Loudspeaker with passively controlled voice coil sections

Abstract

A method for driving a voice coil of a loudspeaker may include providing a magnetic circuit having an air gap, providing a voice coil suspended in the air gap, and applying an audio signal to the voice coil to move the voice coil along a travelling axis. The voice coil comprises a center voice coil section, an upper voice coil section, and a lower voice coil section arranged on respective sides of the center voice coil section. A center driving signal is provided to the center voice coil and an upper rectified driving signal, attenuating a first direction of current, and a lower rectified driving signal, attenuating a second direction of current, are provided respectively to the upper and lower voice coil sections. The invention further relates to a voice coil driving system and a loudspeaker comprising a voice coil driving system.

Claims

1. A method for driving a voice coil of a loudspeaker comprising: providing a magnetic circuit having an air gap and a voice coil suspended in said air gap; and applying an audio signal to said voice coil suspended in said air gap to produce an electromotive force moving said voice coil along a travelling axis; wherein said voice coil comprises a plurality of voice coil sections, arranged along said travelling axis, said plurality of voice coil sections comprising a center voice coil section, an upper voice coil section and a lower voice coil section, said upper voice coil section and said lower voice coil section arranged respectively on either side of said center voice coil section; wherein said applying said audio signal comprises providing a center driving signal based on said audio signal to said center voice coil section and providing an auxiliary driving signal based on said audio signal to said upper voice coil section and said lower voice coil section, said providing an auxiliary driving signal comprising providing an upper rectified driving signal to said upper voice coil section and providing a lower rectified driving signal to said lower voice coil section; wherein said upper rectified driving signal is provided by attenuating or blocking, a first direction of current of said auxiliary driving signal by passive rectification; wherein said lower rectified driving signal is provided by attenuating, such as blocking, a second direction of current of said auxiliary driving signal by passive rectification; and wherein said first and second direction of current of said auxiliary driving signal are opposite directions of current.

2. The method for driving a voice coil according to claim 1, wherein said upper rectified driving signal is provided by rectifying said auxiliary driving signal in said first direction of current and wherein said lower rectified driving signal is provided by rectifying said auxiliary driving signal in said second direction of current.

3. The method for driving a voice coil according to claim 2, wherein said rectifying said auxiliary driving signal is half-wave rectification.

4. The method for driving a voice coil according to claim 1, wherein said providing said upper rectified driving signal comprises processing said auxiliary driving signal using an upper rectifying unit, and said providing said lower rectified driving signal comprises processing said auxiliary driving signal using a lower rectifying unit.

5. The method for driving a voice coil according to claim 4, wherein said upper rectifying unit comprises a passive rectifying circuit, and wherein said lower rectifying unit comprises a passive rectifying circuit.

6. The method for driving a voice coil according to claim 5, wherein each of said upper rectifying unit and said lower rectifying unit comprises a half-wave rectifier circuit.

7. The method for driving a voice coil according to claim 6, wherein neither said upper rectifying unit nor said lower rectifying unit comprises a full-wave rectifier circuit or a full-wave bridge rectifier.

8. The method for driving a voice coil according to claim 6, wherein said upper rectifying unit comprises a diode and wherein said lower rectifying unit comprises a diode.

9. The method for driving a voice coil according to claim 8, wherein said diodes are coupled with forward voltage drop compensation.

10. The method for driving a voice coil according to claim 1, wherein said center driving signal and said auxiliary driving signal are provided by one or more amplifiers.

11. The method for driving a voice coil according to claim 10, wherein said one or more amplifiers is a common amplifier.

12. The method for driving a voice coil according to claim 1, wherein said providing an upper rectified driving signal comprises amplifying said auxiliary driving signal and wherein said providing a lower rectified driving signal comprises amplifying said auxiliary driving signal.

13. The method for driving a voice coil according to claim 1, wherein said auxiliary driving signal has an amplitude different from said center driving signal.

14. The method for driving a voice coil according to claim 1, wherein said auxiliary driving signal has the same amplitude as said center driving signal.

15. The method for driving a voice coil according to claim 1, wherein said upper voice coil section is a first upper voice coil section, said lower voice coil section is a first lower voice coil section, said upper rectified signal is a first upper rectified signal, said lower rectified signal is a second lower rectified signal and said auxiliary driving signal is a first auxiliary driving signal; wherein said plurality of voice coil sections arranged along said travelling axis further comprises a second upper voice coil section and a second lower voice coil section, said second upper voice coil section and said second lower voice coil section arranged respectively on either side of the group of voice coil sections comprising said first upper voice coil section, said center voice coil section, and said first lower voice coil section; wherein said step of applying an audio signal further comprises providing a second auxiliary driving signal based on said audio signal to said second upper voice coil section and said second lower voice coil section, said providing a second auxiliary driving signal comprising providing a second upper rectified driving signal to said second upper voice coil section and providing a second lower rectified driving signal to said second lower voice coil section; wherein said second upper rectified driving signal is provided by attenuating, such as blocking, a first direction of current of said second auxiliary driving signal by passive rectification; and wherein said second lower rectified driving signal is provided by attenuating, such as blocking, a second direction of current of said second auxiliary driving signal by passive rectification.

16. The method for driving a voice coil according to claim 1, wherein said upper voice coil section is displaced with respect to the center voice coil section along a first displacement direction along said travelling axis, said lower voice coil section is displaced with respect to the center voice coil section along a second displacement direction along said travelling axis, and said upper voice coil section and said lower voice coil section are arranged symmetrically around said center voice coil section, wherein said first displacement direction and said second displacement direction are opposite directions along said travelling axis.

17. The method for driving a voice coil according to claim 1, wherein heights of said voice coil sections are individually one or smaller than, greater than, or of the same height as the height of said air gap along said travelling axis.

18. A voice coil driving system of a loudspeaker comprising: a magnetic circuit having an air gap; a voice coil suspended in said air gap, said voice coil comprising a plurality of voice coil sections, arranged along said travelling axis, said plurality of voice coil sections comprising a center voice coil section, an upper voice coil section and a lower voice coil section, said upper voice coil section and said lower voice coil section being arranged respectively on either side of said center voice coil section; two passive rectifying units each arranged to provide respectively either an upper rectified driving signal to said upper voice coil section based on said auxiliary driving signal or a lower rectified driving signal to said lower voice coil section based on said auxiliary driving signal, wherein an upper rectifying unit of said two passive rectifying units is arranged to process said auxiliary driving signal by attenuating or blocking a first direction of current of said auxiliary driving signal to provide said upper rectified driving signal, wherein a lower rectifying unit of said two passive rectifying units is arranged to process said auxiliary driving signal by attenuating, such as blocking, a second direction of current of said auxiliary driving signal to provide said lower rectified driving signal, and wherein said first and second direction of current of said auxiliary driving signal are opposite directions of current.

19. The voice coil driving system according to claim 18, wherein said one or more passive rectifying units comprise one or more rectifying circuits.

20. The voice coil driving system according to claim 19, wherein said one or more passive rectifying units comprise one or more diodes.

21. The voice coil driving system according to claim 18, wherein said voice coil driving system is arranged to receive an audio signal and provide a center driving signal based on said audio signal to said center voice coil section, and provide an auxiliary driving signal based on said audio signal to said one or more passive rectifying units.

22. A loudspeaker comprising a diaphragm; an interface configured to receive an audio signal; and a voice coil driving system according to claim 18.

23. The loudspeaker according to claim 22, wherein said loudspeaker comprises one or more amplifiers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments of the invention will in the following be described with reference to the drawings where

(2) FIG. 1a-1b illustrate a conventional loudspeaker design,

(3) FIG. 2 illustrates a loudspeaker with a voice coil driving system comprising a center voice coil section, an upper voice coil section, and a lower voice coil section according to an embodiment of the invention,

(4) FIG. 3a-3c illustrate the reciprocating translation of the voice coil according to embodiments of the invention comprising three voice coil sections,

(5) FIG. 4a-4b illustrate various configurations for providing a center driving signal and an auxiliary driving signal to the plurality of voice coil sections according to embodiments of the invention,

(6) FIG. 5 illustrates a loudspeaker according to an embodiment of the invention,

(7) FIG. 6a-6c illustrate various configurations for applying a center driving signal and an auxiliary driving signal to the plurality of voice coil sections according to the invention,

(8) FIG. 7a-7b illustrate configurations of the voice coil sections according to various embodiments of the invention, and

(9) FIG. 8 illustrates a preferred processing of the driving signal according to various embodiments of the invention.

DETAILED DESCRIPTION

(10) FIG. 1a shows a cut-through view of a conventional loudspeaker. FIG. 1b illustrates a section view at the line 1b-1b in FIG. 1a. Disposed within the loudspeaker are two concentrically aligned magnetic members 2 forming a magnetic circuit. These magnetic members 2 may be arranged such that a circular air gap 3 is formed within the magnetic circuit 2.

(11) A voice coil 1 comprising a plurality of coil windings may be further suspended within the air gap 3. The windings of the voice coil 1 may be arranged such that when an electric current is passed through the coil 1, an electromotive force may translate the voice coil 1 within the air gap 3, such that a membrane or diaphragm 7 is actuated. An alternating current may cause a reciprocating movement of the diaphragm 7, which generates an acoustic sound signal.

(12) Referring to FIG. 2, a voice coil driving system 51 according to some embodiments of the invention is illustrated. The voice coil driving system 51 comprises a magnetic circuit 2 formed by two concentrically aligned magnetic members 2. The magnetic members may be permanent magnets or metal poles. The magnetic circuit 2 may be arranged such that a circular air gap 3 is formed within the magnetic circuit 2 terminated by the two magnetic members 2. The circular air gap 3 is a volume of air which takes on the form of a volume disposed between two axially aligned cylinders of different widths.

(13) Various alternative voice coil based loudspeaker configurations may be used or incorporated with the principles described herein relative to the disclosed embodiments. For example, such configurations may include variations of magnetic circuits and air gaps, including various configurations of permanent magnets, pole pieces, front and back plates, casing, and various configurations of air gaps, including circular air gaps, as described above, linear, polygonal, irregular, one or several air gaps, etc. The present invention is not limited to the magnetic circuit and air gap configuration illustrated in the drawings but may readily be applied to other voice coil based transducers.

(14) A voice coil 1 may be suspended within the air gap 3. The voice coil 1 comprises a plurality of voice coil sections 21-22b, wherein a center voice coil section 21 may be centrally arranged surrounded by an upper voice coil section 21a and a lower voice coil section 21b arranged respectively on either side of said main voice coil section. The voice coil sections may be axially aligned along a travelling axis 4 of the voice coil 1. Each voice coil section comprises a plurality of metal windings coiling around the inner magnetic member 2 and a travelling axis 4 as seen in FIG. 2. The voice coil sections may be mechanically coupled, but not necessarily electrically coupled, to form the voice coil 1. The mechanical coupling may comprise a support such as a tube, mesh or wire structure of paperboard, plastic or metal, e.g. a foil.

(15) The voice coil sections 21-22b may be configured such that when an electric current is passed through a voice coil section 21-22b, located at least partly within the air gap 3 of the magnetic circuit 2, an electromotive force may translate the particular voice coil section 21-22b along the travelling axis 4. Since voice coil sections 21-22b are mechanically coupled members of the voice coil 1, an electromotive force generated by any of voice coil sections 21-22b may translate the entire voice coil 1 along the travelling axis 4. The translation of the voice coil 1 along the travelling axis 4 may result in the voice coil 1 pushing and pulling a diaphragm 7 of a loudspeaker 50. The movement of the diaphragm 7 generates an acoustic sound signal.

(16) The coupling of the voice coil 1 and the diaphragm 7 may be established by the above-mentioned mechanical coupling of the voice coil section, e.g. a plastic foil tube, or may involve further support members (e.g., a spider and a diaphragm surround, or other configurations). The idle position of the voice coil 1 may be controlled by the support members, such as a spider and/or a diaphragm surround and frame. The total voice coil height may be centered in the air gap when idle, so that with three voice coil sections, as illustrated in the examples, the middle voice coil section may be aligned with the magnetic circuit and air gap.

(17) Since the magnetic field is substantially disposed within the air gap 3 in the magnetic circuit 2, only voice coil sections 21-22b that are at least partly positioned within the air gap may generate a substantive electromotive force upon application of an electric current. Generally, the more of a particular voice coil section is contained within the air gap, the higher the force generated upon electric current application. Referring to the particular arrangement shown in FIG. 2, only voice coil section 21 is fully or partially disposed within the air gap 3, whereas voice coil sections 22a and 22b are disposed completely outside the air gap 3. Since the magnetic field density is highest within the air gap 3, and quickly decreasing outside the air gap 3, voice coil section 21 may generate a substantial electromotive force to translate the voice coil 1, whereas voice coil sections 22a and 22b are located sufficiently far away from the air gap 3 that the efficiency in converting electric driving power into electromotive force is considerably lower, and practically insignificant, compared to the efficiency of converting electric driving power into electromotive force for voice coil section 21.

(18) Windings of voice coil sections 21-22b, located away from the air gap 3 may contribute to voice coil heating when an electric driving power is applied, but may only contribute marginally in translating the voice coil 1 along the travelling axis 4. Therefore, as described above, it may be advantageous to avoid applying power to voice coil sections 21-22b, e.g. 22a-22b, that are not at least partly disposed within the air gap at a particular time.

(19) Generally, a loudspeaker system aims to reproduce an audio signal 30 in the excursion of a voice coil 1, where the excursion refers to the position of the voice coil 1 relative to its resting position. An audio signal 30 may comprise a representation of varying sound intensities, which may require varying excursions for reproduction. Thus, an audio signal 30 may require a range of excursions which can be established by utilizing the electromotive force which can be generated by the center voice coil section 21, for the audio signal 30 to be reproduced. The reproduction of another audio signal 30 may require a range of excursions which can be established by utilizing the center voice coil section 21 together with the upper voice coil section 22a and the lower voice coil section 22b.

(20) Referring to FIG. 2, each voice coil section 21-22b may receive a driving signal 41, 43a-43b. These driving signals may be provided by any means, e.g. by a controller, by an amplifier, or by an external source. The center voice coil section 21 may receive a center driving signal 41, the upper voice coil section 22a may receive an upper rectified driving signal 43a, and the lower voice coil section 22b may receive a lower rectified driving signal 43b. The upper rectified driving signal 43a and the lower rectified driving signal 43b may be based on an auxiliary driving signal 42. The auxiliary driving signal 42 and the center driving signal 41 may be based on, or even identical with, the audio signal 30.

(21) The driving signals 41, 43a-43b may include the actual electric signals that pass through the respective voice coil sections 21-22b to generate electromotive force to translate the voice coil 1 along the travelling axis 4. The driving signals 41-43b may be provided to the voice coil sections 21-22b via channels, e.g. cables or wires, or other electrical connections.

(22) In the exemplary embodiment shown in FIG. 2, the center driving signal 41 and the auxiliary driving signal 42 may include the audio signal 30, but other configurations may be used.

(23) The upper and lower rectified driving signals 43a-43b may be derived by rectification units 16, based on the auxiliary driving signal 42. A rectifying unit or a passive rectifying unit may process current asymmetrically. For example, a rectifying unit may respond in a first way to a current in a first direction and may respond in a second, different way to a current in a second, different direction.

(24) Preferably, a rectification unit may attenuate or block one direction of current and allow the opposite direction of current to pass.

(25) An example of a rectifying unit 16 may include a diode 17, which may include an asymmetric conductance, i.e. low resistance for one direction of current, and higher resistance for the opposite direction of current, within the current and/or voltage limitations of the diode.

(26) The two rectification units 16 of the exemplary embodiment shown in FIG. 2 may be implemented to block opposite directions of current. As such, when current of an auxiliary driving signal 42 flows in a one direction, this current may substantially flow to the lower voice coil section 22b, whereas its flow to the upper voice coil 22a section may be blocked. Similarly, when current of an auxiliary driving signal flows in the opposite direction, this current may substantially flow to the upper voice coil section 22a, whereas its flow to the lower voice coil section 22b may be blocked. This is further described below.

(27) For one direction of driving signal current, the lower voice coil section 22b, whose resting position is completely or partly outside the air gap, may be translated towards the air gap, while for the opposite direction of current, it is translated away from the air gap. By implementation of a rectification unit 16, it is possible to attenuate or block current of the auxiliary driving signal 42 to the lower voice coil section 22b when it is translated away from the air gap, where it is not able to generate a substantial electromotive force. The same principle applies to upper voice coil section 22a, where, for example, the direction in which the current should be attenuated or blocked should be opposite.

(28) Since each direction of current of the driving signals can be related to a certain direction of translation or excursion of the voice coil, depending on the magnetic field orientation, the rectification units may thus be utilized to restrict current to the upper and lower voice sections 22a-22b when they are translated away from the air gap. Notably, the upper rectified driving signal 43a and lower rectified driving signal 43b may be arranged to provide current flowing in an appropriate direction of current to produce an electromotive force on the voice coil 1 such that the applied audio signal 30 is correctly reproduced by the excursion of the voice coil.

(29) The voice coil sections 21-22b of voice coil 1 may be configured in multiple ways, depending on the relative positioning and dimensioning of the voice coil sections 21-22b. In the illustrated embodiments, the voice coil 1 comprises three voice coil sections 21-22b, but various other numbers of voice coil sections and voice coil section geometries may be used. For example, in various embodiments, voice coil geometry may be varied, or the number of included voice coils may be increased (e.g., to five voice coil sections). Voice coil configurations including five voice coils may include one center voice coil section, two upper voice coil sections, and two lower voice coil sections, where each of the upper and lower voice coil sections may be connected to different rectification units.

(30) Referring to FIGS. 3a-3c, the dynamical behavior of the voice coil 1 is illustrated according to some embodiments of the invention.

(31) FIG. 3a shows a voice coil 1 comprising a center voice coil section 21, an upper. voice coil section 22a, and a lower voice coil section 22b, at an instant of time during application of an audio signal. At the represented time, the excursion required to reproduce the audio signal can be generated by the center voice coil section 21 alone. Depending on the direction of current of the auxiliary driving signal 42, current may be attenuated or blocked towards either the upper voice coil section 22a or the lower voice coil section 22b.

(32) In some embodiments, a current and/or voltage threshold may be implemented such that no current is provided to either the upper voice coil section 22a or the lower voice coil section 22b, at instances of time similar as that shown in FIG. 3a.

(33) In FIG. 3b, an audio signal is applied such that the voice coil 1 has been translated in an upward direction 5 at the shown instance of time. As such, the current of the center driving signal 41 and the auxiliary driving signal 42 may have a direction of flow, which is attenuated or blocked towards the upper voice coil section 22a.

(34) Alternatively, in FIG. 3c, an audio signal is applied such that the voice coil 1 has been translated in a downward direction 6 at the represented instant of time. As such, the current of the center driving signal 41 and the auxiliary driving signal 42 may have a direction of flow, which is attenuated or blocked towards the lower voice coil section 22b.

(35) FIGS. 4a-4b show various embodiments, both including amplification means.

(36) In FIG. 4a, an audio signal 30 is provided to an amplifier 12 via an interface 52. The amplifier 12 is powered by a power supply unit 13, e.g. a battery, a DC power supply, or an AC-to-DC power supply. The amplifier 12 has two output channels where the center driving signal 41 and the auxiliary driving signal are provided, respectively. According to the invention, the amplifier 12 may provide two identical amplifications or two different amplifications to provide the two outputs. The center driving signal 41 is sent to the center voice coil section 21, and the auxiliary driving signal 42 is sent to rectification units 16 to generate an upper rectified signal 43a and a lower rectified signal 43b, which are provided to the upper voice coil section 22a and the lower voice coil section 22b, respectively.

(37) In FIG. 4b, an audio signal 30 is provided to an amplifier 12 via an interface 52, where the amplifier 12 is powered by a power supply unit 13. The amplifier 12 has one output channel, where a signal, which serves as both a center driving signal 41 and an auxiliary driving signal 42, is provided. One channel guides the signal to the center voice coil section 21, and two other channels are attached to diodes 17, which act as rectification units 16 in the embodiment shown. The diodes 17 are attached with opposite directionalities, such that one direction of current is primarily provided to the lower voice coil section 22b, while the opposite direction of current is primarily provided to the upper voice coil section 22a.

(38) Utilizing one or more diodes 17 in rectification units 16 is not limited to embodiments where an amplifier 12 with one or more output channels is include, as diodes may be used in rectification units included together with any of the disclosed embodiments.

(39) FIG. 5 represents a loudspeaker 50. The loudspeaker 50 receives an audio signal 30, which is applied to a voice coil driving system 51.

(40) A loudspeaker according to the invention may include a passive loudspeaker which may not require a power source but may include a pre-amplified audio signal, or it may include an active loudspeaker which may include a power source, e.g. for internal amplification for example when receiving a line level or digital audio signal.

(41) FIGS. 6a-6c illustrate various configurations of applying the center driving signal 41 and the auxiliary driving signal 42, according to embodiments of the invention.

(42) In FIG. 6a, an incoming driving signal 46 is provided to the voice coil 1. This driving signal 46 is provided to the center voice coil section 21 as a center driving signal 41 and provided to two diodes 17 to generate an upper rectified driving signal 43a and a lower rectified driving signal 43b. The diodes 17 are mounted with opposite directionality, such that the upper rectified driving signal 43a and a lower rectified driving signal 43b respectively comprise currents flowing in opposite directions. Current delivered to the voice coil sections 21-22b are guided out via output channels 44a-45. These channels are electrically connected to provide a single current output 47 of the voice coil 1.

(43) In FIG. 6b, an incoming driving signal 46 is provided to the voice coil 1. In this case, driving signal 46 is provided only to the center voice coil section 21 as a center driving signal 41. The current of the signal leaves the center voice coil section 21 via an output channel 45. This output channel provides the auxiliary driving signal 42, which is provided to two diodes 17 to generate an upper rectified driving signal 43a and a lower rectified driving signal 43b. The diodes 17 are mounted with opposite directionality, such that the upper rectified driving signal 43a and a lower rectified driving signal 43b respectively comprise currents flowing in opposite directions. The current of the upper and lower rectified driving signals 43a-43b, leaves the upper and lower voice coil sections 22a-22b through output channels 44a-44b, which are electrically connected to provide a single current output 47 of the voice coil 1.

(44) In FIG. 6c, a center driving signal 41 and an auxiliary driving signal 42 may be provided to the voice coil 1. The center driving signal is provided to the center voice coil section 21, whereas the auxiliary driving signal 42 is provided to two diodes 17 to generate an upper rectified driving signal 43a and a lower rectified driving signal 43b. The diodes 17 are mounted with opposite directionality, such that the upper rectified driving signal 43a and a lower rectified driving signal 43b respectively comprise currents flowing in opposite directions.

(45) The current of the center driving signal 41 leaves the center voice coil section 21 through output channel 45, and the current of the upper and lower rectified driving signals 43a-43b, leaves the upper and lower voice coil sections 22a-22b through output channels 44a-44b, which are electrically connected to provide a single auxiliary current output 48.

(46) The embodiments shown in FIGS. 6a-6c may utilize diodes 17 as rectification units 16, but the disclosed embodiments are not limited to using diodes 17. Furthermore, the disclosed embodiments are examples of providing rectified driving signals through passive rectifications, but other configurations may be used.

(47) FIG. 7a shows an alternative embodiment of the invention with a voice coil 1 comprising five distinct voice coil sections, including a second upper voice coil section 24a, a first upper voice coil section 23a, a center voice coil section 21, a first lower voice coil section 23b, and a second lower voice coil section 24b. The upper and lower voice coil sections 23a-24b may be connected to rectifying units 16 similarly as described in the above with relation to a voice coil 1 comprising an upper voice coil section 22a and a lower voice coil section 22b. In other words, a voice coil 1 comprising five distinct voice coil sections may comprise, e.g., four rectifying units 16. In this embodiment, four rectifying units 16, such as diodes 17, may be used, however in other embodiments of the invention, fewer rectifying units 16 may be used for a voice coil 1 comprising five distinct voice coil sections. As an example, one or more rectifying circuits, e.g., two rectifying circuits, may be used to provide rectified driving signals to the voice coil section.

(48) As such, when one or more driving signals are provided to the voice coil 1, a first direction of current may be provided to the first lower voice coil section 23b and the second lower voice coil section 24b, and a second direction of current may be provided to the first upper voice coil section 23b and the second upper voice coil section 24b.

(49) Rectifying units 16 providing rectified signals to upper and lower voice coil sections 23a-24b may have different current and/or voltage thresholds. Such thresholds may ensure that current is not provided to voice coil sections when it is not required. As such, when the voice coil sections 24a-24b are not required to generate an electromotive force to aid in the movement of the voice coil 1, rectification units 16 connected to the outer voice coil sections 24a-24b may attenuate or block current. Furthermore, when the voice coil sections 23a-23b are not required to generate an electromotive force to reproduce an applied audio signal, rectifying units 16 connected to the voice coil sections 23a-23b may attenuate or block current. Current and/or voltage thresholds may be chosen such that the audio signal can be reproduced without distortion within the limitations of the full voice coil 1, while minimal current may be supplied to voice coil sections which are not substantially within the air gap and are therefore not able to generate a significant electromotive force.

(50) FIG. 7b shows another alternative embodiment of the invention, where the height of individual voice coil sections of the voice coil 1 is smaller than the air gap 3 along the direction of the travelling axis 4.

(51) Either the upper voice coil section 22a or the lower voice coil section 22b may be able to generate an electromotive force to translate the voice coil 1 for very large excursions when the center voice coil section 21 is substantially outside the air gap, i.e. when the center voice coil section 21 leaves the air gap and another voice coil section 22a-22b enters. In such embodiments, an audio signal 30 may be reproduced by the excursion without distortion, even though the center voice coil section 21 leaves the air gap 3.

(52) FIG. 8 represents a method of generating an upper rectified driving signal 43a and a lower rectified driving signal 43b based on an auxiliary driving signal 42.

(53) Four panels P1-P4 display representations of the driving signals at different stages. These representations describe the amplitude of the driving signals as a function of time. The zero point shown on the four panels P1-P4 illustrates points in time of a driving signal where no current flows in respective voice coil sections.

(54) In the exemplary embodiment shown in FIG. 8, a center driving signal 41 is provided to a center voice coil section 21, with a representation of the center driving signal shown in panel P1. Additionally, an auxiliary driving signal 42 is provided, with a representation of the auxiliary driving signal 42 shown in panel P2. In this embodiment the representations of the center driving signal in panel P1 and the auxiliary driving signal in panel P2 are displayed to have similar amplitudes. In various other embodiments, the actual current and/or voltage of the center driving signal and of the auxiliary driving signal may not be the same, i.e. the signals may differ in amplitude/gain, etc.

(55) The auxiliary driving signal 42 may be provided to rectification units 16 to generate an upper rectified driving signal 43a and a lower rectified driving signal 43b. Thus, as the auxiliary driving signal 42 is provided, a first direction of current may be provided to the lower voice coil section 22b and a second, different direction of current may be provided to the upper voice coil section 22a. This is illustrated in panel P3 which show a representation of the upper rectified driving signal 43a and in panel P4 which show a representation of the lower rectified driving signal 43b. In panel P3, negative parts of the signal are absent, whereas in panel P4, positive parts of the signal are absent. In this embodiment, substantially no current of the auxiliary driving signal may be provided to either the upper voice coil section 22a or the lower voice coil section 22b, depending on the direction of the current.

LIST OF REFERENCE SIGNS

(56) 1 Voice coil 2 Magnetic circuit 3 Air gap 4 Travelling axis 5 Upward direction 6 Downward direction 7 Diaphragm 12 Amplifier 13 Power supply unit 16 Rectifying unit 17 Diode 21 Center voice coil section 22a Upper voice coil section 22b Lower voice coil section 23a First upper voice coil section 23b First lower voice coil section 24a Second upper voice coil section 24b Second lower voice coil section 30 Audio signal 40 Driving signal 41 Center driving signal 42 Auxiliary driving signal 43a Upper rectified driving signal 43b Lower rectified driving signal 44a Upper rectified driving signal out 44b Lower rectified driving signal out 45 Center driving signal out 46 Driving signal in 47 Driving signal out 48 Auxiliary driving signal out 50 Loudspeaker 51 Voice coil driving system 52 Interface P1 Panel showing representation of center driving signal P2 Panel showing representation of auxiliary driving signal P3 Panel showing representation of upper rectified driving signal P4 Panel showing representation of lower rectified driving signal