of size than this length. More precisely, the ratio between the free length and the thickness of the suspension  34  is preferably smaller than 5 (and here smaller than 3). Minimizing the free length of the suspension  34  allows for stabilizing the moving part  16  and prevents tilting thereof (anti-pitch effect). 
     On the side of its inner edge  35 , the suspension  34  is located within the groove  29  with a slight compression between the flanges  31 ,  32  in order to avoid parasite noises, but without being fixed thereto. In addition, the inner diameter of the suspension is higher than the inner diameter of the groove  29  (i.e. to the outer diameter of the web  33 ), such that an annular space  37  is formed between the suspension  34  and the web  33 . 
     Accordingly, the suspension  34  is floating with respect of the rim  26  of the plate  21 , with a possible radial clearance, whereby the suspension  34  may slip with respect of the flanges  31 ,  32 . In order to contribute to this slipping, a layer of pasty lubricant (such as grease) may be applied onto the flanges  31 ,  32 . The radial clearance defined by the annular space  37  between the suspension  34  and the web  33  (i.e. the bottom of the groove  29 ) is preferably less than 1 mm. In a preferred embodiment, the clearance is of about 0.5 mm. In the drawings, this clearance is exaggerated for the sake of clarity. 
     In addition, it is preferable that the part of suspension  34  located within the groove  29  have a width (measured radially) higher or equal to its thickness, in order to ensure good mechanical link of the planar contact type and minimize any harmful tilting of the suspension  34  with respect of the plate  21 . 
     The suspension  34  thereby extends inside the diaphragm  17 . The suppression of an external peripheral suspension allows for avoiding acoustical interferences which exist in known transducers, between the radiation of the diaphragm and the radiation of its suspension. 
     In addition, as the suspension  34  exerts no radial constraint on the diaphragm  17 , it does not provide any centering function of the diaphragm with respect of the secondary magnetic circuit  2 , thereby improving the simplicity of assembly of the secondary transducer  3 , or of replacement of the diaphragm  17  in case of failure. 
     The centering of the diaphragm  17  is achieved at the level of the movable coil  18 , which is adjusted with a small clearance onto the core  5  and automatically centers with respect thereof as soon as the movable coil  18 , dived into the magnetic field of the air gap  15 , is displaced by a modulation electric current. 
     However, the suspension  34  provides a return function to the moving part  16  toward an intermediate rest position, in which the moving part  16  stands in the absence of any axial constraint on the movable coil  18  (i.e., practically, in the absence of an electrical current therethrough). It is in this intermediate position that the transducer  1  is illustrated in the drawings. 
     The suspension  34  also provides a function of maintaining the trim of the diaphragm  17 , i.e. of maintaining the peripheral edge  19  of the diaphragm  17  in a plane perpendicular to the axis A 2 , in order to avoid tilting (or pitch) of the diaphragm  17  which would affect its good operation. 
     The electric current is provided to the movable coil  18  by two electrical circuits  38  which link the ends of the movable coil  18  to two feeding electrical terminals (not illustrated). 
     As depicted in  FIG. 1 , each electrical circuit  38  comprises:
         an electrical conductor  39  of great diameter, including a copper wire insulated with a plastic jacket, extending through the magnetic circuit  2  and located within a slot formed longitudinally within the rod  2  of the endoskeleton  20 , and a stripped front end  40  of which opens in the inner volume of the diaphragm  17  and protrudes from the magnetic circuit  2  in one hole  28  of the disc  27 ;   an electrical connection element under the form of a metal eye  41  (which may be made of copper or brass) crimped within the hole  28  and to which the stripped end  40  of the conductor  39  is electrically linked (for example by means of a welding point, not illustrated);   a conductor  42  of small diameter, under the form of a resilient metallic braid suitably formed, which extends within the internal volume of the diaphragm  17  and extending over the rim  26  and the suspension  34 , an inner end  43  of which is electrically connected to the eye  41  (for example by means of a welding point, not illustrated), and an opposite outer end of which is electrically connected to an end of the movable coil  18 .       

     Only one conductor  42  of small diameter is visible on  FIG. 1 . The second one, which is diametrically opposite to the latter, is located in front of the section plane of the figure. 
     Due to their arcuate form (U-shape of the conductors  42 , and to their great resilience, the conductors may deform easily and follow the movements of the diaphragm  17  which accompany the vibrations of the movable coil  18 , without adding any radial or axial constraint which might compromise the positioning of the moving part  16 . 
     The transducer  1  comprises an acoustical waveguide  44 , fixed to the magnetic circuit  2 . 
     The waveguide  44  is one piece and is made of a material having a high thermal conductivity, higher than 50 W.m −1 .K −1 , such as in aluminum (or an aluminum alloy). 
     The waveguide  44  has a rotational symmetry, is fixed to the yoke  4  and comprises a substantially cylindrical outer side wall  45  which extends flush with the side wall  8  of the yoke  4 . The waveguide is preferably screwed, by means of at least three screws. In order to maximize thermal contact between both pieces, it is advantageous to complete the screwing by applying a heat conducting paste. 
     As depicted on  FIG. 1  and  FIG. 2 , the waveguide  44  has, on a back peripheral edge, a skirt  46  which adjusts in a shoulder  47  made in the yoke  4 , of complementary shape, whereby a precise centering of the waveguide  44  with respect of the yoke  4 , and more generally with respect of the magnetic circuit  2  and the diaphragm  17 , is provided. In addition, thermal conduction between both pieces  4 ,  44  is enhanced. 
     The waveguide  44  has a back face  48  shaped like a substantially spherical cap, which extends in a concentric way with respect of the diaphragm  17 , facing and in the vicinity of an outer face thereof, which the back face  48  partly covers. 
     In an preferred embodiment depicted in  FIG. 1-4 , the back face  48  is provided with openings and comprises a continuous peripheral portion  49  which extends in the vicinity of the back edge of the waveguide  44 , and a discontinuous central portion  50  carried by a series of wings  51  which radially protrude inwardly (i.e. towards the axis A 2  of the transducer  1 ) from the side wall  45 . The back face  48  is limited inwardly—i.e. on the diaphragm side—by a petaloid shaped edge  52 . 
     As depicted on  FIG. 1 , the wings  51  do not meet at the axis A 2  but are interrupted at an inner end located at a distance from axis A 2 . At its apex, each wing  51  has a curved edge  53 . 
     The side wall  45  of the waveguide  44  is limited inwardly by a discontinuous frusto-conical front face  54  divided into a plurality of angular sectors  55  which extend between the wings  51 . This front face  54  forms a horn initial section extending from the inside to the outside and from a back edge, formed by the petaloid edge  52  which forms a throat of the horn initial section  54  up to a front edge  56  which forms a mouth of the horn initial section  54 . The angular sectors  55  of the horn initial section  54  are portions of a cone with rotational symmetry the axis of which is merged with the axis A 2 , and the generatrix of which is curved (for example following a circular, exponential or hyperbolic law). The horn initial section  54  ensures a continuous acoustical impedance adjustment between the air environment limited by the throat  52  and the air environment limited by the mouth  56 . 
     In an embodiment, the tangent to the horn initial section  54  on the mouth  56  forms, together with a plane perpendicular to the axis A 2  of the transducer  1 , an angle comprised between 30° and 70°. In the depicted example, this angle is of about 50°. 
     Each wing  51 , one function of which is to increase the surface of the waveguide  44  to contribute to dissipation and convection of heat produced by the movable coil  18 , has two side flanges  57  which outwardly connect to the angular sectors  55  of the horn initial section  54  through fillets  58 . 
     The side flanges  57  contribute to guiding the wave generated by the diaphragm  17 . 
     In an alternate embodiment depicted on  FIG. 5 , the waveguide  44  does not form a horn initial section but a whole horn (which may be of rotational symmetry around the axis A 2 ), the throat  52  of which is of circular shape and the mouth  56  of which has a diameter far greater than the diameter of the throat  52 . 
     The waveguide  44  limits on the diaphragm  17  two distinct and complementary zones, namely:
         an uncovered outer zone  59 , of petaloid shape, outwardly limited by the throat  52 ,   a covered outer zone  60 , the shape of which is complementary to the covered zone  59 , inwardly limited by the throat  52 .       

     The back face  48  of the waveguide  44  and the corresponding covered outer zone  60  of the diaphragm  17  together define an air volume  61  called compression chamber, in which the acoustical radiation of the vibrating diaphragm  17  driven by the coil  18  moving in the air gap  15  is not free, but compressed. The uncovered inner zone  49  directly connects to the facing throat  52 , which concentrates acoustical radiation of the whole diaphragm  17 . 
     The compression rate of the transducer  1  is defined by the ratio of the emitting surface, corresponding to the planar surface limited by the overall diameter of the diaphragm  17  (measured on the edge  19 ) and the surface limited by the projection, in a plane perpendicular to the axis A 2 , of the throat  52 . This compression rate is preferably higher than 1.2:1, and for example equal or greater than 1.4:1. Higher compression rates, for example up to 4:1, are possible. 
     The hereabove transducer  1  may be used in an individual matter or coupled to a low range transducer  62  for forming a several-way loudspeaker system  63  designed to cover a large acoustical spectrum, ideally the whole audio bandwidth. 
     Practically, the low range transducer  62  may be designed to reproduce the low range and/or the mid range, and possibly part of the high range. To this end its diameter shall preferably be comprised between 10 cm and 38 cm. Although the main object of the present invention does not include the definition of parameters regarding the spectrum covered by the different transducers of the system  63 , it shall be however noted that the spectrum of the low range transducer  62  may cover the low range, i.e. the range of 20 Hz-200 Hz, or the mid-range, i.e. the rage of 200 Hz-200 Hz, or even at least part of the mid-range and low range (and for example the whole low range and mid-range) and possibly part of the high range. As an example, the low range transducer  62  may be designed to cover a bandwidth of 20 Hz-1 kHz, or 20 Hz-2 kHz, or even 20 Hz-4 kHz. 
     The high range transducer  1  is preferably designed so that its pass band is at least complementary to the low range transducer  62  in high range. One may therefore ensure that the pass band of the high range transducer  1  covers at least part of the mid-range and the whole high range, up to 20 kHz. 
     It is preferable that the linear responses of the transducer  1 ,  62  at least partly cross, and that the sensitivity level of the high range transducer  1  be at least equal to that of the low range transducer  62 , in order to avoid a decrease of the global response of the system  63  at certain frequencies corresponding to the higher part of the spectrum of the low range transducer  62  and to the lower part of the spectrum of the high range transducer  1 . 
     The low range transducer  62  comprises a magnetic circuit  64  which includes an annular magnet  65 , sandwiched between two soft steel pole pieces which form field plates, i.e. a back pole piece  66  and a front pole piece  67 , glued on opposite face of the magnet  65 . 
     The magnet  65  and the pole pieces  66 ,  67  have a rotational symmetry around a common axis A 1 , forming the general axis of the low range transducer  62 . 
     In the depicted embodiment, the back pole piece  66  is of one piece and comprises an annular bottom  68  fixed to a back face  69  of the magnet  65 , and a central cylindrical core  70 , which has a front face  71  opposite the bottom  68  and is provided with a central bore  72  opening on both sides of the pole piece  66 . 
     The pole piece or front plate  67  has the form of an annular washer and has a back face  73 , by means of which it is fixed to a front face  74  of the magnet  65 , and an opposite front face  75  which extends in the same plane as the front face  71  of the core  70 . 
     The front plate  67  has at its center a bore  76  the inner diameter of which is greater than the external diameter of the core  70 , so that between the bore  76  and the core which is located therein is defined an air-gap  77  in which part of the magnetic field generated by the magnet  65  is present. 
     The low range transducer  62  includes a chassis  78  called basket, which includes a base  79  through which the basket  78  is fixed to the magnetic circuit  64 -and more precisely to the front face  75  of the front plate  67 -, a crown  80  through which the transducer  62  is fixed to a holding structure, and a plurality of branches  81  linking the base  79  and the crown  80 . 
     The low range transducer  62  additionally comprises a movable part  82  including a diaphragm  83  and a movable coil  84  comprising a solenoid  85  coiled around a cylindrical support  86  fixed to the diaphragm  83 . 
     The diaphragm  83  is made of a light rigid material such as impregnated cellulose pulp, and has a conical or frusto-conical shape with rotational symmetry around the axis A 1 , with a curved generatrix (such as a circular, exponential or hyperbolic law). 
     The diaphragm  83  is fixed on the surround of the crown  80  by means of a peripheral suspension  87  (also called rim) which may be made of an add-on tore piece glued to the diaphragm  83 . The suspension  87  may be elastomeric (such as of natural or artificial rubber), polymeric (honeycombed or not) or in an impregnated and coated fabric or nonwoven. 
     In its center, the diaphragm  83  defines an opening  88  on the inner edge of which the support  86  is glued by a front end thereof. The geometrical center of the opening  88  is considered, in first approximation, as the acoustical center C 1  of the low range transducer  62 , i.e. the equivalent punctual source from which the acoustical radiation of the low range transducer  62  is generated. 
     A hemispheric dust cap  89 , made of an acoustically non emitting material, may be affixed to the diaphragm  83  in the vicinity of the opening  88  to protect the latter from dust. 
     The solenoid  85 , made of a conductive metal wire (such as copper or aluminum), is rolled on the support  86 , at a back end thereof located within the main air gap  77 . Depending upon the diameter of the low range transducer  62 , the diameter of the solenoid  85  may be comprised between 25 mm and over 100 mm. 
     The centering, the elastic return force and the axial guiding of the movable piece  82  are achieved by the peripheral suspension  87  and by a central suspension  90 , also called spider, of generally annular shape, with concentric corrugations, and having a peripheral edge  91  by which the spider  90  is glued to an edge  92  of the basket  78  in the vicinity of the base  79 , and an inner edge  93  buy which the spider  90  is glued to the cylindrical support  86 . 
     The solenoid  85  is provided with electrical signal in a classical way by means of two electrical conductors (not illustrated) connecting each end of the solenoid  85  to an electrical terminal of the transducer  62 , where the link is made to a power amplifier. 
     As depicted on  FIG. 4 , the high range transducer  1  is housed within the low range transducer  62  within a central frontal room (on the front side of the magnetic circuit  64 ) limited backwards by the front face  71  of the core  70 , and sidewise by the inner wall of the support  86 . 
     As depicted on  FIG. 4  and  FIG. 5 , the high range transducer  1  may be mounted within the low range transducer  62  both:
         In a coaxial way, i.e. the axis A 1  of the low range transducer  62  and the axis A 2  of the high range transducer  1  are merged,   In a frontal way, i.e. the transducer  1  is positioned in the front of the magnetic circuit  64  (i.e. on the side of the magnetic circuit  64  where the diaphragm  83  is located).       

     This so-called “frontal” assembly, which is opposite to the rear assembly in which the transducer is mounted on the back face of the yoke (cf. e.g. U.S. Pat. No. 4,164,631 to Tannoy) is made possible due to the miniaturization of the high range transducer  1 , obtained without reducing the emitting surface of the diaphragm  17 . 
     Such a miniaturization results both from the extra-thin and extra-wide form of the magnetic circuit  2  (which has the overall diameter of the transducer  1 ) and from the manufacturing of the diaphragm  17  which allows for the maximization of its emitting surface. 
     Compactness of the magnetic circuit  2  (in particular its low thickness) is made possible through the use of a neodymium iron boron magnet  3 . However, such compactness would have been vain should the diaphragm  17  have been made in a classical way including a peripheral suspension. 
     Indeed, in such a configuration the diameter of the effective radiating surface of the diaphragm is smaller than the overall diameter of the diaphragm, whereby only an inner part of the suspension contributes to the acoustical radiation whereas its outer part, interconnected to a fixed part of the transducer, is passive. In such a known configuration, the diameter of the frontal radiating surface is insufficient and does not allow the coaxial frontal assembly, since it is not possible to manufacture a short horn initial section capable of being aligned with the diaphragm of the low range transducer in the available room. 
     A known diaphragm has an effective radiating surface smaller than its physical surface, and often insufficient to allow a for a good reproduction of the lower part of high range frequencies or the higher part of mid range frequencies and therefore does not allow the high range transducer to ensure a proper junction with the upper part of the spectrum reproduced by the low range transducer. 
     On the contrary, the diaphragm  17  of the high range transducer  1  with its inner suspension  34  has a 100% radiating surface, i.e. the diameter of the effective radiating surface is equal to the overall diameter of the diaphragm  17 . The gain of radiating surface is higher than about ⅙—i.e. more than 16%—with respect of the known diaphragms. 
     Such a gain allows for decreasing the lower limit of the frequency bandwidth reproduced by the high range transducer  1 , and hence for enhancing the homogeneity of the system  63 . The induced increase of the diameter of the moving coil  18  allows for an increase of the sensitivity and power handling of the transducer of a factor proportional to the gain of radiating surface (i.e. proportional to the square of the diaphragm diameter). 
     Practically, the transducer  1  is fixed to the main magnetic circuit  64  on the front side thereof and is received, as already stated, in a space limited backwards by the front face  71  of the core  70 , and sidewise by the inner wall of the cylindrical support  86 ; the yoke  4  of the magnetic circuit  2  is urged directly, or through a spacer, against the front face  71  of the core  70 . To this end, the transducer  1  has an overall diameter lower than the inner diameter of the cylindrical support  86 . However, it is preferable to minimize the clearance between the transducer  1  and the support  86 , in order to reduce the harmful acoustical effect produced by the annular cavity formed between them. This clearance should however be sufficient to prevent friction of the support  86  onto the transducer  1 . A low clearance, of several tenths of millimeters (comprised e.g. between 0.2 mm and 0.6 mm) is a good compromise (on  FIG. 4  and  FIG. 5  such clearance is exaggerated for the sake of clarity). 
     The rod  22  of endoskeleton  20  is received within the bore  72  of the core  70 , and the transducer  1  is rigidly fixed to the magnetic circuit  2  of the low range transducer  62  by means of a nut  94  screwed onto a threaded portion of the rod  22  and tightened against the yoke  66 , possibly with a washer therebetween, as depicted on  FIG. 4  and  FIG. 5 . 
     In addition to the coaxial frontal positioning of the transducer  1  with respect of the low range transducer  62 , their respective geometries, the thickness of the magnetic circuits  2 ,  64  and the curvature (and hence the depth) of the diaphragm  83 , are preferably adapted to permit at least an approximate coincidence of the acoustic centers C 1 , C 2  of the transducers  1 ,  62 , such that the time offset between the acoustical radiation of the transducer  1 ,  62  be unperceivable (this situation is called time alignment of the transducers  1 ,  62 ). The system  63  may then be regarded as perfectly coherent despite duality of the sound sources. 
     In addition, in the embodiment depicted on  FIG. 4 , the axial positioning of the high range transducer  1  with respect of the low range transducer  62 , together with the geometry of the waveguide  44 , are such that the diaphragm  83  is aligned with the horn initial section  54 . In other words, the tangent to the horn initial section  54  on the mouth  56  merges with the tangent to the diaphragm  83  at its central opening  88 . In such a configuration, the waveguide  44  and the diaphragm of the low range transducer together form a complete horn for the high range transducer  1 , permitting both transducers  1 ,  62  to have homogeneous directivities. 
     In the alternate embodiment of  FIG. 5 , the waveguide  44  forming a whole horn is independent from the diaphragm  83  of the low range transducer  62 . In such configuration, the directivities of the transducers  1 ,  62  are distinct and may be optimized separately, which is advantageous in some applications, such as stage monitor speakers. 
     The system  63  may be mounted on any type of loudspeaker enclosure, such a stage monitor loudspeaker  95 , with an inclined front face, as in the depicted example of  FIG. 6 .