Patent Publication Number: US-8971564-B2

Title: Voice coil support for a coil transducer motor structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §§371 national phase conversion of PCT/EP2009/004804, filed Jul. 2, 2009, which claims priority of European Patent Application No. 08290652.0, filed Jul. 2, 2008, the contents of which are incorporated herein by reference. The PCT International Application was published in the English language. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a voice coil support for a coil transducer motor structure and particularly a voice coil support adapted to be placed in a magnetic field in order for the voice coil support to reciprocate along an axis of displacement. 
     This invention is disclosed in the context of a moving voice-coil transducer motor assembly for a loudspeaker. However, it is believed to be useful in other applications such as microphones, geophones, and shakers. 
     Generally, voice-coil transducer motor assemblies, such as those used in traditional electrodynamic loudspeakers, comprise magnetic field generating means adapted to generate a magnetic field in which a coil fixed on a moving part also called a mandrel or voice coil support, can be driven by a driving current in order to induce vibrations to a diaphragm connected to the voice coil support to produce sound. In order to improve the yield, as well as to reduce the inertia of the loudspeaker, the voice coil support that is the moving part and the diaphragm that is attached to it, are designed to be as light as possible. 
     To meet these requirements, the voice coil support is usually a hollow cylinder and the diaphragm a conical piece of material and both are made of a material such as paper, aluminum, polyimide film such as Kapton®, glass fibre or another light composite material. 
     Reducing the weight of these voice coil supports reduces their rigidity and results in the generation of resonant frequencies. Thus, the frequency response of the voice-coil transducer motor assemblies are affected by nonlinearities. 
     These nonlinearities occur because of mode coupling between mechanical modes and acoustical modes, resulting in a transfer of energy between mechanical waves and acoustic waves. 
     This problem leads to some harmonics of the sound produced by the loudspeakers integrating such voice-coil transducer motor assemblies, to be hardly audible and almost extinguished, especially at high frequencies. At lower frequencies, some energy is absorbed during the excitation of the assembly and radiated when the excitation is stopped, leading to longer trailing edges, and the sound produced in the loudspeaker being somewhat unclear. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved voice coil support component for a coil transducer motor assembly and in particular such an assembly that reduces or extinguishes mode coupling and its resulting drawbacks. 
     Thereto, the present invention provides a voice coil support for a coil transducer motor assembly. 
     Further advantageous features of the invention are disclosed. 
     Preferably, the voice coil support may have a monobloc structure made of one solid piece of material, with a mechanical mode of vibration at a natural frequency outside of a frequency range of interest, preferably the audible frequency range. By providing a monobloc voice coil support with a mechanical mode of vibration at a natural frequency outside of the audible frequency range, mode coupling between mechanical modes and acoustic modes, whereby mechanical energy is exchanged between mechanical modes and acoustic modes, occurs only beyond an upper audible limit frequency, usually around 20 kHz that is outside of the frequency range of interest. Even if some amount of mechanical energy is exchanged, this energy is not transported to an outer surface of the voice coil support. 
     Said monobloc structure of the voice coil support may comprise a material having an infinite or quasi-infinite airflow resistivity. 
     Said monobloc structure of the voice coil support may comprise a closed pore material, such as a carbon mousse compound, or a polystyrene compound, that results in having a rigid as well as a light moving part. 
     The monobloc structure of the voice coil support may be an open pore material such as an elastomeric mousse. 
     The monobloc structure of the voice coil support may comprise a material that is transparent to the magnetic field and preferably is an electrical insulator. 
     According to an embodiment, at least the first surface and the second surface and preferably the first surface, the second surface and the outer surface are coated with at least a partially waterproof material that can comprise a resin or a varnish such as an acrylic or cellulosic varnish. 
     Advantageously, the outer surface of the voice coil support may be coated with a material that is resistant to being wetted through contact with a ferrofluid seal, such as a non-metallic material for limiting the effect of Eddy currents. 
     Preferably, ridges adapted to receive coil windings may be defined in the outer surface around the circumference of the voice coil support. 
     Advantageously, the second surface may be chosen amongst a plane, concave or convex surface. 
     Preferably, the voice coil support may be made in the shape of a solid of revolution. 
     Preferably, the shape of the voice coil support may be chosen amongst:
         a cylindrical shape,   a two circular cone frustum shape, the frustum portions being connected to each other by their smaller surface base side, or   a two circular cone frustum shape portion connected to each other by their smaller surface base side to a cylindrical shape portion, or   a paraboloid of revolution shape.       

     The invention also relates to a method of manufacturing a voice coil support according to the invention, the method including the steps of:
         providing a liquid or a powder of the desired material into a casting die of the desired shape,   setting the material to form said voice coil support,   removing the obtained voice coil support from the casting die.       

     Further advantageous features of the method of manufacturing a voice coil support according to the invention are:
         the method may include the step of cutting ridges in the outer surface of the voice coil support;   the method may include the step of providing coil winding into the casting die before providing the material into the casting die and maintaining the coil winding in position until the material sets.       

     The invention also relates to a coil transducer motor structure incorporating at least one magnetic element arranged in use to provide a path for magnetic flux between the ends of at least one coil, the coil being wound around a reciprocating voice coil support according to the invention. 
     The invention also relates to a loudspeaker incorporating a coil transducer motor structure according to the invention fixed on top of a cabinet providing return stroke means. 
     The loudspeaker may incorporate a suspension wire in the cabinet that may be connected towards one end to the first surface of the voice coil support and towards the other end to the cabinet and may extend preferably along the displacement axis Z. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of a cross-section of a voice-coil transducer motor assembly comprising a monobloc voice coil support according to a first embodiment; 
         FIG. 2  is a schematic representation of a cross-section of a voice-coil transducer motor assembly comprising a monobloc voice coil support according to a second embodiment; 
         FIG. 3  is a schematic representation of a cross-section of a voice-coil transducer motor assembly comprising a monobloc voice coil support according to a third embodiment; 
         FIG. 4  is a schematic representation of a cross-section of a voice-coil transducer motor assembly comprising a monobloc voice coil support according to a fourth embodiment; and 
         FIG. 5A  and  FIG. 5B  represent respectively views in perspective of voice coil supports having concave and convex emissive surfaces. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the figures and for the moment in particular to  FIG. 1 , a cross-section through a loudspeaker  10  is illustrated. 
     This loudspeaker  10  essentially comprises a cabinet  11  on top of which is located a voice-coil transducer motor structure  20  comprising a voice coil support  21 , or moving part, adapted to move along an axis of displacement Z. An emissive surface  22  is located at the top of the voice coil support  21 , at the opposite of a lower surface  26  of the voice coil support  21 , closing in part the top of the cabinet  11 . This emissive surface  22  is adapted to transmit the excitation produced by the voice-coil transducer motor structure  20  to the air. 
     Upper  22 H and lower  22 L voice-coils are wound around a lateral face  27  of the voice coil support  21  and at least one magnetic element  23  is arranged in use to provide concentration of its resultant magnetic field around the location of an upper  22 H and a lower  22 L voice-coil. As shown on the figure, the magnetic element  23  surrounds the voice coil support  21  at a distance. 
     On  FIG. 1 , the upper  22 H and lower  22 L voice-coils are placed in ridges  24  made in the lateral face  27  around the circumference of the voice coil support  21 . 
     By driving the current circulating in the upper  22 H and the lower  22 L voice-coils, the voice coil support  21  can be moved along the axis of displacement Z. 
     The voice coil support  21  is guided along its axis of displacement Z by ferrofluid seals  25  acting as guiding elements. One possible ferrofluid seal is of the type disclosed in the patent document FR2892887 incorporated in its entirety herein by reference. 
     As shown on  FIG. 1 , a ferrofluid seal  25  is placed in between the voice coil support  21  and the magnet element  23 . The ferrofluid seal  25  is placed around the point where the magnetic flux gradient is the largest, here equidistant from the upper  22 H and lower  22 L voice-coils. 
     Use of ferrofluid seals  25  can help avoid nonlinearities in the movements of the moving part  21  in the coil transducer motor structure  20  compared to known suspension elements that are usually made of elastomer. 
     Moreover, ferrofluid seals  25  act as thermal bridges, allowing the heat generated by the current circulating in the coil to flow through and be dissipated in the magnetic element  23  and in the cabinet  11 . 
     If the ferrofluid seals  25  allow the voice coil support  21  to be guided along its axis of displacement Z, return stroke means are provided, for the voice coil support  21  to be able to reciprocate along its axis Z. 
     These means take advantage of the volume change in the cabinet  11  when the voice coil support  21  moves along the axis of displacement Z. The volume defined in the cabinet  11  is delimited at the top by the coil transducer motor structure  20 , and at least partially by the lower surface  26  of the voice coil support  21 . 
     A hole  12  is made in the cabinet  11 , providing a small leakage, the dimensions of the hole being adapted to provide a very long time constant compared to the frequencies at which the coil transducer motor structure  20  operates. This hole  12  causes the pressure in the cabinet  11  for quasi-static or long period movements of the voice coil support  21  to equalize and compensates for barometric pressure changes. 
     For example, the diameter of the hole  12  is between 0.1 and 1 mm for a volume defined in the cabinet  11  of about 10 cubic centiliter. 
     When the voice coil support  21  moves upwards, the pressure in the cabinet  11  decreases, a depression is created and a return stroke force is generated retaining the voice coil support  21  by its lower surface  26 . A small quantity of air is sucked into the cabinet  11  through the hole  12 , to slowly increase the pressure in the cabinet  11 . 
     When the voice coil support  21  moves downwards, the pressure in the cabinet  11  increases, and some air is slowly expelled out of the cabinet  11  through the hole  12 . 
     At usual operating frequency range, the amount of air exchange is negligible. 
     Thus, the voice coil support  21  is retained by its lower surface  26  by an effect of suction. Such a return stroke means has the advantage of not introducing non linearities to the voice-coil transducer motor structure  20  unlike elastomer suspension means. 
     A suspension wire  13  can be connected towards one end to the lower surface  26  of the voice coil support  21  and towards the other end to the cabinet  11  and extends preferably along the displacement axis Z. 
     This suspension wire  13  is adapted to prevent the voice coil support  21  from being pushed out of the top of the voice-coil transducer motor structure  20  in case of failure of the return stroke means, for example when a strong shock occurs along the displacement axis Z. 
     The length of the suspension wire  13  is therefore designed for the suspension wire  13  to enter into action only when the return stroke means are inactive or beyond their working range. 
     Advantageously, the voice coil support  21  has a monobloc structure, preferably made in the shape of a solid of revolution. The monobloc structure is made of one solid piece of material, i.e. that the voice coil support  21  is made of massive material without hollow parts, and preferably obtained by casting. This monobloc structure is adapted to have its natural mechanical mode of vibration outside of the audible frequency range, that is limited from 20 Hz to 20 kHz. Therefore, mode coupling is prevented between mechanical modes and acoustical modes in the frequency range of interest, which is the range of audible frequencies for a loudspeaker. The solid monobloc structure of the voice coil support  21  allows for the mechanical modes to occur beyond an upper frequency of the frequency range of interest, or for example in loudspeakers beyond the upper limit of audible sounds. 
     This monobloc structure allows ssssprevention of coupling between mechanical modes and acoustical modes during the excitation of the voice-coil transducer motor structure  20  in the frequency range of interest. Thus the sound produced by the loudspeaker  10  is made clearer and of higher quality, rising and trailing edges of the acoustic signal being sharper. 
     The monobloc structure should also prevent the transmission of acoustic waves at least between the lower surface  26  and the emissive surface  22 . Thus the voice coil support  21  comprises a material that preferably exhibits a quasi infinite or infinite airflow resistivity. 
     Such a material is therefore adapted to prevent airflow communication between at least the lower surface  26  and its emissive surface  22 , or more generally speaking communication of fluid between at least the lower surface  26  and its emissive surface  22 . Preferably, the material prevents the communication of fluid between any one of the surfaces  22 ,  26 ,  27  to any other one surface  22 ,  26 ,  27  of the voice coil support  21 . 
     Therefore, the minimum absolute value of airflow resistivity of the material is such that it reduces the speed of airflow within the voice coil support  21  by a factor in the range of 2 to 4. 
     To improve yield and efficiency of the voice-coil transducer motor structure  20 , the voice coil support is designed to be as light as possible as well as being rigid enough to prevent mode coupling in a bandwidth of audible sounds. For these reasons, the applicant has noticed that closed pore materials or open pore materials with, preferably, an appropriate waterproof coating on the voice coil support&#39;s  21  outer surface  27  are the most suitable materials for making the voice coil support  21 . 
     The voice coil support&#39;s  21  outer surface  27  is preferably covered with a material adapted not to be wetted by ferrofluid seals  25  and for the ferrofluid seals  25  to slide better on the outer surface  27 , and for the ferrofluid seals  25  not to disappear by absorption into the voice coil support material  21 . 
     By way of example, suitable materials for coating the outer surface  27  comprise non metallic materials, acrylic or cellulosic varnishes. These coatings can be vaporized onto the voice coil support  21  and help to prevent the formation of eddy currents around the voice coil support  21 . These coatings can be applied on the outer surface  27  by a chemical vapour deposition method for example. 
     The closed pore material also prevents acoustic waves from being propagated from the bottom face  26  to the emissive surface  22  of the voice coil support  21  which would otherwise disturb the acoustical signal generated in the loudspeaker  10 . 
     This material should be transparent to the magnetic field generated by the magnet element  23 , and preferably be an electrical insulator, allowing the coil windings  22 H,  22 L to be irradiated. 
     By way of example, suitable closed pore materials comprise carbon mousse compounds, polystyrene compounds or the like. 
     Open pore materials having an infinite or quasi-infinite airflow resistivity are also suitable as constitutive materials of the voice coil support  21 . 
     By way of example, suitable open pore materials comprise elastomeric mousses or foams. 
     When the voice coil support  21  is made of an open pore material, at least the first surface  26  and the second surface  22  and, preferably, the first surface  26 , the second surface  22  and the outer surface  27  are coated with material that can comprise a resin or a varnish such as an acrylic or cellulosic varnish in order to achieve at least a partial waterproof effect. 
     According to the invention, the voice coil support  21  can be obtained by several ways. 
     In a first variant, the voice coil support  21  can be obtained by providing a chunk of the desired solid material, cutting the chunk of solid material to the desired shape and preferably coating the outer surface  27  of the voice coil support with a material chosen from a resin or a varnish and adapted not to be wetted by ferrofluid seals  25 . 
     Ridges  24  are then cut in to the outer surface  27  of the voice coil support  21 , their dimensions and location being adapted to receive coil windings  22 H,  22 L. 
     In a second variant, the voice coil support  21  can be obtained by providing a liquid or a powder of the desired material, pouring or injecting the material into a casting die of the desired shape, waiting for solidification of the material, and removing the obtained voice coil support from the casting die once the material has become solid. 
     Preferably, the second variant includes a step of coating the outer surface  27  of the voice coil support  21  with a material chosen from a resin or a varnish and adapted not to be wetted by ferrofluid seals  25 . 
     Ridges  24  can be provided by the same method as in the first variant. 
     In a third variant, the voice coil support  21  can be obtained by a blowing process. In that case, the voice coil support  21  will have a monobloc structure that will be a solid piece of material that can have hollow parts inside but will be a closed volume structure. That is to say the voice coil support  21  will have upper  22  and lower  26  surfaces. 
     Ridges  24  can be provided by the same method as in the first and second variants. 
     Coil windings  22 H,  22 L can also be placed into the casting die prior to the introduction, preferably by injection, of the material and maintained in position until it solidifies. This method allows for the voice coil support  21  to be made rapidly and efficiently and the coil windings to be integrated during the moulding process. 
     According to the first embodiment of the invention as disclosed in combination with  FIG. 1 , the voice coil support  21  has a cylindrical shape. The voice coil support  21  is able to reciprocate along its displacement axis Z while the ferrofluid seals  25  slide on the outer surface  27 . The return stroke force is mainly exerted by the interaction between the lower surface  26  and the cabinet  11 . 
     According to a second embodiment of the invention shown on  FIG. 2 , the voice coil support  21  has a monobloc structure in the shape of two circular cone frustum portions, these frustum portions being connected to each other by their smaller surface base side. 
     The location of the connection of the two frustum portions is designed to be equidistant from the upper  22 H and lower  22 L voice-coils. Therefore, at resting position of the voice coil support  21 , the ferrofluid seals  25  lie at the location of the connection of the two frustum portions. The slopes designed in the outer surface  27  tend to provide an additional return stroke force tending to bring back the voice coil support  21  to its resting position when the voice coil support  21  moves upwards or downwards. 
     According to a third embodiment of the invention shown on  FIG. 3 , the voice coil support  21  has a monobloc structure in the shape of two circular cone frustum portions connected to each other by their smaller surface base side to a cylindrical portion. The cylindrical portion is located at a position equidistant from the upper  22 H and lower  22 L voice-coils. Therefore, at resting position of the voice coil support  21 , the ferrofluid seals  25  lie against the cylindrical portion. The height of this cylindrical portion sets the excursion of the voice coil support  21  where the movement sees only the return stroke generated by the cabinet  11 . The cylindrical portion allows a wider ferrofluid seal  25 , extending along the cylindrical portion. 
     According to a fourth embodiment of the invention shown on  FIG. 4 , the voice coil support  21  has a monobloc structure, in the shape of a paraboloid of revolution. This embodiment is advantageous in that the ferrofluid seal  25  applies a return stroke force gradually increasing as the voice coil support  21  moves away from its resting position and is particularly adapted to positioning of the voice coil support  21  along its displacement axis Z. 
     The voice coil support  21  according to the invention comprises an emissive surface  22  towards the one end of the voice coil support  21  adapted to be extending outwards from the loudspeaker  10 . 
     This surface replaces the diaphragm that is present in the loudspeakers of the state of the art, in order to prevent the introduction of non linearities. 
     Depending on the characteristic of the field of emission the loudspeaker  10  is intended for, the emissive surface  22  can take several shapes, from flat represented in  FIGS. 1 through 4 ), concave or convex as shown in  FIGS. 5A and 5B . Thus the directivity of the sound produced by the loudspeaker  10  can be tuned. 
       FIG. 5A  illustrates a concave emissive surface  22 . 
       FIG. 5B  illustrates a convex emissive surface  22 .