EP4120694A1

EP4120694A1 - Drive unit for a loudspeaker

Info

Publication number: EP4120694A1
Application number: EP21186045.7A
Authority: EP
Inventors: Morten Lydolf
Original assignee: Harman Becker Automotive Systems GmbH
Current assignee: Harman Becker Automotive Systems GmbH
Priority date: 2021-07-16
Filing date: 2021-07-16
Publication date: 2023-01-18

Abstract

A drive unit (200) for a loudspeaker (100) comprises a first magnet (210) and a second magnet (220) arranged along a longitudinal axis (201) of the drive unit (200) with a gap (230) between the first and second magnets (210, 220). Each magnet of the first and second magnets (210, 220) has, along the longitudinal axis (201), a first end (211, 221) with a first magnetic polarity (N) and an opposing second end (212, 222) with a second magnetic polarity (S). The first and second polarities being different. At the gap (230), the first end (211) of the first magnet (210) faces the first end (221) of the second magnet (220). The drive unit (200) comprises a voice coil (240) movable along the longitudinal axis (201) and positionable at least between the first end (211) of the first magnet (210) and the first end (221) of the second magnet (220), and at least one additional magnetic member (250, 260, 291, 295) providing at least one additional magnetic pole located at a radial distance (253) to the longitudinal axis (201) greater than a radial distance (246) of the voice coil (240) to the longitudinal axis (201). The at least one additional magnetic pole has the second magnetic polarity (S).

Description

The present application relates to the field of drive units for loudspeakers, in particular to the field of so-called dynamic loudspeakers with a moving voice coil.

BACKGROUND ART

Loudspeakers are widely used in various areas, for example in consumer products like radios, televisions, audio players, computers, mobile phones and electronic musical instruments, and commercial applications, for example sound reinforcement in theatres, concert halls, and public address systems.
A loudspeaker may comprise a magnet, in particular a permanent magnet, a voice coil arranged in a magnetic field provided by the magnet, a membrane (also called diaphragm) coupled to the voice coil and elastically coupled via a suspension to a frame of the loudspeaker. When an alternating electrical current of for example an electrical audio signal is applied to the voice coil, the voice coil is forced to move back and force due to the Faraday's law of induction, which causes the membrane attached to the voice coil to move back and forth, pushing on the air to create sound waves. The combination of magnet and voice coil is also called drive unit or electromagnetic motor system. Arrangement and properties of the magnet and voice coil may affect characteristics of a loudspeaker. Characteristics of a loudspeaker may relate to efficiency, i.e. the sound power output divided by the electrical power input, sensitivity, i.e. the sound pressure level at for example 1W electrical input measured at 1 meter, linearity or frequency response, maximum acoustic output power, size and weight.

SUMMARY OF THE INVENTION

The present invention seeks to improve at least some of the above characteristics of a loudspeaker.
According to the present invention, a drive unit for a loudspeaker as defined in the independent claim is provided. The dependent claims define embodiments of the invention.
According to various examples, a drive unit for a loudspeaker comprises a first magnet and second magnet. The first and second magnets may be permanent magnets, e.g. bar magnets. The first magnet and the second magnet are arranged along a longitudinal axis of the drive unit, and the gap is provided between the first and second magnets in the longitudinal axis. Each magnet of the first and second magnets has, in the direction of the longitudinal axis, a first end with a first magnetic polarity and an opposing second end with a second magnetic polarity. The first and second polarities are different. For example, the first polarity is a north pole and the second polarity is a south pole or vice versa. At the gap, the first end of the first magnet faces the first end of the second magnet. In other words, the first and second magnets are arranged such that the ends having the same polarity are facing each other at the gap.
For example, the first and second magnets may each have a cylindrical shape with the first polarity at one base of the cylindrical shape and the second polarity at the other base of the cylindrical shape. An axis of rotational symmetry of each of the first and second cylindrical magnets may be aligned with the longitudinal axis of the drive unit. The gap between the first and second magnets may be considered to have a cylindrical shape with its axial direction aligned to the longitudinal axis. It is to be noticed that, at the gap, same magnetic polarities are facing each other, for example, at the gap, there is the north pole of the first magnet as well as the north pole of the second magnet. In other examples, at the gap, there is the south pole of the first magnet as well as the south pole of the second magnet.
Additionally, the drive unit comprises a voice coil which is movable along the longitudinal axis and capable of being positioned at least between the first end of the first magnet and the first end of the second magnet. In other words, the voice coil may be located, in the longitudinal axis, at the height of the gap. The voice coil may be configured to enclose the first magnet and/or the second magnet in a circumferential direction around the longitudinal axis. For example, the first and second magnets may have a same outer diameter and the voice coil may have an inner diameter at least a little larger than the outer diameter of the first and second magnets. Furthermore, the first magnet may have a first magnet length along the longitudinal axis and the second magnet may have a second magnet length along the longitudinal axis. The gap may have a gap length along the longitudinal axis. The first magnet length may be the same as the second magnet length, whereas the gap length may be shorter than the first and second magnet lengths. For example, the first and second magnet lengths may be in the range of a few centimeters, for example 1 to 10 cm, whereas the gap length may be in a range of a few millimeters, for example 1 to 20 mm. The diameter of the first and second magnets may be in a range of a few millimeters, for example 10 to 50 mm. The voice coil may be movable in the longitudinal axis from the centered position at the height of the gap a few millimeters, for example up to 5 to 30 mm in each direction of the longitudinal axis, i.e. for example up to 5 to 30 mm from the gap along the first magnet or in the opposite direction up to 5 to 30 mm from the gap along the second magnet.
Furthermore, the drive unit comprises at least one additional magnetic member providing at least one additional magnetic pole located at a radial distance to the longitudinal axis greater than a radial distance of the voice coil to the longitudinal axis. The at least one additional magnetic pole has the second polarity. For example, the at least one additional magnetic member may be arranged at a radial distance to the longitudinal axis which is greater than an outer radius of the voice coil. As a result, the voice coil will not get into contact with the at least one additional member even if the voice coil is moved along the longitudinal axis such that it reaches the height of the position of the at least one additional magnetic member. The at least one additional magnetic member may provide the at least one additional magnetic pole along the longitudinal axis within the first magnet length. As the at least one additional magnetic member has a polarity opposite to the polarity provided at the gap by the first and second magnets, the magnetic field (e.g. B-field) in the area of the voice coil is increased by the presence of this additional magnetic force from the at least one additional magnetic pole and thus the speaker sensitivity may be increased. Furthermore, the at least one additional magnetic pole may contribute to increase the magnetic field (B-field) in the area of the voice coil by providing a medium for guiding the magnetic field from the first and second magnets. This may also contribute to increase the speaker sensitivity. Furthermore, the at least one additional pole provides a larger area for the focused magnetic air gap which may provide a more linear behavior of the drive unit, for example provide a more linear excursion.
The at least one additional magnetic member may comprise at least a first additional magnetic member and a second additional magnetic member. For example, the first additional magnetic member may be arranged along the length of the first magnet and the second additional magnetic member may be arranged along the length of the second magnetic member. The first additional magnetic member may be arranged with respect to the first magnet in a same fashion as the second additional magnetic member with respect to the second magnet. As a result, the above described effects of the additional magnetic members may be achieved when the voice coil is moving from centered position at the height of the gap in the direction along the first magnet in the same way as when the voice coil is moving from the centered position at the height of the gap in the opposite direction along the second magnet. Therefore, whenever in the following additional features of "the at least one additional magnetic member" are described, these features may be applied to all additional magnetic members, in particular to the above described first and second additional magnetic members. However, the above described effect of the at least one additional magnetic member may also be achieved, at least in part, when only one additional magnetic member is provided at the drive unit. Furthermore, it is to be noticed that the first and second additional magnetic poles provided by the first and second additional magnetic members, respectively, both have the second polarity, i.e. a polarity different from the polarity provided by the first and second magnets at the gap.
According to various examples, the at least one additional magnetic member is made of a magnetic material, i.e. the at least one additional magnetic member itself is a magnet, for example a permanent magnet. For example, the at least one additional magnetic member comprises a ring magnet. The ring magnet may have an inner diameter larger than an outer diameter of the first magnet. The inner diameter of the ring magnet may also be larger than an outer diameter of the voice coil. For example, the ring magnet may have cylindrical hollow shape and an axis of rotation of the cylindrical hollow shape may extend along the longitudinal axis of the drive unit. At the ends in the longitudinal direction, the ring magnet may have its poles, for example a north pole at one end and a south pole at the opposing other end. The ends of the cylindrical hollow shape represent the bases of the ring magnet. The at least one additional magnetic pole may be at a first base of the ring magnet. For example, the second polarity may be at the first base and the first polarity may be at the second base.
The first base of the ring magnet may be arranged along the first magnet, i.e. the first base is arranged at a height of the first magnet and not at a height of the gap. A distance between the first base of the ring magnet and the gap may be smaller than a distance between the second base of the ring magnet and the gap. In other words, the ring magnet is extending from the first base away from the gap along the first magnet.
As noted above, the drive unit may comprise also an additional magnetic member arranged along the second magnet, for example a corresponding second ring magnet. This second ring magnet may also have a base with the second polarity arranged closer to the gap than a base with the first polarity such that this second ring magnet extends away from the gap along the second magnet.
It is to be noticed that the at least one additional magnetic member may have the above described ring shape or any other shape, for example a polygonal shape. The additional magnetic member may be rotationally symmetrical and aligned with the longitudinal axis of the drive unit. The additional magnetic member may be non-rotationally symmetrical, e.g. ellipsoidal or polygonal. Additional magnetic poles provided by the additional magnetic member may have a same radial distance or a varying radial distance with respect to the longitudinal axis of the drive unit. However, independent of the shape of the at least one additional magnetic member, the at least one additional magnetic member provides one or more magnetic poles having the second polarity at a radial distance to the longitudinal axis greater than a radial distance of the voice coil to the longitudinal axis.
According to various examples, the ring magnet is attached to the first magnet via a spacer of non-magnetic material. The non-magnetic material may comprise for example plastics. However, the non-magnetic material may comprise any other paramagnetic, diamagnetic or antiferromagnetic material. The spacer may have a hollow cylindrical shape and an inner diameter of the spacer may be glued or press fitted on a circumferential surface of the first magnet and the ring magnet may be glued or press fitted on an outer circumferential surface of the spacer.
According to further examples, the ring magnet may be coupled to the first magnet via a connecting element of magnetic material. The magnetic material may comprise any ferromagnetic material, for example iron. For example, the connecting element may be disc-shaped with an outer diameter corresponding to the outer diameter of the ring magnet. The connecting element may be coupled to the second base of the ring magnet and a base at the second end of the first magnet. The second end of the first magnet has the second polarity and the second base of the ring magnet has the first polarity. Thus, the connecting element may magnetically be attracted to the second end of the first magnet and the second base of the ring magnet and may guide a magnetic field between the ring magnet and the first magnet. The connecting element may be provided in combination with the above described spacer or may be provided without the spacer.
In further examples, the at least one magnetic member is not a magnet by itself, but may comprise a ferromagnetic member coupled to the second end of the first magnet. As a result, the magnetic pole at the second and of the first magnet, i.e. the magnetic pole having the second polarity, is in contact with the at least one additional magnetic member such that the at least one additional magnetic member can provide this magnetic pole having the second polarity as the at least one additional magnetic pole to a location at a radial distance to the longitudinal axis greater than the radial distance of the voice coil to the longitudinal axis.
For example, the ferromagnetic member may be made of iron, cobalt, nickel or a combination thereof.
For example, the ferromagnetic member may have a cup-like shape. A bottom of the cup-like shape may be coupled to the second end of the first magnet and an annular edge of the cup-like shape has an inner diameter larger than an outer diameter of the first magnet, in particular the annular edge of the cup-like shape has an inner diameter larger than an outer diameter of the voice coil. As a result, the bottom of the cup-like shape of the ferromagnetic member is in contact with the second polarity of the first magnet and thus a magnetic pole having the second polarity is provided at the annular edge of the cup-like shape of the ferromagnetic member. The cup-like shape of the ferromagnetic member may be arranged with respect to the first and second magnets such that a distance between the annular edge and the gap is smaller than a distance between the bottom of the cup-like shape and the gap.
An axis of rotational symmetry of the cup-like shape may be aligned with the longitudinal axis of the drive unit. The cup-like shape may be non-rotationally symmetrical, e.g. ellipsoidal or polygonal. Additional magnetic poles provided by the cup-like shape may have a same radial distance or a varying radial distance with respect to the longitudinal axis of the drive unit.
It is clear that this "passive" ferromagnetic member may have lower effects concerning improving sensitivity and linearity as the "active" permanent magnet, e.g. the above described ring magnet. However, this "passive" ferromagnetic member provides a magnetic pole having the second polarity at a radial distance to the longitudinal axis greater than a radial distance of the voice coil to the longitudinal distance such that the magnetic field in the area of the voice coil may be increased and linearity improved.
In general, according to various examples, the at least one additional magnetic member may have an at least partly oblique surface facing the gap. For example, the oblique surface may be configured such that a normal on the oblique surface may essentially direct to the gap between the first and second magnets. This may contribute to improve linearity of the drive unit. However, other shapes of the edge of the at least one additional magnetic member may be used to improve linearity of the drive unit.
As mentioned above, the drive unit may comprise several additional magnetic members as the above described at least one additional member. For example, the drive unit may comprise at least one further magnetic member providing at least one additional magnetic pole located at radial distance to the longitudinal axis greater than a radial distance of the voice coil to the longitudinal axis, e.g. the above described second ring magnet or a second cup-like shaped ferromagnetic member. The at least one additional magnetic pole has the second polarity. The at least one further magnetic member has corresponding features with respect to the second magnet as the at least one additional magnetic member with respect to the first magnet described above.
In various examples, the axis of rotational symmetry of the voice coil is aligned with the longitudinal axis of the drive unit.
The first and second magnets may each have a cylindrical shape. An axis of rotational symmetry of each of the cylindrical first and second magnets may be aligned with the longitudinal axis of the drive unit.
As a result, the voice coil may slide along the first magnet, the gap and the second magnet and vice versa along the longitudinal axis. In particular, an inner diameter of the voice coil may be at least slightly larger than an outer diameter of the first and second magnets such that the voice coil can move along the first magnet, the gap and the second magnet without being in contact with any of the first and the second magnets. Furthermore, an inner diameter of the at least one additional magnetic member and, if present, an inner diameter of the at least one further magnetic member may be larger than an outer diameter of the voice coil such that the voice coil can slide along the longitudinal axis within the at least one additional magnetic member and, if present, the at least one further magnetic member.
A loudspeaker comprises a frame, the above-described drive unit, and a membrane coupled to the frame and the drive unit. For example, the voice coil comprises a tubular carrier on which coil windings are arranged. The membrane may be mounted at the tubular carrier, for example at one end of the tubular carrier or at a center of the tubular carrier in a circumferential direction.
It is to be understood that the features mentioned above and those described in detail below may be used not only in the described combinations, but also in other combinations or in isolation without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a sectional view of a drive unit for a loudspeaker according to various examples.
FIG. 2 schematically illustrates a top view of the drive unit of FIG. 1.
FIG. 3 schematically illustrates a sectional view of a loudspeaker according to various examples.
FIG. 4 schematically illustrates a sectional view of a drive unit for a loudspeaker according to further examples.
FIG. 5 schematically illustrates a sectional view of a drive unit for a loudspeaker according to further examples.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.
The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling.
Some examples of the present disclosure generally provide for a plurality of mechanical and electrical components. All references to the components and the functionality provided by each are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various components disclosed, such labels are not intended to limit the scope of operation for the components. Such components may be combined with each other and/or separated in any manner based on the particular type of implementation that is desired.
FIG. 1 shows a sectional view of a drive unit 200 for a loudspeaker. The sectional view is taken along a longitudinal axis 201 of the drive unit 200. Several of the below described components may have an axis of rotational symmetry, for example cylindrical or tubular components, and the axis of rotational symmetry of such a component may be aligned to the longitudinal axis 201.
The drive unit 200 comprises a first magnet 210 and a second magnet 220. Each of the first and second magnets 210, 220 may be a cylindrical bar magnet having, in an axis of rotational symmetry of a cylindrical shape of the bar magnet, a first end and an opposing second end. For example, the first magnet 210 has a first end 211 and an opposing second end 212, and the second magnet 220 has a first end 221 and an opposing second end 222. Each of the first and second magnets 210, 220 may have a first magnetic polarity at the first end and a second polarity different from the first polarity at the second end. The first and second magnets 210, 220 are arranged along the longitudinal axis 201 with the axis of rotational symmetry being aligned to the longitudinal axis 201. The first end 211 of the first magnet 210 faces the first end 221 of the second magnet 220 with a gap 230 between the first ends 211, 221 of the first and second magnets 210, 220. As shown in FIG. 1, the polarity at the first ends 211, 221 may be a magnetic north pole N and the polarity at the second ends 212, 222 may be a magnetic south pole S. As a result, the ends 211, 221 at the gap 230 have the same magnetic polarity. A length 213 of the first magnet 210 may be a few centimeters, for example in a range of 1 to 10 cm. A length 223 of the second magnet 220 may be essentially the same as the length 213 of the first magnet 210. A diameter of the first and second magnets 210, 220 may be a few millimeters or centimeters, for example in a range of 1 to 5 cm. A length 233 of the gap 230 may be a few millimeters, for example in a range of 1 to 20 mm. The first and second magnets 210, 220 may be permanent magnets comprising ferromagnetic materials, for example iron, nickel, cobalt and/or neodymium.
The drive unit 200 comprises furthermore a voice coil 240. The voice coil 240 comprises a tubular carrier 241 on which a plurality of coil windings 242 are arranged. The carrier 241 may be made of a non-magnetic material, for example paper, aluminium or plastics, like polyimide, for example Kapton. An inner diameter of the carrier 241 is larger than an outer diameter of the first and second magnets 210, 220. The voice coil 240 is movable in the direction of the longitudinal axis 201 as indicated by double arrow 243. The voice coil 240 may be movable along a part of the length 213 of the first magnet 210, the length 233 of the gap 230, and a part of the length 223 of the second magnet 220. The voice coil 240 may be elastically held in position along the longitudinal axis 201 by a membrane (not shown in FIG: 1) of the loudspeaker in which the drive unit 200 is mounted. In other examples, the voice coil 240 may be held in position by a suspension system (not shown) to a frame (not shown) of the loudspeaker.
The drive unit 200 comprises a first magnetic member 250 and a second magnetic member 260. Each of the first and second magnetic members 250, 260 may be a ring magnet with an essentially tubular shape. An inner diameter of the first and second magnetic members 250, 260 is larger than an outer diameter of the first and second bar magnets 210, 220. As shown in FIG. 1, an inner diameter of the first and second magnetic members 250, 260 may also be larger than an outer diameter of the voice coil 240. The first ring magnet 250 has an axis of rotational symmetry aligned to the longitudinal axis 201. Along the longitudinal axis 201, the first ring magnet has a first base 251 and an opposing second base 252. At the first base 251, a magnetic pole having the second magnetic polarity is provided. At the second base 252, a magnetic pole having the first magnetic polarity is provided. In the example shown in FIG. 1, the first ring magnet 250 provides a south pole S at the first base 251 and a north pole N at the second base 252. Likewise, the second ring magnet 260 has an axis of rotational symmetry aligned to the longitudinal axis 201. Along the longitudinal axis 201, the second ring magnet 260 has a first base 261 and an opposing second base 262. At the first base 261, a magnetic pole having the second magnetic polarity is provided. At the second base 262, a magnetic pole having the first magnetic polarity is provided. In the example shown in FIG. 1, the second ring magnet 260 provides a south pole S at the first base 261 and a north pole N at the second base 262.
The first ring magnet 250 may be arranged coaxially with the first magnet 210. As shown in FIG. 1, the first ring magnet 250 may be arranged along the length 213 of the first magnet 210. The first ring magnet 250 is arranged such that the first end 251 of the first ring magnet 250 is arranged closer to the first end 211 of the first magnet 210 than the second end 252 of the first ring magnet 250. As a result, the magnetic north pole N of the first ring magnet 250 is essentially at the same height as the magnetic south pole S of the first magnet 210, and the magnetic south pole S of the first ring magnet 250 is near the magnetic north pole N of the first magnet 210.
In a similar way, the second ring magnet 260 may be arranged with respect to the second magnet 220. The second ring magnet 260 may be arranged along the length 223 of the second magnet 220. The second ring magnet 260 is arranged such that the first end 261 of the second ring magnet 260 is arranged closer to the first end 221 of the second magnet 220 than the second end 262 of the second ring magnet 260. Thus, the magnetic north pole N of the second ring magnet 260 is at essentially the same height as the magnetic south pole S of the second magnet 220, and the magnetic south pole S of the second ring magnet 260 is near the magnetic north pole N of the second magnet 220.
The first and second ring magnets 250, 260 may be permanent magnets, comprising ferromagnetic materials, for example iron, nickel, cobalt and/or neodymium.
FIG. 2 shows a top view of the drive unit 200. As can be seen, the first magnet 210, the voice coil 240 and the first ring magnet 250 are arranged coaxially with respect to the longitudinal axis 201, i.e. the axes of rotational symmetry of each of the first magnet 210, the voice coil 240 and the first ring magnet 250 are aligned with the longitudinal axis 201. Although not visible in FIG. 2, the second magnet 220 and the second ring magnet 260 are arranged coaxially with respect to the longitudinal axis 201 also. As can be seen from FIG. 2, a radial distance 245 of an inner surface of the voice coil 240 is larger than a radial distance 214 of an outer surface of the first magnet 210. A radial distance 253 of an inner surface of the first ring magnet 250 is larger than a radial distance 246 of an outer surface of the voice coil 240.
The resulting magnetic fields are schematically shown by arrows in FIG. 1. The magnetic field will be concentrated in the area around the gap 230 between the two same magnetic poles N of the first and second magnets 210, 220 and are directed towards the opposite magnetic pole S of the ring magnets 250, 260. The voice coil 240 is located in the focused magnetic field around the gap 230 between the first and second magnets 210, 220 and will introduce a driving force on the voice coil 240 along the longitudinal axis 201 when an electrical current is running through the coil windings 242 of the voice coil 240. The orientation of the driving force depends on the direction of the electrical current which allows oscillating movements of the voice coil 240 in the direction of arrow 243 in response to an electrical signal input, for example an alternating current.
The ring magnets 250, 260 may increase the magnetic field (B field) in the area of the voice coil 240 due to the additional magnetic force provided by the magnetic poles S of the ring magnets 250, 260. Furthermore, the magnetic field in the area of the voice coil 240 may be increased by guiding the magnetic field from magnetic poles N the first and second magnets 210, 220 to the magnetic poles S of the ring magnets 250, 260. These effects may contribute to increase the sensitivity of a loudspeaker including the drive unit 200. Furthermore, a larger area for the focused magnetic air gap may be provided which may enable a more linear behavior of the drive unit, for example the drive unit 200 may provide a more linear excursion. Additionally, as shown in FIG. 1, the edges of the first ends 251, 261 of the ring magnets 250, 260 may be formed in order to optimize linearity of the drive unit 200, for example, the inner edges of the first ends 251, 261 may be slanted or curved.
FIG. 3 shows the drive unit 200 in connection with a loudspeaker 100. The loudspeaker 100 comprises a frame 110, a membrane 120 and the drive unit 200. For reasons of clarity, reference signs of several of the components discussed above in connection with FIG. 1 are omitted in FIG. 3. However, those components may also be present in the drive unit 200 shown in FIG. 3 with essentially the same properties and functions as discussed above unless noted otherwise. In the following description, reference to those components may be made by referring to the corresponding reference signs. An inner edge of the membrane 120, which is also called diaphragm, is attached to the carrier 241 of the voice coil 240. At an outer edge of the membrane 120, the membrane 120 is coupled to the frame 110 via a suspension 130 as it is known in the art. A tubular support 270 may enclose the first and second magnets 210, 220 to keep the first and second magnets 210, 220 in position with the gap 230 between them. The support 270 may be made of non-magnetic material, for example a paramagnetic, diamagnetic, or antiferromagnetic material. Near the second end 212 of the first magnet 210 and the second end 252 of the first ring magnet 250, a ring-shaped spacer 271 may be provided to keep the first ring magnet 250 in position with respect to the first magnet 210. In the same way, near the second end 222 of the second magnet 220 and the second end 262 of the second ring magnet 260, a ring shaped spacer 272 may be provided to keep the second ring magnet 260 in position with respect to the second magnet 220. The frame 110 may be mounted at the second ring magnet 260, the spacer 272 and/or the second magnet 220. A cover (not shown) may be provided at the membrane 120 which hides the first ring magnet 250, the first magnet 210 and the spacer 271 from being seen from an outside of the loudspeaker 100.
FIG. 4 shows a further drive unit 200. For reasons of clarity, reference signs of several of the components discussed above in connection with FIG. 1 are omitted in FIG. 4. However, those components may also be present in the drive unit 200 shown in FIG. 4 with essentially the same properties and functions as discussed above unless noted otherwise. In the following description, reference to those components may be made by referring to the corresponding reference signs. In addition to the drive unit 200 shown in FIG. 1 and FIG. 3, the drive unit 200 of FIG. 4 comprises a first connecting element 281 and a second connecting element 282. Each of the first and second connecting elements 281, 282 may comprise, for example, a disk shaped plate of ferromagnetic material, for example iron, cobalt or nickel. An axis of rotational symmetry of the disk shaped plate of each of the first and second connecting elements 281, 282 may be aligned to the longitudinal axis 201. The first connecting element 281 essentially covers the second end 252 of the first ring magnet 250 and the second end of the first magnet 210 such that a magnetic field is guided between the magnetic poles at the second ends 252, 212 of the first ring magnet 250 and the first magnet 210, i.e. in FIG. 4 between the north pole N of the first ring magnet 250 and the south pole S of the first bar magnet 210. The second connecting element 282 essentially covers the second end 262 of the second ring magnet 260 and the second end 222 of the second magnet 220 such that the magnetic field is guided between the magnetic poles at the second ends 262, 222 of the second ring magnet 260 and the second magnet 220,i.e. in FIG. 4 between the north pole N of the second ring magnet 260 and to the south pole S of the second magnet 220. The first and second connecting elements 281, 282 may reduce the magnetic field outside the drive unit 100.
FIG. 5 shows another example of a drive unit 200. For reasons of clarity, reference signs of several of the components discussed above in connection with FIG. 1 are omitted in FIG. 5. However, those components may also be present in the drive unit 200 shown in FIG. 5 with essentially the same properties and functions as discussed above unless noted otherwise. In the following description, reference to those components may be made by referring to the corresponding reference signs. The drive unit 200 shown in FIG. 5 does not comprise the first and second ring magnets 250, 260. Instead, the drive unit 200 comprises a first ferromagnetic member 291 and a second ferromagnetic member 295. Each of the ferromagnetic members 291, 295 has a cup-like shape and is made of a ferromagnetic material, for example iron, cobalt or nickel. In other words, each cup-like shaped ferromagnetic member 291, 295 has a hollow cylindrical shape with a corresponding bottom plate 292, 296 at one end of the hollow cylindrical shape and an opposing open end 293, 297. The cup-like shaped ferromagnetic members 291, 295 may each have an axis of rotational symmetry which is aligned to the longitudinal axis 201. The bottom plate 292 of the cup-like shaped first ferromagnetic member 291 is in contact with the second end 212 of the first magnet 210 and a cylindrical sidewall of the cup-like shaped first ferromagnetic member 291 extends into the direction of the gap 230. The bottom plate 296 of the cup-like shaped second ferromagnetic member 295 is in contact with the second end 222 of the second magnet 220 and a cylindrical sidewall of the cup-like shaped second ferromagnetic member 295 extends into the direction of the gap 230.
In the example shown in FIG. 5, the bottom plates 292, 296 of the first and second ferromagnetic members 291, 295 are in contact with the magnetic south poles S of the corresponding first and second magnets 210, 220. As a result, the edges at the open ends 293, 297 of the cup-like shaped first and second ferromagnetic members 291, 292 provide a magnetic south pole S.
The effects achieved by the magnetic poles at the open ends 293, 297 of the cup-like shaped first and second ferromagnetic members 291, 295 are in principle the same as the effects of the magnetic poles at the first ends 251, 261 of the first and second ring magnets 250, 260. However, the effects of the magnetic poles at the first ends 251, 261 of the first and second ring magnets 250, 260 may be stronger than the effects achieved by the magnetic poles at the open ends 293, 297 of the cup-like shaped first and second ferromagnetic members 291, 295.

Claims

A drive unit for a loudspeaker, the drive unit (200) comprising:
- a first magnet (210) and a second magnet (220) arranged along a longitudinal axis (201) of the drive unit (200) with a gap (230) between the first and second magnets (210, 220), each magnet of the first and second magnets (210, 220) having, along the longitudinal axis (201), a first end (211, 221) with a first magnetic polarity (N) and an opposing second end (212, 222) with a second magnetic polarity (S), the first and second polarities being different, wherein, at the gap (230), the first end (211) of the first magnet (210) faces the first end (221) of the second magnet (220),

- a voice coil (240) movable along the longitudinal axis (201) and positionable at least between the first end (211) of the first magnet (210) and the first end (221) of the second magnet (220), and

- at least one additional magnetic member (250, 260, 291, 295) providing at least one additional magnetic pole located at a radial distance (253) to the longitudinal axis (201) greater than a radial distance (246) of the voice coil (240) to the longitudinal axis (201), wherein the at least one additional magnetic pole has the second magnetic polarity (S).
The drive unit of claim 1, wherein the first magnet (210) has a first magnet length (213) along the longitudinal axis (201), the second magnet (220) has a second magnet length (223) along the longitudinal axis (201), and the gap (230) has a gap length (233) along the longitudinal axis (201),
wherein at least one (250) of the at least one additional magnetic member (250, 260, 291, 295) provides the at least one additional magnetic pole along the longitudinal axis (201) within the first magnet length (213).
The drive unit of claim 1 or claim 2, wherein the at least one additional magnetic member (250, 260, 291, 295) comprises a ring magnet (250) having an inner diameter larger than an outer diameter of the first magnet (210), wherein the at least one additional magnetic pole is at a first base (251) of the ring magnet.
The drive unit of claim 3, wherein the ring magnet (250) has a second base (252) opposite to the first base (251), wherein a further magnetic pole at the second base (252) has the first polarity (N).
The drive unit of claim 4, wherein a distance between the first base (251) of the ring magnet (250) and the gap (230) is smaller than a distance between the second base (252) of the ring magnet (250) and the gap (230).
The drive unit of any one of claims 3-5, wherein the ring magnet (250) is attached to the first magnet (210) via a spacer (271) of non-magnetic material.
The drive unit of any one of claims 3-6, wherein the ring magnet (250) is coupled to the first magnet (210) via a connecting element (281) of magnetic material.
The drive unit of any one of claims 3-7, wherein an axis of rotational symmetry of the ring magnet (250) is aligned with the longitudinal axis (201) of the drive unit (200).
The drive unit of any one of the preceding claims, wherein the at least one additional magnetic member (250, 260, 291, 295) comprises a ferromagnetic member (291) coupled to the second end (212) of the first magnet (210).
The drive unit of claim 9, wherein the ferromagnetic member (291) has a cup-like shape with a bottom (292) of the cup-like shape being coupled to the second end (212) of the first magnet (210) and an annular edge (293) of the cup-like shape having an inner diameter larger than an outer diameter of the first magnet (210).
The drive unit of claim 9 or claim 10, wherein a distance between the annular edge (293) and the gap (230) is smaller than a distance between the bottom (292) and the gap (230).
The drive unit of any one of claims 9-11, wherein an axis of rotational symmetry of the cup-like shape is aligned with the longitudinal axis (201) of the drive unit (200).
The drive unit of any one of the preceding claims, wherein the at least one additional magnetic member (250, 260, 291, 295) has an oblique surface facing the gap (230).
The drive unit of any one of the preceding claims, further comprising
- at least one further magnetic member (260, 295) providing at least one additional magnetic pole located at a radial distance to the longitudinal axis (201) greater than a radial distance of the voice coil (240) to the longitudinal axis (201), wherein the at least one additional magnetic pole has the second polarity (S), wherein the at least one further magnetic member (260, 295) has corresponding features with respect to the second magnet (220) as the at least one additional magnetic member (250, 291) with respect to the first magnet (210) as defined in any one of claims 2-13.
The drive unit of any one of the preceding claims, wherein an axis of rotational symmetry of the voice coil (240) is aligned with the longitudinal axis (201) of the drive unit (200).
The drive unit of any one of the preceding claims, wherein the first and second magnets (210, 220) each have a cylindrical shape, wherein an axis of rotational symmetry of each of the cylindrical first and second magnets (210, 220) is aligned with the longitudinal axis (201) of the drive unit (200).
A loudspeaker comprising:
- a frame (110),

- a drive unit (200) as defined in an one of the preceding claims, the drive unit (200) being mounted at the frame (110), and

- a membrane (120) coupled to the frame (110) and the drive unit (200).
The loudspeaker of claim 17, wherein the voice coil (240) comprises a tubular carrier (241) and coil windings (242) on the tubular carrier (241), wherein the membrane (120) is mounted at the tubular carrier (241).