阅读说明：本技术 具有改善线性度的扬声器电机 (Loudspeaker motor with improved linearity ) 是由 拉尔斯·里斯波 卡斯滕·廷加德 莫滕·哈尔沃森 布鲁诺·普兹 于 2020-02-12 设计创作，主要内容包括：本发明涉及一种用于电动扬声器的电机,包括绕电机轴线布置的磁路组件。磁路组件包括：外磁体、导磁顶板、导磁底板、中心极片和用于接收音圈的气隙。气隙由面向中心极片的轴向延伸的周壁部分的导磁顶板的轴向延伸的内壁形成,以限定气隙的宽度、底部、顶部和高度。磁路组件还包括向外突出的导磁构件,该导磁构件布置在气隙的顶部上方。中心极片包括磁性构件,该磁性构件至少从气隙的底部轴向延伸到导磁底部构件或导磁底板。磁性构件表现出小于10的相对交流磁导率,例如小于5或小于2,例如约1,其对应于自由空气的相对交流磁导率。(The invention relates to a motor for an electrodynamic loudspeaker comprising a magnetic circuit assembly arranged about a motor axis. The magnetic circuit assembly includes: the outer magnet, magnetic conduction roof, magnetic conduction bottom plate, central pole piece and the air gap that is used for receiving the voice coil loudspeaker voice coil. The air gap is formed by an axially extending inner wall of the magnetically permeable top plate facing the axially extending peripheral wall portion of the central pole piece to define a width, a bottom, a top and a height of the air gap. The magnetic circuit assembly further comprises an outwardly projecting magnetically permeable member arranged above the top of the air gap. The central pole piece includes a magnetic member extending axially at least from the bottom of the air gap to a magnetically conductive bottom member or plate. The magnetic member exhibits a relative ac permeability of less than 10, such as less than 5 or less than 2, such as about 1, which corresponds to the relative ac permeability of free air.)

1. A motor for an electro-dynamic loudspeaker, comprising:

a magnetic circuit assembly disposed about an axis of the machine, the magnetic circuit assembly comprising:

the voice coil comprises an outer magnet, a magnetic top plate, a magnetic bottom plate, a central pole piece and an air gap for receiving the voice coil;

wherein the air gap is formed by axially extending inner walls of the magnetically permeable top plate facing the axially extending peripheral wall portion of the central pole piece to define a width, a bottom, a top and a height of the air gap;

an outwardly projecting magnetically permeable member disposed over the top of the air gap;

said central pole piece including a magnetic member extending axially at least from said bottom of said air gap to either a magnetically permeable bottom member or said magnetically permeable baseplate; wherein the content of the first and second substances,

the magnetic member exhibits a relative alternating magnetic permeability of less than 10, for example less than 5 or less than 2.

2. A motor for an electrodynamic loudspeaker according to claim 1 wherein the magnetic member of the central pole piece comprises a permanent magnet, such as a neodymium magnet or a ferrite magnet.

3. An electric motor for an electrodynamic loudspeaker according to claim 1, wherein the magnetic member of the central pole piece comprises a magnetically permeable material, such as an isotropic, high resistance Soft Magnetic Composite (SMC) material, driven into direct current magnetic saturation by at least one of a permanent magnet and a field coil.

4. A motor for an electrodynamic loudspeaker according to any one of claims 1 to 3, wherein the magnetic member of the central pole piece extends outwardly to the top of the air gap to define the axially extending peripheral wall portion of the central pole piece.

5. An electric motor for an electrodynamic loudspeaker according to any one of claims 1 to 3, wherein the central pole piece includes a magnetically conductive top member;

the magnetically permeable pole top extends axially from the bottom of the air gap to the top of the air gap to define the axially extending peripheral wall portion of the central pole piece.

6. An electric motor for an electrodynamic loudspeaker according to any one of the preceding claims, wherein the outwardly projecting magnetically permeable member is disposed within an outwardly projecting plane or surface defined by the axially extending peripheral wall portion of the central pole piece.

7. A motor for an electrodynamic loudspeaker according to claims 5 and 6, wherein the magnetically permeable pole tip and the outwardly projecting magnetically permeable member are integrally formed from a single piece of magnetically permeable material, such as a ferromagnetic material, for example CR1010 steel.

8. An electric motor for an electrodynamic loudspeaker according to claim 7 wherein the magnetically conductive pole top comprises a disc or cylindrical element defining the axially extending peripheral wall portion of the central pole piece and the outwardly projecting magnetically conductive member.

9. An electric motor for an electrodynamic loudspeaker according to claim 8 wherein the outwardly projecting magnetically permeable member defines a recessed outer wall relative to the axially extending peripheral wall portion of the central pole piece.

10. An electric motor for an electrodynamic speaker according to claim 6, further comprising a non-magnetic spacer disposed between a top of the central pole piece and the outwardly projecting magnetically permeable member.

11. An electric motor for an electrodynamic loudspeaker according to any one of claims 1 to 6, wherein the outwardly projecting magnetically permeable member is arranged outwardly of an outwardly projecting plane defined by the axially extending inner wall of the magnetically permeable top plate.

12. A motor for an electrodynamic loudspeaker according to any one of the preceding claims, wherein the height of the magnetic member of the central pole piece is at least 0.5 times the height of the outer magnets, for example more than 0.7 or 0.9 times the height of the outer magnets.

13. A motor for a dynamic loudspeaker according to any of the preceding claims, further comprising at least one of:

-an electrically conductive ring arranged below the bottom of the air gap and surrounding the central pole piece; and

-an electrically conductive ring surrounding said outwardly projecting magnetically permeable member, arranged above said top of said air gap and inside said outwardly projecting plane or surface defined by said axially extending peripheral wall portion of said central pole piece.

14. An electrodynamic loudspeaker comprising:

-a frame;

-an electric machine according to any of the preceding claims;

-a movable diaphragm attached to the voice coil; the voice coil is disposed in the air gap of the motor.

15. An electrodynamic loudspeaker according to claim 14, wherein the magnetic circuit assembly is configured such that the change in 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 measured at 31Hz is less than 10%, such as less than 7.5%, or even less than 5%;

wherein the displacement range corresponds to 0.5 times the difference between the height of the voice coil and the height of the air gap.

Technical Field

The present invention relates in one aspect to a motor for a dynamic loudspeaker and in another aspect to a dynamic loudspeaker comprising the motor. The present invention relates in a first aspect to a motor for an electrodynamic loudspeaker, the motor comprising a magnetic circuit assembly arranged about a motor axis. The magnetic circuit assembly includes: the outer magnet, magnetic conduction roof, magnetic conduction bottom plate, central pole piece and the air gap that is used for receiving the voice coil loudspeaker voice coil. The magnetic circuit assembly further comprises an outwardly projecting magnetically permeable member arranged above the top of the air gap. The central pole piece includes a magnetic member extending axially at least from the bottom of the air gap to a magnetically conductive bottom member or plate. The magnetic member exhibits a relative ac permeability of less than 10, such as less than 5 or less than 2, such as about 1, which corresponds to the relative ac permeability of free air.

Background

Electro-dynamic loudspeakers have a motor that converts electrical energy into mechanical motion. The most common principle of operation is the moving coil, where an electrical input or drive current flows in the voice coil of an electro-dynamic loudspeaker. The voice coil is levitated in a permanent magnetic field having a strong radial component. The drive current and the radial magnetic field through the voice coil generate a so-called lorentz force along the axis of the voice coil. The voice coil is typically rigidly attached to the diaphragm or diaphragm of the electro-dynamic loudspeaker. Accordingly, the lorentz force moves the diaphragm based on the outward and inward movement to generate the sound pressure.

The lorentz force or driving force on the diaphragm is the product of the drive current I, the magnetic flux density B in the air gap and the wire length l in the radial magnetic field. More precisely, it is the integral of the radial component of I times B over the voice coil wire length.

This integral is often designated as the BL product or force factor of the motor. Thus, the electric machine transfers (converts) energy in both directions between the electrical and mechanical domains. Thus, the motor also acts as a generator, causing mechanical motion to produce electrical energy. The magnetic field induces a voltage (EMF) in the voice coil that is proportional to the speed of the voice coil and diaphragm assembly. The scale factor is again a force factor. In practice the motor of an electro-dynamic loudspeaker has several distinct non-linear mechanisms that produce undesirable linear and non-linear distortions in the generated sound pressure.

One non-linear distortion mechanism is caused by position/displacement dependent changes in the BL product, such that the B x L product varies with the position of the voice coil in the magnetic gap. The force factor decreases gradually from a maximum at the rest position of the voice coil, typically when the drive current in the voice coil is zero. This first non-linear distortion mechanism is static, i.e. depends only on the position of the voice coil.

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

The force on the voice coil and the graph is no longer strictly proportional to the voice coil current (i.e., the drive current) because the force factor itself has become dependent on the voice coil current. This effect depends on the position of the voice coil, but the non-linearity is present due to the superposition of the two magnetic fields, not due to the movability of the voice coil. According to the description of the problem, the force factor modulation is also referred to as position dependent inductance, magnetic flux modulation and reluctance force. This is described in detail in the AES paper "force factor modulation in electrodynamic loudspeakers" published at 141 th convention on 2016, 29 th month to 3 th month.

The force factor modulation causes a second order nonlinear distortion in the form of a force component proportional to the voice coil current squared:

where L is the position dependent generalized inductance of the coil defined in the AES paper, x is the coil position, and i is the coil current.

In other words, the second order nonlinear distortion is proportional to the square of the voice coil current and the spatial derivative of the coil inductance. The variable voice coil inductance also distorts in another manner. The voice coil inductance is a part of the electrical impedance of the voice coil such that when it is driven by a voltage source (in most cases this is the case), the voice coil current depends in a position dependent manner on the applied drive voltage. In the 2016 AES paper above, it was shown that the force nonlinear component equation can be generalized to include the frequency dependence of the voice coil inductance. As mentioned before, the magnetic circuit acts as a core of the voice coil, which means that when the magnetic permeability of the part of the magnetic circuit is frequency dependent, the voice coil inductance becomes frequency dependent.

The reason for the frequency dependent permeability is that when the voice coil magnetic flux changes, eddy currents flow into all conductive parts or components of the magnetic circuit or system, such as ferrous parts, due to current changes or coil movement. The eddy currents will flow in a manner that counteracts the magnetic flux variations (lenz's law) -or in other words, the eddy currents act as short-circuited coil turns, thereby reducing the inductance of the voice coil.

Since the electrical conductivity of the material in which the eddy currents flow is limited, the current is reduced when the coil flux remains stationary for a period of time, i.e., at dc or 0hz and very low frequencies, there are no eddy currents to counteract the inductance. Thus, the dc voice coil inductance is determined entirely by the geometry and permeability of the magnetic circuit material. At higher frequencies, eddy currents become more pronounced, thereby reducing the inductance to a level lower than at dc.

Some prior art electro-dynamic loudspeakers have included so-called shorting rings surrounding the pole pieces and voice coil. The rings are made of an electrically conductive but non-magnetic material, such as copper or aluminum. The aim is to reduce the inductance of the coil at least at higher frequencies. Due to the lower resistivity of copper or aluminum compared to iron, most eddy currents flow in the short circuit ring, not in iron. For the same reason, the eddy currents are also larger, thus more strongly counteracting the magnetic field variations that the voice coil attempts to induce or generate in the magnetic circuit. This reduces the force factor modulation at least at higher frequencies. Further side benefits include reduced inductance, which means greater sensitivity to a given voltage applied to the voice coil, and reduced non-linear inductance caused by hysteresis in the iron. This does not mean that the short-circuiting rings, no matter how placed, will unconditionally improve the linearity. Since force factor modulation is equivalent to the position dependence of the generalized voice coil inductance, at elevated frequencies it is likely to reduce the inductance while increasing the spatial gradient (absolute change per millimeter of motion) of the inductance. At low frequencies, the prior art shorting ring has no effect. The lower the frequency of the desired effect, the larger the cross-section of the shorting ring must be at the lower frequencies, which becomes too large for the amount of space available within the magnetic circuit of a practical loudspeaker.

The present inventors have realized that if the motor and magnetic circuit of an electro-dynamic loudspeaker are designed or configured such that the voice coil inductance is independent of displacement/position, both non-linear distortion due to force factor modulation and non-linear distortion due to voice coil current modulation are cancelled. Thus, an ideal motor for an electro-dynamic loudspeaker has a voice coil inductance that does not vary with voice coil displacement, i.e., is position independent.

It is therefore an object or an object of the present invention to provide an electrodynamic loudspeaker motor which substantially eliminates the detrimental displacement dependence of the voice coil inductance, or at least significantly reduces the displacement/position dependence of the voice coil inductance, compared to prior art loudspeaker motors. For the reasons mentioned above, this reduction will improve the linearity of the motor and thereby reduce several types of non-linear distortion of the electrodynamic loudspeaker. Thereby improving the objective and subjective tone quality of the speaker.

Disclosure of Invention

A first aspect of the invention relates to a motor for an electrodynamic loudspeaker comprising:

a magnetic circuit assembly disposed about the motor axis. The magnetic circuit assembly may include: the outer magnet, magnetic conduction roof, magnetic conduction bottom plate, central pole piece and the air gap that is used for receiving the voice coil loudspeaker voice coil. The air gap is formed by an axially extending inner wall of the magnetically permeable top plate facing the axially extending peripheral wall portion of the central pole piece to define a width, a bottom, a top and a height of the air gap. The magnetic circuit assembly further comprises an outwardly projecting magnetically permeable member arranged above the top of the air gap. The central pole piece includes a magnetic member extending axially at least from the bottom of the air gap to a magnetically conductive bottom member or plate. The magnetic member exhibits a relative ac permeability of less than 10, such as less than 5 or less than 2, such as about 1, which corresponds to the relative ac permeability of free air.

In the present description, the term "alternating magnetic permeability" of the magnetic member refers to the slope of the tangent of the curve/graph of the magnetic flux density B versus the magnetic field strength H at zero voice coil current. The term "relative alternating magnetic permeability" mu_rIs referred to as magnetic vacuum permeability μ₀"alternating magnetic permeability" of a multiple of. The tangent line can be viewed as a linearized small signal or alternating current model of the dc operating point around the magnetic component. The slope of the tangent line is the permeability of the small-signal model of the magnetic component, i.e. the "alternating permeability" of the magnetic component. At larger magnetic field strengths, e.g. above 1.5 tesla, the B-H curve becomes flatter, which means that the ac permeability decreases with material saturation of the magnetic member. Permanent magnets are highly magnetically saturated in nature and therefore typically have an ac permeability that is not much greater than air. Neodymium magnets may exhibit a relative ac permeability of less than 1.5 or less than 1.1.

Thus, the small ac permeability of the magnetic member in combination with the outwardly projecting magnetically permeable member provides a synergistic effect by significantly reducing the increase in the voice coil inductance upon inward displacement of the voice coil, and by arranging the outwardly projecting magnetically permeable member above the top of the air gap, additionally compensating for the increase in the small remaining voice coil inductance. This geometry ensures that the voice coil inductance also increases at approximately the same rate as the inductance increases when the voice coil is displaced outwardly, thus making the displacement-related changes in the voice coil inductance very small, as discussed in further detail below with reference to the figures.

As mentioned above, the magnetic member of the central pole piece may comprise a permanent magnet, such as a neodymium magnet or a ferrite magnet, which is highly magnetically saturated in nature. Alternatively, the magnetic members of the central pole piece may comprise magnetically permeable material, such as isotropic, high resistance Soft Magnetic Composite (SMC) material, which is driven into direct current magnetic saturation by at least one of the permanent magnets and the field coils.

The outwardly projecting magnetically permeable member may be disposed generally inside or outside of an outwardly projecting plane or surface defined by the axially extending peripheral wall portion of the central pole piece, as discussed in further detail below with reference to the figures (e.g., in connection with the motor embodiments of fig. 2 and 4).

In one embodiment of the electrical machine, the central pole piece comprises a magnetically permeable top member extending axially from the bottom of the air gap to the top of the air gap, thereby forming or defining an axially extending peripheral wall portion of the central pole piece. The outwardly projecting magnetically permeable member may be disposed on top of and integrally formed with the magnetically permeable top member or provided as a separate element, in conjunction with or adjacent to the top surface of the magnetically permeable top member, as discussed in further detail below with reference to the figures. The magnetically permeable top member and/or the outwardly projecting magnetically permeable member may be formed of or comprise a highly magnetically permeable material, for example a ferromagnetic material, such as AISI CR1010 steel or an isotropic, high resistance Soft Magnetic Composite (SMC) material, as will be discussed in further detail below with reference to the drawings.

According to one embodiment of the electric machine, the outer magnet comprises an annular permanent magnet coaxially arranged around a cylindrical central pole piece centered on the machine axis.

According to another embodiment of the motor, the height of the outwardly protruding magnetically permeable member exceeds the height of the magnetically permeable top plate, e.g. 1.5 times the height of the magnetically permeable top plate.

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

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

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

-a frame for supporting the frame,

-a motor according to any embodiment of the motor described above and/or any embodiment of the motor described below in connection with the figures. The electrodynamic loudspeaker further comprises a movable diaphragm or membrane attached to the voice coil, wherein the voice coil is arranged in, e.g. freely suspended in, the air gap of the motor.

The magnetic circuit assembly of the electrodynamic loudspeaker is preferably configured such that the change in inductance of the voice coil within a predetermined displacement range of the voice coil defined by the outward displacement limit and the inward displacement limit, measured at 31Hz, is less than 10%, such as less than 7.5%, or even less than 5%; wherein the displacement range corresponds to 0.5 times the difference between the height of the voice coil and the height of the air gap. Those skilled in the art will recognize that the outward displacement limits and the inward displacement limits may be symmetric about the resting or neutral position of the voice coil. The magnetic circuit assembly of the electrodynamic loudspeaker is preferably configured such that the variation of the inductance of the voice coil within the predetermined displacement range also falls within the same percentage limit at one or more additional test frequencies selected from the group consisting of: 1Hz, 100Hz, 316Hz, 1kHz and 3.16 kHz.

Drawings

Preferred embodiments of the present invention are described in further detail below with reference to the attached drawing figures, wherein:

figure 1 is a schematic cross-sectional view of a motor of a prior art electro-dynamic loudspeaker,

figure 2 is a schematic axial cross-sectional view of a motor for an electro-dynamic loudspeaker according to a first embodiment of the invention,

figure 3 is a schematic axial cross-sectional view of a motor for an electro-dynamic loudspeaker according to a second embodiment of the present invention,

figure 4 is a schematic axial cross-sectional view of a motor for an electro-dynamic loudspeaker according to a third embodiment of the invention,

figure 5 is a schematic axial cross-sectional view of a motor for an electro-dynamic loudspeaker according to a fourth embodiment of the present invention,

figure 6 is a schematic axial cross-sectional view of a motor for an electro-dynamic loudspeaker according to a fifth embodiment of the present invention,

figure 7 is a schematic axial cross-sectional view of a motor for an electro-dynamic loudspeaker according to a sixth embodiment of the present invention,

figure 8 is a schematic axial cross-sectional view of a motor for an electro-dynamic loudspeaker according to a seventh embodiment of the invention,

figure 9 shows a schematic axial cross-sectional view of an exemplary electro-dynamic loudspeaker incorporating a motor according to any one of the above-described embodiments of the motor,

figure 10 shows a graph of voice coil inductance at 1Hz versus inward and outward displacement for an exemplary motor design or structure according to the present invention simulated by finite element analysis,

figure 11 shows a graph of voice coil inductance versus inward displacement and outward displacement at different frequencies for an exemplary motor design or structure according to the present invention simulated by finite element analysis,

fig. 12 shows a graph of B x L product versus inward displacement and outward displacement for an exemplary motor design or structure according to the present invention simulated by finite element analysis; and

fig. 13 shows a graph of magnetic field lines at the air gap of a magnetic circuit assembly of a motor design or structure according to an exemplary embodiment of the present invention simulated by finite element analysis.

Detailed Description

Various exemplary embodiments of the motor of the present invention for an electrodynamic loudspeaker are described below with reference to the accompanying drawings. It will be appreciated by persons skilled in the art that the drawings are diagrammatic and simplified for clarity and that details which are essential to an understanding of the invention are shown, while other details are omitted. Throughout this application, like reference numerals refer to like elements or components. Therefore, similar elements or components need not be described in detail with respect to each figure. It will also be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to the described order is not actually required.

Fig. 1 is a schematic cross-sectional view of a motor 100 of a prior art electro-dynamic loudspeaker. The voice coil inductance is proportional to the square of the number of windings of the voice coil and inversely proportional to the reluctance of the magnetic circuit. The number of windings is fixed, wherein the reluctance of the magnetic circuit may change with the displacement of the voice coil, and therefore the diaphragm attached to the position of the voice coil may also be displaced due to the change in the amount of material having a higher magnetic permeability than air near the voice coil. The other description mode is as follows: the effective permeability is modulated by the displacement of the voice coil. The voice coil surrounds the central iron pole piece. When the voice coil is moved inward toward the base plate, i.e., from the negative X position (-X) from the rest position 0 shown 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. In contrast, when the voice coil is moved outward in the diaphragm direction of the motor, i.e., the positive X (+ X) position moved outward from the rest position 0 as shown in the drawing, the voice coil inductance is low because the voice coil moves in free air exhibiting low magnetic permeability.

Fig. 2A shows a schematic axial cross-sectional view of a motor 200 for an electrodynamic loudspeaker (not shown) according to a first embodiment of the invention. The size of the electro-dynamic loudspeaker may be a so-called 6.5 inch size with a diaphragm diameter of about 120 mm. The present motor 200 and its magnetic circuit assembly, as well as other motor embodiments discussed below, are configured or designed such that displacement-related changes in the inductance of the voice coil are minimized or reduced as compared to prior art speaker motors. Thus, for the reasons discussed in detail above in the background section of the invention, both the non-linear distortion due to the force factor modulation and the non-linear distortion due to the voice coil current modulation are minimized.

The motor 200 may be rotationally symmetric about a central motor axis 205 of the motor 200. The motor 200 includes a magnetic circuit assembly configured to generate a radially oriented substantially static magnetic field in the annular air gap 220. The magnetic circuit assembly includes an outer annular permanent magnet 240, a magnetically permeable top plate 235, e.g., formed as an annular disk, a magnetically permeable bottom plate or yoke 230, and a central pole piece 245. Air gap 220 is configured to receive an annular or ring-shaped voice coil 225, which may form part of a diaphragm assembly of an electro-dynamic loudspeaker. An annular or ring-shaped voice coil 225 is freely suspended in the annular air gap 220 and is therefore movable along the central motor axis 205 outwardly away from the magnetic circuit assembly and inwardly into the magnetic circuit assembly about the voice coil's rest position 0. The rest position corresponds to zero dc current in the voice coil 225 and preferably corresponds to a centered position of the voice coil 225 in the air gap 220. The rest position of the annular voice coil 225 is schematically indicated by "0" on the arrow "X" in the drawing, whereas an outward displacement of the voice coil 225 away from the magnetic circuit assembly corresponds to the positive/+ direction of X, whereas an inward displacement of the voice coil 225 into the magnetic circuit assembly corresponds to the negative/-direction of X.

Magnetically permeable top plate 235 may be formed of a highly magnetically permeable material, such as a ferromagnetic material (e.g., CR1010 steel), and has a height between one-sixth and two-thirds of the height of annular voice coil 225. The magnetically conductive bottom plate or yoke 230 may be formed of a highly magnetically conductive material, such as a ferromagnetic material (e.g., AISI CR1010 steel), and has a height or thickness between 4mm and 16mm, depending on the external dimensions of the motor 200.

The central pole piece or central pole assembly includes a magnetic member 250 extending from the bottom 220b of the air gap 220 to a magnetically permeable bottom member 245, which magnetically permeable bottom member 245 may be formed as an upwardly projecting cylindrical protrusion 245 integrally formed with the magnetically permeable bottom plate or yoke 230. The magnetically permeable base member 245 is physically and magnetically coupled to the lower surface of the magnetic member 250. Thus, the magnetic member 250 in this embodiment of the motor 200 is disposed between the magnetically permeable pole top 210 (which may be a flat disk) and the magnetically permeable bottom member 245. In other embodiments of the magnetic circuit assembly, the magnetic member 250 may extend axially from the bottom 220b of the air gap 220 all the way to the magnetically conductive bottom plate or yoke 230. The height of the magnetic member 250 is preferably at least 0.5 times the height of the annular permanent magnet 240, for example, more than 0.7 times or 0.9 times the height of the annular permanent magnet 240. Alternatively or additionally, the height of the magnetic member 250 is greater than the difference between the height of the voice coil and the height of the air gap 220. Each of these limitations will generally ensure that the height of the magnetic member 250 is large enough to significantly reduce the inductance of the voice coil upon inward displacement, as the amount of magnetically permeable material within the voice coil is reduced.

The top 210 of the magnetically permeable pole extends axially from the bottom 220B (see fig. 2B)) of the air gap 220 to the top 220a of the air gap 220 to define an axially extending peripheral wall portion 236 of the central pole piece 245 that forms an inner wall (e.g., circular or oval) or surface of the air gap 220. The opposing walls of the air gap 220 are formed by axially extending inner walls (e.g., circular or oval) 242 of the magnetically permeable top plate 235, wherein the axially extending walls 242 face axially extending peripheral wall portions of the central pole piece 245 to define the width, bottom 220b, top 220a and height of the air gap 220. Those skilled in the art will appreciate that the height and/or width of the air gap 220 may be scaled according to the overall size of the motor 200 and voice coil 225. The magnetically permeable pole tip 210 may be formed from a highly magnetically permeable material, such as a ferromagnetic material, for example, AISI CR1010 steel.

The magnetically permeable pole top 210 includes an outwardly projecting portion or projection 215 or "cap" 215 disposed above the top 220a of the air gap 220, i.e., externally. Thus, in this embodiment, the outwardly projecting portion or protrusion 215 is also disposed above the upper planar surface 237 of the magnetically permeable top member 235. An outwardly projecting "cap" 215 is disposed inwardly of an outwardly projecting plane or surface (not shown) defined by the axially extending peripheral wall portion 217 of the central pole piece 245, i.e., toward the central motor axis 205. Thus, unlimited axial displacement of the voice coil 225 is allowed.

Thus, the magnetically permeable pole top 210 can include a first cylindrical portion or segment 212 that defines the aforementioned inner wall (axially extending peripheral wall portion) 236 of the air gap 220. The magnetically permeable pole tip 210 of the central pole piece 245 also includes the above-described outwardly projecting protrusion 215, and in this embodiment, this protrusion 215 is formed by the second cylindrical portion of the magnetically permeable pole tip 210, is disposed on top of the first cylindrical portion 212, and is integrally formed therewith, or is provided as a separate element bonded to or abutting the top surface of the first cylindrical portion 212. Those skilled in the art will appreciate that the outwardly projecting protrusion 215 need not be cylindrical. The first and second cylindrical portions 212, 215, respectively, of the magnetically permeable pole tip 210 may be integrally formed, such as by milling or machining a suitably shaped cylindrical ferrite member or other high magnetic permeability material, such as AISI CR1010 steel or an isotropic, high resistance Soft Magnetic Composite (SMC) material, such as fromAB manufactured and soldMaterials, examplesSuch as Somaloy1P, Somaloy3P or Somaloy 5P. The cross-sectional area of the second cylindrical portion 215 can be smaller than the cross-sectional area of the first cylindrical portion or segment 212 to define a recessed upper outer circular wall 215A relative to the inner wall 236 of the permeable pole tip 210, the recessed upper outer circular wall 215A defining the inner surface or wall 236 of the magnetic gap 220. In other words, the outwardly protruding protrusion 215 extends outwardly in the axial direction 205 of the motor 200 above the magnetic gap 220.

In certain alternative embodiments, first cylindrical portion 212 and second cylindrical portion 215 may each have the same diameter to eliminate the concave nature of upper outer circular wall 215A.

The magnetic member 250 may exhibit a relative ac permeability of less than 10, such as less than 5 or less than 2. In certain embodiments, the magnetic member 250 comprises or is formed from a permanent magnet, such as a neodymium magnet or a ferrite magnet. In other embodiments of the electric machine 200, as discussed in further detail below, the magnetic member 250 comprises a magnetically permeable material, such as an isotropic, high-resistance Soft Magnetic Composite (SMC) material, that is driven into direct current magnetic saturation by at least one of the permanent magnets and the field coils. SMC materials may include those discussed aboveA material.

Each of the outer annular permanent magnet 240 and the magnetic member 250 is axially magnetized as schematically shown by the magnetic field lines used to drive the magnetic flux through the magnetic circuit assembly and across the air gap, which thus carries a radially oriented magnetic field. The outer ring permanent magnet 240 may comprise a ferrite magnet or a neodymium magnet.

The arrangement of the magnetically conductive, outwardly projecting protrusion or cap 215 increases the inductance of the voice coil 225 upon outward displacement, i.e., the positive "X" value of the voice coil 225, such that the increase in inductance effectively offsets or compensates for the increased inductance of the voice coil 225 upon inward displacement thereof.

The reduced cross-sectional area of the magnetically permeable cap 215 directs the DC magnetic flux of the magnetic circuit assembly, i.e., the static DC magnetic flux, to flow in the air gap 220. This feature ensures that the DC magnetic flux is concentrated in the air gap 220 and that the magnetic field strength in the magnetically conducting cap 215 is low. This feature, in turn, ensures that magnetically permeable cap 215 is not affected by magnetic saturation, resulting in a high permeability and more effectively compensating for the displacement dependent inductance l (x) of voice coil 225.

In contrast, the magnetic member 250 disposed below the bottom 220b of the air gap 220, e.g., having an upper end surface substantially aligned with the bottom 220b of the air gap 220, preferably exhibits or has a small relative alternating current permeability as described above to reduce the displacement dependence of the voice coil inductance. As will be explained below, a small ac relative permeability can be achieved in several ways, for example by means of high dc or static magnetic saturation, for example by using permanent magnets or by using soft magnetic materials, for example ferromagnetic materials driven into dc saturation by permanent magnets or magnetic field coils. In both cases, the ac relative permeability may be very small, e.g. below 10 or below 5.

The above-described increase in the voice coil inductance at inward displacement of the voice coil 225 is caused, on the one hand, by a decrease in the distance from the voice coil 225 to the magnetically conductive bottom plate or yoke 230 comprising the upwardly projecting cylindrical protrusion 245. Another significant contribution to increasing the voice coil inductance at the inward displacement of the voice coil 225 in prior art motor designs is the high permeability of the ferromagnetic material of the center pole piece.

Those skilled in the art will appreciate that the combined nature of the magnetic member 250 and the magnetically permeable cap 215 largely eliminates or at least significantly reduces such undesirable increases in voice coil inductance at inward displacement of the voice coil 225 of the present motor 200. The small AC relative permeability of the magnetic member 250 (which may be comparable to free air, i.e., μ, in some embodiments)_r1.0) at least reduces the presence of magnetically permeable material inside voice coil 225 at the inward displacement. When voice coil 225 is fully pulled inward, the voice coil inductance is still at its maximum because magnetically permeable top member 210 and yoke 235 still help to shorten the magnetic field lines compared to free air. However, it is critical that the voice coil inductance be significantly reduced compared to designs with a magnetically conductive center pole piece near the coil.

Thus, the magnetic member 250 and magnetically permeable cap 215 provide a synergistic effect by first significantly reducing the voice coil inductance at inward displacement of the voice coil 225 by the magnetic member 250, and additionally compensating for a small residual voice coil inductance increase at inward displacement by disposing the magnetically permeable cap 215 over the top of the air gap 220, so that the voice coil inductance also increases at outward displacement of the voice coil 225. In other words, the combination of the magnetically permeable cap 215 with the magnetic member 250 in the central pole piece 245 enables it to achieve this precisely thanks to its low ac permeability.

Fig. 3 shows a schematic axial cross-sectional view of a motor 300 for an electrodynamic loudspeaker (not shown) according to a second embodiment of the invention. The outwardly projecting magnetically permeable member 315 is supported by a non-magnetic spacer 343 disposed between the top surface of the magnetically permeable pole top 310 of the central pole piece 345 and the magnetically permeable cap 315. Even though magnetically conductive cap 315 is not directly physically or magnetically coupled to the center pole piece, its high magnetic permeability still compensates for the displacement-related inductance of voice coil 325 at the outward displacement or position for the reasons described above.

Fig. 4 shows a schematic axial cross-sectional view of a motor 400 for an electrodynamic loudspeaker (not shown) according to a third embodiment of the invention. The magnetically conductive top plate 435 of the magnetic circuit assembly includes an annular magnetically conductive disk-like protrusion 415. The annular magnetically permeable disk 415 may be integrally formed with the magnetically permeable top plate 435. The inner circular peripheral wall 415a of the annular magnetically permeable disk 415 is disposed outside of the outwardly projecting plane defined by the inner axially extending wall 436 of the magnetically permeable pole tip 410. Even if the annular magnetically permeable cap 415 is disposed entirely outside of the outwardly projecting plane defined by the inner axially extending wall 436 and thus outside of the voice coil 425, its proximity and high permeability still compensate for the displacement-related inductance of the voice coil 425 at the outward displacement or position for the reasons described above.

Fig. 5 shows a schematic axial cross-sectional view of a motor 500 for an electrodynamic loudspeaker (not shown) according to a fourth embodiment of the invention. The electric machine 500 is substantially the same as the electric machine 200 according to the first embodiment discussed above, but additionally comprises a first electrically conductive ring 547, which is arranged below the bottom of the air gap 520 and surrounds the central pole piece. The center pole includes a magnetic member 500 and magnetically permeable pole top 510 and magnetically permeable bottom member 545. Motor 500 can further include a second conductive ring 546, second conductive ring 546 being positioned on the outwardly directed surface of magnetically permeable pole top 510 and surrounding outwardly projecting magnetically permeable cap 515. Thus, the second electrically conductive ring 546 is disposed above the top of the air gap 520 and inside the outwardly projecting plane or surface defined by the axially extending peripheral wall portion 517 of the center pole piece. Each of the first and second conductive rings 547 and 546 functions as a so-called short-circuit ring, and is preferably made of an electrically conductive but non-magnetically conductive material, such as copper or aluminum. For the above reasons, the shorting rings 547, 546 have the advantage of reducing the increase in voice coil impedance at higher frequencies (e.g., above 10 Hz) by reducing eddy currents flowing in the magnetic circuit assembly.

Fig. 6 shows a schematic axial cross-sectional view of a motor 600 for an electrodynamic loudspeaker (not shown) according to a fifth embodiment of the invention. The central pole piece of motor 600 includes a magnetic member 650, which magnetic member 650 extends axially from the top of air gap 620 all the way to a magnetically conductive bottom plate or yoke 630. The central pole piece 645 of the present magnetic circuit does not have the previously discussed magnetically permeable pole tips 210, 310, 410, 510. Thus, in this motor embodiment 600, the axially extending peripheral wall portion 636 of the magnetic member 650 forms an inner (e.g., circular or oval) axially extending peripheral wall portion 636 of the central pole piece. The opposing walls of the air gap 620 are formed by an inner (e.g., circular or oval) axially extending wall portion 642 of the magnetically permeable top plate 635 such that the two axially extending wall portions collectively define the dimensions of the air gap 620. The magnetic member 650 is preferably formed of a dc magnetically saturated isotropic, high resistance Soft Magnetic Composite (SMC) material as previously discussed. The outer ring magnet 635 drives the magnetic member 650 into dc magnetic saturation by generating an appropriate magnetic flux such that the relative ac permeability of the member 650 is preferably less than 5 or less than 2. The motor 600 includes an outwardly projecting magnetically permeable member 615 supported by and preferably bonded to the upper surface of the magnetic member 650 and disposed above the top of the magnetic gap 620. In a variation of the present motor embodiment 600, the magnetic member 650 comprises an axially extending, i.e. along the axis 605, through opening or hole (not shown) for reducing the effective cross-sectional area of the magnetic member 650 by, for example, more than 30% or 50%. This axially extending through opening or hole of the magnetic member 650 may be used to facilitate dc magnetic saturation of the material (e.g., SMC material previously discussed) of the magnetic member 650.

Fig. 7 shows a schematic axial cross-sectional view of a motor 700 for an electrodynamic loudspeaker (not shown) according to a sixth embodiment of the invention. The central pole piece 745 of the motor 700 includes an annular cylindrical magnetic member 750 disposed about the central motor axis 705. The annular cylindrical magnetic member 750 is made of a magnetically permeable material, preferably an isotropic high resistance Soft Magnetic Composite (SMC) material as discussed above. The central pole piece additionally includes a disk-shaped permanent magnet 750a, such as a neodymium magnet, that extends from the bottom of the air gap 720 down to the top surface of the annular cylindrical magnetic member 750. The disc-shaped permanent magnet 750a is configured to drive the annular cylindrical magnetic member 750 into direct current magnetic saturation. The dc magnetic saturation of the annular cylindrical magnetic member 750 provides a small relative ac permeability of the annular cylindrical magnetic member 750, for example less than 5 or less than 2. The axially oriented through hole 751 of the annular cylindrical magnetic member 750 reduces the effective cross-sectional area of the magnetic member 750 and thus helps to induce proper dc magnetic saturation in the magnetic member 750.

The center pole piece of the present magnetic circuit additionally includes a magnetically permeable pole top 710 that radially conducts and directs magnetic flux through an air gap 725. The magnetically permeable pole top 710 is preferably integrally formed with an outwardly projecting and recessed portion or protrusion 715 or "cap" disposed over the top of the air gap 720 in a manner similar to the first embodiment of the invention described above, i.e., the outer.

Fig. 8 shows a schematic axial cross-sectional view of a motor 800 for an electrodynamic loudspeaker (not shown) according to a seventh embodiment of the invention. The central pole piece of the motor 800 includes an annular cylindrical magnetic member 850 arranged about a central motor axis 805. The annular cylindrical magnetic member 850 is made of a magnetically permeable material, preferably an isotropic high resistance Soft Magnetic Composite (SMC) material as discussed above. The upper top surface of the annular cylindrical magnetic member 850 is disposed at the bottom of the air gap 820 and extends axially downward to the bottom member 845 of the central pole piece. The bottom member 845 of the central pole piece may be integrally formed with the magnetically conductive bottom plate or yoke 830. The center pole piece has an axially oriented through hole or bore 851 extending through the bottom plate or yoke 830, the annular cylindrical magnetic member 850, and through the magnetically permeable pole top 810. The electric machine 800 additionally includes an annular or toroidal magnetic field coil 850b that carries a suitable direct current, at least during operation of the electric machine 800, to generate a direct current or static magnetic field through the magnetic circuit assembly. The magnetic field and magnetic flux generated by the toroidal field coil 850b are configured to drive the toroidal cylindrical magnetic member 850 into direct current magnetic saturation. Thus, the toroidal field coil 850b has substantially the same purpose as the disk-shaped permanent magnet 750a of the sixth embodiment of the present motor discussed previously. The dc magnetic saturation of the annular cylindrical magnetic member 850 provides a small relative ac permeability of the annular cylindrical magnetic member 850, for example less than 5 or less than 2. The axially oriented through hole 851 of the annular cylindrical magnetic member 850 reduces its effective cross-sectional area and thus helps to induce proper dc magnetic saturation in the magnetic member 850.

Fig. 9 shows a schematic axial cross-sectional view of an exemplary electro-dynamic loudspeaker 979 incorporating a motor according to any of the above-described embodiments of motors 200, 300, 400, 500, 600, 700, and 800. An electro-dynamic loudspeaker 979 or driver generally includes a frame 971 and a diaphragm 975 mounted to a motor 900. The diaphragm 975 is attached or connected to the frame 971 by a flexible surround 976, the flexible surround 976 may include an outer edge 984, the outer edge 984 being bonded or otherwise fixedly attached to a peripheral upwardly-directed circular surface of the frame 971. This causes the diaphragm 975 to vibrate according to the vibration of the voice coil 925. The voice coil 925 may be supported by a hollow cylindrical former 974, the former 974 also being attached to the star 973. The star 973 is a flexible, corrugated support that holds the voice coil 925 in the center of the air gap 920 of the motor 900, while allowing the voice coil 925 to move freely in the upward and downward directions. The star 973 may be attached to the outer surface of the former 974 and a star platform 983 located on an interior portion of the frame 971 by different means (e.g., adhesives). In the embodiments described herein, the frame 971 has a generally circular shape. However, in other embodiments, the frame 971 and other elements of the speaker 979 may be in different forms, such as rectangular or elliptical profiles or forms. The former 974 may be fixedly attached to the inner circular surface area of the diaphragm 975 by an adhesive or other bonding mechanism. The diaphragm 975 may be made of any suitable material with sufficient rigidity and weight, such as fabric, plastic, paper, or lightweight metal. The frame 971 may be made of any suitable material, such as a metallic or non-metallic material.

FIG. 10 shows a graph of voice coil inductance versus inward displacement and outward displacement at 1Hz for a finite element analysis modeling simulation of the motor design 200 according to the first embodiment of the present invention. The x-axis represents the displacement of sound in millimeters relative to its rest or neutral position. The y-axis represents the inductance of the voice coil at 1Hz in mH. The motor structure includes a voice coil 225 having a diameter of 39 mm, a height of 23.7 mm, and a winding or turn of 220. The height of the air gap 220 is 4 mm.

A first graph 1010 represents the simulated inductance of the motor design 200, which includes an outwardly protruding portion or protrusion 215 or "cap" 215 disposed above the top 220a of the air gap 220, i.e., the outer portion. The second graph 1020 represents the simulated inductance of the same motor design 200, but without the magnetically conductive "cap" 215.

As shown in the first graph 1010, the inductance of the voice coil 225 varies by only about 0.06mH/2.45 mH-2.5% for a peak-to-peak displacement range of 10mm near the rest position (x-0). Furthermore, this level of performance can also be achieved at higher frequencies, e.g. 31 Hz. For the present motor design, the 10mm displacement range corresponds to about 0.5 times the difference between the height of the voice coil 225 and the height of the air gap 220. As is evident from the second plot 1020 without the "cap", the inductance change of the voice coil is much greater, about 0.25mH/2.2 mH-11%, for the same peak-to-peak displacement range of 10mm near the rest position (X-0).

Fig. 11 shows a series of five separate graphs of voice coil inductance versus inward displacement and outward displacement at various frequencies for an exemplary motor design or structure modeled by finite element analysis modeling of a motor design 500 according to a fourth embodiment of the present invention. The motor structure is generally similar to the motor structure described above in connection with fig. 10, but additionally includes first and second electrically conductive shorting rings: the latter loop results in a beneficial reduction in the inductance of the voice coil at higher frequencies (e.g., above 10Hz or 31Hz) for the reasons discussed above. The x-axis represents the displacement of sound in millimeters relative to its rest or neutral position. The y-axis represents the inductance of the voice coil at 1Hz in mH.

The series of graphs of the voice coil inductance include a first graph 1110 simulated at 31Hz, a second graph 1120 simulated at 100Hz, a third graph 1130 simulated at 316Hz, a fourth graph 1140 simulated at 1kHz, and a fifth graph 1150 simulated at 3.16 kHz. As is evident from each of these voice coil inductance plots, the change in voice coil inductance is very small for all frequencies tested. For example, at 31Hz, the inductance change is about 2% -3% for a peak-to-peak displacement range of 10mm of voice coil 225 about the rest position (x ═ 0). In addition, substantially similar performance levels are also achieved at higher frequencies such as 316Hz, 1kHz and 3.16 kHz.

Fig. 12 shows a graph of the B x L product versus inward displacement and outward displacement simulated by finite element analysis modeling of the motor design 200 according to the first embodiment of the invention. The x-axis represents the displacement of sound in millimeters relative to its rest or neutral position. The y-axis represents the force factor. The first graph 1210 represents a simulated B x L product of the motor design 200, which includes an outwardly protruding portion or protrusion 215 or "cap" 215 disposed above the top 220a of the air gap 220, i.e., the outer portion. The second graph 1220 represents the simulated B x L product of the same motor design 200, but without the magnetically conductive "cap" 215.

Fig. 13 shows a graph of AC magnetic field lines at the air gap 220 of the magnetic circuit assembly of the motor design or structure 200 according to the first embodiment of the invention simulated by finite element analysis. To avoid the effects of eddy currents, simulations were performed at very low frequencies (e.g., 1 μ Hz). The graph shows that the AC field lines that pass through the voice coil 220 (i.e., the AC field lines generated by the voice coil 225 rather than the permanent magnets 240) do so twice in opposite directions. These lines of magnetic force thus counteract the lorentz force of the voice coil. The result of this mechanism is that the derivative of the voice coil inductance L' (x) is very close to zero.