METHOD FOR AVOIDING AN OFFSET OF A MEMBRANE OF A ELECTRODYNAMIC ACOUSTIC TRANSDUCER

Abstract

A method for avoiding an offset of a membrane (3) of an electrodynamic acoustic transducer (1) having two voice coils (7, 8) is presented, wherein a control voltage (U.sub.CTRL) is applied to at least one of the voice coils (7, 8) until the electromotive force (U.sub.emf1) of the first coil (7) or a parameter derived thereof and the electromotive force (U.sub.emf2) of the second coil (8) or a parameter derived thereof substantially reach a predetermined relation. Furthermore, an electronic offset compensation circuit (12) is presented, which performs the above application of a control voltage (U.sub.CTRL). Finally, the invention relates to a transducer system with a transducer (1) and an electronic offset compensation circuit (12) connected to the transducer (1).

Claims

1. Method for avoiding an offset of a membrane of an electrodynamic acoustic transducer having two voice coils, wherein a control voltage U.sub.CTRL is applied to at least one of the voice coils and altered until the electromotive force U.sub.emf1 of the first coil or a parameter derived thereof and the electromotive force U.sub.emf2 of the second coil or said parameter derived thereof substantially reach a predetermined relation.

2. Method as claimed in claim 1, wherein the electromotive force U.sub.emf1 of the first coil and the electromotive force U.sub.emf2 of the second coil are calculated by the formulas
U.sub.emf1=U.sub.in1(t)Z.sub.C1.Math.I.sub.in(t)
U.sub.emf2=U.sub.in2(t)Z.sub.C2.Math.I.sub.in(t) wherein Z.sub.C1 is the coil resistance of the first coil, U.sub.in1(t) is the input voltage to the first coil at the time t and I.sub.in(t) is the input current to the first coil at the time t and wherein Z.sub.C2 is the coil resistance of the second coil, U.sub.in2(t) is the input voltage to the second coil at the time t and I.sub.in(t) is the input current to the second coil at the time t.

3. Method as claimed in claim 2, wherein a parameter derived from the electromotive force U.sub.emf1, U.sub.emf2 is an absolute value of the electromotive force U.sub.emf1, U.sub.emf2, a square value of the electromotive force U.sub.emf1, U.sub.emf2 or a root mean square value of the electromotive force U.sub.emf1, U.sub.emf2.

4. Method as claimed in claim 3, wherein the control voltage U.sub.CTRL is applied to at least one of the voice coils and altered until the low pass filtered electromotive force U.sub.emf1 of the first coil or a parameter derived thereof and the low pass filtered electromotive force U.sub.emf2 of the second coil or said parameter derived thereof substantially reach a predetermined relation.

5. Method as claimed in claim 4, wherein a delta sigma modulation is used for applying a control voltage U.sub.CTRL to at least one of the voice coils.

6. Method as claimed in claim 5, wherein a signal output of the delta sigma modulator is filtered before it is applied to the coil arrangement.

7. Method as claimed in claim 4, wherein a control voltage U.sub.CTRL is applied to both the first coil and the second coil.

8. Method as claimed in any one of claim 7, wherein a sound signal is applied to the first coil and/or the second coil during application of a control voltage U.sub.CTRL.

9. Method as claimed in claim 8, wherein the sound signal is applied just to an outer tap of the serially connected voice coils.

10. Method as claimed in any one of claim 1, comprising the steps of: a) calculating a velocity of the membrane based on an input voltage U.sub.in and an input current I.sub.in to a coil of the transducer and based on an idle driving force factor of the transducer in an idle position of the membrane; b) calculating a position of the membrane by integrating said velocity; c) calculating the velocity of the membrane based on the input voltage U.sub.in and the input current I.sub.in to the coil of the transducer and based on a driving force factor of the transducer at the position of the membrane calculated in step b) and d) recursively repeating steps b) and c).

11. Method as claimed in claim 10, characterized in that the velocity, the input voltage U.sub.in, the input current I.sub.in, the idle driving force factor, the driving force factor and the position are related to the same point in time.

12. Method as claimed in claim 10, characterized in that the velocity, the input voltage U.sub.in, the input current I.sub.in, the idle driving force factor, the driving force factor and the position are related to different points in time.

13. Method as claimed in claim 12, comprising the steps of: a) calculating a velocity v(t) of the membrane based on an input voltage U.sub.in(t) and an input current I.sub.in(t) to a coil of the transducer and based on an idle driving force factor of the transducer in an idle position of the membrane; b) calculating a position x(t) of the membrane by integrating said velocity v(t); c) calculating the velocity v(t+1) of the membrane based on the input voltage U.sub.in(t+1) and the input current I.sub.in(t+1) to the coil of the transducer and based on a driving force factor BL(x(t) of the transducer at the position x(t) of the membrane calculated in step b) and d) recursively repeating steps b) and c) wherein t gets t+1.

14. Method as claimed in any one of claim 10, wherein the position x(t) of the membrane is calculated by the formula
x(t)=x(t1)+v(t).Math.t

15. Method as claimed in any one of claim 14, wherein the velocity v of the membrane is calculated by the formula
v(t)=(U.sub.in(t)Z.sub.C.Math.I.sub.in(t))/BL(0) in step a) or by
v(t+1)=(U.sub.in(t+1)Z.sub.C.Math.I.sub.in(t+1))/BL(x(t)) in step c)

16. Method as claimed in any one of claim 14, wherein the velocity v of the membrane is calculated by the formula
v(t+1)=v.sub.(t+1).Math.BL(0)/BL(x(t)) in step c) wherein
v.sub.(t+1)=(U.sub.in(t+1)Z.sub.C.Math.I.sub.in(t+1))/BL(0)

17. Method as claimed in claim 14, wherein the velocity v of the membrane is calculated by use of the electromotive force U.sub.emf1 of the first coil or the electromotive force U.sub.emf2 of the second coil or the sum of the electromotive force U.sub.emf1 of the first coil and the electromotive force U.sub.emf2 of the second coil.

18. Electronic offset compensation circuit, which is designed to be connected to a coil arrangement of an electrodynamic acoustic transducer, wherein the coil arrangement comprises two voice coils and wherein the transducer comprises a membrane, the coil arrangement attached to the membrane and a magnet system being designed to generate a magnetic field transverse to a longitudinal direction of a wound wire of the coil arrangement, and wherein the an electronic offset compensation circuit is designed to apply a control voltage U.sub.CTRL to at least one of the voice coils and to alter said control voltage U.sub.CTRL until the electromotive force U.sub.emf1 of the first coil or a parameter derived thereof and the electromotive force U.sub.emf2 of the second coil or a parameter derived thereof substantially reach a predetermined relation.

19. Electronic offset compensation circuit as claimed in claim 18, which electronic offset compensation circuit is furthermore designed to a) calculate a velocity of the membrane based on an input voltage U.sub.in and an input current I.sub.in to a coil of the transducer and based on an idle driving force factor of the transducer in an idle position of the membrane; b) calculate a position of the membrane by integrating said velocity; c) calculate the velocity of the membrane based on the input voltage U.sub.in and the input current I.sub.in to the coil of the transducer and based on a driving force factor of the transducer at the position of the membrane calculated in step b) and to d) recursively repeat steps b) and c).

20. Transducer system, comprising an electrodynamic acoustic transducer with a membrane, a coil arrangement attached to the membrane, wherein the coil arrangement comprises two voice coils, and a magnet system being designed to generate a magnetic field transverse to a longitudinal direction of a wound wire of the coil arrangement and an electronic offset compensation circuit as claimed in claim 18 being electrically connected to the coil arrangement.

21. Transducer system, comprising an electrodynamic acoustic transducer with a membrane, a coil arrangement attached to the membrane, wherein the coil arrangement comprises two voice coils, and a magnet system being designed to generate a magnetic field transverse to a longitudinal direction of a wound wire of the coil arrangement and an electronic offset compensation circuit as claimed in claim 19 being electrically connected to the coil arrangement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] These and other aspects, features, details, utilities, and advantages of the invention will become more fully apparent from the following detailed description, appended claims, and accompanying drawings, wherein the drawings illustrate features in accordance with exemplary embodiments of the invention, and wherein:

[0055] FIG. 1 shows a cross sectional view of an exemplary transducer;

[0056] FIG. 2 shows a simplified circuit diagram of the transducer 1 shown in FIG. 1;

[0057] FIG. 3 shows exemplary graphs of the driving force factors of the first and the second coil of the transducer shown in FIG. 1 and

[0058] FIG. 4 a more detailed embodiment of a transducer system.

[0059] Like reference numbers refer to like or equivalent parts in the several views.

DETAILED DESCRIPTION OF EMBODIMENTS

[0060] Various embodiments are described herein to various apparatuses. Numerous specific details are set forth to provide a thorough understanding of the overall structure, function, manufacture, and use of the embodiments as described in the specification and illustrated in the accompanying drawings. It will be understood by those skilled in the art, however, that the embodiments may be practiced without such specific details. In other instances, well-known operations, components, and elements have not been described in detail so as not to obscure the embodiments described in the specification. Those of ordinary skill in the art will understand that the embodiments described and illustrated herein are non-limiting examples, and thus it can be appreciated that the specific structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments, the scope of which is defined solely by the appended claims.

[0061] Reference throughout the specification to various embodiments, some embodiments, one embodiment, or an embodiment, or the like, means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases in various embodiments, in some embodiments, in one embodiment, or in an embodiment, or the like, in places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Thus, the particular features, structures, or characteristics illustrated or described in connection with one embodiment may be combined, in whole or in part, with the features, structures, or characteristics of one or more other embodiments without limitation given that such combination is not illogical or non-functional.

[0062] It must be noted that, as used in this specification and the appended claims, the singular forms a, an and the include plural referents unless the content clearly dictates otherwise.

[0063] The terms first, second, and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms include, have, and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0064] All directional references (e.g., plus, minus, upper, lower, upward, downward, left, right, leftward, rightward, front, rear, top, bottom, over, under, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader's understanding of the present disclosure, and do not create limitations, particularly as to the position, orientation, or use of the any aspect of the disclosure. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

[0065] As used herein, the phrased configured to, configured for, and similar phrases indicate that the subject device, apparatus, or system is designed and/or constructed (e.g., through appropriate hardware, software, and/or components) to fulfill one or more specific object purposes, not that the subject device, apparatus, or system is merely capable of performing the object purpose.

[0066] Joinder references (e.g., attached, coupled, connected, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims.

[0067] All numbers expressing measurements and so forth used in the specification and claims are to be understood as being modified in all instances by the term about or substantially, which particularly means a deviation of 10% from a reference value.

[0068] FIG. 1 shows an example of an electrodynamic acoustic transducer 1, which may be embodied as a loudspeaker, in cross sectional view. The transducer 1 comprises a housing 2 and a membrane 3 having a bending section 4 and a center section 5, which is stiffened by a plate in this example. Furthermore, the transducer 1 comprises a coil arrangement 6 attached to the membrane 3. The coil arrangement 6 comprises a first coil 7 and a second coil 8. The first coil 7 is arranged on top of the second coil 8 and concentric to the second coil 8 in this example. Furthermore, the transducer 1 comprises a magnet system with a magnet 9, a pot plate 10 and a top plate 11. The magnet system generates a magnetic field B transverse to a longitudinal direction of a wound wire of the coil arrangement 6.

[0069] Additionally, the electrodynamic acoustic transducer 1 comprises three connection terminals T1 . . . T3 electrically connected to the coils 7, 8 and connected to an electronic offset compensation circuit 12. The electrodynamic acoustic transducer 1 and the electronic offset compensation circuit 12 form a transducer system.

[0070] The excursion of the membrane 3 is denoted with x in the example shown in FIG. 1, its velocity with v. As known, a current through the coil arrangement 6 causes a movement of the membrane 3 and thus sound, which emanates from the transducer 1.

[0071] FIG. 2 shows a simplified circuit diagram of the transducer 1 shown in FIG. 1. Concretely, FIG. 2 shows a voltage source, generating the voltage U.sub.in, which is fed to a serial connection of a first inductance L1, which is formed by the first voice coil 7, and a second inductance L2, which is formed by the second voice coil 8.

[0072] Finally, FIG. 3 shows a graph of a first driving force factor BL1 of the first voice coil 7 and a graph of a second driving force factor BL2 of the second voice coil 8. The driving force factors BL7 and BL8 may be measured as it is known in prior art. In particular, FIG. 3 also shows the magnetic zero position MP of the membrane 3 and its desired idle position IP, which differs from the magnetic zero position MP in this example.

[0073] A method for calculating the excursion x of membrane 3 is now as follows:

In a first step a), a velocity v of the membrane 3 is calculated based on an input voltage U.sub.in and an input current I.sub.in to the coils 7, 8 of the transducer 1 and based on an idle driving force factor BL1(0), BL2(0) of the transducer 1 in an idle position IP (where x=0 or assumed to be 0) of the membrane 3.

[0074] The velocity v of the membrane 3 may be calculated by the formula

v(t)=(U.sub.in(t)Z.sub.C.Math.I.sub.in(t))/BL(0)

wherein Z.sub.C is the coil resistance.

[0075] Generally, the velocity v of the membrane 3 can be calculated by use of

[0076] the electromotive force U.sub.emf1 of the first coil 7 or

[0077] the electromotive force U.sub.emf2 of the second coil 8 or

[0078] the sum of the electromotive force U.sub.emf1 of the first coil 7 and the electromotive force U.sub.emf2 of the second coil 8.

[0079] In a first example the electromotive force U.sub.emf1 of the first coil 7 is used as a basis for the calculation. The electromotive force U.sub.emf1 is calculated as follows:

U.sub.emf1=U.sub.in1(t)Z.sub.C1.Math.I.sub.in(t)

[0080] Accordingly, the velocity is

v(t)=(U.sub.in1(t)Z.sub.C1I.sub.in(t))/BL1(0)

[0081] In a second step b), the position x of the membrane 3 is calculated by integrating said velocity v. Either by

x(t)=v(t).Math.dt

or by

x(t)=x(t1)+v(t).Math.t

[0082] In a next step c), the velocity v of the membrane 3 is calculated based on the input voltage U.sub.in and the input current I.sub.in to the coil 7 of the transducer 1 and based on a driving force factor BL(x) of the transducer 1 at the position x of the membrane 3 calculated in step b). In our example the velocity v is calculated by the formula

v(t)=(U.sub.in1(t)Z.sub.C1.Math.I.sub.in(t))/BL1(x(t))

Steps b) and c) are recursively repeated until a desired accuracy is obtained.

[0083] In the above example, the velocity v, the input voltage U.sub.in, the input current I.sub.in, the idle driving force factor BL(0), the driving force factor BL(x) and the position x are related to the same point in time t. That means, that a sample of the input voltage U.sub.in, the input current I.sub.in is taken once, and the position x is calculated in several iterations.

[0084] However, the velocity v, the input voltage U.sub.in, the input current I.sub.in, the idle driving force factor BL(0), the driving force factor BL(x) and the position x may also be related to different points in time t. If so, steps c) and d) are altered. In step c), the velocity v(t+1) of the membrane 3 based on the input voltage U.sub.in(t+1) and the input current I.sub.in(t+1) to the coil 7 of the transducer 1 and based on a driving force factor BL(x(t)) of the transducer 1 at the position x(t) of the membrane 3 is calculated. In our example using the first coil 7 this means

v(t+1)=(U.sub.in(t+1)Z.sub.C.Math.I.sub.in(t+1))/BL(x(t))

[0085] Accordingly, steps b) and c) are recursively repeated wherein t gets t+1. In this way, the calculation of the position x is an ongoing process, whose accuracy basically depends on how fast the calculation is in relation to the velocity v of the membrane 3. In simple words this means that the calculation of the position x is the more accurate the lower the frequency of the signal driving the membrane 3 is.

[0086] As an alternative to the methods presented hereinbefore, the calculation of the velocity v of the membrane 3 may be done with the idle driving force factor BL(0) in the idle position IP of the membrane 3 in a first step, which is corrected then by a factor showing the relation between BL(0) and BL(x). Accordingly, the velocity v of the membrane 3 can be calculated by the formula

v(t+1)=v.sub.(t+1).Math.BL(0)/BL(x(t)) in step c) wherein

v.sub.(t+1)=(U.sub.in(t+1)Z.sub.C.Math.I.sub.in(t+1))/BL(0)

[0087] Here, v is a rough approximation of the velocity of the membrane 3 calculated with the use of the idle driving force factor BL(0) in the idle position IP of the membrane 3. This velocity then is corrected by use of the factor BL(0)/BL(x(t)).

[0088] In real applications, the idle position IP of the membrane 3 (x=0) often does not coincide with the point where the electromotive force U.sub.emf1 of the first coil 7 equals the electromotive force U.sub.emf2 of the second coil 8. This leads to a deviation of the calculated position x of the membrane 3 from the real position of the membrane 3.

[0089] In other words, the conjunction area between the first coil 7 and the second coil 8 is not in the same plane as the top plate 11. This deviation may be caused by a specific design and/or tolerances during manufacturing.

[0090] To avoid or reduce this deviation, a control voltage is applied to at least one of the voice coils 7, 8 and altered until the electromotive force U.sub.emf1 of the first coil 7 and the electromotive force U.sub.emf2 of the second coil 8 substantially reach a predetermined relation and until the coil arrangement reaches a desired idle position IP. The electromotive force U.sub.emf1 of the first coil 7 and the electromotive force U.sub.emf2 of the second coil 8 can be calculated by the formulas

U.sub.emf1=U.sub.in1(t)Z.sub.C1.Math.I.sub.in(t)

U.sub.emf2=U.sub.in2(t)Z.sub.C2.Math.I.sub.in(t)

[0091] Generally, said relation can be a particular ratio or a difference between said values. Particularly, the desired idle position IP can be the magnetic zero position MP, in which the idle position IP of the membrane (x=0) coincides with the point where the electromotive force U.sub.emf1 of the first coil equals the electromotive force U.sub.emf2 of the second coil. In this particular point a ratio between said values is substantially 1, respectively a difference between said values is substantially 0.

[0092] The application of the control voltage may also be based on a parameter derived from the electromotive force U.sub.emf1, U.sub.emf2. Beneficially, said parameter is an absolute value of the electromotive force U.sub.emf1, U.sub.emf2, a square value of the electromotive force U.sub.emf1, U.sub.emf2 or a root mean square value of the electromotive force U.sub.emf1, U.sub.emf2.

[0093] Accordingly, the control voltage may be applied to at least one of the voice coils 7, 8 and altered until a (root mean) square value of the electromotive force U.sub.emf1 of the first coil 7 and a (root mean) square value of the electromotive force U.sub.emf2 of the second coil 8 substantially reach a predetermined relation. Alternatively, the control voltage may be applied to at least one of the voice coils 7, 8 and altered until an absolute value of the electromotive force U.sub.emf1 of the first coil 7 and an absolute value of the electromotive force U.sub.emf2 of the second coil 8 reach a predetermined relation. It should be noted that the offset compensation method may also be based on a relation of other parameters derived from the electromotive forces U.sub.emf1, U.sub.emf2.

[0094] Particularly, the electromotive forces U.sub.emf1 and U.sub.emf2/parameters derived thereof are determined in the whole audio band in a first step, the energy of the electromotive forces U.sub.emf1 and U.sub.emf2 respectively a parameter thereof is determined in a second step, and the result of the second step is low pass filtered by a first filter, which may be part of the offset calculation module 13. Finally, the signals obtained in the third step are used for application of the control voltage U.sub.CTRL. For example, the cut off frequency of said low pass filter is 50 Hz in case of a micro speaker and 10 Hz case of other speakers. Preferably, the cut off frequency is 20 Hz in case of a micro speaker and 5 Hz case of other speakers. Thus, a frequency of an alternating component of the control voltage U.sub.CTRL is low in comparison to the frequencies of the sound output by the transducer 1. Generally, the control voltage U.sub.CTRL may comprise a constant component and an alternating component. In special cases, the control voltage U.sub.CTRL may also be a pure DC-voltage. The control voltage is applied to at least one of the voice coils 7, 8 and altered until the electromotive force U.sub.emf1 of the first coil 7/a parameter derived thereof substantially equals the electromotive force U.sub.emf2 of the second coil 8/said parameter derived thereof below the above frequencies.

[0095] The above-mentioned filter structures illustrate the inertial behavior of the control loop. A realization of the control loop may be based on state of the art control loop theory based on PID controller (proportional-integral-derivative controller) of arbitrary order.

[0096] In the examples presented hereinbefore, the electromotive force U.sub.emf1 of the first coil 7 was used to determine an excursion x of the membrane 3. However, in the same way the electromotive force U.sub.emf2 of the second coil 8 or the sum of the electromotive force U.sub.emf1 of the first coil 7 and the electromotive force U.sub.emf2 of the second coil 8 may be used for this reason. If so,

v(t)=(U.sub.in2(t)Z.sub.C2.Math.I.sub.in(t))/BL2

or

v(t)=(U.sub.in1(t)+U.sub.in2(t)(Z.sub.C1+Z.sub.C2).Math.I.sub.in(t))/BL12

may be used for the calculation of the velocity v of the membrane 3, wherein BL12 is the driving force factor of the complete coil arrangement 6.

[0097] The calculations presented hereinbefore as well as the application of a control voltage to the coil arrangement 6 generally may be done by the offset compensation circuit 12. The offset compensation circuit 12 may be a standalone device or may be integrated into another device.

[0098] The presented method for calculating the position x of the membrane 3 can be used to compensate non-linearities of the transducer 1. For example, the non-linear graph of the driving force factor BL (see FIG. 3) leads to a non-linear conversion of the electric signals fed to the coil arrangement 6 into a movement of the membrane 3. Knowing the position x of the membrane 3, this non-linearity can be compensated by altering the electric signals.

[0099] FIG. 4 now shows a more concrete embodiment of a transducer system, particularly of the electronic offset compensation circuit 12 connected to the coil arrangement 6, which is shown by the inductances L1 and L2 in FIG. 4. The electronic offset compensation circuit 12, comprises an offset calculation module 13, a position calculation module 14, a sound signal changing module 15, a mixer 16 and a power amplifier 17.

[0100] The offset calculation module 13 is connected to a current measuring device A, and a first voltage measuring device V1 and a second voltage measuring device V2. As explained above, the electromotive force U.sub.emf1 of the first coil 7 and the electromotive force U.sub.emf2 of the second coil 8 can be calculated based on the input current I.sub.in(t) to the first coil 7 and the second coil 8, which is measured with the current measuring device A, the input voltage U.sub.in1(t) to the first coil 7, which is measured with the first voltage measuring device V1, the input voltage U.sub.in2(t) to the second coil 8, which is measured with the second voltage measuring device V2, and the coil resistance Z.sub.C1 of the first coil 7 and the coil resistance Z.sub.C2 of the second coil 8, which are considered to be known from a separate measurement. Based on this information, the offset calculation module 13 calculates a control voltage U.sub.CTRL, which is applied to the coils 7 and 8.

[0101] The offset calculation module 13 especially may comprise a delta sigma modulator which does the offset compensation according to a delta sigma modulation. In this case, a deviation from the target relation between the electromotive force U.sub.emf1 of the first coil 7 and the electromotive force U.sub.emf2 of the second coil 8 is summed with opposite sign and applied to the coil arrangement 6 thus compensating the above deviation and thus heading for the desired idle position IP. A delta sigma modulator can also be considered as an integral controller, and other integration controllers may be used in the offset calculation module 13 as well. The application of the control voltage U.sub.CTRL by the offset calculation module 13 may also be based on a parameter derived from the electromotive force U.sub.emf1, U.sub.emf2 as disclosed hereinbefore.

[0102] In addition to an optional first filter in the offset calculation module 13 a second filter 18 may be arranged downstream of the offset calculation module 13. The first filter avoids that the offset calculation module 13 interferes with the sound output of the transducer 1. The second filter 18 reduces or avoids instability in the control loop.

[0103] As explained above, also the position x can be calculated by use of the input current I.sub.in(t) to the first coil 7 and the second coil 8, the input voltage U.sub.in1(t) to the first coil 7, the input voltage U.sub.in2(t) to the second coil 8 as well as the driving force factor BL(x) of the transducer 1. This job is performed by the position calculation module 14, which calculates the position x of the membrane 3 and in this example outputs it to the sound signal changing module 15. The sound signal changing module 15 compensates non-linearity in the driving force factor BL(x) (see FIG. 3) based on the membrane position x. Concretely, the sound signal changing module 15 alters the input sound signal U.sub.Sound based on the membrane position x and the driving force factor BL(x) and outputs an altered sound signal U.sub.Sound so that sound emanating from the transducer 1 fits to the sound signal U.sub.Sound as best as possible, and distortions are kept low. Alternatively or in addition, the level of the sound signal U.sub.Sound may be limited, or it may be cut off by the sound signal changing module 15 at high membrane excursions x so as to avoid damages of transducer 1. Of course, the membrane position x may also be used for other controls and output to external electronic circuits.

[0104] It should be noted at this point that shifting the idle position IP of the membrane 3 does not necessarily involve the position calculation as presented above. Shifting the idle position IP of the membrane 3 may simply be based on altering the desired relation between the electromotive force U.sub.emf1 of the first coil 7 and the electromotive force U.sub.emf2 of the second coil 8 or based on altering a desired relation of parameters derived from the electromotive forces U.sub.emf1, U.sub.emf2.

[0105] It should also be noted that in the example shown in FIG. 4 both the position calculation module 14 and the sound signal changing module 15 comprise information about the driving force factor BL(x). In the position calculation module 14 this information is used to calculate the membrane position x, whereas in the sound signal changing module 15 the sound signal U.sub.Sound is altered by use of the driving force factor BL(x). Of course, both functions can be integrated into a single module, and of course the sound signal changing module 15 can also comprise other information about the transducer 1 up to a complete model so as to avoid distortions when converting the sound signal U.sub.Sound into sound.

[0106] In the example shown in FIG. 4, the control voltage U.sub.CTRL is mixed with the altered sound signal U.sub.Sound by the mixer 16. Finally, the mixed signal is amplified by the power amplifier 17 and applied to the transducer 1. Because of the mixer 16, the altered sound signal U.sub.Sound is applied during application of a control voltage U.sub.CTRL.

[0107] It should be noted that the electronic offset compensation circuit 12 just shows the general function by use of functional blocks for illustrating purposes. Putting the disclosed functions into practice may need amendments of the electronic offset compensation circuit 12 and more detailed electronics. Functional blocks do not necessarily coincide with physic blocks in a real offset compensation circuit 12. A real physic block may incorporate more than one of the functions shown in FIG. 4. Moreover, dedicated functions of the functions shown in FIG. 4 may also be omitted in a real offset compensation circuit 12, and a real offset compensation circuit 12 may also perform more than the discloses functions.

[0108] For example, the position calculating module 14 and the sound signal changing module 15 may be omitted. In this case, the sound signal U.sub.Sound is applied to the transducer unchanged. In a further example, just the sound signal changing module 15 is omitted. In this case the position calculating module 14 may output the position x to an external sound signal changing circuit. One skilled in the art will also easily realize that the power amplification and the mixing can be done with just one amplifier.

[0109] In this example, both the control voltage U.sub.CTRL and the altered sound signal U.sub.Sound are applied to both the first coil 7 and the second coil 8, i.e. to an outer tap of the coil arrangement 6. Nevertheless, this is an advantageous solution, it is not the only one. In an alternate embodiment, the control voltage U.sub.CTRL is applied just to the first coil 7 and the (altered) sound signal U.sub.Sound is applied to just the second coil 8. In this case, a mixer 16 can be omitted as the control voltage U.sub.CTRL and the altered sound signal U.sub.Sound are superimposed by the movement of the membrane 3.

[0110] In summary, the electronic offset compensation circuit 12, depending on which functions it comprises, provides a proper solution for feeding a sound signal U.sub.Sound to a transducer 1 while keeping distortions low and while avoiding damage of the transducer 1. In combination with the transducer 1 an advantageous transducer system is presented which allows for easy operation. A user just needs to feed a signal to be converted into sound to the transducer system and does not need to care about distortions and/or avoiding damage of the transducer 1. Preferably, the electronic offset compensation circuit 12 and the transducer 1 are embodied as a single device or module. For example, the electronic offset compensation circuit 12 can be arranged in the housing 2 of the transducer 1.

[0111] Generally, the transducer 1 respectively the membrane 3 may have any shape in a top view, in particular a rectangular, circular or ovular shape. Furthermore, the coils 7 and 8 may have the same height or different heights, the same diameter or different diameters as well as the same number of winding or different numbers of windings.

[0112] It should be noted that although avoiding an offset of the membrane 3 was just disclosed in the advantageous context with the calculation of a membrane position x, avoiding an offset of the membrane 3 is not limited to this particular application. In contrast, it may also be used for simply shifting the membrane 3 into that position, which is intended as the idle position IP by design thereby compensating tolerances and improving the performance of the transducer 1 in general. Accordingly, distortions of the audio output of the transducer 1 can be reduced and/or symmetry may be improved thereby allowing for the same membrane stroke in forward and backward direction. The membrane 3 may also be shifted to an altered desired idle position IP so as to alter the sound characteristics of the transducer 1.

[0113] It should be noted that the invention is not limited to the above mentioned embodiments and exemplary working examples. Further developments, modifications and combinations are also within the scope of the patent claims and are placed in the possession of the person skilled in the art from the above disclosure. Accordingly, the techniques and structures described and illustrated herein should be understood to be illustrative and exemplary, and not limiting upon the scope of the present invention. The scope of the present invention is defined by the appended claims, including known equivalents and unforeseeable equivalents at the time of filing of this application. Although numerous embodiments of this invention have been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this disclosure.

[0114] Particularly, it should be noted that the position calculation method and the position calculation module 14 for calculating a membrane position x as well as a transducer system comprising such a position calculation module 14 (i.e. the features of any one of claims 10-17, 19 and 20) can form the basis of an independent invention without the limitations of claims 1 and 18.

[0115] The very same counts for the application of a sound signal just to an outer tap of the serially connected voice coils 7, 8 (i.e. the features of claim 9) as well as a transducer system with those features, which can form the basis of an independent invention without the limitations of claims 1 and 18.

LIST OF REFERENCES

[0116] 1 electrodynamic acoustic transducer [0117] 2 housing [0118] 3 membrane [0119] 4 bending section [0120] 5 stiffened center section [0121] 6 coil arrangement [0122] 7 first coil [0123] 8 second coil [0124] 9 magnet [0125] 10 pot plate [0126] 11 top plate [0127] 12 electronic offset compensation circuit [0128] 13 offset calculation module (with optional first filter) [0129] 14 position calculation module [0130] 15 sound signal changing module [0131] 16 mixer [0132] 17 power amplifier [0133] 18 second filter [0134] A current measuring device [0135] B magnetic field [0136] BL driving force factor [0137] BL1 driving force factor of the first coil [0138] BL2 driving force factor of the second coil [0139] I.sub.in input current [0140] L1 inductance of the first coil [0141] L2 inductance of the second coil [0142] MP magnetic zero position [0143] IP desired idle position [0144] T1 . . . T3 connection terminals [0145] U1 voltage at the first coil [0146] U2 voltage at the second coil [0147] U.sub.CTRL control voltage [0148] U.sub.In input voltage [0149] U.sub.Sound sound signal [0150] U.sub.Sound altered sound signal [0151] v membrane velocity [0152] V1 first voltage measuring device [0153] V2 second voltage measuring device [0154] x membrane excursion