Patent Description:
An electrodynamic loudspeaker has a motor that converts electrical energy into mechanical motion. The most common operating principle is a moving-coil, wherein an electrical input or drive current flows in a voice coil of the electrodynamic loudspeaker. The voice coil is suspended in a permanent magnetic field with a strong radial component. The drive current through the voice coil and the radial magnetic field produce a so-called Lorentz force along an axis of the voice coil. The voice coil is typically rigidly attached to a diaphragm or membrane of the electrodynamic loudspeaker. The Lorentz force thereby displaces the diaphragm in an outwardly and inwardly based motion to create sound pressure.

The Lorentz force or drive force on the diaphragm is the product of the drive current I, flux density B in the air gap and a length of the wire I that is inside the radial magnetic field. More accurately, it is I times the integral of the radial component of B over the length of the wire of the voice coil.

This integral is often designated as the BI product or force factor of the motor. The motor accordingly transduces (converts) energy in both directions between electrical and mechanical domains. Consequently, the motor also acts as a dynamo so that mechanical motion produces electrical energy. The magnetic field induces a voltage (EMF) in the voice coil being proportional to a velocity of the voice coil and the diaphragm assembly. The proportionality factor is again the force factor. Practical motors of electrodynamic loudspeakers possess several pronounced non-linear mechanisms which produces undesired linear and non-linear distortions in the generated sound pressure.

One non-linear distortion mechanism is caused by a position/displacement-dependent variation of the Bl-product such that the B*I product varies with the position of the voice coil in the magnetic gap. The force factor falls off gradually from a maximum that is typically found at the rest position of the voice coil at zero drive current in the voice coil. This first non-linearity distortion mechanism is static, i.e. only depends on the position of the voice coil.

Another dynamic non-linear distortion mechanism also exists. The drive current in the voice coil creates its own magnetic field in response to the flow of current. Part of the generated magnetic field by the voice coil circulates through the magnetic circuit, i.e. the voice coil behaves as a cored inductor with the magnetic circuit acting as the core. The magnetic flux generated by the voice coil current is superimposed on the permanent magnetic flux in the magnetic gap such that the magnetic flux in the magnetic gap varies with coil current in an undesirable manner.

The force on the voice coil and diagram is no longer strictly proportional to the voice coil current, i.e. drive current, since the force factor itself has become dependent on the voice coil current. This effect depends on the position of the voice coil but non-linearity exists because of the superposition of the two magnetic fields, not because of the movability of the voice coil. Depending on how the problem is described, force factor modulation is also known as position-dependent inductance, flux modulation and reluctance force. It is described in detail in <NPL>.

The force-factor modulation causes a <NUM>nd order non-linear distortion in the form of a force component proportional to the voice coil current squared: <MAT> where L is the position dependent generalized inductance of the coil as defined in the subject AES paper, x is the coil position and i is the coil current.

In other words, the <NUM>nd order non-linear distortion is proportional to the voice coil current squared and the spatial derivative of the coil inductance. The variable voice coil inductance also produces distortion in yet another way. The voice coil inductance is part of the electrical impedance of the voice coil such that when it is driven by a voltage source (as it is in the vast majority of cases), the voice coil current becomes dependent on the applied drive voltage in a position dependent manner. It is shown in the above-mentioned <NUM> AES paper that the equation for the non-linear component of the force can be generalized to include frequency dependency of the voice coil inductance. As mentioned before, the magnetic circuit acts as a core for the voice coil which means that the voice coil inductance becomes frequency dependent when the permeability of parts of the magnetic circuit is frequency dependent.

The cause of frequency dependent permeability is the introduction of eddy currents which flow in all parts or member of the magnetic circuit or system that are electrically conductive, such as iron parts, when the voice coil magnetic flux changes, either because current changes or because the coil moves. The eddy currents will flow in such a manner as to counteract changes in magnetic flux (Lenz's law) - or stated alternatively, the eddy currents act as shorted coil turns which reduce the inductance of the voice coil.

Because the conductivity of the materials in which those eddy current flows is finite, the current will die down when the coil flux remains static for some time, i.e. there are no eddy currents to counteract the inductance at DC, or <NUM>, and at very low frequencies. Consequently, the voice coil inductance at DC is solely determined by the geometry and permeability of the materials of the magnetic circuit. At higher frequencies the eddy currents become more pronounced so as to reduce inductance below that found at DC.

Certain prior art electrodynamic loudspeakers have included so-called shorting rings around the pole piece and the voice coil. These rings are made of an electrically conductive, but non-magnetic material, such as copper or aluminum. The aim is to reduce the voice coil inductance, at least at higher frequencies. Thanks to the lower resistivity of copper or aluminum compared to iron, most of the eddy currents flow in the shorting rings instead of in the iron. For the same reason the eddy currents are also larger, and therefore more strongly counteract the magnetic field variation that the voice coil tries to induce or create in the magnetic circuit. This reduces force factor modulation, at least at higher frequencies. Further side benefits include reduced inductance, meaning higher sensitivity for a given voltage applied across the voice coil, and a reduction in the non-linear inductance caused by magnetic hysteresis in the iron. That does not mean that a shorting ring, however placed, will unconditionally improve linearity. Since force factor modulation is equivalent to a position dependency of the generalized voice coil inductance, it is quite possible, at elevated frequencies, to reduce the inductance whilst at the same time driving up the spatial gradient of that inductance (absolute change per millimeter of motion). At low frequencies, the prior art shorting rings have no effect. The lower the frequency at which an effect is desired, the greater the section of the shorting ring has to be at lower frequencies that section becomes too large for the amount of space available inside the magnetic circuit of a practical loudspeaker.

The present inventors have realized that if the motor and magnetic circuit of an electrodynamic loudspeaker is designed, or configured, such that the voice coil inductance is displacement/position-independent, then non-linear distortion due to the force factor modulation and non-linear distortion due to voice coil current modulation are both eliminated. Therefore, an ideal motor for an electrodynamic loudspeaker exhibits a voice coil inductance that does not change with displacement of the voice coil, i.e. it is position independent.

<CIT> discloses an audio speaker with a motor structure in which a shorting ring that sinks eddy currents also serves as a permanent fixture to provide coaxial alignment of two or more components within the speaker.

<CIT> discloses a loudspeaker with a magnet circuit which includes a center plate, first and second magnets disposed respectively on a lower surface and an upper surface of the center plate and magnetized to be repulsive with each other.

Document <CIT> discloses an electromagnetic transducer motor structure comprising: a yoke; a magnet and plate stack of a plurality of components including, a first permanent magnet magnetically coupled to the yoke, a first magnetically conductive plate magnetically coupled to the first permanent magnet(s) and defining a first magnetic air gap with the yoke; and a first shorting ring fixture coupled to, and coaxially aligning, a set of fixtured components, wherein the set of fixtured components includes at least two of the yoke, the first permanent magnet, and the first plate.

Consequently, one aim or objective of the present invention is to provide an electrodynamic loudspeaker motor which substantially eliminates the harmful displacement dependency of the voice coil inductance or at least markedly reduces displacement/position dependency of the voice coil inductance compared to prior art loudspeaker motors. This reduction will improve linearity of the motor and thereby reduce several types of non-linear distortion of the electrodynamic loudspeaker for the reasons described above. Thus, improving the objective and subjective sound quality of the loudspeaker.

A first aspect of the invention relates to a motor for an electrodynamic loudspeaker, comprising:
a magnetic circuit assembly arranged about a motor axis. The magnetic circuit assembly comprises: an outer magnet, a magnetically permeable top plate, a magnetically permeable bottom plate, a center pole piece and an air gap for receipt of a voice coil. The air gap is formed by an inner axially extending wall of the magnetically permeable top plate facing an axially extending peripheral wall section of the center pole piece to define a width, a bottom, a top and height of the air gap. The magnetic circuit assembly additionally comprises outwardly projecting magnetically permeable member arranged above the top of the air gap. The center pole piece comprises a magnetic member extending axially from at least the bottom of the air gap to a magnetically permeable bottom member or to the magnetically permeable bottom plate. The magnetic member exhibits a relative AC magnetic permeability smaller than <NUM>, such as smaller than <NUM> or smaller than <NUM>, such as about <NUM> which corresponds to the relative AC magnetic permeability of free air.

In the present specification, the term "AC magnetic permeability" of the magnetic member refers to a slope of a tangent of a curve/plot of flux density, B, versus magnetic field strength, H, at zero voice coil current. The term "relative AC magnetic permeability", µr, refers to the "AC magnetic permeability" expressed as a multiple of the magnetic vacuum permeability µ<NUM>. The tangent can be viewed as a linearized small signal, or AC, model around a DC operating point of the magnetic member. The slope of the tangent is the permeability of the small-signal model of the magnetic member, i.e. the "AC magnetic permeability" of the magnetic member. At large magnetic field strengths, for example above <NUM> Tesla, this B-H curve becomes flatter, meaning that the AC magnetic permeability decreases as the material of the magnetic member saturates. A permanent magnet is by nature highly magnetically saturated and therefore typically possesses an AC magnetic permeability that is not much larger than that of air. Neodymium magnets may exhibit a relative AC magnetic permeability below <NUM> or below <NUM>.

Hence, the small AC magnetic permeability of the magnetic member in combination with the outwardly projecting magnetically permeable member provides a synergistic effect by markedly reducing the increase of the voice coil inductance at inwards displacements of the voice coil, and additionally compensating a small residual voice coil inductance increase by the arrangement of the outwardly projecting magnetically permeable member above the top of the air gap. This geometry ensures that the voice coil inductance also increases at outwards displacement of the voice coil in nearly the same proportion as the inductance increases at inwards displacement of the voice coil, hence making the displacement dependent variation of inductance of the voice coil extremely small as discussed in additional detail below with reference to the appended drawings.

The magnetic member of the center pole piece may comprise a permanent magnet such as a Neodymium magnet or a Ferrite magnet which by nature are highly magnetically saturated as discussed above. Alternatively, the magnetic member of the center pole piece may comprise magnetically permeable material, such as an isotropic, high resistive Soft Magnetic Composite (SMC) material, driven into DC magnetic saturation by at least one of: a permanent magnet and a field coil.

The outwardly projecting magnetically permeable member may generally be arranged inside, or outside, an outwardly projecting plane or surface defined by the axially extending peripheral wall section of the center pole piece as discussed in additional detail below with reference to the appended drawings - for example in connection with the motor embodiments of <FIG> and <FIG>.

In one embodiment of the motor, the center pole piece comprises a magnetically permeable top member which is extending axially from the bottom of the air gap to the top of the air gap and thereby forms or defines the axially extending peripheral wall section of the center pole piece. The outwardly projecting magnetically permeable member may be arranged on top of the magnetically permeable top member and either integrally formed therewith or provided as a separate element bonded or abutted to a top surface of the magnetically permeable top member as discussed in additional detail below with reference to the appended drawings. The magnetically permeable top member and/or the outwardly projecting magnetically permeable member may formed by or comprise, a highly permeable material, e.g. a ferromagnetic material such as AISI CR1010 steel or an isotropic, high resistive Soft Magnetic Composite (SMC) material discussed in additional detail below with reference to the appended drawings.

According to one embodiment of the motor, the outer magnet comprises an annular permanent magnet co-axially arranged around a cylindrical center pole piece centered about the motor axis.

According to another embodiment of the motor, a height of the outwardly projecting magnetically permeable member exceeds a height of the magnetically permeable top plate for example <NUM> times the height of the magnetically permeable top plate.

According to another embodiment of the motor, a height of the magnetic member of the center pole piece is larger than a difference between a height of the voice coil and the height of the air gap.

Additional embodiments of the invention are set out in the below-appended dependent patent claims.

A second aspect of the invention relates to an electrodynamic loudspeaker comprising:.

The magnetic circuit assembly of the electrodynamic loudspeaker is preferably configured such that a variation of inductance of the voice coil over a predetermined displacement range of the voice coil defined by an outward displacement limit and an inward displacement limit is less than <NUM> %, such as less than <NUM> %, or even less than <NUM> %, measured at <NUM>; wherein said displacement range corresponds to <NUM> times a difference between a height of the voice coil and a height of the air gap. The skilled person will appreciate that the outward and inward displacement limits may be symmetrical about a rest or neutral position of the voice coil. The magnetic circuit assembly of the electrodynamic loudspeaker is preferably configured such that also the variation of inductance of the voice coil over the predetermined displacement range falls within the same percentage limits at one or more additional test frequencies selected from a group of: <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

Preferred embodiments of the invention are described below in additional detail in connection with the appended drawings, in which:.

In the following, various exemplary embodiments of the present motor for an electrodynamic loudspeaker are described with reference to the appended drawings. The skilled person will understand that the accompanying drawings are schematic and simplified for clarity and therefore merely show details which are essential to the understanding of the invention, while other details have been left out. Similar reference numerals refer to like elements or components throughout the application. Similar elements or components will therefore not necessarily be described in-detail with respect to each figure.

<FIG> is a schematic cross-sectional view of a prior art motor <NUM> of an electrodynamic loudspeaker. The voice coil inductance is proportional to the number of windings of the voice coil squared, and inversely proportional to the magnetic reluctance of the magnetic circuit. The number of windings is fixed, wherein the reluctance of the magnetic circuit may change with displacement of the voice coil, and therefore also displacement of the diaphragm attached to the voice coil position due to varying amount of materials with higher permeability than air that are close to the voice coil. Another way to describe it: the effective permeability is modulated by the displacement of the voice coil. The voice coil surrounds a central iron pole piece. When the voice coil is inwardly displaced towards the bottom plate, i.e. negative X position (-X) from the rest position <NUM> as indicated on the drawing, the voice coil inductance is high due to the increased amount of iron material inside the voice coil. This position of the voice coil increases the effective permeability. Conversely, the voice coil inductance is low when the latter is displaced outwardly in the direction of the diaphragm of the motor, i.e. positive X (+X) position from the rest position <NUM> as indicated on the drawing, since the voice coil moves out in the free air which exhibits a low magnetic permeability.

<FIG> shows a schematic axial cross-sectional view of a motor <NUM> for an electrodynamic loudspeaker (not shown) in accordance with a first embodiment of the invention. The size of the electrodynamic loudspeaker may be so-called <NUM> inch dimension with a diaphragm diameter of about <NUM>. The present motor <NUM> and its magnetic circuit assembly, as well as the other motor embodiments discussed below, are configured or designed such that the displacement-dependent variation of inductance of the voice coil is minimized or reduced compared to prior art loudspeaker motors. Consequently, non-linear distortion due to force factor modulation and non-linear distortion due to voice coil current modulation are both minimized for the reasons discussed in detail above in the background of the invention section.

The motor <NUM> may be rotationally symmetrical about a central motor axis <NUM> of the motor <NUM>. The motor <NUM> comprises a magnetic circuit assembly which is configured to generate a radially oriented essentially static magnetic field in a ring-shaped air gap <NUM>. The magnetic circuit assembly comprises an outer annular permanent magnet <NUM>, a magnetically permeable top plate <NUM>, e.g. formed as an annular disc, a magnetically permeable bottom plate or yoke <NUM> and a center pole piece <NUM>. The air gap <NUM> is configured for receipt of a ring-shaped or annular voice coil <NUM>, which may form part of a diaphragm assembly of the electrodynamic loudspeaker. The annular or ring-shaped voice coil <NUM> is suspended freely in the ring-shaped air gap <NUM> and therefore displaceable along the central motor axis <NUM> outwardly away from the magnetic circuit assembly and inwardly into the magnetic circuit assembly about a rest position <NUM> of the voice coil. The rest position corresponds to DC zero current in the ring-shaped voice coil <NUM>, and preferably corresponds to a centered position of the ring-shaped voice coil <NUM> in the air gap <NUM>. The rest position of the ring-shaped voice coil <NUM> is schematically indicated by "<NUM>" on the "X" arrow of the drawing, while the outward displacement of the voice coil <NUM> away from the magnetic circuit assembly corresponds to positive/+ direction of X, and inward displacement of the voice coil <NUM> into the magnetic circuit assembly corresponds to negative/- direction of X.

The magnetically permeable top plate <NUM> may be formed from a highly permeable material, e.g. a ferromagnetic material such as CR1010 steel and a have height between one-sixth and two-thirds of the height of the ring shaped voice coil <NUM>. The magnetically permeable bottom plate or yoke <NUM> may be formed from a highly permeable material, e.g. a ferromagnetic material such as AISI CR1010 steel, and a have height or thickness between <NUM> and <NUM> depending on the outer dimensions of the motor <NUM>.

The center pole piece or center pole assembly comprises a magnetic member <NUM> which extends from a bottom 220b of the air gap <NUM> to a magnetically permeable bottom member <NUM> which may be formed as an upwardly projecting cylindrical protrusion <NUM> integrally formed with the magnetically permeable bottom plate or yoke <NUM>. The magnetically permeable bottom member <NUM> is physically and magnetically coupled to the lower surface of the magnetic member <NUM>. Hence, the magnetic member <NUM> in the present embodiment of the motor <NUM> is arranged in-between a magnetically permeable pole top <NUM>, which may be a flat disc, and the magnetically permeable bottom member <NUM>. In other embodiments of the magnetic circuit assembly, the magnetic member <NUM> may extend axially all the way from the bottom 220b of the air gap <NUM> to the magnetically permeable bottom plate or yoke <NUM>. The height of the magnetic member <NUM> is preferably at least <NUM> times the height of the annular permanent magnet <NUM>, for example more than <NUM> times, or <NUM> times the height of the annular permanent magnet <NUM>. Alternatively, or additionally, the height of the magnetic member <NUM> is larger than a difference between a height of the voice coil and the height of the air gap <NUM>. Each of these limitations will typically ensure that the height of the magnetic member <NUM> is sufficiently large to markedly reduce the inductance of the voice coil at inward displacements because of the reduction of the amount of magnetically permeable material inside the voice coil.

The magnetically permeable pole top <NUM> is extending axially from a bottom 220b of the air gap <NUM> (refer to <FIG>)) to a top 220a of the air gap <NUM> to define an axially extending peripheral wall section <NUM> of the center pole piece <NUM> which forms an inner, e.g. circular or elliptical, wall or surface of the air gap <NUM>. The opposing wall of the air gap <NUM> is formed by an inner, e.g. circular or elliptical, axially extending wall <NUM> of the magnetically permeable top plate <NUM>, wherein the axially extending wall <NUM> is facing the axially extending peripheral wall section of the center pole piece <NUM> so as to define a width, a bottom 220b, a top 220a and height of the air gap <NUM>. The skilled person will appreciate that the height and/or width of the air gap <NUM> may be scaled according to the overall dimensions of the motor <NUM> and voice coil <NUM>. The magnetically permeable pole top <NUM> may formed from a highly permeable material e.g. a ferromagnetic material such as AISI CR1010 steel.

The magnetically permeable pole top <NUM> comprises an outwardly projecting portion or protrusion <NUM> or "hat" <NUM> arranged above, i.e. outwardly of, the top 220a of the air gap <NUM>. Hence, in the present embodiment, the outwardly projecting portion or protrusion <NUM> is also arranged above an upper flat surface <NUM> of the magnetically permeable top member <NUM>. The outwardly projecting "hat" <NUM> is arranged inside, i.e. towards the central motor axis <NUM>, an outwardly projecting plane or surface (not shown) defined by the axially extending peripheral wall section <NUM> of the center pole piece <NUM>. Hence, allowing unrestricted axial displacement of the voice coil <NUM>.

The magnetically permeable pole top <NUM> may therefore comprise a first cylindrical portion or section <NUM> that defines the above-discussed inner wall (axially extending peripheral wall section) <NUM> of the air gap <NUM>. The magnetically permeable pole top <NUM> of the center pole piece <NUM> additionally comprises the above-mentioned outwardly projecting protrusion <NUM>, which in the present embodiment is formed by a second cylindrical portion of the magnetically permeable pole top <NUM>, arranged on top of the first cylindrical portion <NUM> and either integrally formed therewith or provided as a separate element bonded or abutted to a top surface of the first cylindrical portion <NUM>. The skilled person will appreciate that the outwardly projecting protrusion <NUM> need not be cylindrical. The first and second cylindrical portions <NUM>, <NUM>, respectively, of the magnetically permeable pole top <NUM> may be integrally formed - for example by milling or machining a suitably shaped cylindrical Ferrite member or other highly magnetically permeable material such as AISI CR1010 steel or an isotropic, high resistive Soft Magnetic Composite (SMC) material like Somaloy® material such as Somaloy 1P, Somaloy 3P or Somaloy 5P manufactured and sold by Höganäs AB. A cross-sectional area of the second cylindrical portion <NUM> may be smaller than a cross-sectional area of the first cylindrical portion or section <NUM> to define a recessed upper outer circular wall 215a relative to the inner wall <NUM> of the magnetically permeable pole top <NUM>, which defines the inner surface or inner wall <NUM> of the magnetic gap <NUM>. In other words, the outwardly projecting protrusion <NUM> extends outwards above the magnetic gap <NUM> in the axial direction <NUM> of the motor <NUM>.

In certain alternative embodiments, the first and the second cylindrical portions <NUM>, <NUM>, respectively, may have identical diameters to eliminate the recessed properties of the upper outer circular wall 215a.

The magnetic member <NUM> may exhibit a relative AC magnetic permeability smaller than <NUM>, such as smaller than <NUM>, or smaller than <NUM>. In certain embodiments, the magnetic member <NUM> comprises, or is formed by, a permanent magnet such as a Neodymium magnet or a Ferrite magnet. In other embodiments of the motor <NUM> as discussed in additional detail below, the magnetic member <NUM> comprises a magnetically permeable material, for example an isotropic, high resistive Soft Magnetic Composite (SMC) material, which material is driven into DC magnetic saturation by at least one of: a permanent magnet and a field coil. The SMC material may comprise the above-discussed Somaloy® material.

Each of the outer annular permanent magnets <NUM> and the magnetic member <NUM> are axially magnetized as schematically illustrated by the magnetic field lines, which are used to drive magnetic flux through the magnetic circuit assembly and across the air gap which therefore carries a radially oriented magnetic field. The outer annular permanent magnets <NUM> may comprise a Ferrite magnet or Neodymium magnet.

The arrangement of the magnetically permeable outwardly projecting protrusion or hat <NUM> increases the inductance of the voice coil <NUM> at outwards displacement, i.e. positive "X" values, of the voice coil <NUM>, such that the increase of inductance is effectively counteracting, or compensating for, the increased inductance of the voice coil <NUM> at inwards displacements thereof.

The reduced cross-sectional area of the magnetically permeable hat <NUM> directs the DC magnetic flux, i.e. static DC magnetic flux, of the magnetic circuit assembly to flow in the air gap <NUM>. This feature ensures that the DC magnetic flux is focused in the air gap <NUM> and that the magnetic field strength is low in the magnetically permeable hat <NUM>. This feature in turn ensures that the magnetically permeable hat <NUM> is kept out of magnetic saturation leading to a high permeability and a more effective compensation of the displacement dependent inductance L(x) of the voice coil <NUM>.

In contrast, the magnetic member <NUM> which is arranged below the bottom 220b of the air gap <NUM>, e.g. having an upper end surface substantially aligned with the bottom 220b of the air gap <NUM>, preferably exhibits or possesses a small relative AC magnetic permeability as specified above in order to reduce the displacement dependency of the voice coil inductance. The small AC relative magnetic permeability can be achieved in several ways, for example by means of high DC or static magnetic saturation e.g. by the use of a permanent magnet or using a soft magnetic material such as ferromagnetic material driven into DC saturation by a permanent magnet or field coil as explained below. In both cases the AC relative magnetic permeability may be very small, e.g. below <NUM> or below <NUM>.

The above-mentioned increase of the voice coil inductance at inwards displacements of the voice coil <NUM> is caused at one hand by the reduced distance from the voice coil <NUM> to the magnetically permeable bottom plate or yoke <NUM> including the upwardly projecting cylindrical projection <NUM>. Another significant contribution to the increase of voice coil inductance in prior art motor designs at inwards displacements of the voice coil <NUM> is the high magnetic permeability of ferromagnetic material of the center pole piece.

The skilled person will appreciate that the combined properties of the magnetic member <NUM> and the magnetically permeable hat <NUM> largely eliminate, or at least markedly reduce, this undesired increase of the voice coil inductance at inwards displacements of the voice coil <NUM> of the present motor <NUM>. The small AC relative magnetic permeability of the magnetic member <NUM>, which in some embodiments may be comparable to free air, i.e. µr =<NUM>, at least reduces the presence of magnetically permeable material inside the voice coil <NUM> at inwards displacements. The voice coil inductance may still be at its maximum when the voice coil <NUM> is fully drawn inwards, because the magnetically permeable top member <NUM> and yoke <NUM> still help to shorten the magnetic field lines compared to free air. Crucially though, that voice coil inductance is markedly reduced compared to the design with the magnetically permeable center pole piece near to the coil.

Hence, the magnetic member <NUM> and the magnetically permeable hat <NUM> provide a synergistic effect by firstly markedly reduce the voice coil inductance at inward displacements of the voice coil <NUM> by the magnetic member <NUM>, and in addition compensate the small residual voice coil inductance increase at inward displacements by the arrangement of the magnetically permeable hat <NUM> above the top of the air gap <NUM>, such that the voice coil inductance also increases at outwards displacement of the voice coil <NUM>. In other words, to combine the magnetically permeable hat <NUM> with the magnetic member <NUM> in the center pole piece <NUM> which thanks to its low AC magnetic permeability, makes it amenable for precisely this purpose.

<FIG> shows a schematic axial cross-sectional view of a motor <NUM> for an electrodynamic loudspeaker (not shown) in accordance with a second embodiment of the invention. An outwardly projecting magnetically permeable member <NUM> is supported by a non-magnetic spacer <NUM> disposed in-between a top surface of a magnetically permeable pole top <NUM> of the center pole piece <NUM> and magnetically permeable hat <NUM>. Even though the magnetically permeable hat <NUM> is not directly physically or magnetically coupled to the center pole piece its high magnetic permeability still compensates for the displacement-dependent inductance of the voice coil <NUM> at outwards displacements or positions for the reasons discussed above.

<FIG> shows a schematic axial cross-sectional view of a motor <NUM> for an electrodynamic loudspeaker (not shown) in accordance with a third embodiment of the invention. A magnetically permeable top plate <NUM> of the magnetic circuit assembly comprises an annular magnetically permeable disc-like protrusion <NUM>. The annular magnetically permeable disc <NUM> may be integrally formed with the magnetically permeable top plate <NUM>. An inner circular peripheral wall 415a of the annular magnetically permeable disc <NUM> is arranged outside an outwardly projecting plane defined by the inner axially extending wall <NUM> of the magnetically permeable pole top <NUM>. Even though the annular magnetically permeable hat <NUM> is arranged entirely outside of the outwardly projecting plane defined by the inner axially extending wall <NUM> and therefore outside the voice coil <NUM>, its proximity and high magnetic permeability still compensates the displacement dependent inductance of the voice coil <NUM> at outwards displacements or positions for the reasons discussed above.

<FIG> shows a schematic axial cross-sectional view of a motor <NUM> for an electrodynamic loudspeaker (not shown) in accordance with a <NUM>th embodiment of the invention. The motor <NUM> is largely identical to the previously discussed motor <NUM> according to the first embodiment, but additionally comprises a first electrically conductive ring <NUM> arranged below the bottom of the air gap <NUM> and surrounding the center pole piece. The center pole comprises a magnetic member <NUM> and a magnetically permeable pole top <NUM> and a magnetically permeable bottom member <NUM>. The motor <NUM> may furthermore comprise a second electrically conductive ring <NUM> resting on an outwards oriented surface of the magnetically permeable pole top <NUM> and surrounding the outwardly projecting magnetically permeable hat <NUM>. Hence, the second electrically conductive ring <NUM> is arranged above a top of the air gap <NUM> and inside an outwardly projecting plane or surface defined by the axially extending peripheral wall section <NUM> of the center pole piece. Each of the first and second electrically conductive rings <NUM>, <NUM> operates as the so-called shorting rings and are preferably made from an electrically conductive, but non-magnetically permeable, material such as copper or aluminum. The benefit of the shorting rings <NUM>, <NUM> is a reduction in the increase of the voice coil impedance at higher frequencies, e.g. above <NUM> by reduction of eddy currents flowing in the magnetic circuit assembly for the reasons discussed above.

<FIG> shows a schematic axial cross-sectional view of a motor <NUM> for an electrodynamic loudspeaker (not shown) in accordance with a <NUM>th embodiment of the invention. The center pole piece of the motor <NUM> comprises a magnetic member <NUM> which extends axially all the way from a top of the air gap <NUM> to a magnetically permeable bottom plate or yoke <NUM>. The center pole piece <NUM> of the present magnetic circuit lacks the previously discussed magnetically permeable pole tops <NUM>, <NUM>, <NUM>, <NUM>. Hence, in the present motor embodiment <NUM> it is an axially extending peripheral wall section <NUM> of the magnetic member <NUM> which forms an inner, e.g. circular or elliptical, axially extending peripheral wall section <NUM> of the center pole piece. The opposing wall of the air gap <NUM> is formed by an inner, e.g. circular or elliptical, axially extending wall section <NUM> of the magnetically permeable top plate <NUM> such that these two axially extending wall sections jointly define dimensions of the air gap <NUM>. The magnetic member <NUM> is preferably formed by the previously discussed DC magnetically saturated isotropic, high resistive Soft Magnetic Composite (SMC) material. The outer annular magnet <NUM> drives the magnetic member <NUM> into DC magnetic saturation by generating an appropriate magnetic flux such that the relative AC magnetic permeability of the member <NUM> preferably is smaller than <NUM> or smaller than <NUM>. The motor <NUM> comprises an outwardly projecting magnetically permeable member <NUM> which is supported by, and preferably bonded to, an upper surface of the magnetic member <NUM> and arranged above a top of the magnetic gap <NUM>. In one variant of the present motor embodiment <NUM>, the magnetic member <NUM> comprises an axially extending, i.e. along axis <NUM>, through-going opening or aperture (not shown) which serves to reduce an effective cross-sectional area of the magnetic member <NUM> for example by more than <NUM> % or <NUM> %. This axially extending through-going opening or aperture of the magnetic member <NUM> may serve to facilitate DC magnetic saturation of the material of the the magnetic member <NUM> such as the previously discussed SMC material.

<FIG> shows a schematic axial cross-sectional view of a motor <NUM> for an electrodynamic loudspeaker (not shown) in accordance with a <NUM>th embodiment of the invention. The center pole piece <NUM> of the motor <NUM> comprises an annular cylindrical magnetic member <NUM> arranged about a central motor axis <NUM>. The annular cylindrical magnetic member <NUM> is fabricated from a magnetically permeable material, preferably the isotropic high resistive Soft Magnetic Composite (SMC) material discussed above. The center pole piece additionally comprises a disc shaped permanent magnet 750a, for example a Neodymium magnet, which is from a bottom of the air gap <NUM> and extends down to a top surface of the annular cylindrical magnetic member <NUM>. The disc shaped permanent magnet 750a is configured to drive the annular cylindrical magnetic member <NUM> into DC magnetic saturation. The DC magnetic saturation of the annular cylindrical magnetic member <NUM> provides a small relative AC magnetic permeability, such as smaller than <NUM>, or smaller than <NUM>, of the annular cylindrical magnetic member <NUM>. The axially oriented through going aperture <NUM> of the annular cylindrical magnetic member <NUM> reduces the effective cross-sectional area of the magnetic member <NUM> and therefore helps to provokes appropriate DC magnetic saturation in the magnetic member <NUM>.

The center pole piece of the present magnetic circuit additionally comprises a magnetically permeable pole top <NUM> which conducts and directs magnetic flux radially through the air gap <NUM>. The magnetically permeable pole top <NUM> is preferably integrally formed with an outwardly projecting, and recessed, portion or protrusion <NUM> or "hat" arranged above, i.e. outwardly of, the top of the air gap <NUM> in a similar manner as the first embodiment of the invention discussed above.

<FIG> shows a schematic axial cross-sectional view of a motor <NUM> for an electrodynamic loudspeaker (not shown) in accordance with a <NUM>th embodiment of the invention. The center pole piece of the motor <NUM> comprises an annular cylindrical magnetic member <NUM> arranged about a central motor axis <NUM>. The annular cylindrical magnetic member <NUM> is fabricated in a magnetically permeable material, preferably the isotropic high resistive Soft Magnetic Composite (SMC) material discussed above. An upper top surface of the annular cylindrical magnetic member <NUM> is arranged at the bottom of air gap <NUM> and extends axially down to a bottom member <NUM> of the center pole piece. The bottom member <NUM> of the center pole piece may be integrally formed with a magnetically permeable bottom plate or yoke <NUM>. The center pole piece has an axially oriented thoroughgoing aperture or hole <NUM> which extends through the bottom plate or yoke <NUM>, the annular cylindrical magnetic member <NUM> and through a magnetically permeable pole top <NUM>. The motor <NUM> additionally comprises an annular, or toroid, field coil 850b, which carries a suitable DC current, at least during operation of the motor <NUM>, to generate a DC or static magnetic field through the magnetic circuit assembly. The magnetic field and flux generated by the annular field coil 850b is configured to drive the annular cylindrical magnetic member <NUM> into DC magnetic saturation. Hence, the annular field coil 850b serves essentially the same purpose as the disc shaped permanent magnet 750a of the previously discussed <NUM>th embodiment of the present motor. The DC magnetic saturation of the annular cylindrical magnetic member <NUM> provides a small relative AC magnetic permeability, such as smaller than <NUM>, or smaller than <NUM> of the annular cylindrical magnetic member <NUM>. The axially oriented through going aperture <NUM> of the annular cylindrical magnetic member <NUM> reduces its effective cross-sectional area and therefore helps to provokes appropriate DC magnetic saturation in the magnetic member <NUM>.

<FIG> shows a schematic axial cross-sectional view of an exemplary electrodynamic loudspeaker <NUM> incorporating a motor according to any of the above-mentioned embodiments of the motor <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. The electrodynamic loudspeaker <NUM> or driver generally comprises a frame <NUM> mounted to the motor <NUM> and a diaphragm <NUM>. The diaphragm <NUM> is attached or connected to the frame <NUM> through a flexible surround <NUM> which may comprise an outer rim <NUM> which is glued or otherwise fixedly attached to a peripheral upwards oriented circular surface of the frame <NUM>. This provides for the diaphragm <NUM> to vibrate in accordance with vibrations of the voice coil <NUM>. The voice coil <NUM> may be supported by a hollow, cylindrical-shaped former <NUM> which is also attached to a spider <NUM>. The spider <NUM> is a flexible, corrugated support that holds the voice coil <NUM> centered in the air gap <NUM> of the motor <NUM> while allowing the voice coil <NUM> to move freely in upward and downward directions. The spider <NUM> may be connected to an outer surface of the former <NUM> and to a spider plateau <NUM> located on the interior part of the frame <NUM> by different means such as adhesives. The frame <NUM> has a generally circular shape in the embodiment described herein. However, in other embodiments, the frame <NUM>, and other elements of the loudspeaker <NUM>, may be of a different form, e.g. a rectangular or elliptical outline or form. The former <NUM> may be fixedly attached to an inner circular surface area of the diaphragm <NUM> by adhesives or other bonding mechanisms. The diaphragm <NUM> can be made of any suitable material with sufficient rigidity and weight such as fabric, plastic, paper or lightweight metal. The frame <NUM> can be made of any suitable material such as metal or non-metal materials.

<FIG> shows a plot of voice coil inductance at <NUM> versus inwards and outwards displacement of as simulated by Finite Element Analysis modelling of the motor design <NUM> according to the first embodiment of the invention. The x-axis shows the voice displacement relative to its rest or neutral position in millimeters. The y-axis shows the inductance of the voice coil at <NUM> in mH. The present motor construction included a voice coil <NUM> with a diameter of <NUM>, a height of <NUM> and <NUM> turns or windings. The height of the air gap <NUM> is <NUM>.

A first plot <NUM> represents the simulated inductance of the motor design <NUM> which includes the magnetically permeable outwardly projecting protrusion or "hat" <NUM> arranged above, i.e. outwardly of, the top 220a of the air gap <NUM>. A second plot <NUM> represents the simulated inductance of the same motor design <NUM>, but without the magnetically permeable "hat" <NUM>.

As evident from first plot <NUM> for a peak-peak displacement range of <NUM> of the voice coil about the rest position (x=<NUM>), the inductance variation of the voice coil <NUM> is merely about <NUM> mH/<NUM> mH = <NUM> %. Furthermore, this level of performance is also achieved at somewhat higher frequencies such as <NUM>. The <NUM> displacement range corresponds to about <NUM> times the difference between the height of the voice coil <NUM> and the height of the air gap <NUM> for the present motor design. As evident from second plot <NUM> without the "hat" for the same peak-peak displacement range of <NUM> about the rest position (X=<NUM>), the inductance variation of the voice coil is much larger and about <NUM> mH/<NUM> mH = <NUM> %.

<FIG> shows a series of five individual plots of the voice coil inductance at various frequencies versus inwards and outwards displacement of an exemplary motor design or structure as simulated by Finite Element Analysis modelling of the motor design <NUM> according to the <NUM>th embodiment of the invention. This motor construction is overall similar to the above-mentioned motor construction in connection with <FIG>, but additionally comprises the first and second electrically conductive shorting rings: The latter rings cause a beneficial reduction of voice coil inductance at higher frequencies, e.g. above <NUM> or <NUM>, for the reasons discussed above. The x-axis shows the voice displacement relative to its rest or neutral position in millimeters. The y-axis shows the inductance of the voice coil at <NUM> in mH.

The series of plots of the voice coil inductance includes a first plot <NUM> simulated at <NUM>, a second plot <NUM> simulated at <NUM>, a third plot <NUM> simulated at <NUM>, a fourth plot <NUM> simulated at <NUM> and a fifth plot <NUM> simulated at <NUM>. As evident from each of these voice coil inductance plots, the variation of the inductance of the voice coil is very small for all test frequencies. For example, at <NUM> the inductance variation is about <NUM>-<NUM> % for a peak-peak displacement range of <NUM> of the voice coil <NUM> about the rest position (x=<NUM>). Furthermore, a substantially similar level of performance is also achieved at the higher frequencies such as at <NUM>, <NUM> and <NUM>.

<FIG> shows plots of the B*I products versus inward and outward displacement as simulated by Finite Element Analysis modelling of the motor design <NUM> according to the first embodiment of the invention. The x-axis shows the voice displacement relative to its rest or neutral position in millimeters. The y-axis shows the force factor. A first plot <NUM> represents the simulated B*I product of the motor design <NUM> which includes the magnetically permeable outwardly projecting protrusion or "hat" <NUM> arranged above, i.e. outwardly of, the top 220a of the air gap <NUM>. A second plot <NUM> represents the simulated B*I product of the same motor design <NUM>, but without the magnetically permeable "hat" <NUM>.

Claim 1:
A motor (<NUM>) for an electrodynamic loudspeaker, comprising:
a magnetic circuit assembly arranged about a motor axis (<NUM>), comprising:
an outer magnet (<NUM>), a magnetically permeable top plate (<NUM>), a magnetically permeable bottom plate (<NUM>), a center pole piece (<NUM>) and an air gap (<NUM>) for receipt of a voice coil (<NUM>);
wherein the air gap (<NUM>) is formed by an inner axially extending wall (<NUM>) of the magnetically permeable top plate (<NUM>) facing an axially extending peripheral wall section (<NUM>) of the center pole piece (<NUM>) to define a width, a bottom, a top and height of the air gap (<NUM>); characterized in that said magnetic circuit assembly comprises:
an outwardly projecting, along the motor axis, magnetically permeable member (<NUM>) arranged above the top of the air gap (<NUM>); and
said center pole piece (<NUM>) comprises a magnetic member (<NUM>) extending axially from at least the bottom of the air gap (<NUM>) to a magnetically permeable bottom member or to the magnetically permeable bottom plate (<NUM>); wherein
said magnetic member (<NUM>) exhibits a relative AC magnetic permeability smaller than <NUM>, such as smaller than <NUM> or smaller than <NUM>.