Method and apparatus for reducing reaction forces in loudspeakers

Abstract

A method and apparatus for reducing reaction forces in a loudspeaker is provided. The method includes supplying a current to the loudspeaker from a power source connected to the loudspeaker. The method further includes measuring the current through the loudspeaker using a current-sensing resistor connected in series with the loudspeaker. The method further includes supplying a current through the loudspeaker to an actuator connected in series with the loudspeaker using a voltage-controlled power source connected in series with the loudspeaker that is substantially proportional to the measured current.

Description

[Technical Field]

[0001] The present invention relates generally to improvements in loudspeaker apparatus and methods, and more particularly to methods for reducing reaction forces in loudspeakers by using a voltage-controlled current source to drive an actuator connected in series with a motor assembly used to generate forces on a diaphragm, and to active loudspeaker apparatus employing these methods. [Background technology]

[0002] According to Newton's third law of motion, the diaphragm exerts an equal and opposite reaction force on the motor assembly. This vibration reaction force is also transmitted to the air through the mechanical connection between the loudspeaker's frame and enclosure assembly. These undesirable vibrations and the resulting sound waves adversely affect the sound intended to be transmitted by the speaker's diaphragm. Therefore, it is desirable to reduce or eliminate the vibrations caused by the reaction force. Summary of the Invention [Problem to be solved by the invention]

[0003] The present disclosure provides a method and apparatus for reducing reaction forces in a loudspeaker. A loudspeaker motor generates an acoustic force on a diaphragm to generate audio, which has the undesirable side effect of an opposite reaction force that degrades the audio quality. An actuator is connected to the loudspeaker in a manner that produces an actuator force on a mass that generates a response force in the opposite direction to the reaction force. The actuator is applied in series with the loudspeaker to properly operate the actuator so that the reaction force opposes the response force. A sense resistor and power control current source are placed after the loudspeaker and are designed to supply a current to the actuator that is proportional to the current passing through the loudspeaker. [Means for solving the problem]

[0004] In a first embodiment, a method for reducing a reaction force in a loudspeaker includes supplying a current to the loudspeaker from a power supply connected to the loudspeaker, the method further including measuring the current through the loudspeaker using a current sensing resistor connected in series with the loudspeaker, the method further including supplying a current through the loudspeaker to an actuator connected in series with the loudspeaker that is proportional to the measured current using a voltage controlled power supply connected in series with the loudspeaker.

[0005] In a second embodiment, an apparatus for reducing a reaction force in a loudspeaker includes a loudspeaker, an actuator, a power supply, and a current sensing resistor. The actuator is connected in series with the loudspeaker. A power supply is connected to the loudspeaker and supplies a current to the loudspeaker, and a current sensing resistor is connected in series with the loudspeaker and measures the current through the loudspeaker. A voltage controlled power supply is connected in series with the loudspeaker and supplies a current through the loudspeaker to the actuator that is proportional to the measured current.

[0006] In some embodiments, a voltage controlled power supply is in parallel with a current sensing resistor.

[0007] In some embodiments, the voltage controlled power supply includes a current sensing level potentiometer that provides a voltage drop proportional to the voltage caused by the current through the loudspeaker, the current sensing level potentiometer having a resistance value at least an order of magnitude higher than the current sensing resistor.

[0008] In some embodiments, the voltage controlled power supply includes a low pass filter that filters high frequency transients in the voltage drop across the current sense level divider. The low pass filter may be formed by a low pass filter resistor and a low pass filter capacitor.

[0009] In some embodiments, the voltage controlled power supply includes a current source operational amplifier that provides an output current proportional to the voltage across a current sense resistor. The low pass filter is connected to the non-inverting input of the current source operational amplifier. The inverting terminal of the current source operational amplifier is connected to the actuator current sense resistor. The coefficient of the output current proportional to the voltage across the current sense resistor is determined by the resistance of the actuator current sense resistor.

[0010] In some embodiments, the voltage controlled power supply includes an actuator power amplifier that amplifies the output current to produce a current suitable for driving an actuator.

[0011] Other technical features will be readily apparent to those skilled in the art from the following figures, descriptions, and claims.

[0012] For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts, and in which: [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 shows a schematic diagram of a loudspeaker in which an electromechanical actuator is attached to the speaker motor assembly to remove reaction forces. [Figure 2A] 1 shows an exemplary graph illustrating the impedance response of a bass reflex loudspeaker system according to an embodiment of the present disclosure. [Figure 2B] 1 shows an exemplary graph illustrating the force response of a bass reflex loudspeaker system according to an embodiment of the present disclosure. [Figure 3A] FIG. 1 shows an exemplary diagram of a loudspeaker with series-driven actuators according to an embodiment of the present disclosure. [Figure 3B] 1 shows an exemplary schematic diagram of a loudspeaker for reducing reaction forces in a loudspeaker according to an embodiment of the present disclosure; [Figure 4] 1 shows an exemplary schematic diagram of a loudspeaker with analog signal processing circuitry according to an embodiment of the present disclosure. [Figure 5] FIG. 1 shows an exemplary diagram of a loudspeaker with series-driven actuators according to an embodiment of the present disclosure. [Figure 6] FIG. 1 shows an exemplary diagram of a loudspeaker with series-driven actuators according to an embodiment of the present disclosure. [Figure 7] FIG. 1 shows a schematic diagram of a current source topology for a loudspeaker according to an embodiment of the present disclosure. [Figure 8] FIG. 1 shows a schematic diagram of a current source topology for a loudspeaker according to an embodiment of the present disclosure. [Figure 9] FIG. 1 shows a schematic diagram of a current source topology for a loudspeaker according to an embodiment of the present disclosure. [Figure 10] 1 shows an exemplary diagram of current measurements for a loudspeaker according to an embodiment of the present disclosure. [Figure 11] 1 shows an exemplary diagram of current measurements for a loudspeaker according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0014] The use of the same reference symbols in different drawings indicates similar or identical items.

[0015] 1-11 described below, and the various embodiments used to explain the principles of the present invention in this patent document, are for illustrative purposes only and should not be construed as limiting the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any type of suitably arranged device or system.

[0016] Figure 1 shows a schematic diagram of a loudspeaker 100 in which an electromechanical actuator is attached to a speaker motor assembly to remove reaction forces. The embodiment of loudspeaker 100 shown in Figure 1 is for illustrative purposes. Figure 1 does not limit the scope of the present disclosure to any particular implementation of a loudspeaker.

[0017] Dynamic loudspeaker 100 may include an acoustic assembly 102 and a reaction assembly 104. Reaction assembly 104 may also be referred to as an actuator. Acoustic assembly 102 includes the components of loudspeaker 100 used to generate audio output. Acoustic assembly 102 may include a diaphragm 106, a basket 108, a voice coil 110, an acoustic magnet 112, an acoustic yoke 114, an acoustic front plate 116, a spider 118, a surround 120, and a cap 122.

[0018] The dynamic loudspeaker 100 functions by applying an acoustic force 140 to a diaphragm 106 to excite acoustic waves in a medium such as air. The diaphragm 106 acts as a transducer that converts mechanical vibrations into sound or acoustic vibrations. The diaphragm 106 may be connected at a first end to a basket 108 and at a second end to a voice coil 110. The stiffness and damping characteristics of the diaphragm 106 largely determine the precision of the sound waves generated. The diaphragm 106 may be formed of any suitable material. In some embodiments, the diaphragm 106 may be formed of paper, paper composites and laminates, plastic materials, or other materials capable of generating acoustic vibrations from mechanical vibrations.

[0019] The basket 108 is a frame that provides structural support for the loudspeaker 100. The basket 108 is attached to a speaker case or other structural component in which the loudspeaker 100 is mounted. The components of the loudspeaker 100 are attached to the basket 108. The basket 108 provides a rigid structure that is precisely manufactured to ensure proper alignment of all components of the loudspeaker 100. The basket 108 may provide additional functionality, such as heat dissipation from the other components. The basket 108 may be made of any suitable material to minimize additional vibrations caused by the diaphragm 106. In some embodiments, the basket 108 may be appropriately designed for proper alignment of the other components of the loudspeaker 100 and may be formed of pressed steel, cast aluminum, plastic, or other materials that can minimize vibration of the basket 108 due to its connection with the diaphragm 106.

[0020] Voice coil 110 is a coil of wire that receives an alternating current (AC) from an audio signal, which creates an electromagnetic field through voice coil 110. The electromagnetic field of voice coil 110 opposes a permanent magnetic field generated by acoustic magnet 112. The opposing electromagnetic field causes vibration in voice coil 110 and diaphragm 106. An acoustic force 140 acting on diaphragm 106 is generated by a motor assembly that passes a current through voice coil 110, which is positioned within the static magnetic field generated by acoustic magnet 112. The acoustic force 140 acting on diaphragm 106 is equal to the current through voice coil 110 times the strength of the magnetic field generated by acoustic magnet 112 times the length of voice coil 110 within the magnetic field, and acoustic force 140 can be expressed as: (1) F=iBL In the formula, F represents the acoustic force 140 applied to the diaphragm 106 , i represents the current flowing through the voice coil 110 , B represents the magnetic field strength generated by the acoustic magnet 112 , and L represents the length of the voice coil 110 .

[0021] While a single voice coil 110 is described, it is possible to implement multiple voice coils 110 in the loudspeaker 100 to provide various wiring options and the ability to connect two different signals simultaneously. Note that while the voice coil 110 is described as underhung, the voice coil 110 may also be integrated into the loudspeaker 100 as an overhung voice coil. An underhung voice coil 110 will cost more than an overhung voice coil 110, but may provide more linear motor strength across the excursion range. The voice coil 110 may be formed of any suitable material for making wire that converts alternating current into an electromagnetic field. In some embodiments, the voice coil 110 may be formed of copper, aluminum, or any other material that converts alternating current into an electromagnetic field.

[0022] The acoustic magnet 112, acoustic yoke 114, and acoustic front plate 116 cumulatively generate a permanent static magnetic field. The acoustic magnet 112 provides a static magnetic field that cancels the alternating electromagnetic field of the voice coil 110. As described above, the opposing fields cause the diaphragm 106 to move inward and outward. The acoustic magnet 112 may be any shape and size. In certain embodiments, the acoustic magnet 112 may be ring-shaped. The acoustic magnet 112 may be made of any suitable material. In some embodiments, the acoustic magnet 112 may be made of a ferroceramic material or other material to generate the static magnetic field.

[0023] The acoustic yoke 114 may have a backplate portion and a center pole portion. The center pole portion of the acoustic yoke 114 and the acoustic front plate 116 may form an air gap in the magnetic circuit. The backplate portion of the acoustic yoke 114 and the acoustic front plate 116 are attached to opposite sides of the acoustic magnet 112. In some embodiments, a magnetic fluid may be contained in the air gap for additional cooling and resonance damping of the voice coil 110. The acoustic yoke 114 and the acoustic front plate 116 also aid in heat dissipation from the voice coil 110. The acoustic yoke 114 and the acoustic front plate 116 may be formed of any suitable material. In some embodiments, the acoustic yoke 114 and the acoustic front plate 116 may be formed of iron or other suitable permeable material and form a magnetic circuit with the acoustic magnet 112. The acoustic yoke 114 and the acoustic front plate 116 may be formed of the same material or different materials.

[0024] The spider 118 and surround 120 together form a suspension for the diaphragm 106. The spider 118 is connected between the inside of the diaphragm 106 and the basket 108. The surround 120 is connected between the outside of the diaphragm 106 and the basket 108. The suspension axially and radially centers the voice coil 110 within the air gap of the magnetic circuit. The suspension provides a restoring force that maintains the position of the voice coil 110 within the air gap, limiting undesired movement of the diaphragm 106 and voice coil 110. The spider 118 may be made of any suitable material engineered to provide uniform movement of the voice coil 110 without promoting movement in any one direction. In some embodiments, the spider 118 may be formed of a cloth impregnated with a rigid resin or other suitable material that benefits controlled damping. The surround 120 may be formed as part of the diaphragm 106 or as a separate component. In embodiments in which the surround 120 is a separate component, the surround 120 may be formed of the same or a similar material as the diaphragm 106, or may be formed of a different material that provides adequate suspension for the diaphragm 106.

[0025] The cap 122 covers the central opening of the diaphragm 106. The cap 122 prevents dust and dirt from entering the air gap of the magnetic circuit. The cap 122 increases the strength of the diaphragm 106 while helping to maintain the shape of the diaphragm 106. The added mass of the cap 122 can reduce resonance of the driver. In certain embodiments, the cap 122 may include a screen or vents to allow air flow to further cool the voice coil 110. The cap 122 may be formed convex or concave. The cap 122 may be formed of any suitable material. In certain embodiments, the cap 122 is formed of the same material as the diaphragm 106.

[0026] When an acoustic force 140 is applied to the diaphragm 106, a reaction force 142 is applied in the opposite direction. As previously mentioned, the reaction force 142 can cause undesirable vibrations that can degrade the quality of the sound produced by the diaphragm 106. The reaction assembly 104 generates a response force 146 to oppose the reaction force 142. The reaction assembly 104 may include a mass 124, a support plate 126, a reactive coil 128, a reactive magnet 130, a reactive yoke 132, a reactive front plate 134, a first mass vibration absorber 136, and a second mass vibration absorber 138.

[0027] The reaction assembly 104 generates an actuator force 144 that counteracts the reaction force generated by the loudspeaker motor by moving a mass 124, which has a small surface area compared to the movable diaphragm 106. The mass 124 is moved in a direction opposite to the diaphragm 106, with the intention of counteracting the reaction force 142 exerted on the motor by the diaphragm 106. The mass 124 may be connected at a first end to the support plate 126 and at a second end to a reactive coil 128. The properties of the mass 124 are designed to generate a response force 146 equal to the generated reaction force 142. The mass 124 may be formed of any suitable material. In some embodiments, the mass 124 may be formed of iron or lead, or other materials capable of increasing specific gravity, to enhance sound insulation from the mass 124.

[0028] The support plate 126 is a frame that provides structural support for the reaction assembly 104. The support plate 126 is attached to the speaker case, basket 108, or other structural component in which the loudspeaker 100 is mounted. The components of the reaction assembly 104 are attached to the support plate 126. The support plate 126 provides a rigid structure that is precisely manufactured to ensure proper alignment of all components of the loudspeaker 100. The support plate 126 may provide additional functions, such as heat dissipation from other components. The support plate 126 may be made of any suitable material to minimize additional vibrations caused by the mass 124. In some embodiments, the support plate 126 may be formed of pressed steel, cast aluminum, plastic, or other material that is appropriately designed for proper alignment of the other components of the loudspeaker 100 and that can minimize vibrations of the support plate 126 due to its connection to the mass 124. The support plate 126 may be formed of the same material as the basket 108 or a different material.

[0029] Reactive coil 128 is a coil of wire that receives an alternating current from an audio signal, which creates an electromagnetic field through reactive coil 128. The electromagnetic field of reactive coil 128 opposes a permanent magnetic field generated by reactive magnet 130. The opposing electromagnetic field induces vibration in reactive coil 128 and mass 124. An actuator force 144 acting on mass 124 is generated by a motor assembly that passes a current through reactive coil 128, which is positioned within the static magnetic field generated by reactive magnet 130. The acoustic force 140 acting on mass 124 is equal to the current through reactive coil 128 times the magnetic field strength generated by reactive magnet 130 times the length of reactive coil 128 within the magnetic field, and actuator force 144 can be expressed as: (2) F a =i r B r L r In the formula, F a represents the actuator force 144 applied to the mass 124, and i r represents the current flowing through the reactive coil 128, and B r represents the magnetic field strength generated by the reactive magnet 130, and L r represents the length of the reactive coil 128.

[0030] While a single reactive coil 128 is described, multiple reactive coils 128 may be implemented in the reaction assembly 104 to provide various wiring options and the ability to connect two different signals simultaneously. Note that while the reactive coil 128 is described as underhanging, the reactive coil 128 may also be integrated into the loudspeaker 100 as an overhanging coil. The reactive coil 128 may be formed of any suitable material for making a wire that converts alternating current into an electromagnetic field. In some embodiments, the reactive coil 128 may be formed of copper, aluminum, or any other material that converts alternating current into an electromagnetic field. The reactive coil 128 may be formed of the same or a different material as the voice coil 110.

[0031] The reactive magnet 130, reactive yoke 132, and reactive front plate 134 cumulatively generate a permanent static magnetic field for the reaction assembly 104. The reactive magnet 130 provides a static magnetic field that cancels the alternating electromagnetic field of the reactive coil 128. As described above, the opposing fields cause the mass 124 to move inward and outward. The reactive magnet 130 may be any shape and size. In certain embodiments, the reactive magnet 130 may be ring-shaped. The reactive magnet 130 may be made of any suitable material. In some embodiments, the reactive magnet 130 may be made of a ferroceramic material or other material to generate a static magnetic field. The reactive magnet 130 may be formed of the same or different material and in the same or different shape as the acoustic magnet 112.

[0032] The reactive yoke 132 may have a backplate portion and a center pole portion. The center pole portion of the reactive yoke 132 and the reactive front plate 134 may form an air gap in the magnetic circuit. The backplate portion of the reactive yoke 132 and the reactive front plate 134 are attached to opposite sides of the reactive magnet 130. In some embodiments, the reactive yoke 132 may be integrally formed and integrated with the acoustic yoke 114. In some embodiments, a magnetic fluid may be included in the air gap for additional cooling and resonance damping of the voice coil 110. The reactive yoke 132 and the reactive front plate 134 also aid in heat dissipation from the reactive coil 128. The reactive yoke 132 and the reactive front plate 134 may be formed of any suitable material. In some embodiments, the reactive yoke 132 and the reactive front plate 134 may be formed of iron or other suitable permeable material and form a magnetic circuit with the reactive magnet 130. The reactive yoke 132 and the reactive front plate 134 may be formed of the same or different materials and shapes as the acoustic yoke 114 and the acoustic front plate 116, respectively.

[0033] The first mass vibration absorber 136 and the second mass vibration absorber 138 together form a suspension for the mass 124. The first mass vibration absorber 136 connects the inside of the mass 124 to the support plate 126. The second mass vibration absorber 138 connects the outside of the mass 124 to the support plate 126. The suspension axially and radially centers the reactive coil 128 within the air gap of the magnetic circuit. The suspension provides a restoring force that maintains the position of the reactive coil 128 within the air gap, limiting undesired movement of the mass 124 and the reactive coil 128. The first mass vibration absorber 136 may be made of any suitable material engineered to provide uniform movement of the reactive coil 128 without promoting movement in any one direction. In some embodiments, the first mass vibration absorber 136 may be formed of a fabric impregnated with a rigid resin or other suitable material that provides controlled damping. The second mass vibration absorber 138 may be formed as part of the mass 124 or as a separate component. In embodiments in which the second mass vibration absorber 138 is a separate component, the second mass vibration absorber 138 may be formed of the same or a similar material as the mass 124, or may be formed of a different material that provides suitable suspension for the mass 124. The first mass vibration absorber 136 and the second mass vibration absorber 138 may be formed of the same or a different material as the spider 118.

[0034] While Figure 1 illustrates an example of a loudspeaker 100 that employs an electromechanical actuator to eliminate reaction forces, various modifications may be made to Figure 1. For example, the number and arrangement of various components of loudspeaker 100 may be varied as needed or desired. Furthermore, loudspeaker 100 may be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0035] Figure 2A shows an example graph illustrating an impedance response 200 of a bass-reflex loudspeaker system according to an embodiment of the present disclosure. Figure 2B shows an example graph illustrating a reaction force 142 of a bass-reflex loudspeaker system according to an embodiment of the present disclosure. The embodiments of the impedance response 200 illustrated in Figure 2A and the reaction force 142 illustrated in Figure 2B are for illustrative purposes. Figures 2A and 2B do not limit the scope of the present disclosure to any particular implementation of a loudspeaker.

[0036] For the reaction assembly 104 to be effective at canceling vibrations, the instantaneous acoustic force 140 acting on the diaphragm 106 must be tracked. Determining the acoustic force 140 is a complex geometry of the entire loudspeaker 100, including the magnetic field strength of the acoustic assembly 102, the length of the voice coil 110 within the magnetic field, the mass of the diaphragm 106, back EMF, suspension compliance, compliance of the air within the acoustic assembly 102, mechanical damping, etc. The complexity of the system is reflected in the example impedance response 200 of a bass-reflex loudspeaker shown in Figures 2A and 2B.

[0037] The force (F) 206 acting on the diaphragm 106 is proportional to the current (i) flowing through the voice coil 110. According to Ohm's law, the current is inversely proportional to the impedance (z) 204, which can be expressed as: (3) F∝BL / z where F represents force 206, B represents magnetic field strength, L represents the length of voice coil 110 within the magnetic field, and z represents impedance 204. Thus, acoustic force 140 acting on diaphragm 106 is a complex function of frequency 202. Reaction force 142 has a magnitude approximately equal to acoustic force 140 but in the opposite direction.

[0038] To effectively cancel reaction force 142 with response force 146, actuator force 144 must have the same frequency response as acoustic force 140 of loudspeaker 100. Tuning the electromechanical parameters of the actuator mechanism so that response force 146 is approximately equal to reaction force 142 is difficult. If reaction force 142 of acoustic assembly 102 is substantially different from response force 146 of reaction assembly 104, the response force 146 caused by movement of reaction assembly 104 may increase, rather than reduce or cancel, the vibration caused by reaction force 142. Furthermore, because loudspeaker 100 can be mounted in different enclosures, acoustic force 140 and actuator force 144 may be modified in different ways. For example, a sealed enclosure will have a very different force response than a bass-reflex enclosure, which means that the electromechanical parameters of reaction assembly 104 will need to be tuned differently for each acoustic assembly 102 and each enclosure. Tuning loudspeaker 100 differently for different applications requires significant engineering resources and application expertise and is not suitable for mass production.

[0039] While Figures 2A and 2B illustrate an exemplary graph showing an impedance response 200 of a bass reflex loudspeaker system, various modifications can be made to Figures 2A and 2B. For example, the dimensions of the impedance response 200 and the response force 146, and their individual components, can be modified as needed or desired.

[0040] FIG. 3A shows an exemplary diagram of a loudspeaker assembly 300 with serially driven actuators according to an embodiment of the present disclosure. FIG. 3B shows an exemplary schematic 302 of a loudspeaker assembly 300 for reducing reaction forces in a loudspeaker. The embodiment of the loudspeaker assembly 300 and the schematic 302 of the loudspeaker assembly 300 shown in FIG. 3A are for illustrative purposes. FIGs. 3A and 3B do not limit the scope of the present disclosure to any particular implementation of a loudspeaker. For simplicity, the active component driving the functionality of the acoustic assembly 102 will be referred to as the loudspeaker 304, and the active component driving the functionality of the reaction assembly 104 will be referred to as the actuator 306.

[0041] 2A and 2B can be mitigated to some extent by a power supply 312 driving the actuator 306 in series with the loudspeaker 304. The currents through the loudspeaker 304 and the actuator 306 are necessarily identical in a series configuration and are converted into a proportional force by the loudspeaker 304. A significant drawback of this configuration is that the actuator's impedance forms a voltage divider with the loudspeaker's impedance. As a result, the actuator's impedance changes the frequency response of the loudspeaker 304. Furthermore, any nonlinearity in the actuator 306 is reflected in the actuator's 306 impedance response 200, and the actuator's nonlinearity is converted into the acoustic force 140 output by the loudspeaker 304.

[0042] To overcome the above difficulties, the current through the loudspeaker 304 is measured directly and a proportional drive current is supplied to the actuator 306. The force supplied by the actuator 306 is independent of the actuator's own impedance response 200. The system is shown schematically in Figure 3B.

[0043] Sense resistor Rs 308 has a resistance significantly less than the minimum impedance of loudspeaker 304. The voltage across sense resistor Rs 308 is proportional to the current through loudspeaker 304. The voltage across sense resistor Rs 308 drives voltage-controlled current source 310. Voltage-controlled current source 310 generates a current that drives actuator 306. The actuator drive current is proportional to the current through loudspeaker 304 and is independent of the impedance of actuator 306.

[0044] 3A and 3B illustrate an example loudspeaker assembly 300 and circuit diagram 302, various modifications may be made to Figures 3A and 3B. For example, the number and arrangement of various components of loudspeaker assembly 300 and circuit diagram 302 may be changed as needed or desired. Furthermore, loudspeaker assembly 300 and circuit diagram 302 may be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0045] Figure 4 shows an exemplary schematic diagram 400 of a loudspeaker with analog signal processing circuitry, one embodiment of the present invention. The embodiment of loudspeaker 400 shown in Figure 4 is for illustrative purposes only. Figure 4 does not limit the scope of the present disclosure to any particular implementation of a loudspeaker.

[0046] The circuit of Figure 4 shows one implementation of the voltage-controlled current source 310 of Figure 3B. A loudspeaker power amplifier AR1 402 (not shown in Figure 3B) is connected between the power supply 312 and the loudspeaker LS1 304 and amplifies the power supply drive signal applied to the loudspeaker LS1 304. A loudspeaker current sense resistor Rs 308, connected between the loudspeaker LS1 304 and ground, corresponds to the sense resistor Rs 308 shown in Figure 3 and generates a voltage proportional to the current through the loudspeaker LS1 304.

[0047] The current sense level divider R1 410, low pass filter resistor R2 412 and low pass filter capacitor C1 414, current source op amp U1 416, actuator power amplifier AR2 418, and actuator current sense resistor R3 420 form the voltage controlled current source 310 of Figure 3B. The current sense level divider R1 410 provides a voltage drop proportional to the voltage caused by the current through the loudspeaker LS1 304, which is approximately the same as the current through the current sense resistor Rs 308 when the current sense level divider R1 410 has a resistance much larger (e.g., at least an order of magnitude larger) than the resistance of the current sense resistor Rs 408. The voltage drop across the current sense level divider R1 410 is filtered of high frequency transients using a low pass filter formed by low pass filter resistor R2 412 and low pass filter capacitor C1 414, which are connected to the non-inverting input of current source op amp U1 416. The inverting terminal of current source op amp U1 416 is connected to ground through actuator current sense resistor R3 420. Thus, current source op amp U1 416 generates an output current that is proportional to the voltage across current sense resistor Rs 408 (and the voltage across current sense level divider R1 410), with a factor determined by the resistance of actuator current sense resistor R3 420. The current output of current source op amp U1 416 is amplified by actuator power amplifier AR2 418 to generate a current signal suitable for driving actuator TR1 306.

[0048] While Figure 4 illustrates an example of a loudspeaker 400 with serially driven actuators, various modifications can be made to Figure 4. For example, the number and arrangement of the various components of loudspeaker 400 can be changed as needed or desired. Furthermore, loudspeaker 400 can be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0049] FIG. 5 shows an exemplary diagram of a loudspeaker with series-driven actuators according to an embodiment of the present disclosure.

[0050] The dynamic loudspeaker 500 may include an acoustic assembly 102 and a reaction assembly 504. The reaction assembly 504 may also be referred to as an actuator. The acoustic assembly 102 includes the components of the loudspeaker 500 used to generate audio output. The acoustic assembly 102 may include a diaphragm 106, a basket 108, a voice coil 110, an acoustic magnet 112, an acoustic yoke 114, an acoustic front plate 116, a spider 118, a surround 120, and a cap 122. These components are described in detail above.

[0051] When an acoustic force 140 is applied to the diaphragm 106, a reaction force 142 is applied in the opposite direction. As previously mentioned, the reaction force 142 can cause undesirable vibrations that can degrade the quality of the sound produced by the diaphragm 106. The reaction assembly 504 generates a response force 146 to oppose the reaction force. The reaction assembly 504 may include a first support plate 526, a reactive coil 528, a reactive magnet 530, a reactive yoke 532, a reactive front plate 534, a first mass vibration absorber 536, a second mass vibration absorber 538, and a second support plate 548.

[0052] The hi 504 substantially cancels the actuator force 144 generated by the loudspeaker motor by moving the reactive yoke 532, reactive magnet 530, reactive front plate 534, and second support plate. The movement of these components is best understood by describing the movement of the reactive yoke 532. The reactive yoke 532 is moved in a direction opposite to the diaphragm 106 to cancel the reaction force 142 exerted on the motor by the diaphragm 106. The reactive yoke 532 may be connected at a first end to the first support plate 526 and at a second end to the reactive coil 528. The characteristics of the reactive yoke 532 are designed to generate a response force 146 equal to the generated reaction force 142. The reactive yoke 532 may be formed of any suitable material. In some embodiments, the reactive yoke 532 may be formed of iron or lead, or other materials capable of increasing specific gravity, to enhance sound insulation from the reactive yoke 532.

[0053] The first support plate 526 is a frame that provides structural support for the reaction assembly 504. The first support plate 526 is attached to the speaker case, basket 108, or other structural component on which the loudspeaker 500 is mounted. The components of the reaction assembly 504 are attached to the first support plate 526. The first support plate 526 provides a rigid, precision-engineered structure to ensure proper alignment of all components of the loudspeaker 500. The first support plate 526 may provide additional functionality, such as heat dissipation from other components. The first support plate 526 may be made of any suitable material to minimize additional vibrations induced in the reactive yoke 532. In some embodiments, the first support plate 526 may be formed of pressed steel, cast aluminum, plastic, or other material that is appropriately designed for proper alignment of the other components of the loudspeaker 500 and that can minimize vibrations of the first support plate 526 due to its connection to the reactive yoke 532. The first support plate 526 may be formed of the same material as the basket 108 or a different material.

[0054] Reactive coil 528 is a coil of wire that receives an alternating current (AC) from an audio signal, which creates an electromagnetic field through reactive coil 528. The electromagnetic field of reactive coil 528 opposes a permanent magnetic field generated by reactive magnet 530. The opposing electromagnetic field induces vibration in reactive coil 528 and reactive yoke 532. Actuator force 144 acting on reactive yoke 532 is generated by a motor assembly that applies a current to reactive coil 528, which is positioned in the static magnetic field generated by reactive magnet 530. Acoustic force 540 acting on reactive yoke 532 is equal to the current through reactive coil 528 times the magnetic field strength generated by reactive magnet 530 times the length of reactive coil 528 within the magnetic field.

[0055] While a single reactive coil 528 is described, multiple reactive coils 528 may be implemented in the reaction assembly 504 to provide various wiring options and the ability to connect two different signals simultaneously. Note that while the reactive coil 528 is described as underhanging, the reactive coil 528 may also be integrated into the loudspeaker 500 as an overhanging coil. The reactive coil 528 may be formed of any suitable material for making a wire that converts alternating current into an electromagnetic field. In some embodiments, the reactive coil 528 may be formed of copper, aluminum, or any other material that converts alternating current into an electromagnetic field. The reactive coil 528 may be formed of the same or a different material than the voice coil 110.

[0056] The reactive magnet 530, reactive yoke 532, and reactive front plate 534 cumulatively generate a permanent static magnetic field for the reaction assembly 504. The reactive magnet 530 provides a static magnetic field that cancels the alternating electromagnetic field of the reactive coil 528. As described above, the opposing fields cause the reactive yoke 532 to move inward and outward. The reactive magnet 530 may be any shape and size. In certain embodiments, the reactive magnet 530 may be ring-shaped. The reactive magnet 530 may be made of any suitable material. In some embodiments, the reactive magnet 530 may be made of a ferroceramic material or other material to generate a static magnetic field. The reactive magnet 530 may be formed of the same or different material and in the same or different shape as the acoustic magnet 112.

[0057] The second support plate 548 is a frame that provides a support structure for the reaction assembly 504. The second support plate 548 is attached to the reactive front plate 534. The second support plate 548 can provide additional functions, such as heat dissipation from other components. The second support plate 548 can be made of any suitable material to minimize additional vibrations caused by the reactive yoke 532. In some embodiments, the first support plate 526 can be formed of pressed steel, cast aluminum, plastic, or other material that is appropriately designed for proper alignment of the other components of the loudspeaker 500 and can minimize vibrations of the first support plate 526 due to its connection with the reactive yoke 532. The second support plate 548 can be formed of the same or a different material from the basket 108 and the first support plate 526.

[0058] The first mass vibration absorber 536 and the second mass vibration absorber 538 together form a suspension for the reactive yoke 532. The first mass vibration absorber 536 connects the inside of the support for the reactive coil 528 to the second support plate 548. The second mass vibration absorber 538 connects the outside of the reactive yoke 532 to the first support plate 526. The suspension axially and radially centers the reactive coil 528 within the air gap of the magnetic circuit. The suspension provides a restoring force that maintains the position of the reactive coil 528 within the air gap, limiting undesired movement of the reactive yoke 532 and reactive coil 528. The first mass vibration absorber 536 can be made of any suitable material designed to provide uniform movement of the reactive coil 528 without promoting movement in any one direction. In some embodiments, the first mass vibration absorber 536 may be formed of a fabric impregnated with a stiff resin or other suitable material that provides controlled damping. The second mass vibration absorber 538 may be formed as part of the reactive yoke 532 or as a separate component. In embodiments in which the second mass vibration absorber 538 is a separate component, the second mass vibration absorber 538 may be formed of the same or a similar material as the reactive yoke 532, or may be formed of a different material that provides suitable suspension for the reactive yoke 532. The first mass vibration absorber 536 and the second mass vibration absorber 538 may be formed of the same or a different material as the spider 118.

[0059] While Figure 5 illustrates an example of a loudspeaker 500 with serially driven actuators, various modifications can be made to Figure 5. For example, the number and arrangement of the various components of loudspeaker 500 can be changed as needed or desired. Furthermore, loudspeaker 500 can be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0060] FIG. 6 shows an exemplary diagram of a loudspeaker with series-driven actuators according to an embodiment of the present disclosure.

[0061] The dynamic loudspeaker 600 may include an acoustic assembly 102 and a reaction assembly 604. The reaction assembly 604 may also be referred to as an actuator. The acoustic assembly 102 includes the components of the loudspeaker 600 used to generate audio output. The acoustic assembly 102 may include a diaphragm 106, a basket 108, a voice coil 110, an acoustic magnet 112, an acoustic yoke 114, an acoustic front plate 116, a spider 118, a surround 120, and a cap 122. These components are described in detail above.

[0062] When an acoustic force is applied to the diaphragm 106, a reaction force 142 is applied in the opposite direction. As previously mentioned, the reaction force 142 can cause undesirable vibrations that can degrade the quality of the sound produced by the diaphragm 106. The reaction assembly 604 generates a response force 146 to oppose the reaction force. The reaction assembly 604 may include a first support plate 626, a reactive coil 628, a reactive magnet 630, a first reactive front plate 634, a second reactive plate 635, a first mass vibration absorber 636, a second mass vibration absorber 637, a second support plate 648, a third support plate 649, and a cylinder 650.

[0063] Cylinder 650 can support the “mass” of reaction assembly 604 and can include first reactive front plate 634, second reactive plate 635, first mass vibration absorber 636, second mass vibration absorber 637, second support plate 648, and third support plate 649. Cylinder 650 can be coupled to first support plate 626. Cylinder 650 can provide additional functionality, such as heat dissipation from other components. Cylinder 650 can be made of any suitable material to minimize additional vibrations caused by the mass. In some embodiments, cylinder 650 can be formed of pressed steel, cast aluminum, plastic, or other material that is appropriately designed for proper alignment with other components of loudspeaker 600 and can minimize vibrations of cylinder 650 due to its connection to the mass.

[0064] The reaction assembly 604 moves a mass to substantially counteract the actuator force 144 generated by the loudspeaker motor. The reaction assembly 604 includes a first reactive front plate 634, a second reactive plate 635, a first vibration absorber mass 636, a second vibration absorber mass 637, a second support plate 648, and a third support plate 649. The mass is moved in a direction opposite to the diaphragm 106 to counteract the reaction force 142 exerted on the motor by the diaphragm 106. The mass may be connected at a first end to the first support plate 626 and at a second end to a reactive coil 628. The mass's properties are designed to generate a response force 146 equal to the generated reaction force 142. The mass may be formed of any suitable material. In some embodiments, the mass may be formed of iron or lead, or other materials capable of increasing specific gravity, to enhance sound isolation from the mass.

[0065] The first support plate 626 is a frame that provides structural support for the reaction assembly 604. The first support plate 626 is attached to the speaker case, basket 108, or other structural component on which the loudspeaker 600 is mounted. The components of the reaction assembly 604 are attached to the first support plate 626. The first support plate 626 provides a rigid, precision-engineered structure to ensure proper alignment of all components of the loudspeaker 100. The first support plate 626 may provide additional functionality, such as heat dissipation from other components. The first support plate 626 may be made of any suitable material to minimize additional vibrations caused by the mass. In some embodiments, the first support plate 626 may be formed of pressed steel, cast aluminum, plastic, or other material that is appropriately designed for proper alignment of the other components of the loudspeaker 600 and that can minimize vibrations of the first support plate 626 due to its connection to the mass 124. The first support plate 626 may be formed of the same material as the basket 108 or a different material.

[0066] Reactive coil 628 is a coil of wire that receives an alternating current from an audio signal, which creates an electromagnetic field through reactive coil 628. The electromagnetic field of reactive coil 628 opposes a permanent magnetic field generated by reactive magnet 630. The opposing electromagnetic field causes vibration in reactive coil 628 and the mass. Actuator force 144 acting on the mass is generated by a motor assembly that applies current to reactive coil 628, which is located in the static magnetic field generated by reactive magnet 630. Acoustic force 640 acting on the mass is equal to the current through reactive coil 628 times the magnetic field strength generated by reactive magnet 630 times the length of reactive coil 628 within the magnetic field.

[0067] It should be noted that although the reactive coil 628 is described as underhanging, the reactive coil 628 may also be integrated in the loudspeaker 600 as an overhanging coil. The reactive coil 628 may be made of any suitable material for making a wire that converts alternating current into an electromagnetic field. In some embodiments, the reactive coil 628 may be made of copper, aluminum, or any other material that converts alternating current into an electromagnetic field. The reactive coil 628 may be made of the same or a different material than the voice coil 110.

[0068] The reactive magnet 630, reactive yoke 632, and first reactive front plate 634 cumulatively generate a permanent static magnetic field for the reaction assembly 604. The reactive magnet 630 provides a static magnetic field that cancels the alternating electromagnetic field of the reactive coil 628. As described above, the opposing fields cause masses to move inward and outward. The reactive magnet 630 may be any shape and size. In certain embodiments, the reactive magnet 630 may be ring-shaped. The reactive magnet 630 may be made of any suitable material. In some embodiments, the reactive magnet 630 may be made of a ferroceramic material or other material to generate a static magnetic field. The reactive magnet 630 may be formed of the same or different material and in the same or different shape as the acoustic magnet 112.

[0069] The second support plate 648 forms a frame that provides structural support for the reaction assembly 604. The second support plate 648 is attached to the first reactive front plate 634. The second support plate 648 can provide additional functionality, such as heat dissipation from other components. The second support plate 648 can be made of any suitable material to minimize additional vibrations caused by the mass. In some embodiments, the second support plate 648 can be formed of pressed steel, cast aluminum, plastic, or other material that is appropriately designed for proper alignment of the other components of the loudspeaker 600 and can minimize vibrations of the second support plate 648 due to its connection with the mass 124. The second support plate 648 can be formed of the same or a different material from the basket 108 and the first support plate 626.

[0070] The third support plate 649 forms a frame that provides structural support for the reaction assembly 604. The third support plate 649 is attached to the second reactive plate 635. The third support plate 649 can provide additional functionality, such as heat dissipation from other components. The third support plate 649 can be made of any suitable material to minimize additional vibrations caused by the mass. In some embodiments, the third support plate 649 can be formed of pressed steel, cast aluminum, plastic, or other material that is appropriately designed for proper alignment of the other components of the loudspeaker 600 and can minimize vibrations of the third support plate 649 due to its connection with the mass 124. The third support plate 649 can be formed of the same or a different material than the basket 108, the first support plate 626, and the second support plate 648.

[0071] The first and second mass vibration absorbers 636 and 637 together form a suspension for the mass. The first and second mass vibration absorbers 636 connect between the first and second support plates 626 and 648. The second and third support plates 649 connect between the first and second support plates 626 and 649. The suspension axially and radially centers the reactive coil 628 within the air gap of the magnetic circuit. The suspension provides a restoring force that maintains the position of the reactive coil 628 within the air gap, limiting undesired movement of the mass and reactive coil 628. The first and second mass vibration absorbers 636 may be made of any suitable material designed to provide uniform movement of the reactive coil 628 without promoting movement in any one direction. In some embodiments, the first and second mass vibration absorbers 636, 637 may be formed of a fabric impregnated with a stiff resin or other suitable material that provides controlled damping. The first and second mass vibration absorbers 636, 637 may be formed of the same or a different material than the spider 118.

[0072] While Figure 6 illustrates an example of a loudspeaker 600 with serially driven actuators, various modifications can be made to Figure 6. For example, the number and arrangement of the various components of loudspeaker 600 can be changed as needed or desired. Furthermore, loudspeaker 600 can be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0073] 7 shows a schematic diagram of a loudspeaker current source topology 700 according to an embodiment of the present disclosure. As mentioned above, tuning issues can be alleviated to some extent by a power supply 712 driving an actuator 706 in series with the loudspeaker 704. As shown in FIG. 7, a modified Howland current source 720 can be used to measure the current and provide a proportional current to the actuator 706.

[0074] While Figure 7 illustrates one example of a current source topology 700, various modifications may be made to Figure 7. For example, the number and arrangement of various components of current source topology 700 may be changed as needed or desired. Furthermore, current source topology 700 may be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0075] FIG. 8 shows a schematic diagram of a loudspeaker current source topology 800 according to an embodiment of the present disclosure. As mentioned above, tuning issues can be mitigated to some extent by a power supply 812 driving an actuator 806 in series with a loudspeaker 804. As shown in FIG. 8 , a digital signal processor algorithm 820 can be used to measure the current and provide a proportional current to the actuator 806. The digital signal processor algorithm 820 may include a loudspeaker current sense voltage input 822, an actuator current sense feedback gain 824, and an actuator control voltage output gain 826. The loudspeaker current sense voltage input 822 can detect the current voltage input to the loudspeaker 804. The actuator current sense feedback gain 824 can determine the proportional current to apply to the actuator 806. The actuator control voltage output gain 826 can apply the proportional current to the actuator 806.

[0076] While Figure 8 illustrates one example of current source topology 800, various modifications may be made to Figure 8. For example, the number and arrangement of various components of current source topology 800 may be changed as needed or desired. Furthermore, current source topology 800 may be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0077] FIG. 9 shows a schematic diagram of a current source topology 900 for a loudspeaker according to an embodiment of the present disclosure. Unlike a loudspeaker 904, an actuator 906 can be completely contained within or outside a speaker enclosure. When placed inside or outside a speaker enclosure, the actuator 906 can experience consistent acoustic loading because all sides of the reactive moving mass are contained within the same acoustic environment. As a result, the electrical impedance response 200 of the actuator 906 is independent of the specific loudspeaker enclosure implementation (bass reflex, closed, etc.) and is therefore relatively consistent and predictable. A feedforward control topology is provided in which the actuator impedance response 200 is mathematically modeled, which may include frequency, amplitude, temperature dependence, and other predictable parameters affecting the actuator 906. The mathematical impedance model filters the response of the control voltage derived from the loudspeaker current so that the feedforward output voltage makes the actuator current directly proportional to the loudspeaker current. Protection algorithms, such as voltage limiters, may also be implemented to prevent excessive actuator excursions. The current source topology 900 may include an actuator control voltage output gain 920. The actuator control voltage output gain 920 may control the voltage applied to the actuator 906 to offset the loudspeaker 904.

[0078] While Figure 9 illustrates an example current source topology 900, various modifications can be made to Figure 9. For example, the number and arrangement of various components of current source topology 900 can be changed as needed or desired. Furthermore, current source topology 900 can be used in any other suitable loudspeaker process and is not limited to the particular process described above.

[0079] FIG. 10 shows an example diagram 1000 for measuring current in a loudspeaker according to an embodiment of the present disclosure. As shown in FIG. 10, a wire 1002 carrying current passing through the loudspeaker may pass through a Rogowski coil 1004. The Rogowski coil 1004 may include a toroid around the wire 1002. The Rogowski coil 1004 can provide voltage control to a current source. Similarly, the Rogowski coil 1004 can be used to measure actuator current and provide negative feedback to the current source.

[0080] Although Figure 10 illustrates an example current measurement diagram 1000, various modifications may be made to Figure 10. For example, the number and arrangement of various components of current measurement diagram 1000 may be changed as needed or desired. Furthermore, current measurement diagram 1000 may be used with any other suitable loudspeaker process and is not limited to the particular process described above.

[0081] FIG. 11 shows an exemplary diagram 1100 for measuring current in a loudspeaker according to an embodiment of the present disclosure.

[0082] 11, a wire 1102 carrying current passing through a loudspeaker may pass through a Hall effect current sensing loop 1104. The Hall effect current sensing loop 1104 may provide voltage control to a current source. Similarly, the Hall effect current sensing loop 1104 may be used to measure the actuator current and provide negative feedback to the current source.

[0083] Although Figure 11 illustrates an example current measurement diagram 1100, various modifications can be made to Figure 11. For example, the number and arrangement of various components of current measurement diagram 1100 can be changed as needed or desired. Furthermore, current measurement diagram 1100 can be used with any other suitable loudspeaker process and is not limited to the particular process described above.

[0084] It may be advantageous to provide definitions of certain terms used throughout this patent document. The terms "include" and "comprise," and their derivatives, mean including without limitation. The term "or" is inclusive and means "and / or." The term "associated with" and its derivatives may mean include, be included within, interconnect, contain, be contained within, connect, couple, be in communication with, cooperate with, interleave, juxtapose, be adjacent to, coupled, have, be in a state of, have a relationship with, and the like. When used with a list of items, the phrase "at least one" means that different combinations of one or more of the listed items may be used, or that only one item in the list may be required. For example, "at least one of A, B, and C" includes any of the following combinations: A, B, C, A and B, A and C, B and C, A, B, and C.

[0085] Nothing in this application should be read as implying that any particular element, step, or function is essential or critical to the scope of any claim. The scope of patented subject matter is defined solely by the allowed claims. Moreover, no claim is subject to 35 U.S.C. §112(f) with respect to any of the appended claims or claim elements unless the precise words "a means for" or "a step for" are expressly used in a particular claim, followed by a participial phrase identifying the function. The use of terms such as "mechanism," "module," "device," "unit," "component," "element," "member," "apparatus," "machine," "system," "processor," or "controller" in the claims, including but not limited to, is understood and intended to refer to structures known to those skilled in the art, as further modified or enhanced by the features of the claims themselves, and is not intended to invoke 35 U.S.C. §112(f).

[0086] While this disclosure has described particular embodiments and generally associated methods, modifications and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of exemplary embodiments does not define or constrain the disclosure. Other changes, substitutions, and alterations are possible without departing from the spirit and scope of the disclosure, as defined by the following claims.

Claims

1. 1. A method for reducing reaction forces in a loudspeaker, comprising: providing current to the loudspeaker from a power supply connected to the loudspeaker; measuring the current through the loudspeaker using a current sensing resistor connected in series with the loudspeaker; using a voltage controlled power supply connected in series with said loudspeaker to supply a current substantially proportional to said measured current through said loudspeaker to an actuator connected in series with said loudspeaker; A method comprising:

2. The method of claim 1 , wherein the voltage controlled power supply is in parallel with the current sense resistor.

3. Providing the current using the voltage controlled power supply comprises: using a current detection level divider to provide a voltage drop proportional to the voltage caused by the current through said loudspeaker; The method of claim 1 , comprising:

4. 4. The method of claim 3, wherein the current sense level divider has a resistance value at least an order of magnitude higher than the current sense resistor.

5. Providing the current using the voltage controlled power supply comprises: filtering high frequency transients in the voltage drop across the current detection level divider using a low pass filter; The method of claim 3 further comprising:

6. The method of claim 5 , wherein the low pass filter is formed by a low pass filter resistor and a low pass filter capacitor.

7. Providing the current using the voltage controlled power supply comprises: using a current source operational amplifier to generate an output current proportional to the voltage across said current sense resistor; The method of claim 5 further comprising:

8. The method of claim 7 , wherein the low pass filter is connected to the non-inverting input of the current source operational amplifier.

9. the inverting terminal of the current source operational amplifier is connected to an actuator current sensing resistor; the coefficient of the output current proportional to the voltage across the current sense resistor is determined by the resistance of the actuator current sense resistor; The method of claim 7.

10. Providing the current using the voltage controlled power supply comprises: amplifying the output current using an actuator power amplifier to generate the current suitable for driving the actuator; The method of claim 7 further comprising:

11. A loudspeaker; an actuator connected in series with the loudspeaker; a power source connected to the loudspeaker for supplying current to the loudspeaker; a current sensing resistor connected in series with the loudspeaker to measure the current through the loudspeaker; a voltage controlled power supply connected in series with the loudspeaker for supplying a current to the actuator that is substantially proportional to the measured current through the loudspeaker; An apparatus comprising:

12. 12. The apparatus of claim 11, wherein the voltage controlled power supply is in parallel with the current sense resistor.

13. The voltage controlled power supply a current-sensing level divider that provides a voltage drop proportional to the voltage caused by the current through the loudspeaker; 12. The apparatus of claim 11.

14. 14. The apparatus of claim 13, wherein the current sense level divider has a resistance value at least an order of magnitude higher than the current sense resistor.

15. The voltage controlled power supply a low pass filter for filtering high frequency transients in the voltage drop across the current detection level divider; 14. The apparatus of claim 13.

16. 16. The apparatus of claim 15, wherein the low pass filter is formed by a low pass filter resistor and a low pass filter capacitor.

17. The voltage controlled power supply a current source operational amplifier that generates an output current proportional to the voltage across the current sense resistor; 16. The apparatus of claim 15.

18. 18. The apparatus of claim 17, wherein the low pass filter is connected to a non-inverting input of the current source operational amplifier.

19. the inverting terminal of the current source operational amplifier is connected to an actuator current sensing resistor; the coefficient of the output current proportional to the voltage across the current sense resistor is determined by the resistance of the actuator current sense resistor; 18. The apparatus of claim 17.

20. The voltage controlled power supply amplifying the output current using an actuator power amplifier to generate a current suitable for driving the actuator; The apparatus of claim 17 further comprising:

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