EP1474952B1 - Moving-coil electrodynamic motor particularly for a loudspeaker, loudspeaker and corresponding pole piece - Google Patents

Abstract

The invention relates to a moving-coil electrodynamic motor and a loudspeaker. A moving-coil includes a winding with a given number of turns. A motor comprises a magnet arranged between a front pole piece and a rear pole piece. The front pole piece and the rear pole piece enclose a magnetic field in a gap and the moving coil is arranged in the gap. The gap may include a groove arranged essentially parallel to the turns. The coil has a height less than or equal to the height of the gap and the groove forms a recessed zone with an internal ring made from electrically conducting material.

Description

The present invention relates to an electrodynamic motor with voice coil intended in particular for producing a loudspeaker. A loudspeaker and a pole piece adapted to the motor are also considered within the scope of the invention.

Speakers with electrodynamic voice coil motor are known. They conventionally comprise a motor with a two-pole magnet for producing a magnetic field which is looped in an air gap by means of two pole pieces, each pole piece being in relation with one of the poles of the magnet. The pole pieces are usually made of soft iron or low carbon steel. The gap is a zone of free space in which the magnetic field is substantially constant and it corresponds in practice to an area where the pole pieces are closest to each other. A voice coil consisting of turns of a conductive wire is arranged in the gap. When a current flows in the coil, the latter being subjected to the magnetic field, a force is created which causes the displacement of the coil according to the vector formula F = BI / in which B is the induction or magnetic field, I the intensity of the current and / the length of the conductor subjected to the magnetic field. The minimum width of the gap depends on the thickness of the coil because it must be able to move freely, with sufficient clearance, in said air gap taking into account the constraints of manufacture, expansion of materials ...

In order to convert the moving force of the voice coil into an acoustic pressure wave, the coil is secured to an acoustic membrane which can thus move the air into its environment. The structural and dimensional characteristics of the membrane are adapted to the envisaged applications of the loudspeakers. The motor has a circular symmetry with respect to a central antero-posterior axis and the coil is a circular winding of turns. The membrane generally presents such circular symmetry, but there are, however, elliptical diaphragm loudspeakers.

In the most general case of the restitution of sounds audible by the human ear, or even somaesthetic for the lowest frequencies, the frequency range to be restored is very wide, from about ten Hertz to a few tens from KiloHertz. It is generally considered that the range of frequencies to be restored must be between 20Hz and 20KHz. The wavelengths associated with this wide range do not allow the realization of a single speaker with good rendering characteristics, for example in terms of distortion, sensitivity and directivity throughout the range. It is therefore necessary to use adapted loudspeakers whose reduced rendering range makes it possible to obtain quality rendering characteristics over said reduced range. We consider the speakers adapted to the low frequencies, so-called "boomers", whose membrane is extended, and displacement or significant excursion and those adapted to the highest frequencies, called "tweeters", whose membrane is of reduced surface . Finally, there are speakers more specifically adapted to intermediate frequencies, also called "mediums".

The difference in size of the membranes between the "boomers" and the "tweeters" has led to different technical achievements of the engine. For example, in DUNOD's Pierre Loyez's "Techniques of loudspeakers and loudspeakers", a general description of the different types of loudspeakers can be found.

In the typical case of a "boomer", the membrane is a cone and the motor magnet is a ring or crown, the two terms being synonymous here, usually ferrite. The magnetic field is then looped in an air gap between an inner edge of an anterior pole piece disposed on a front pole of the magnet and a central core extending forwardly the rear pole piece in relation to the rear pole of the pole. magnet. The membrane is secured at one end to the coil and the other to a peripheral suspension, or edge, integral with a rigid frame also attached to the engine. An inner suspension or "spider" makes it possible to axially maintain the voice coil during its antero-posterior movements so that it remains free, without lateral friction, in the gap. Such a structure is said to be double-guided because, on the one hand, the peripheral suspension or edge and, on the other hand, the "spider" both contribute to the axial retention even for large excursions of the coil. A core cover or dust cover can also be implemented in the center of the cone.

In the typical case of a "tweeter", the membrane is usually a dome that can be concave or convex. A core magnet, generally a neodymium metal cylinder, iron, boron, is disposed by a first of its two poles on a rear pole piece or cylinder head which, by a lateral extension from rear to front, loops the magnetic field in a gap. The magnet may also be ferrite or metal type with other components and, for example, TICONAL. An anterior pole piece, or so-called pellet or anterior field plate, is disposed on the second pole of the magnet with an outer peripheral edge which limits the gap to the opposite of the yoke. A suspension at the periphery of the dome, allows the maintenance of the voice coil along the anteroposterior axis during its movements within the air gap, without lateral friction. Such a structure is said to have a simple guide as opposed to the "boomer" previously described which is double-guided.

Finally, we know loudspeakers combining characteristics of the two previous types, and in particular for a "medium" dome whose magnet is a ferrite ring and whose looping of the field to the air gap by the rear pole piece is performed by an extension in central crown forward.

As we have seen, it is therefore possible to classify the speakers either according to the structure of the magnet, or depending on the material of the magnet, or depending on the shape of the membrane. For the structure of the magnet, it can be either a magnet solid core or pierced with a central orifice and disposed at the center of the motor, a magnet ring or ring then disposed outwardly relative to the central axis and the coil. In the case of a core magnet, the coil is disposed outwardly with respect to the magnet. In the case of a corona magnet, the coil is generally disposed inwards, towards the center, with respect to the magnet. For the material, the magnet may be ferrite type, TICONAL or ALNICO metal alloy type or ALCOMAX® (aluminum, nickel, titanium, cobalt, iron), or based on rare earth (samarium, cobalt or neodymium, iron , boron). For the shape of the membrane, we know the domes, cones and mixed forms, still called W.

In order to limit the harmonic distortion during the restitution, it is necessary to have a good linearity of response. The latter is in particular obtained when the coil intercepts a flow of the homogeneous magnetic field during its movements. The maximum linear displacement distance of the coil is generally called linear maximum excursion or X _Max which can be abbreviated to X _M. Two alternatives have been proposed for this purpose. The first is to achieve a homogeneous winding with a height (along the anteroposterior axis of displacement) important, greater than the height of the air gap. In this way, as long as the coil remains entirely in the gap, there will be proportionality between the force and the current in the coil. This configuration, called long coil, is preferably implemented in the "boomers". The second consists, on the contrary, in making a coil whose height is reduced relative to the height of the gap. This configuration, called short coil, is preferably implemented in the "tweeters" or "mediums".

While improvements have improved the performance characteristics of voice-coil electrodynamic loudspeakers, limitations in linearity are still present due to the asymmetry of the fields towards the ends of the air gap which, in practice , limits the maximum linear excursion of the coil. In addition, there are phenomena complex electromagnetic fields created by the variable electric fields, by the displacement of conductors in a magnetic field, the modulation of the static magnetic field of the magnet in the air gap by the variable current flowing in the coil, ie offset of the coil called "DC shift "(DC slip) and the generation of eddy currents. These phenomena are also sources of distortion in the restitution.

US 3,867,587 describes a motor according to the preamble of claim 1.

The invention proposes to reduce the effect of this type of problem by a particular configuration of the gap of a moving coil electrodynamic motor. The invention therefore relates to an electrodynamic motor with a voice coil, in particular for a loudspeaker, the voice coil consisting of a winding of a determined number of turns of an electrical conductor being integral with an acoustic membrane, the motor comprising at least a two-pole magnet disposed between an anterior pole piece and a posterior pole piece, the anterior pole piece and the posterior pole piece looping the magnetic field of the magnet in an air gap, said gap being delimited on a first side by a first peripheral edge of the anterior pole piece and a second side by a second peripheral edge of the posterior pole piece, the voice coil being disposed in said gap, the turns being perpendicular to the magnetic field so that when a current is sent into the coil, the latter moves along an anteroposterior axis.

According to the invention, the gap is split, at least one of the edges of said gap having a groove, the groove being oriented substantially parallel to the turns, said edge having a first posterior surface of height E1 separated from a second anterior surface height E2 through the groove of height C, the groove forming a zone recessed with respect to the first surface and the second surface, E1 determining a first gap space of the posterior magnetic field B1 and E2 determining a second field gap space prior magnet B2, the coil having a height H _B less than or equal to the height E1 + C + E2 of the split air gap, and in that the groove comprises internally a continuous and closed ring of electrically conductive material. In addition E1 is substantially equal to E2, the coil is substantially homogeneous and the coil has a height H _B such that H _B is substantially equal to E1 + C or E2 + C or E1 / 2 + C + E2 / 2.

It is understood that the ring of conductive material may occupy all or part of the groove and that the width of the gap may be the same or not along the height of the air gap, the latter alternative corresponding to the case where the first surface is away from the second or vice versa. Similarly, the groove may be disposed on a peripheral edge of an anterior pole piece, the groove may be disposed on a peripheral edge of a posterior pole piece and, finally, the two peripheral edges respectively corresponding to the posterior pole piece and to the anterior pole piece may each have a groove, in the latter case, the realized structure may be symmetrical or not with respect to the median plane of the gap. The winding can be homogeneous or not. The height E1 may be equal or not to the height E2.

• au repos, la bobine est disposée en regard de la gorge et intercepte par une première extrémité avec NB1 spires le champ magnétique postérieur B1 et par une seconde extrémité avec NB2 spires le champ magnétique antérieur B2, de manière à ce que le produit B1.NB1 soit sensiblement égal au produit B2.NB2 le long de la hauteur, (cette dernière formule correspond à une moyenne des deux précédentes)
• l'entrefer a une largeur sensiblement constante au moins le long des hauteurs E1 et E2,
• l'excursion maximale linéaire X_M du bobinage de part et d'autre de sa position de repos est sensiblement égale en valeur absolue à (E1 / 2) + C ou (E2 / 2) + C ou une moyenne des deux,
• le bobinage est sensiblement homogène et E1=E2=E et H_B =E+C,
• l'excursion maximale X_M du bobinage de part et d'autre de sa position de repos est en valeur absolue X_M = (E / 2) + C,
• gorge est disposée sur le premier bord périphérique de la pièce polaire antérieure,
• la gorge est disposée sur le second bord périphérique de la pièce polaire postérieure,
• chacun des deux cotés délimitant l'entrefer comporte une gorge,
• la pièce polaire antérieure est prolongée vers l'avant,
• la pièce polaire postérieure n'est pas prolongée vers l'avant en regard de la zone de bord périphérique correspondant audit prolongement de la pièce polaire antérieure,
• l'extrémité libre de la pièce polaire postérieure adjacente à la zone d'entrefer présente une conicité,
• le prolongement antérieur de la pièce polaire antérieure présente une conicité vers son bord périphérique,
• le moteur comporte vers l'avant de la pièce polaire antérieure au moins un contre-aimant, ledit contre-aimant ayant ses pôles orientés d'une manière opposée à l'orientation des pôles de l'aimant,
• le moteur comporte en outre vers l'avant, sur le contre-aimant, une pastille d'un matériau ferromagnétique et en ce que la pièce polaire postérieure est prolongée vers l'avant,
• le prolongement vers l'avant de la pièce polaire est jusqu'au plus au niveau supérieur de la pastille,
• le moteur comporte en outre vers l'avant, sur le contre-aimant, une pastille d'un matériau ferromagnétique et la pièce polaire postérieure n'est pas prolongée vers l'avant jusqu'au niveau supérieur de la pastille,
• le matériau ferromagnétique est du fer doux,
• le moteur présente une symétrie de révolution circulaire autour d'un axe central antéro-postérieur,
• les bords E1 et E2 sont droits et parallèles entre-eux,
• les bords E1 ou E2 sont droits et inclinés,
• le coté de l'entrefer ne comportant pas de gorge est droit et sensiblement parallèle à l'axe antéro-postérieur de déplacement de la bobine,
• le diamètre du bord E1 de l'entrefer est sensiblement égal au diamètre du bord E2 de l'entrefer afin que la largeur de l'entrefer soit sensiblement constante le long de sa hauteur,
• la largeur de l'entrefer dans le premier espace d'entrefer de champ magnétique postérieur B1 est supérieur à la largeur de l'entrefer dans le second espace d'entrefer de champ magnétique antérieur B2,
• l'aimant est un aimant noyau, la pièce polaire antérieure est une pastille sensiblement plane, la pièce polaire postérieure est une culasse en forme de U en coupe avec un fond sur lequel repose l'aimant et des bords montants, l'entrefer dédoublé étant délimité par le bord périphérique externe de la pastille et la zone adjacente du bord montant de la culasse,
• E1 et E2 sont chacun approximativement égaux à 3 mm, C est approximativement égal à 4,175 mm, le prolongement de la pièce polaire antérieure approximativement égal à 3 mm et H_B est approximativement égal à 7,025 mm,
• l'extrémité libre de la pièce polaire postérieure adjacente à la zone d'entrefer est abaissée par rapport à l'extrémité antérieure de la pièce polaire antérieure de hauteur E2, l'abaissement de l'extrémité libre de la pièce polaire postérieure étant d'environ 0,5 mm,
• la largeur de l'entrefer dans la zone postérieure délimitée par E1 est supérieure à la largeur de l'entrefer dans la zone antérieure délimitée par E2,
• la largeur de l'entrefer dans la zone de la gorge avec le matériau conducteur de l'électricité est intermédiaire entre la largeur de l'entrefer en E1 et en E2,
• l'aimant a un diamètre de 37 mm et une épaisseur de 6 mm, le bord E1 de l'entrefer est sur un diamètre de 37 mm, le bord E2 de l'entrefer est sur un diamètre de 37,5 mm,
• le bord C du matériau est sur un diamètre de 37,40 mm,
• le bord interne de la gorge est sur un diamètre de 22,3 mm,
• le bord de l'entrefer sur la pièce polaire postérieure est sur un diamètre de 40,7 mm,
• le bord interne de la bobine est sur un diamètre de 38,0 mm et le bord externe de la bobine est sur un diamètre de 40,2 mm,
• le jeu de la bobine par rapport à chacun des deux bords de l'entrefer est d'environ 250 µm,
• la conicité de l'extrémité libre de la pièce polaire postérieure adjacente à la zone d'entrefer est approximativement de 27,5° par rapport à l'horizontale,
• le bord externe de la pièce polaire postérieure est sur un diamètre approximatif de 50 mm,
  (les dimensions indiquées pour le moteur optimisé doivent être considérées comme approximatives car il faut tenir compte des contraintes de fabrication, d'usinage, moulage...)
• l'aimant est un aimant couronne, la pièce polaire antérieure est une bague sensiblement plane présentant un bord périphérique interne vers le centre de la couronne, la pièce polaire postérieure est formée d'une plaque arrière sur laquelle repose l'aimant et d'un noyau central s'étendant d'arrière en avant, l'entrefer dédoublé étant délimité par le bord périphérique interne de la bague et la zone adjacente du noyau central,
• le matériau conducteur de l'électricité est choisi parmi l'or, l'argent, le cuivre, l'aluminium, le carbone graphite en combinaison ou non et est, de préférence, en cuivre,
• le matériau conducteur est isolé électriquement du matériau de sa pièce polaire à gorge,
• le matériau conducteur n'est pas isolé électriquement du matériau de sa pièce polaire à gorge,
• l'isolant électrique du matériau conducteur présente un coefficient de conduction thermique élevé,
• de préférence, le fond de la gorge présente deux zones de raccordement arrondies aux parois supérieure et inférieure de la gorge,
• la bobine comporte un support de bobine,
• la bobine est homogène sur sa hauteur,
• la bobine n'est pas homogène sur sa hauteur, deux zones de spires séparées par un espace étant mises en oeuvre,
• les spires de la bobine sont des pistes conductrices déposées sur un support de bobine,
• l'aimant noyau comporte une ouverture centrale,
• la pièce polaire antérieure de l'aimant noyau à ouverture centrale comporte également une ouverture centrale,
• le prolongement antérieur de la pièce polaire antérieure de l'aimant noyau à ouverture centrale comporte également une ouverture centrale,
• le contre-aimant d'un aimant noyau à ouverture centrale comporte également une ouverture centrale,
• la pastille d'un contre-aimant à ouverture centrale comporte également une ouverture centrale,
• la pastille est une pièce plane essentiellement continue et pleine,
• la pièce polaire postérieure est monobloc,
• la pièce polaire postérieure résulte de la réunion d'au moins deux éléments,
• la pièce polaire antérieure est monobloc,
• la pièce polaire antérieure résulte de la réunion d'au moins deux éléments.

In various embodiments of the invention, the following means can be combined according to all technically feasible possibilities, are considered:

• at rest, the coil is arranged facing the groove and intercepts by a first end with NB1 turns the posterior magnetic field B1 and by a second end with NB2 turns the magnetic field B2, so that the product B1.NB1 is substantially equal to the product B2.NB2 along the height, (the latter formula corresponds to an average of the two previous ones)
• the gap has a substantially constant width at least along the heights E1 and E2,
• the linear maximum excursion X _M of the winding on either side of its rest position is substantially equal in absolute value to (E1 / 2) + C or (E2 / 2) + C or an average of the two,
• the winding is substantially homogeneous and E1 = E2 = E and H _B = E + C,
• the maximum excursion X _M of the winding on either side of its rest position is in absolute value X _M = (E / 2) + C,
• groove is disposed on the first peripheral edge of the anterior pole piece,
• the groove is disposed on the second peripheral edge of the posterior pole piece,
• each of the two sides delimiting the gap has a groove,
• the anterior pole piece is extended forward,
• the posterior pole piece is not extended forward facing the peripheral edge area corresponding to said extension of the anterior pole piece,
• the free end of the posterior pole piece adjacent to the gap zone has a taper,
• the anterior extension of the anterior pole piece has a taper towards its peripheral edge,
• the motor comprises, towards the front of the front pole piece, at least one counter-magnet, said counter-magnet having its poles oriented in a manner opposite to the orientation of the poles of the magnet,
• the motor further comprises, towards the front, on the counter-magnet, a pellet of a ferromagnetic material and in that the rear pole piece is extended towards the front,
• the forward extension of the pole piece is up to the upper level of the pellet,
• the motor further comprises, towards the front, on the counter-magnet, a pellet of a ferromagnetic material and the rear pole piece is not extended forwards to the upper level of the pellet,
• the ferromagnetic material is soft iron,
• the motor has a symmetry of circular revolution around a central anteroposterior axis,
• the edges E1 and E2 are straight and parallel to each other,
• the edges E1 or E2 are straight and inclined,
• the side of the gap having no groove is straight and substantially parallel to the antero-posterior axis of movement of the coil,
• the diameter of the edge E1 of the gap is substantially equal to the diameter of the edge E2 of the air gap so that the width of the air gap is substantially constant along its height,
• the width of the gap in the first gap space of the posterior magnetic field B1 is greater than the width of the gap in the second gap space of the magnetic field B2,
• the magnet is a core magnet, the anterior pole piece is a substantially flat pad, the rear pole piece is a U-shaped yoke in section with a bottom on which the magnet rests and rising edges, the split gap being bounded by the outer peripheral edge of the pellet and the adjacent area of the rising edge of the bolt,
• E1 and E2 are each approximately 3 mm, C is approximately 4.175 mm, the extension of the anterior pole piece approximately 3 mm and H _B is approximately 7,025 mm,
• the free end of the posterior pole piece adjacent to the gap zone is lowered relative to the anterior end of the anterior pole piece of height E2, the lowering of the free end of the posterior pole piece being about 0.5 mm,
• the width of the gap in the posterior zone delimited by E1 is greater than the width of the gap in the anterior zone delimited by E2,
• the width of the gap in the throat zone with the electrically conductive material is intermediate between the width of the air gap at E1 and E2,
• the magnet has a diameter of 37 mm and a thickness of 6 mm, the edge E1 of the gap is on a diameter of 37 mm, the edge E2 of the gap is on a diameter of 37.5 mm,
• the edge C of the material is on a diameter of 37.40 mm,
• the inner edge of the groove is on a diameter of 22.3 mm,
• the edge of the gap on the posterior pole piece is on a diameter of 40.7 mm,
• the inner edge of the coil is on a diameter of 38.0 mm and the outer edge of the coil is on a diameter of 40.2 mm,
• the clearance of the coil with respect to each of the two edges of the gap is about 250 microns,
• the taper of the free end of the posterior pole piece adjacent to the gap zone is approximately 27.5 ° to the horizontal,
• the outer edge of the posterior pole piece is on an approximate diameter of 50 mm,
  (The dimensions indicated for the optimized motor must be considered as approximate because we must take into account the constraints of manufacture, machining, molding ...)
• the magnet is a crown magnet, the anterior pole piece is a substantially planar ring having an inner peripheral edge towards the center of the crown, the rear pole piece is formed of a back plate on which the magnet rests and a central core extending from back to front, the split air gap being delimited by the inner peripheral edge of the ring and the adjacent zone of the central core,
• the electrically conductive material is chosen from among gold, silver, copper, aluminum, graphite carbon, in combination or otherwise, and is preferably made of copper,
• the conductive material is electrically insulated from the material of its throat piece,
• the conductive material is not electrically isolated from the material of its throat piece,
• the electrical insulator of the conductive material has a high coefficient of thermal conduction,
• preferably, the bottom of the groove has two rounded connection areas at the upper and lower walls of the groove,
• the coil has a coil holder,
• the coil is homogeneous over its height,
• the coil is not homogeneous over its height, two zones of turns separated by a space being implemented,
• the coils of the coil are conductive tracks deposited on a coil support,
• the core magnet has a central opening,
• the anterior pole piece of the center opening core magnet also has a central opening,
• the anterior extension of the anterior pole piece of the core opening core magnet also has a central opening,
• the counter magnet of a core opening core magnet also has a central opening,
• the pellet of a center-opening counter-magnet also has a central opening,
• the pellet is a flat piece that is essentially continuous and solid,
• the posterior polar piece is monobloc,
• the posterior polar piece results from the union of at least two elements,
• the anterior polar piece is monobloc,
• the anterior polar piece results from the union of at least two elements.

The invention also relates to a voice coil electrodynamic loudspeaker comprising a motor according to one or more of the preceding features.

Thus, the invention, thanks to a means of remarkable simplicity, allows the realization of a moving coil electrodynamic motor with improved linearity. Thanks to the split structure of the air gap it is possible to obtain the forces to which it is subjected the coil during its movements equilibrate and compensate between the two zones defined by E1 and E2. For example, when the force created by the field area B1 decreases because the number of exposed turns decreases, the coil moving forward, an equivalent force created by the field B2 is added because more turns are entering B2 field. In practice, the product B. / is adapted so that there is actually compensation during the displacement of the coil: the fields B1 and B2 may be identical or different with a homogeneous coil or not. The configuration of the invention makes it possible in particular to reduce odd-order distortions which are the most unpleasant to the ear. This configuration also makes it possible to reduce the effects of DC offset, "DC shift". The engines thus obtained can also be extremely compact while having a large excursion of the voice coil. The use of a counter-magnet also makes it possible to increase the efficiency of the motor and to produce a low leakage loudspeaker particularly suitable for television applications in which the magnetic fields must be reduced to avoid distortions of the images produced by cathode ray tubes. Moreover, thanks to the implementation of computer modeling tools for magnetic and electromagnetic fields, it is also possible to optimize the basic configuration of the invention to obtain even better results while minimizing the mass of materials to be used, the volumes being determined to be in the low limit of magnetic saturation. With optimization, the width of the gap is different between the posterior field area limited by E1 and the area of the previous field limited by E2 because E1 is closer to the magnet than E2. Similarly, the front extension of the posterior pole piece is shortened relative to the front end of E2 which is vis-à-vis. The application of the invention has, for example, allowed the production of a loudspeaker which has a reduced distortion of about three times in a low frequency range compared to a speaker of the state of the technical.

• la Figure 1 représentant schématiquement un moteur selon l'invention dans un haut-parleur à cône et à aimant couronne,
• la Figure 2 représentant schématiquement un moteur dans un haut-parleur à dôme et à aimant couronne,
• la Figure 3 représentant schématiquement un moteur dans un haut-parleur à aimant noyau,
• la Figure 4 représentant une partie d'une coupe transversale et axiale d'un moteur,
• la Figure 5 représentant une partie d'une coupe transversale et axiale d'un moteur optimisé, et
• la Figure 6 représente un haut-parleur avec moteur de l'invention à fuites magnétiques réduites.

The present invention will now be exemplified by the description which follows and in connection with:

• the Figure 1 schematically showing a motor according to the invention in a cone loudspeaker and crown magnet,
• the Figure 2 schematically showing a motor in a dome and crown magnet,
• the Figure 3 schematically showing a motor in a core magnet speaker,
• the Figure 4 representing part of a cross section and axial of an engine,
• the Figure 5 representing part of a cross-sectional and axial section of an optimized engine, and
• the Figure 6 represents a speaker with a motor of the invention with reduced magnetic leakage.

On the Figure 1 a loudspeaker 1 with an electrodynamic motor with a crown or ring magnet and with a cone-type membrane 6 is shown in a transverse section passing through the axis of central symmetry of revolution of the motor. Such a type of loudspeaker is generally adapted to the restitution of the low frequencies of the spectrum and is generally called "boomer". The motor here consists of a crown magnet 3 of ferrite type with two opposite polar faces. The magnet rests with its posterior polar face on a rear pole piece 4, or rear plate, and which has a front central extension 4 'in the central opening of the ring 3. A front pole piece 2 is disposed on the polar face This anterior pole piece 2 is in the general shape of a ring or ring with a central opening just like the magnet. The anterior pole piece 2 is therefore an extended metal piece in the general shape of a ring. An air gap is formed between the inner peripheral edge of the anterior pole piece 2 and a corresponding area of the front extension 4 'of the rear pole piece 4. The two pole pieces are preferably made of soft iron. The posterior pole piece may also include a central anterior-posterior opening.

The edge of the pole piece at the origin of the looping of the magnetic field in the gap is here represented schematically with a reduced thickness compared to its other dimensions, which thickness corresponds to the height of the gap. However, in practice, the height of the air gap may be greater, as will be seen in the detailed detailed description of an engine, because the height of the gap is adapted to the material used and the magnetic field generated by the magnet to, inter alia, avoid magnetic saturation of said material. The inner peripheral edge of the anterior pole piece 2 outwardly defining the air gap comprises a substantially central groove for the creation of a split air gap. In the zone of the gap, on the front 4 'central extension of the posterior pole piece 4, a groove is also made. The doubling of the gap by a groove makes it possible to achieve two zones of substantially equal heights and defining a first rear field area and a second area of the front field in the air gap. However, as described later, the shape of the split air gap can be adapted to the particular structure used to optimize the operation of the speaker. In particular, the heights of the posterior and anterior field areas may be different. Similarly, in this case where a groove is present on each of the two edges delimiting the gap, this structure may or may not be symmetrical with respect to the gap. In variants not shown, it is also possible to make a groove only on the anterior pole piece or only on the posterior pole piece.

A coil 5 formed of turns of an electrical conductor is disposed in the gap. Preferably, the coil is homogeneous in that at each level along the height of the coil there is the same number of turns. As a result, the force generated for a given magnetic field is constant at any point in the height of the coil. The height of the coil 5 is less than the height of the gap. At rest, in the absence of current in the coil, the latter is placed at the level of the groove and so that its anterior end intercepted the anterior field and that its posterior end intercepts the posterior field. At rest, the coil arrangement may be symmetrical or not with respect to the groove along an anteroposterior axis along which it moves, the coil height intercepting the posterior field may or may not be equal to the height coil intercepting the front field.

A cone 6 is fixed by a first central end to a coil support which is part of the coil 5 and a second outer end to a peripheral suspension 8 secured to a rigid frame 7 itself fixed to the engine. An inner suspension or "spider" 10 transversely retaining the central end of the cone 6 and the coil during its antero-posterior movements so that the coil does not rub against motor elements. Conventionally, a dust cover 9 is disposed towards the center of the cone.

On the Figure 2 the loudspeaker 11 also uses a ring magnet 13 and an anterior pole piece 12 as well as a two-part rear pole piece 14, 14 '. However, the frequency range to be restored being in the middle or high part of the spectrum, the membrane is a dome 16 which is secured peripherally to the coil 15 disposed in the air gap. The magnet is preferably of the ferrite type. The dome and the coil are connected to a frame 17 by a peripheral suspension 18 or edge. Absorbent materials 19 make it possible to improve the damping of the system. The front central extension 14 'of the rear pole piece 14 is an insert ring, open at its center for the passage of the connecting wires 40 to the coil 15, the son passing through a portion 17' of the frame. The posterior pole piece 14 itself and the anterior pole piece 12 are metal pieces, preferably of soft iron, generally ring-shaped to be arranged at each pole of the magnet 13. The inner peripheral edge of the pole piece previous 12 delimiting outwardly the gap has a substantially central groove for the creation of a split air gap. The edge of the extension 14 'of the posterior pole piece delimiting the gap also has a groove. The The groove can split the air gap into two zones of substantially equal or non-equal heights to define a first rear field area and a second area anterior gap area. The splitting by the groove makes it possible to produce two zones of substantially equal or non-equal heights defining a first rear field area and a second area of the front field in the air gap. However, as described later, the shape of the split air gap can be adapted to the particular structure used to optimize the operation of the speaker. In particular, the heights of the posterior and anterior field areas may be different. Similarly, in this case where a groove is present on each of the two edges delimiting the gap, this structure may or may not be symmetrical with respect to the gap. In variants not shown, it is also possible to make a groove only on the anterior pole piece or only on the posterior pole piece.

On the Figure 3 the magnet is a core magnet 23 which is arranged at the center of the motor of a loudspeaker 21. Preferably, the magnet is a pellet or a cylindrical ring of rare earth or ferrite or alloy-based aluminum, nickel, titanium, cobalt and iron, or TICONAL or ALNICO or ALCOMAX. The magnet has two poles and rests with a first of its poles on a rear pole piece 24, or breech, looping the magnetic field through an air gap on an outer peripheral edge of an anterior pole piece 22. A voice coil 25 is arranged in the gap. Depending on the application, cone speaker, the membrane is then a cone 26 classically suspended with dust cover 29, or dome speaker, the membrane is then a dome 29 with suspension 26. The other conventional elements of the speaker, whose chassis and eventual "spider" depending on the type of loudspeaker, have not been represented for reasons of simplification. The gap is split by a groove formed on the outer peripheral edge of the anterior pole piece 22. Symmetrically a groove has also been made on the posterior pole piece. In an alternative embodiment not shown, the groove of the posterior pole piece is omitted or that of the anterior pole piece, the air gap is only duplicated on one side. In an alternative, not shown, of the core magnet motor, the motor may include a central anteroposterior opening.

On the Figures 1 , 2 and 3 the elements forming the motor, the magnet, the two pole pieces and the coil, have a circular symmetry about an axis of central anteroposterior revolution. The invention is however applicable to other forms of engines that are not circular revolution.

The Figure 4 , not to scale, specifies the structure of the engine of the invention. Only the right part of the engine seen in section along the axis of symmetry 31 anteroposterior is shown. The magnet 33 is a core magnet which has a central opening along the axis 31. This central opening may allow the passage of means for securing the motor elements, for example by injection of a hot plastic material or resin. Preferably, a free passage will be left to allow decompression of the volume of air included behind the membrane and in front of the engine itself. In a variant, as had been shown on the Figure 3 , the magnet can be full. The magnet 33 has a first anterior polar face on which is placed an anterior pole piece (or field plate) 32. The anterior pole piece is here open towards its center but in a variant, as on the Figure 3 she can be full.

The magnet 33 has a second rear polar face, opposite the previous one, on which is placed a rear pole piece 34, or breech, having a peripheral extension 34 'forwardly allowing the looping of the magnetic field in a gap. The posterior polar piece is here open towards its center but in a variant, as on the Figure 3 she can be full. The gap is delimited on one side by an outer peripheral edge of the anterior pole piece 32 and, on the other side, by the corresponding surface area of the extension 34 'of the posterior pole piece 34 which is located facing said peripheral edge. A groove 30 is formed on the peripheral edge of the anterior pole piece to split the air gap into two field areas. The throat has a height C.

Preferably the groove is formed on the peripheral edge in a substantially symmetrical manner so that the height E1 of the posterior field zone 37 is substantially equal to the height E2 of the anterior field zone 36. In alternatives, it is It is also possible to adapt these heights by computer optimization methods depending on the materials and the structure used. The groove 30 internally comprises an electrically conductive material which, in a preferred embodiment, is copper or graphite carbon. Preferably, but not shown, the bottom of the groove is rounded, the connections between the bottom wall and the side walls, (upper and lower walls in this case), the groove are rounded. Indeed, a connection edge or corner of a magnetized pole piece creates singular points in the distribution of the magnetic field, which can be detrimental.

Preferably, for reasons of cost, the pole piece which comprises a groove is formed of at least two elements allowing easy placement in the groove of a closed ring of conductive material. For example, the anterior pole piece groove is in two elements, a first element corresponds to the anterior air gap zone of height E2 and a second element corresponds to the groove and the rear air gap zone of height E1. Then insert the conductive ring in the second element, at the throat and then place the first element on this set. It is understood that all other arrangements conventionally known to facilitate the establishment of the ring both in the anterior and posterior pole piece are applicable without departing from the scope of the invention and in particular the two elements may, for example, be identical or substantially identical in height E1 + C / 2 and E2 + C / 2 respectively.

The coil 35 is disposed in the air gap and its height is less than or equal to the height of the air gap and, preferably, lower. For reasons of simplification, the bobbin support which is part of the coil and which is normally fixed forward to a cone or dome type membrane according to the application of the motor has not been shown. At rest, the coil is disposed at the groove and the motor is configured so that the height of the anterior portion of the coil intercepting the anterior field zone 36 is substantially equal to the height of the rear portion of the coil that intercepts the posterior field area 37. Thus in the case shown where E1 = E2 = E, the height H _B of the turns of the coil is preferably equal to E + C, which allows a maximum excursion X _M of the coil with good linearity at +/- ((E / 2) + C) relative to the rest position. Preferably, at rest, the posterior end of the coils of the coil is at the half-height of E1 and the anterior end of the coils of the coil is at the half-height of E2.

As previously mentioned, optimization tools such as MAGNET® or OPERA® have made it possible to adapt the engine. In particular, and by way of example, the heights E1 and E2 may be different, the groove disposed offset with respect to the center of the height of the gap, the width of the airgap different depending on whether one is in the zone posterior field, throat or anterior field area.

We have shown on the Figure 4 , in dashed lines, general variants. The first adaptation is to extend forward 39, the anterior pole piece 32. It will be noted that the extension 34 'of the posterior pole piece 34 is not extended in a manner equivalent to 39 and the anterior field area 36 is only slightly modified. However, in a preferred variant, the extension 39 has a frustoconical edge 40 in order to further reduce this field modification provided by said extension 39. Similarly, alternatively or in combination with the preceding optimization means, it is possible to implement an extension 34 'of posterior polar piece 34 whose anterior free end 38 is also frustoconical. In the latter case, as shown, the most anterior end of the extension 34 'terminates at a level below the anterior end of E2. This latter configuration corresponds to the case where the free end of the posterior pole piece adjacent to the gap zone is lowered relative to the anterior end of the anterior pole piece. In a variant not shown, the free end 38 ends at the same level as E2. This split air gap structure with anterior pole piece with a peripheral groove edge can be adapted to various types of loudspeakers (dome or cone) as seen on the Figures 1 to 3 .

The Figure 5 , not to scale, gives the result of an optimization of an engine. In a variant, the extension 39 of the anterior pole piece 32 may be frustoconical 40. It will be noted that the width of the air gap is not the same according to the considered zone, the width being greater towards the rear than towards the the front to reflect the fact that the magnet is closer to the posterior field area than the anterior field area. In a variant not shown, one or both edges delimiting the gap are inclined to better reflect this effect, contrary to what is shown, where the edges are parallel to each other. It is also possible to implement a coil which is not homogeneous so as to take account of differences in magnetic fields according to the zone of the gap, the product B being preferably constant at any point in the height of the coil. static and during its displacements so that there is compensation of forces reduction (the number of exposed turns decreasing at one end of the coil) by equivalent force additions (the number of exposed turns increasing at the other end of the coil). the coil). It is thus possible to adapt the invention to any type of coil, homogeneous or not by adaptation of the field along the height of the gap.

The Figure 6 is an application of the motor of the invention to the realization of a speaker with low magnetic leakage. Such a high-speaker is more particularly intended to be used in applications where a magnetic field may interfere with the operation of apparatus, for example a television cathode ray tube, a magnetic resonance (NMR) measuring device or the risk of "demagnetizing" objects, for example computer diskette or magnetic tapes a radio cassette tape or ticket or magnetic strip payment. The engine represented on this Figure 6 is magnet core 53 and has, in addition, an improved efficiency by the implementation of a counter-magnet 60 arranged in magnetic opposition on the anterior extension 59 of the anterior pole piece 52 which allows better channeling the lines of magnetic field to and in the air gap in which is the coil 55 which, at rest, by its rear part intercepts a rear magnetic field and its front part an earlier magnetic field of the air gap split by the groove 50. The field The main magnet of the motor is generated by the magnet 53. The two magnetic poles of opposite polarity (north or south or vice versa) of each magnet have been indicated by P + and P- in order to show that the magnet and against magnet are oriented in opposition. The motor has a central symmetry axis 51 of circular revolution. An orifice may be provided in the center of the engine. The looping of the magnetic field produced by the magnet 53 in the gap is obtained by a rear pole piece 54, of the breech type for this core magnet assembly, which has a forward extension 54 ', 54 "and 54' A pellet 61 of ferromagnetic material disposed in front of the counter magnet 60 also enables the magnetic field to be looped with the end 54 "'of the anterior extension of the posterior pole piece 54 with very small lines of magnetic field leaks. . It can even be expected that the width of the space between the peripheral edge of the pellet 61 and the corresponding edge of 54 "'is smaller than the minimum width of the air gap because only the coil carrier (not shown) is present At this point, however, if the excursion of the reel has to be very large until the level of the pellet 61 is reached, there will be sufficient free width so that the reel can pass unhindered. may omit the anterior pellet 61 and the portion 54 '' of the posterior pole piece.In a second simplified alternative, possibly combined with others, the anterior extension 59 of the anterior pole piece 52 may be omitted and the counterpart Finally, in another simplified mode, possibly combined with others, parts 54 "and 54" of the posterior pole piece can be omitted.

As indicated, the counter-magnet also makes it possible to improve the magnetic field in the gap and, in particular, to tend to the symmetrization, or equalization, of the fields between the posterior field zone in relation with E1 and the anterior field area in relation to E2. Thanks to this improvement, during the optimization of the motor, the difference in width of the air gap between the posterior field area and the anterior field area can be reduced or even eliminated. Indeed, in all configurations, the posterior field area, closer to the magnet, is subject to greater magnetization than the field of the former field. In addition, in the absence of a counter magnet, the anterior field area has a larger spread of field lines. This explains that during the optimization it is necessary to further widen the gap in the posterior field area with respect to the anterior field area in the absence of counter magnetization. On the other hand, with a counter-magnet, the field lines are better channeled in the gap and the magnetization better distributed between the two field zones. Thus, depending on the degree of optimization chosen, it is possible to implement or not a difference in width between the anterior and posterior gap zones as a function of a counter magnet present or not present. It is recalled here that the height of the air gap is measured on an axis substantially parallel to the displacement of the coil and the width of the air gap by the distance between an anterior pole piece edge and a corresponding rear pole piece edge on an axis perpendicular to the height.

Although we have represented and described in detail on the Figures 4 , 5 , 6 a core magnet motor, the invention can be applied in a manner equivalent to a magnet motor crowned. Similarly, if it is shown in detail that an air gap with groove, we can implement the invention with a groove on each of the two gap edges or arrange the groove on the other piece in this case, on the Figures 4 , 5 , 6 , on the posterior polar piece instead of the anterior polar piece.

The present invention is not limited by the described embodiments and it can also implement all magnet type combinations with respect to the shape (crown or core or other) as regards the material ( ferrite, metallic, rare earths or others), or membrane type (dome or cone) can be used. The motor of the invention can also implement means for reducing the magnetic radiation such as, for example a shield, one or against magnets. It may also include one or more sensors for providing information to the amplifier to which it is connected in order to enslave it.

Claims (9)

1. Electrodynamic motor with a moving coil, in particular for a loudspeaker, wherein the moving coil is formed of a winding (5, 15, 25, 35, 55) of a specific number of turns of an electric conductor which are connected to an acoustic diaphragm (6, 16), the motor comprising at least one magnet (3, 13, 23, 33, 53) having two poles and being disposed between a front pole piece (2, 12, 22, 32, 52) and a rear pole piece (4, 4', 14, 14', 24, 34, 34', 54, 54', 54", 54"'), the front pole piece and the rear pole piece enclosing a magnetic field of the magnet in a gap, the gap being delimited on a first side by a first peripheral edge of the front pole piece and on a second side by a second peripheral edge of the rear pole piece, the mobile coil being disposed in the gap, the windings being perpendicular to the magnetic field so that when a current is supplied into the coil, the latter moves along a front-rear axis, the gap being divided into two parts, at least one of the edges of the gap comprising a groove (30, 50), the groove being oriented substantially parallel to the turns, the edge having a first rear surface having a height E1 which is separated from a second front surface having a height E2 by the groove having a height C, the groove forming a zone receding with respect to the first surface, E1 defining a first gap space (37) of a rear magnetic field B1 and E2 defining a second gap space (36) of a front magnetic field, the coil having a height H_B equal to or less than a height E1 + C + E2 of the gap which is divided into two parts, and the groove (30, 50) comprising a continuous and closed ring made from an electrically conductive material,
   characterized in that E1 is substantially equal to E2, the winding of the coil is substantially homogenous, and in that the coil has a height H_B such that H_B is substantially equal to E1 + C or E2 + C or E1/2 + C + E2/2.
2. Motor according to claim 1, characterized in that at rest the coil is disposed facing the groove and intercepts by a first end with NB1 turns the rear magnetic field B1 and by a second end with NB2 turns the front magnetic field in such a way that, along the height, the product B1.NB1 is substantially equal to the product B2.NB2.
3. Motor according to claim 1 or 2, characterized in that the front pole piece is extended toward the front (39, 59).
4. Motor according to any one of claims 1 to 3, characterized in that it comprises toward the front of the front pole piece (32) at least one counter-magnet (60), the counter-magnet having poles oriented in a way opposite to the orientation of the poles of the magnet (3, 13, 23, 33, 53).
5. Motor according to claim 4, characterized in that it further comprises toward the front, on the counter-magnet, a pellet (61) made of a ferromagnetic material, and in that the rear pole piece (54) is extended toward the front (54', 54", 54"') up to the level of the pellet.
6. Motor according to any one of claims 1 to 5, characterized in that the magnet is a core magnet (23, 53), the front pole piece (22, 52) is a substantially flat pellet, the rear pole piece is a yoke (24 ,34, 54) having a U shape with a base on which the magnet rests and upright edges (34' 54', 54", 54"'), the split gap being delimited by the external peripheral edge of the pellet and the adjacent portion of the upright edge of the yoke.
7. Motor according to any one of claims 1 to 5, characterized in that the magnet is a crown magnet (3, 13), the front pole piece is a substantially flat ring (2, 12) having an inner peripheral edge toward the center of the crown, the rear pole piece (4, 14) is formed by a rear plate on which the magnet rests and by a central yoke (4', 14') extending from rear to front, the split gap being delimited by inner peripheral edge of the ring and the adjacent portion of the central yoke.
8. Motor according to any one of the preceding claims, characterized in that the electrically conductive material is selected from gold, silver, copper, aluminum, graphite carbon, in combination or not, and is preferably made of copper.
9. Electrodynamic loudspeaker with a mobile coil, characterized in that it comprises a motor according to any one of the preceding claims.

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