Magnet assembly for an electrodynamic speaker driver, electrodynamic speaker driver including the same, and related electrodynamic speaker

Abstract

The present invention relates to a magnet assembly (2) for an electrodynamic speaker driver, comprising a first outer pair of axially magnetized, permanent ring magnets (3a, 3b) having a first thickness and a second inner pair of axially magnetized, permanent ring magnets (4a, 4b) having a second thickness smaller than the first thickness, wherein a face of the first inner magnet (4a) facing the second inner magnet (4b) is offset by a predefined offset distance (Δy) with respect to a face of the first outer magnet (3a) facing the second outer magnet (3b), and a face of the second inner magnet (4b) facing the first inner magnet (4a) is offset by a predefined offset distance (Δy) with respect to a face of the second outer magnet (3b) facing the first outer magnet (3a).

Description

[Technical field]

[0001] The present invention relates to the field of electrodynamic speakers, and in particular to a magnet assembly for an electrodynamic speaker driver, an electrodynamic speaker driver comprising said magnet assembly, and an electrodynamic speaker comprising said driver. [Background technology]

[0002] A conventional electrodynamic loudspeaker driver comprises an electromagnetic actuator, most often consisting of a winding placed on a moving element in a magnetic field generated by a permanent magnet assembly, the configuration of which presents a so-called radial symmetry between the north and south poles of the permanent magnet assembly. When an amplitude- and frequency-modulated current is passed through the winding, the mechanical displacements induced at audible frequencies are converted into a sound field with a diaphragm acting as an emission surface, also known as an acoustic radiator. The sound quality of an electrodynamic loudspeaker depends on the frequency response curve, which must be as invariant as possible over the entire bandwidth (e.g. from 16 Hz to 20,000 Hz), and on the linearity of the system, as indicated by the presence of minimal harmonic distortion and intermodulation.

[0003] If an electrodynamic loudspeaker supports all frequencies equally, it seems possible, a priori, to ensure the reproduction of the timbre of an instrument, which constitutes useful harmonics. However, the reality is more complicated when one considers the need to adequately reproduce the attack transients of a sound, which represent the acoustic signature of a good instrument. The response of the loudspeaker to the transients is a prerequisite for "fidelity" and can be tested by detecting the "dragging" of the diaphragm as the loudspeaker is subjected to a series of pulses. Inertia and self-induced forces of the moving elements are responsible for this deficiency.

[0004] Acoustic, optical and electrical measurements reveal that there is no such thing as an ideal electrodynamic loudspeaker, and that each design exhibits drawbacks in terms of bandwidth limitations, various resonance peaks and inertia. In principle, the concatenation of several loudspeakers allows many of these drawbacks to be overcome, but on the other hand, the cumulative effect of these drawbacks can sometimes be unacceptable for good quality music reproduction.

[0005] In addition, existing electro-dynamic loudspeakers generally have mediocre electro-acoustic efficiencies, with values ​​ranging from 0.5% to 5%. As an example, a room with a piano that delivers about 150mW requires about 100W to ensure the reproduction of a fortissimo. RMS You need to make a sound with .

[0006] The useful driving force behind the displacement of the moving element arises from the interaction of the induced magnetic field, represented by B, with each length element of the winding carrying a current, represented by i(t). Locally, the elementary force F (in Newtons) exerted on a displacing charge carrier in the induced magnetic field is called the Lorentz force, and is exerted in a direction perpendicular to the plane defined by the magnetic field and the carrier velocity. Evaluation in elementary charge carrying capacity according to the above phenomenon leads to the following equation:

[0007]

number

[0008] All this is as if the unwound length of the winding, denoted l, were exposed to a uniform induced magnetic field, and the force coefficient B l =Bl (unit N / A or Tm).

[0009] This intensity-modulated force acts on a moving element, whose mechanical behavior is determined by three components: the inertial force, i.e. the mass of the moving part (M m ) times the product of the imposed acceleration, f m (unit: N / (m / s) or kg / s) (f mIn most cases, R is used in terms of mechanical resistance. m ), which is generally considered to be proportional to the displacement velocity via m The restoring force associated with the suspension mechanism is assigned a stiffness (units N / m) denoted as . For translation guided on the axis x, the equation of behavior of such an idealized electrodynamic loudspeaker can be written as:

[0010]

number

[0011] This general description of an arbitrary damped oscillator can be found in many physical systems.

[0012] Equation [2], in all its generality, presents on its left side the oscillation control requirements of the system described by the right side. In this sense, the application of the Lorentz force is historically evident in the invention of the loudspeaker, motivated by the intensive development of the telephone (tube amplifiers (also known as electron tubes, thermionic vacuum tubes) had not yet been developed and the concept of electrical impedance was still unknown to those skilled in the art).

[0013] Ideally, whatever the current, the force factor B l =Bl should remain unchanged whatever the position of the moving element during operation.

[0014] Therefore, to get an idea of ​​the quality of the speaker, lIt is useful to plot φ as a function of x, the latter parameter representing the displacement of the diaphragm according to the longitudinal direction of the loudspeaker. Ideally, it would be important to have a flat plot over the entire operating range for optimal reproduction of the required signal. For electrodynamic loudspeakers according to the state of the art, the representation of this plot takes the form of an approximate Gaussian, and numerous developments have attempted to improve this behavior (although success is very relative). The typical layout of the moving element in a conventional loudspeaker shows the close proximity of the coil to the fixed part of the permanent magnet assembly. Such a configuration gives rise to the generation of eddy currents in the fixed conductive part when the moving element is actuated, especially at high frequencies.

[0015] In addition, the general construction of conventional electrodynamic loudspeakers requires the implementation of a magnetic assembly, which generally includes a steel material at its center, to achieve a radial configuration of the magnetic field with respect to the driving part. These materials are relatively inexpensive, however, they lead to high-frequency signal degradation in view of eddy currents resulting from the displacement of the current-carrying windings of the moving element. To compensate for this, various inventors have attempted to implement different materials while scrupulously respecting a "monolithic" construction suitable for establishing the radial magnetic field lines. Although relatively efficient, radially shaped field assemblies are technically difficult to manufacture and therefore generally very expensive.

[0016] European patent application EP3634013A1 describes a magnet system for an electromechanical converter, comprising a first outer pair of axially magnetized permanent ring magnets and a second inner pair of axially magnetized permanent ring magnets of opposite polarity to the first pair of permanent magnets, with a voice coil arranged in an air gap formed between the first pair of permanent magnets and the second pair of permanent magnets. However, the thicknesses of the first and second pairs of permanent magnets are identical, and this existing magnet system does not allow a constant magnetic field to be obtained, which can be observed over the entire stroke of the moving element of the electrodynamic loudspeaker driver, and therefore does not allow the force coefficient of the driver to be linearized over the entire useful stroke of the voice coil. In fact, in this existing magnet system, the evolution of the magnetic field in the space associated with the stroke of the moving element is highly distorted. Moreover, in EP3634013A1, there are two solid elements between the upper and lower magnets, which are metals or alloys with very high electrical conductivity values. However, the presence of these metallic elements generates eddy currents (which are generated in any conductive mass placed in the vicinity of the (current carrying) coil during the movement of the moving element), leading to a degrading effect on the high frequency signal.

[0017] The magnet system described in US Patent Application Publication No. 2018 / 132041A1 also exhibits the same drawbacks. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] European patent application EP3634013A1 [Patent Document 2] U.S. Patent Application Publication No. 2018 / 132041A1 Summary of the Invention [Problem to be solved by the invention]

[0019] The present invention aims to solve the shortcomings of the prior art by providing a magnet assembly for an electrodynamic speaker driver, comprising a first outer pair of axially magnetized permanent ring magnets and a second inner pair of axially magnetized permanent ring magnets, wherein a thickness of the permanent magnets of the second inner pair is less than a thickness of the permanent magnets of the first outer pair, and each permanent magnet of the second inner pair is offset by a predefined non-zero offset distance within an associated permanent magnet of the first outer pair, thereby enabling a field compensated electrodynamic ring speaker with low electrical and mechanical losses, and therefore enabling a good quality sound reproduction with much less loss and nonlinear distortion than conventional speakers. [Means for solving the problem]

[0020] The present invention therefore has as its object a magnet assembly for an electrodynamic speaker driver, comprising a first outer pair of magnets and a second inner pair of magnets, the first outer pair of magnets comprising a first outer axially magnetized permanent ring magnet and a second outer axially magnetized permanent ring magnet, said first and second outer permanent ring magnets being axially arranged facing each other, spaced apart from each other and having the same first thickness, the second inner pair of magnets comprising a first inner axially magnetized permanent ring magnet and a second inner axially magnetized permanent ring magnet, said first and second inner permanent ring magnets being axially arranged facing each other, spaced apart from each other and having the same second thickness, the first and second inner permanent ring magnets and the first and second outer permanent ring magnets being coaxial, in other words having the same central axis of symmetry, such that an air gap is formed between the first outer pair of magnets and the second inner pair of magnets. the first inner permanent magnet is disposed inside the first outer permanent magnet and the second inner permanent magnet is disposed inside the second outer permanent magnet, the direction of the NS magnetic field of the first inner permanent magnet is opposite to the direction of the NS magnetic field of the first outer permanent magnet, the direction of the NS magnetic field of the first inner permanent magnet is opposite to the direction of the NS magnetic field of the second inner permanent magnet, and the direction of the NS magnetic field of the first outer permanent magnet is opposite to the direction of the NS magnetic field of the second outer permanent magnet, the second thickness is smaller than the first thickness, and a face of the first inner permanent magnet facing the second inner permanent magnet is offset by a predetermined non-zero offset distance with respect to a face of the first outer permanent magnet facing the second outer permanent magnet, and a face of the second inner permanent magnet facing the first inner permanent magnet is offset by a predetermined offset distance with respect to a face of the second outer permanent magnet facing the first outer permanent magnet.

[0021] Each of the four permanent magnets forming the magnet assembly is in the form of a ring such that each permanent magnet in the form of a ring has a central axis of symmetry, so that if the permanent ring magnets are coaxial, they have the same central axis of symmetry.

[0022] By axially magnetized permanent ring magnet is understood to mean a permanent magnet in the form of a ring which exhibits an axial magnetic flux field relative to the central axis of symmetry of the ring. The permanent ring magnet is therefore magnetized over its thickness (in other words, height), the north pole of the permanent ring magnet being found on one of the two circular faces located at the ends of the ring and the south pole of the permanent ring magnet being found on the other of the two circular faces located at the ends of the ring.

[0023] Each ring (in other words a permanent ring magnet) is defined by the following dimensions: its outer diameter, its inner diameter and its thickness. The cross section of a ring is therefore defined by the difference between its outer diameter and its inner diameter multiplied by its thickness. By thickness of a permanent ring magnet is understood to mean the distance between its lower circular surface and its upper circular surface, in other words the height of the ring according to its central axis of symmetry.

[0024] Therefore, the magnet assembly according to the present invention has a unique arrangement of permanent ring magnets exhibiting an axial flux magnetic field, and a second pair of axially magnetized permanent magnets is inserted within the first pair of axially magnetized permanent magnets while maintaining the same central axis of symmetry and with opposite polarity with respect to the first pair of magnets to ensure the magnetic field compensation function.

[0025] Indeed, a predefined offset distance (denoted Δy≠0) between the plane of the second inner pair of permanent magnets and the plane of each of the first outer pair of permanent magnets allows to obtain a substantially constant value of the radial component of the magnetic field in the air gap of the magnet assembly. The magnet assembly of the present invention therefore allows to obtain a substantially constant value of the physical quantity power coefficient (Bl) at any point along the useful stroke of the loudspeaker coil in the air gap of the magnet assembly. The offset of the second inner pair of permanent magnets relative to the first outer pair of permanent magnets therefore allows to introduce a so-called compensation (rectification) function of the magnetic field strength pattern nonlinearity in the magnet assembly.

[0026] In the present invention, the coupling of a pair of different axially magnetized ring magnets therefore makes it possible to obtain a usable area spatially in which the coil of the electrodynamic loudspeaker can move (in a direction perpendicular to the plane of the ring magnets, marked y) with a magnetic field that is as constant as possible. Since the coil windings have a fixed developed length, this results in a force coefficient B l This leads to the desired invariance of

[0027] The magnet assembly of the present invention can therefore be used in any ironless electrodynamic speaker driver requiring an optimum compensation field configuration obtained by proper placement of axially magnetized permanent ring magnets.

[0028] According to a particular feature of the invention, the first and second outer permanent magnets are separated by at least one of air and a non-metallic material such as a polymer.

[0029] Therefore, the lack of metallic material or alloy (and therefore conductive material) between the outer permanent magnets allows the generation of eddy currents to be minimized, ensuring minimal energy losses.

[0030] According to a particular feature of the invention, the first and second inner permanent magnets are separated by at least one of air and a non-metallic material, such as a polymer.

[0031] Therefore, the lack of metallic materials or alloys between the inner permanent magnets allows the generation of eddy currents to be minimized to ensure minimal energy losses.

[0032] In fact, any type of polymer suitable for mechanically attaching magnets to a speaker cabinet can be used to separate magnets of the same pair, for example, machined polyvinyl chloride (PVC). It should be noted that the separation pieces can also be made by three-dimensional (3D) printing without departing from the scope of the present invention.

[0033] According to a particular feature of the invention, each of the first and second outer permanent magnets consists of two axially magnetized permanent ring sub-magnets which are superimposed and have the same inner diameter, the same outer diameter, the same N-S magnetic orientation and are of different thicknesses.

[0034] This unique configuration of the first outer pair of permanent magnets therefore makes it possible to obtain a substantially constant value of the radial component of the magnetic field in the air gap of the magnet assembly over a larger range in the direction y, in other words over a larger stroke of the coils in the air gap.

[0035] According to one particular feature of the invention, for each of the first and second outer permanent magnets, the permanent sub-magnet facing the other of the first and second outer permanent magnets has a thickness equal to a predefined offset distance.

[0036] According to one particular feature of the invention, the ratio of the first thickness to the second thickness is between 1 and 5.

[0037] According to one particular feature of the invention, the predetermined offset distance is between 2% and 50% of the first thickness.

[0038] Note that in all cases the second thickness must be less than or equal to the first thickness minus the predefined offset distance.

[0039] According to a particular feature of the invention, the air gap between the first outer pair of magnets and the second inner pair of magnets has a spacing between 0.5mm and 6mm.

[0040] The aim of the invention is therefore to advance the displacement of the coil within a magnetic field that is as uniform as possible, but at a significant distance from each of the elements of the pair of ring magnets that generate this field. This distance and the lack of steel material ensures that energy losses associated with eddy currents are kept to a minimum.

[0041] In the present invention, the air gap spacing is therefore increased with respect to current standard technology, thus allowing for possible lateral displacements of the coil of the moving element (e.g. by default due to residual non-linearities), thus avoiding any risk of snagging between the moving element and the permanent magnet during temporary fault operation of the electrodynamic loudspeaker.

[0042] According to a particular feature of the invention, the permanent magnets of the first and second pairs of magnets are made from at least one material from the group consisting of neodymium, iron, boron, cobalt, nickel, and ferromagnetic ceramics comprising at least one from the group consisting of iron oxide, samarium, zinc and aluminum.

[0043] The invention also has as its object an electrodynamic loudspeaker driver comprising a magnet assembly as described above and a movable element comprising a cylindrical coil support partially inserted into the air gap of the magnet assembly and wound with a coil located in the air gap of the magnet assembly.

[0044] Thus, injection of an amplitude- and frequency-modulated current into the coil carried by the moving element causes displacement of the moving element, and thus the coil, into the air gap of the magnet assembly, where the radial component of the generated magnetic field is substantially constant (in other words, the force coefficient Bl is substantially invariant over the entire useful stroke of the coil), allowing high quality sound reproduction with much less loss and nonlinear distortion than conventional speakers.

[0045] The driver according to the invention can be voltage driven (in the conventional manner), but is also particularly suitable for following a current control mode (for a drive regime imposed by an electronic regulator of the voltage / current converter type).

[0046] According to one particular feature of the invention, the moving element is made from a polyimide film (such as Kapton®).

[0047] The invention additionally has as its object an electrodynamic loudspeaker comprising an electrodynamic loudspeaker driver as described above, and a chassis on which is located a diaphragm connected to a movable element of the driver.

[0048] Thus, the mechanical displacement of a moving element induced at audio frequencies is converted into a sound field by means of a diaphragm acting as an emission surface (also known as an acoustic radiator).

[0049] In order to better illustrate the objects of the present invention, preferred embodiments will now be described, by way of example and not by way of limitation, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0050] [Figure 1] FIG. 2 is a cross-sectional view of an axially magnetized permanent ring magnet. [Diagram 2] FIG. 2 is a perspective view of a partially cut magnet assembly according to a first embodiment of the present invention. [Diagram 3] 1 is a cross-sectional view of a magnet assembly according to a first embodiment of the present invention; [Figure 4] 4 represents various curves of the radial component of the magnetic field of the magnet assembly of the first embodiment along the axis y for different values ​​of the offset distance. [Diagram 5] 1 is a cross-sectional view of an electrodynamic speaker driver according to a first embodiment of the present invention. [Figure 6] 11 is an example curve representing the radial component of the magnetic field of the driver of the first embodiment along the axis y for an optimal offset distance value of 1.7 mm. [Figure 7] FIG. 4 is a cross-sectional view of a magnet assembly according to a second embodiment of the present invention. [Figure 8] 13 is an example curve representing the radial component of the magnetic field of the driver of the second embodiment along the axis y for an optimal offset distance value of 1.5 mm. [Figure 9] FIG. 2 is a perspective view of a movable element of a driver according to the present invention; [Figure 10] 1 is an exploded perspective view of an electrodynamic speaker according to the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] With reference to FIG. 1, it can be seen that an axially magnetized permanent ring magnet 1 is represented.

[0052] The axially magnetized permanent ring magnet 1 has a north magnetic pole (N) 1a and a south magnetic pole (S) 1b arranged one above the other to generate an axial magnetic flux magnetic field, and the dotted lines in FIG. 1 represent the magnetic field strength pattern associated with this axially magnetized permanent magnet 1.

[0053] The permanent ring magnet 1 is therefore in the form of a ring and presents an axial magnetic flux field with respect to its central axis of symmetry. The permanent ring magnet 1 is therefore magnetized with respect to its thickness (in other words, its height along its central axis of symmetry) with its north pole (N) being found on one of its two end faces and its south pole (S) being found on the other of its two end faces. With reference to Figures 2 and 3, it can be seen that a magnet assembly 2 is represented according to a first embodiment of the present invention.

[0054] The magnet assembly 2 comprises a first outer pair of magnets comprising a first outer axially magnetized permanent ring magnet 3a and a second outer axially magnetized permanent ring magnet 3b, the first and second outer permanent magnets 3a and 3b being identical and axially arranged facing each other and spaced apart from each other by a distance b, and the first and second outer permanent magnets 3a and 3b having the same first thickness.

[0055] The magnet assembly 2 further comprises a second inner pair of magnets comprising a first inner axially magnetized permanent ring magnet 4a and a second inner axially magnetized permanent ring magnet 4b, the first and second inner permanent magnets 4a and 4b being identical and axially arranged facing each other and spaced apart from each other by a distance b+2Δy, the first and second inner permanent magnets 4a and 4b having the same second thickness which is smaller than the first thickness of the first and second outer permanent magnets 3a and 3b.

[0056] The first and second inner permanent magnets 4a and 4b and the first and second outer permanent magnets 3a and 3b are coaxial.

[0057] The first inner permanent magnet 4a is arranged inside the first outer permanent magnet 3a and the second inner permanent magnet 4b is arranged inside the second outer permanent magnet 3b, so that an air gap e is formed between the first outer pair of magnets and the second inner pair of magnets.

[0058] The magnetic polarity of the first inner permanent magnet 4a (in other words, the N-S magnetic field direction) is opposite to the magnetic polarity of the first outer permanent magnet 3a, the magnetic polarity of the first inner permanent magnet 4a is opposite to the magnetic polarity of the second inner permanent magnet 4b, and the magnetic polarity of the first outer permanent magnet 3a is opposite to the magnetic polarity of the second outer permanent magnet 3b.

[0059] 2 and 3, the N poles of the first and second outer permanent magnets 3a and 3b face each other and the S poles of the first and second inner permanent magnets 4a and 4b face each other. However, without departing from the scope of the invention, a reverse arrangement may also be envisaged, with the S poles of the first and second outer permanent magnets 3a and 3b facing each other and the N poles of the first and second inner permanent magnets 4a and 4b facing each other.

[0060] The face of the first inner permanent magnet 4a facing the second inner permanent magnet 4b is offset by a predefined non-zero offset distance Δy relative to the face of the first outer permanent magnet 3a facing the second outer permanent magnet 3b.

[0061] Similarly, the face of the second inner permanent magnet 4b facing the first inner permanent magnet 4a is offset by a predefined offset distance Δy relative to the face of the second outer permanent magnet 3b facing the first outer permanent magnet 3a.

[0062] The magnet assembly 2 therefore has a unique arrangement of permanent ring magnets 3a, 3b, 4a and 4b exhibiting an axial flux magnetic field, with a second pair of axially magnetized permanent magnets 4a, 4b inserted within the first pair of axially magnetized permanent magnets 3a, 3b maintaining the same central axis of symmetry and with opposite polarity relative to the first pair of magnets 3a, 3b.

[0063] A predefined offset distance Δy between the plane of the second inner pair of permanent magnets 4a, 4b and the plane of each of the first outer pair of permanent magnets 3a, 3b allows a substantially constant value of the radial component Bx of the magnetic field in the air gap e of the magnet assembly 2. The offset of the second inner pair of permanent magnets 4a, 4b with respect to the first outer pair of permanent magnets therefore allows a so-called compensation (rectification) function for non-linearities of the magnetic field strength pattern in the magnet assembly 2 to be introduced.

[0064] According to the specific embodiment represented in Figures 2 and 3, the ring magnets 3a and 3b of the first outer pair of magnets have, by way of example, a rectangular cross section, an outer diameter of 30 mm, an inner diameter of 25 mm, a thickness of 4 mm, a distance b between the inner surfaces of 6 mm, and a material of the Nd-Fe-B (neodymium-iron-boron) type.

[0065] Furthermore, according to the specific embodiment depicted in Figures 2 and 3, the ring magnets 4a and 4b of the second inner pair of magnets have, by way of example, a rectangular cross section, an outer diameter of 17 mm, an inner diameter of 10.25 mm, a thickness of 2.45 mm, a distance b+2Δy between the inner surfaces of 6 mm+2Δy, and a material of Nd-Fe-B type.

[0066] It should be noted that the ratio of the first thickness of the outer magnets 3a and 3b to the second thickness of the inner magnets 4a and 4b can also be between 1 and 5 without departing from the invention.

[0067] The predetermined offset distance Δy can be between 2% and 50% of the first thickness of the outer magnets 3a and 3b, but the second thickness of the inner magnets 4a and 4b must be less than or equal to the first thickness minus the predetermined offset distance Δy.

[0068] In the specific embodiment depicted in Figures 2 and 3, the air gap e therefore has a spacing of 4 mm, however, the air gap e could have a spacing between 0.5 mm and 6 mm without departing from the scope of the invention.

[0069] Additionally, the permanent magnets 3a, 3b, 4a and 4b may also be made from at least one of neodymium, iron, boron, cobalt, nickel, and ferromagnetic ceramics comprising at least one of iron oxide, samarium, zinc and aluminum without departing from the scope of the present invention.

[0070] With reference to FIG. 4, it can be seen that various curves of the measured radial component Bx of the magnetic field generated by the magnet assembly 2 along the axis y (represented in FIGS. 2 and 3) are plotted for different values ​​of the offset distance Δy, located at x=4 mm on the axis x (represented in FIGS. 2 and 3).

[0071] 2 and 3, the zero point of the axis y is defined on the upper surface of the first outer magnet 3a of the magnet assembly 2. The probe is then moved from top to bottom (at x=4 mm, where x=0 mm corresponds to the left outer surface of the first pair of magnets 3a, 3b) to the y axis defined by the lower surface of the second outer magnet 3b of the magnet assembly 2. max FIG. 4 then presents the values ​​of the radial component Bx of the magnetic field collected during the trajectory of the probe, translated by reading from left to right on the axis y placed on the abscissa.

[0072] The Δy offset distance measured in successive steps results in a set of measurements, the various values ​​of which are represented in FIG.

[0073] It can be seen that without offset (Δy=0) it is not possible to obtain a constant force coefficient.

[0074] In addition to Δy=0, three values ​​of Δy are represented in Figure 4 (Δy=1.7 mm, 3.4 mm and 5.1 mm). The best configuration of the magnetic field was observed for Δy=1.7 mm, with a usable stroke of about 5 mm (when y is between about 10 mm and about 15 mm), thus ensuring a constant force coefficient over this stroke.

[0075] The magnet assembly 2 according to the invention therefore makes it possible to obtain a magnetic field strength pattern, the line of which is idealized with respect to the relative invariance of the product Bxl, when the latter is observed as a function of the quasi-static position of the moving element of the electrodynamic loudspeaker.

[0076] With reference to FIG. 5, it can be seen that there is depicted an electrodynamic speaker driver 5 comprising a magnet assembly 2 according to a first embodiment of the present invention.

[0077] The first and second outer permanent magnets 3a and 3b are separated by a ring 6 of a non-metallic material, such as a polymer.

[0078] It should be noted that the ring 6 could also be replaced by air without departing from the scope of the present invention.

[0079] Although not depicted in FIG. 5, the first and second inner permanent magnets 4a and 4b are also separated by a ring of non-metallic material or by air.

[0080] The absence of metallic materials or alloys between the outer permanent magnets 3a and 3b and between the inner permanent magnets 4a and 4b minimises the generation of eddy currents so as to ensure minimum energy losses.

[0081] In fact, any type of polymer suitable for mechanically attaching magnets to a speaker housing to separate magnets of the same pair can be used, for example machined polyvinyl chloride (PVC).

[0082] The magnet assembly 2 of the present invention therefore enables an ironless electrodynamic speaker driver 5 to be used with a compensated optimum magnetic field configuration.

[0083] The electrodynamic loudspeaker driver 5 comprises a magnet assembly 2 and a cylindrical support type mobile element 7 wound with a coil 8 which is inserted into the air gap of the magnet assembly 2 and which is located in the air gap of the magnet assembly 2 .

[0084] The magnetic field B generated by the magnet assembly 2 has a radial component Bx, relative to the axis x, and an axial component By, relative to the axis y.

[0085] This general representation of the electrodynamic speaker driver 5 illustrates that the coil 8 moves along an axis y and is exposed to a radial component Bx of the magnetic field B generated by the magnet assembly 2, such that the magnetic field B is directed towards the centre of the magnet assembly 2, in other words towards the coil 8.

[0086] An amplitude and frequency modulated current i is injected into a coil 8 carried by a movable element 7 .

[0087] Application of Maxwell's laws enables the direction of the Lorentz force F driving the displacement of the movable element 7 to be determined.

[0088] Injection of a current i into the coil 8 therefore causes a displacement of the movable element 7, and therefore the coil 8, according to the axis y in the air gap of the magnet assembly 2, in which the generated radial component Bx of the magnetic field B is substantially constant (in other words the force coefficient Bl is substantially constant at any point of the useful stroke of the coil 8 in the air gap of the magnet assembly 2), allowing good quality sound reproduction with much less losses and non-linear distortions than conventional loudspeakers.

[0089] The driver 5 according to the invention can be voltage driven (in the conventional manner), but is also particularly suitable to follow a current control mode (for a drive regime imposed by an electronic regulator of the voltage / current converter type).

[0090] The air gap spacing e of the magnet assembly 2 is increased with respect to the state of the art to allow for possible lateral displacements of the coil 8 of the moving element 7 (e.g. due to residual non-linearities) and thus avoid any risk of snagging between the moving element 7 and the permanent magnets 3a, 3b, 4a, 4b during temporary fault operation of the driver 5.

[0091] The driver 5 according to the invention therefore allows a displacement of the coil 8 along an axis y in a magnetic field B which is as uniform as possible, but at a significant distance from each of the permanent magnets 3a, 3b, 4a, 4b which generate this magnetic field B. The large air gap spacing e and the lack of steel material ensure minimal energy losses associated with eddy currents, these latter being kept to a minimum.

[0092] With reference to FIG. 6, it can be seen that the radial component Bx of the magnetic field B of the driver 5 of the first embodiment measured along the axis y for an optimum offset distance value Δy=1.7 mm is represented.

[0093] An optimal offset distance value Δy is first experimentally found for the magnet assembly 2 according to the first embodiment. It should be noted that this optimal value could also be automatically found without departing from the scope of the present invention. As illustrated in FIG. 4, an optimal value Δy=1.7 mm is found.

[0094] FIG. 6 represents a measurement of the radial component Bx of the magnetic field B generated by the magnet assembly 2 along the axis y (located at x=4 mm) when Δy=1.7 mm.

[0095] It is clear that the working stroke D1 of the coil 8 held by the movable element 7 with a substantially constant radial component Bx (and therefore a substantially constant force coefficient Bl) is approximately 5 mm and that the useful stroke at constant force coefficient of the coil 8 in the air gap is approximately 5 mm.

[0096] With reference to FIG. 7, it can be seen that a magnet assembly 2' according to a second embodiment of the invention is represented.

[0097] The magnet assembly 2' according to the second embodiment in FIG. 7 is identical to the magnet assembly 2 according to the first embodiment in FIG. 3, with the exception that the first outer permanent magnet 3a is composed of two axially magnetized permanent ring sub-magnets 31a and 32a which are superimposed and have the same inner diameter, the same outer diameter, the same magnetic polarity and different thicknesses, and the second outer permanent magnet 3b is composed of two axially magnetized permanent ring sub-magnets 31b and 32b which are superimposed and have the same inner diameter, the same outer diameter, the same magnetic polarity and different thicknesses.

[0098] The permanent sub-magnets 32a of the first outer permanent magnet 3a are arranged facing the second outer permanent magnet 3b, and the permanent sub-magnets 32b of the second outer permanent magnet 3b are arranged facing the first outer permanent magnet 3a.

[0099] The permanent sub-magnets 31a and 31b have the same thickness, and the permanent sub-magnets 32a and 32b also have the same thickness.

[0100] In FIG. 7, the thickness of the permanent sub-magnets 32a and 32b is equal to the predefined offset distance Δy.

[0101] However, the permanent sub-magnets 32a and 32b may also be of different thicknesses without departing from the scope of the invention.

[0102] With reference to FIG. 8, it can be seen that this represents the radial component Bx of the magnetic field B generated by the magnet assembly 2' according to the second embodiment measured along the axis y (located at x=4 mm) when Δy=1.5 mm.

[0103] If the offset distance value chosen during the measurements is Δy=1.5 mm, this results in a thickness of the sub-magnets 32a and 32b of 1.5 mm and a thickness of the sub-magnets 31a and 31b of 2.5 mm.

[0104] With regard to the magnet assembly 2' according to the second embodiment, it is clear that the operating stroke D2 of the coil 8 with a substantially constant radial component Bx (and therefore a substantially constant force coefficient Bl) is approximately 6 mm, and that the useful stroke at constant force coefficient of the coil 8 in the air gap is approximately 6 mm, which, taking into account the overall dimensions of the device, is significant from the point of view of the current state of the art.

[0105] With reference to FIG. 9, it can be seen that a movable element 7 of the driver 5 is represented.

[0106] A movable element 7 in the form of a cylindrical support carries a coil 8 wound around it.

[0107] One end of the movable element 7 is connected to a diaphragm 11 which is in turn connected to a rectangular frame 9 by means of a number of flexible arms 10 cut from the diaphragm material.

[0108] The movable element 7 is therefore commonly called a "piston".

[0109] The movable element 7 and the diaphragm 11 may be made, for example, from a polyimide film such as Kapton®.

[0110] The frame 9 can be made, for example, from epoxy glass.

[0111] A dome 12 closing the opening of the movable element 7 is connected to the diaphragm 11 at its edge.

[0112] The frame 9 also includes electrical tracks 13 making it possible to supply power to the coils 8 .

[0113] The structure of the moving element 7 is optimized to provide minimum energy losses in terms of the power supply serving to drive the device.

[0114] Referring to FIG. 10, it can be seen that an electrodynamic loudspeaker 14 according to the present invention is depicted.

[0115] The electrodynamic speaker 14 comprises a chassis 15 consisting of a first housing part 15a and a second housing part 15b on which the driver 5 (in other words the movable element 7 holding the magnet assembly 2 or 2' and the coil 8), the diaphragm 11 and the frame 9 are mounted.

[0116] The first outer permanent magnet 3a is mounted inside the second housing part 15b, while the second outer permanent magnet 3b and the first and second inner permanent magnets 4a and 4b are mounted inside the first housing part 15a.

[0117] A disk or ring 16 of a non-metallic material (e.g., a polymer) is positioned above the first inner permanent magnet 4a, below the second inner permanent magnet 4b, and between the first and second inner permanent magnets 4a and 4b so as to separate the first and second inner permanent magnets 4a and 4b by an appropriate distance b+2Δy.

[0118] The first and second housing parts 15a and 15b have holes 17 on their edges to enable the two housing parts 15a and 15b to be attached together by screwing when positioned relative to one another to place the first outer permanent magnet 3a at a distance b from the second outer permanent magnet 3b.

[0119] In the embodiment depicted in Fig. 10, when the first and second housing parts 15a, 15b are assembled, air separates the first and second outer permanent magnets 3a and 3b. However, the first and second outer permanent magnets 3a and 3b could also be separated by a ring 6 of non-metallic material (as depicted in Fig. 5) without departing from the scope of the invention.

[0120] Once the two housing parts 15a and 15b are assembled (so that the magnet assembly 2 is positioned as represented in FIG. 3), the movable element 7 is then inserted into the air gap of the magnet assembly 2 with the upper surface of the second housing part 15b until the frame 9 abuts against the upper surface of the second housing part 15b. The frame 9 is then attached by screwing to the second housing part 15b using holes 18 formed in the four corners of the frame 9.

[0121] The coil 8 is therefore located in the air gap between the two inner permanent magnets 4 a and 4 b of the magnet assembly 2 , in other words in the constant force coefficient region of the magnet assembly 2 .

[0122] Injection of an amplitude and frequency modulated current into the coil 8 by means of the electrical track 13 of the frame 9 causes a displacement of the movable element 7 and thus of the diaphragm 11 (the displacement is made possible by means of a flexible arm 10 connected to the frame 9).

[0123] The diaphragm 11 then acts as an emission surface (or sound generating surface or acoustic radiator) allowing the mechanical displacement of the movable element 7, induced at audio frequencies, to be transformed into a sound field.

[0124] 9 and 10 are specifically of the piston type, in alternative embodiments the coil 8 could be attached directly to the rear surface of the diaphragm 11 without departing from the scope of the invention. Furthermore, the entire periphery of the diaphragm 11 could be connected to the frame 9 (in other words, lacking the flexible arm 10) without departing from the scope of the invention.

[0125] Two electrodynamic loudspeaker prototypes14 were built and characterized: - Series 1 with a sound generating surface diameter of 17 mm: Outer permanent magnets 3a and 3b: outer diameter 30 mm, inner diameter 25 mm, thickness 4 mm, Inner permanent magnets 4a and 4b: outer diameter 17 mm, inner diameter 10.25 mm, thickness 2.45 mm, ●Δy=1.7mm, and - Series 2 with a diameter for the sound generating surface of 40 mm: Outer permanent magnets 3a and 3b: outer diameter 60 mm, inner diameter 45 mm, thickness 8.03 mm, Inner permanent magnets 4a and 4b: outer diameter 39 mm, inner diameter 31 mm, thickness 6.4 mm, ●Δy=3mm. Table 1 below summarizes the characteristics of the coils 8 used for the various prototypes.

[0126] [Table 1]

[0127] Speakers No. 1 and No. 4 belong to family 1 (17 mm sound generating surface diameter), while speakers No. 2, No. 3 and No. 5 belong to family 2 (40 mm sound generating surface diameter).

[0128] Regarding the speakers No. 1 and No. 4 of the first system, they all have in common the size of their frames 9 (25 x 25 mm 2 ), it has a coil 8 of 50 turns of copper wire (diameter 0.07 mm) presenting an overall electrical resistance of approximately R=9 Ω. The first type of construction is developed with their vibrating Kapton® diaphragm 11 stretched across an epoxy glass frame 9. Their resonant natural frequencies will be those of the diaphragm modes. The Kapton® thicknesses are 25 μm, 50 μm and 125 μm, respectively. In contrast, according to the second type of construction (developed on a 125 μm thick Kapton® polyimide film), the piston part is kept in place by means of four flexible arms 10 connected to the frame 9. The diaphragm resonant modes tend to disappear with the piston construction. The pistons can be made of cardboard or Kapton® polyimide.

[0129] Regarding speakers No. 2, No. 3 and No. 5 of the second system, they all have in common the size of their frame 9 (60 x 60 mm 2 ) and a coil 8 of 25 turns of copper wire whose electrical resistance conforms to the information in Table 1. One version has a Kapton® diaphragm that is 25 μm thick, another has a Kapton® piston that is 125 μm thick, another has a cardboard piston on Kapton® that is 125 μm thick, and another has a piston constructed of foam and carbon on a carbon fiber material that is 200 μm thick.

[0130] Regarding the frequency evolution of the impedance coefficients, it is worth noting the invariance that characterizes the high frequency values ​​for each of the prototypes. This characteristic clearly indicates the absence of dissipative eddy currents. In fact, the prototypes have minimal dissipation.

[0131] The processing of impedance measurements allows access to the dissociation of the dissipative part (apparent resistance Re) and the stored energy part (apparent inductance Le).

[0132] Therefore, in state of the art loudspeakers, it is known that eddy currents appear at high frequencies and these eddy currents cause Re and Le to vary with frequency.

[0133] In contrast, the prototype of the present invention is characterized by the absence of eddy currents, and Re and Le do not vary with frequency.

[0134] When designing the electrodynamic loudspeaker 14, the parameters to be considered for dimensioning the electrodynamic loudspeaker 14 are the area of ​​the sound generating surface (or diaphragm 11) and the amplitude of the displacement, on which the sound level generated by the electrodynamic loudspeaker 14 directly depends. Assuming spherical radiation from the radiation source, the acoustic power is given by the sound level L dB and the distance from the radiation source l source Related to:

[0135]

number

[0136] This relationship is true in the far field, i.e. at a measurement distance l source is valid when is greater than the radius r of the emitting surface of the radiation source. According to the above equation, and considering a desired sound level of 80 dB SPL at 10 cm, the speaker 14 must generate an acoustic power of 12.6 μW. The acoustic power produced by a single face of a piston is related to its surface area and displacement as follows:

[0137]

number

[0138] quantity ρ air is the density of air (1.2 kg / m3 at 20°C), c is the speed of sound in air (343 m / s at 20°C), f is the vibration frequency of the sound-emitting surface, S is its surface area, and x is its effective displacement. This equation is only valid if the piston acts like a point source.

[0139] Substitute the values ​​for air density and speed of sound in the above equation, and use the peak displacement x instead of the effective displacement x. max By substituting, we get: P acoustique =0,27f 4 d 4 x max 2

[0140] This equation shows two degrees of freedom to act on the amount of air displaced: the diameter of the emission surface, d, and its displacement amplitude, x max .

[0141] For a given piston stroke (x max ) at low operating frequency f c There is: - 17mm sound generating surface speaker (system 1):

[0142]

number

[0143] - 40mm sound generating surface speaker (system 2):

[0144]

number

[0145] Therefore, the greater the displacement of the piston in the region of constant magnetic field, the more likely the speaker 14 will exhibit a low frequency response.

[0146] It will be understood that the particular embodiments described are illustrative and not limiting, and that modifications may be made without departing from the invention. [Explanation of symbols]

[0147] 1 Axially magnetized permanent ring magnet 1a N magnetic pole 1b S magnetic pole 2, 2' Magnet Assembly 3a First outer axially magnetized permanent ring magnet 3b Second outer axially magnetized permanent ring magnet 4a First inner axially magnetized permanent ring magnet 4b Second inner axially magnetized permanent ring magnet 5. Electrodynamic speaker driver 6 Rings of non-metallic materials 7 Moving Elements 8 Coils 9 Frame 10 Flexible Arm 11 Diaphragm 12. Dome 13. Electric Truck 14 Electrodynamic speaker 15 Chassis 15a first housing part 15b Second housing part 16 Discs or rings of non-metallic material 17, 18 holes 31a, 31b, 32a, 32b Axially magnetized permanent ring sub-magnets b distance B magnetic field Bx radial component By axial component D1, D2 operation stroke e Air gap distance i Amplitude and frequency modulated current Δy offset distance

Claims

1. A magnet assembly (2; 2') for an electrodynamic speaker driver, comprising a first outer pair of magnets and a second inner pair of magnets, the first outer pair of magnets comprises a first outer axially magnetized permanent ring magnet (3a) and a second outer axially magnetized permanent ring magnet (3b), the first and second outer permanent ring magnets (3a, 3b) being axially arranged facing each other, spaced apart from each other and having the same first thickness; the second inner pair of magnets comprises a first inner axially magnetized permanent ring magnet (4a) and a second inner axially magnetized permanent ring magnet (4b), the first and second inner permanent ring magnets (4a, 4b) are axially arranged facing each other, spaced apart from each other and have the same second thickness, the first and second inner permanent ring magnets (4a, 4b) and the first and second outer permanent ring magnets (3a, 3b) are coaxial, in other words have the same central axis of symmetry, the first inner permanent magnet (4a) is arranged inside the first outer permanent magnet (3a) and the second inner permanent magnet (4b) is arranged inside the second outer permanent magnet (3b) such that an air gap is formed between the first outer pair of magnets and the second inner pair of magnets, a NS magnetic field direction of the first inner permanent magnet (4a) opposite to the NS magnetic field direction of the first outer permanent magnet (3a), the NS magnetic field direction of the first inner permanent magnet (4a) opposite to the NS magnetic field direction of the second inner permanent magnet (4b), and the NS magnetic field direction of the first outer permanent magnet (3a) opposite to the NS magnetic field direction of the second outer permanent magnet (3b), the second thickness being less than the first thickness; and 1. A magnet assembly (2; 2') for an electrodynamic speaker driver, characterized in that a face of the first inner permanent magnet (4a) facing the second inner permanent magnet (4b) is offset by a predetermined non-zero offset distance (Δy) with respect to a face of the first outer permanent magnet (3a) facing the second outer permanent magnet (3b), and a face of the second inner permanent magnet (4b) facing the first inner permanent magnet (4a) is offset by the predetermined offset distance (Δy) with respect to a face of the second outer permanent magnet (3b) facing the first outer permanent magnet (3a).

2. 2. A magnet assembly (2; 2') for an electrodynamic speaker driver as claimed in claim 1, characterized in that the first and second outer permanent magnets (3a, 3b) are separated by at least one of air and non-metallic materials such as polymers.

3. A magnet assembly (2; 2') for an electrodynamic speaker driver as claimed in claim 1 or 2, characterized in that the first and second inner permanent magnets (4a, 4b) are separated by at least one of air and a non-metallic material such as a polymer.

4. 4. A magnet assembly (2') for an electrodynamic speaker driver according to claim 1, characterized in that each of the first and second outer permanent magnets (3a, 3b) is composed of two axially magnetized permanent ring sub-magnets (31a, 32a; 31b, 32b) which are superimposed and have the same inner diameter, the same outer diameter, the same N-S magnetic field direction and different thicknesses.

5. 5. A magnet assembly (2') for an electrodynamic speaker driver as claimed in claim 4, characterized in that for each of the first and second outer permanent magnets (3a, 3b), the permanent sub-magnets (32a, 32b) facing the other of the first and second outer permanent magnets (3a, 3b) have a thickness equal to the predetermined offset distance (Δy).

6. A magnet assembly (2; 2') for an electrodynamic speaker driver according to any one of claims 1 to 5, characterized in that the ratio of the first thickness to the second thickness is between 1 and 5.

7. A magnet assembly (2; 2') for an electrodynamic speaker driver as claimed in any one of claims 1 to 6, characterized in that the predetermined offset distance (Δy) is between 2% and 50% of the first thickness.

8. A magnet assembly (2; 2') for an electrodynamic speaker driver as claimed in any one of claims 1 to 7, characterized in that the air gap between the first outer pair of magnets and the second inner pair of magnets has a spacing between 0.5 mm and 6 mm.

9. 9. A magnet assembly (2; 2') for an electrodynamic speaker driver according to any one of claims 1 to 8, characterized in that the permanent magnets (3a, 3b, 4a, 4b) of the first and second pairs of magnets are made from at least one of the following: neodymium, iron, boron, cobalt, nickel, and ferromagnetic ceramics comprising at least one of the following: iron oxide, samarium, zinc and aluminium.

10. 10. An electrodynamic speaker driver (5) comprising a magnet assembly (2; 2') according to any one of claims 1 to 9, and a movable element (7) comprising a cylindrical coil support partially inserted into the air gap of the magnet assembly (2; 2') and on which a coil (8) arranged in the air gap of the magnet assembly (2; 2') is wound.

11. Electrodynamic speaker driver (5) according to claim 10, characterized in that the movable element (7) is made from a polyimide film.

12. 12. An electrodynamic speaker (14) comprising an electrodynamic speaker driver (5) according to claim 10 or 11 and a frame (15) on which is arranged a diaphragm (11) connected to the movable element (7) of the driver (5).

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