Acoustic system

Abstract

To provide an "acoustic system" which obtains required drive power with low power consumption.SOLUTION: A first voice coil 2051 of which the height is larger than a gap in which magnetic flux is propagated and a second voice coil 2052 of which the height is equal with the gap are provided in such a manner that centers in a height direction coincide with each other (a1). An amplification factor of an audio signal applied to the first voice coil 2051 is defined as A1, an amplification factor of an audio signal applied to the second voice coil 2052 is defined as A2, and a displacement of a speaker 2 is detected. In a range of the displacement where the second voice coil 2052 is located at a position where the second voice coil can act properly with the magnetic flux of the gap, A1 is made into 0 and A2 is made into a predetermined amplification factor A2st. In a range of the displacement where the A2 is not made into A2st, A2 is made into 0 and A1 is made into a predetermined amplification factor A1st. It leads A2st<A1st and a range where the second voice coil 2052 is excluded from the gap is included in the range of the displacement where A1 is made into A1st (a1-a3, b1 and b2).SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a technique for driving a speaker. [Background technology]

[0002] A known technology related to the present application is to provide a sensor that detects the displacement of the vibration system of a speaker, and based on the response of the displacement to the input signal, correct the input signal so that the output distortion of the speaker is reduced, and output the corrected signal to the speaker (for example, Patent Document 1).

[0003] Furthermore, a technology related to the present application is known in which a speaker is provided with multiple voice coils with different DC resistance values ​​and the voice coil to be driven can be switched, thereby making it possible to vary the Q characteristics of the speaker in the low frequency range (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-81815 [Patent Document 2] Japanese Utility Model Application Publication No. 62-139192 Summary of the Invention [Problem to be solved by the invention]

[0005] When suppressing speaker malfunctions such as speaker output distortion and over-amplitude by manipulating the signal applied to the voice coil based on the displacement of the speaker's vibration system detected by a sensor, a driving force acts on the voice coil due to interaction with the magnetic flux passing through the gap between the top plate and the center pole. Therefore, if the voice coil is displaced to a position where it leaves the gap, the driving force acting on the voice coil is lost, and it becomes impossible to suppress the occurrence of defective operation.

[0006] On the other hand, if the voice coil height is made sufficiently large relative to the gap height (the axial length of the speaker), it is possible to prevent the voice coil from displacing to a position where it falls outside the gap, but in this case the driving force acting on the voice coil for the same input will be smaller than that of a voice coil whose height is about the same as the gap height, resulting in a deterioration in initial sensitivity, etc. On the other hand, if the amplification factor of the signal output to the voice coil is increased, the driving force acting on the voice coil can be increased, but this will result in increased power consumption.

[0007] Therefore, an object of the present invention is to obtain the required driving force for the voice coil while minimizing power consumption. [Means for solving the problem]

[0008] To achieve the above object, the present invention provides an acoustic system including a speaker and a driver that drives the speaker with an audio signal. The acoustic system also includes a displacement detection unit that detects axial displacement of the speaker's vibration system. The speaker also includes a first voice coil and a second voice coil disposed in a gap through which magnetic flux propagates in the radial direction of the speaker, wherein the axial range of the first voice coil is greater than the axial range of the gap, the axial range of the second voice coil is smaller than the axial range of the first voice coil, and the axial range of the second voice coil is within the axial range of the first voice coil. The driving unit includes a first driving means that applies an audio signal amplified by a set first amplification factor to the first voice coil, a second driving means that applies an audio signal amplified by a set second amplification factor to the second voice coil, and an amplification factor setting means that, based on the axial position of the second voice coil represented by the displacement detected by the displacement detection means, sets the second amplification factor to 0 when the position of the second voice coil is within an ineffective range where the second voice coil cannot interact with magnetic flux propagating through the gap, and sets the first amplification factor to a first standard amplification factor, which is a predetermined amplification factor, when the position of the second voice coil is within an effective range where the second voice coil can properly interact with magnetic flux propagating through the gap, sets the first amplification factor to 0 and sets the second amplification factor to a second standard amplification factor, which is a predetermined amplification factor.

[0009] Here, in this acoustic system, the ineffective range may be a range in the axial direction in which the position of the second voice coil is outside the range of the gap, and the effective range may be a range in the axial direction in which the position of the second voice coil is not outside the range of the gap.

[0010] Furthermore, the above acoustic system may be configured such that, when no audio signal is applied to either the first voice coil or the second voice coil, the gap and the midpoint of the axial range of the first voice coil and the second voice coil are equal in the axial direction.

[0011] Furthermore, in the above acoustic system, the first standard amplification factor may be greater than the second standard amplification factor, the first driving means may have a first amplifier that outputs an audio signal to be applied to the first voice coil, the second driving means may have a second amplifier that outputs an audio signal to be applied to the second voice coil, and the power supply voltage of the second amplifier may be smaller than the power supply voltage of the first amplifier.

[0012] Furthermore, the amplification factor setting means of the above acoustic system may be replaced with one that, based on the axial position of the second voice coil represented by the displacement detected by the displacement detection means, sets the second amplification factor to a smaller amplification factor when the second voice coil is in a position where it cannot interact with the magnetic flux propagating through the gap than when it is in a position where it can interact with the magnetic flux, and sets the first amplification factor to a larger amplification factor when the second voice coil is in a position where it cannot interact with the magnetic flux propagating through the gap than when it is in a position where it can interact with the magnetic flux.

[0013] Here, the driving section of the above-mentioned acoustic system may be provided with a malfunction suppression means for manipulating the audio signal that drives the first voice coil based on the displacement detected by the displacement detection means so as to prevent malfunction of the speaker. The acoustic system described above includes a first voice coil having an axial range larger than the gap, and a second voice coil having an axial range smaller than the first voice coil. Typically, when the second voice coil is located within a range where it can properly interact with the magnetic flux propagating through the gap, an audio signal is applied only to the second voice coil to drive the speaker, and when the second voice coil is located within a range where it cannot interact with the magnetic flux propagating through the gap, an audio signal is applied only to the first voice coil to drive the speaker.

[0014] Here, if the second voice coil can properly interact with the magnetic field propagating through the gap, the driving force for the same input will be greater for the second voice coil, which has a smaller axial range, than for the first voice coil.Therefore, the amplification factor of the audio signal when applying an audio signal only to the second voice coil to drive the speaker can be smaller than the amplification factor of the audio signal when applying an audio signal only to the first voice coil to drive the speaker.As a result, the power consumption when applying an audio signal only to the second voice coil can be smaller than the power consumption when applying an audio signal only to the first voice coil.

[0015] Furthermore, even if the second voice coil becomes disengaged from the gap and can no longer interact with the magnetic flux propagating through the gap, the first voice coil, which has a wider axial range than the second voice coil, is still able to interact with the magnetic flux propagating through the gap. Therefore, by driving this first voice coil, the driving force required for proper playback of audio signals and for preventing malfunctions can be obtained. [Effects of the Invention]

[0016] As described above, according to the present invention, it is possible to obtain the required driving force for the voice coil while minimizing power consumption. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram showing a configuration of an audio system according to an embodiment of the present invention. [Figure 2] 1 is a diagram showing a configuration of a speaker according to an embodiment of the present invention; [Figure 3] FIG. 10 is a diagram illustrating an example of control of an amplification factor according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described. FIG. 1 shows the configuration of an audio system according to this embodiment. As shown in the figure, the acoustic system includes a sound source device 1 that outputs an audio signal, a speaker 2, a displacement sensor 3 provided in the speaker 2, a signal processing device 4, a first amplifier 5, and a second amplifier 6. The power supply voltage V1 of the first amplifier 5 is greater than the power supply voltage V2 of the second amplifier 6, and the rated output / maximum output amplitude of the first amplifier 5 is greater than the rated output / maximum output amplitude of the second amplifier 6. If the audio system is installed in a car, the car battery can be used as the power source for the second amplifier 6, and in this case the power supply voltage V2 will be approximately 12 V to 13 V. When the car battery is used as the power source for the second amplifier 6, the battery power supply voltage is boosted to, for example, 20 V and used as the power supply voltage V1.

[0019] The signal processing device 4 can be configured, for example, using a DSP (Digital Signal Processor), and includes a first amplification factor adjustment unit 41, a second amplification factor adjustment unit 42, a first distortion suppression unit 43, a second distortion suppression unit 44, an over-amplitude protection unit 45, a displacement detection unit 46, and a control unit 47. Next, FIG. 2a shows the configuration of the speaker 2. As shown in the figure, the speaker 2 includes a yoke 201, a magnet 202, a top plate 203, a voice coil bobbin 204, a voice coil 205, a frame 206, a damper 207, a diaphragm 208, an edge 209, a dust cap 210, and a displacement detection magnet 211. If we consider the upward direction of speaker 2 in the axial direction of speaker 2 in the figure as the upward direction of speaker 2 and the downward direction as the downward direction of speaker 2, yoke 201 has center pole 2011 protruding upward in the center, with ring-shaped magnet 202 provided on the outer periphery of center pole 2011, and ring-shaped top plate 203 provided on magnet 202. Top plate 203 is made of a conductive material such as iron. Yoke 201, magnet 202, and top plate 203 form magnetic circuit 220.

[0020] Here, as shown in Figures 2a and 2b, the voice coil bobbin 204 has a hollow cylindrical shape, and as shown in Figures 2b and 2c, a first voice coil 2051, whose coil wire is indicated by a white circle, and a second voice coil 2052, whose coil wire is indicated by a black circle, are wound around the outer periphery of the voice coil bobbin 204 in a stacked form with the second voice coil 2052 wrapped around the outer periphery of the first voice coil 2051.

[0021] Furthermore, the center pole 2011 of the yoke 201 is inserted into the hollow of the voice coil bobbin 204 from below, and the voice coil bobbin 204 is movable up and down relative to the yoke 201 . 2b and 2c, in a neutral state in which no signal is applied to the first voice coil 2051 and the second voice coil 2052, the first voice coil 2051 and the second voice coil 2052 are arranged so that the vertical center positions of the first voice coil 2051 and the second voice coil 2052 coincide with the vertical center position (axial direction of the speaker 2) of the gap, which is the gap through which magnetic flux propagates radially (left and right direction in the figure) of the speaker 2 between the center pole 2011 of the yoke 201 and the top plate 203.

[0022] Also, as shown in FIG. 2c, the height (vertical length) H1 of the first voice coil 2051 is significantly larger than the gap height H0 (for example, H1≧1.5×H0), and the height H2 of the second voice coil 2052 is approximately equal to the gap height H0. Therefore, when matching the impedance of the first voice coil 2051 and the second voice coil 2052, the first voice coil 2051 has a larger wire diameter and a longer wire length than the second voice coil 2052. Diaphragm 208 has a shape roughly similar to the side surface of a truncated cone whose height direction is the vertical direction of speaker 2, and its outer peripheral edge is connected to the upper end of frame 206 by edge 209. In addition, the inner peripheral edge of diaphragm 208 is fixed to the upper end of voice coil bobbin 204.

[0023] In such a configuration of the speaker 2, when a signal is applied to the first voice coil 2051 or the second voice coil 2052, the voice coil bobbin 204 vibrates up and down according to the amplitude of the applied signal due to the electromagnetic interaction between the magnetic flux passing through the gap in the radial direction and the signal flowing through the first voice coil 2051 or the second voice coil 2052. When the voice coil bobbin 204 vibrates, the diaphragm 208 connected to the voice coil bobbin 204 vibrates, generating sound according to the applied signal.

[0024] Next, as shown in Figures 2a and 2b, the displacement detection magnet 211 is fixed to the outer periphery of the voice coil bobbin 204 so as to move up and down together with the voice coil bobbin 204, and generates a magnetic flux in a direction perpendicular to the magnetic flux generated by the magnetic circuit 220. Next, the above-mentioned displacement sensor 3 is fixed to a position close to the displacement detecting magnet 211 in a non-vibration system of the speaker 2, such as the top plate 203. The displacement sensor 3 is a magnetic angle sensor, and as shown in FIG. 2d, it detects and outputs the arctangent Qs / Qc of the angle of the resultant vector Q of the magnetic flux vector Qc acting from the magnetic circuit 220 and the magnetic flux vector Qs acting from the displacement detecting magnet 211 as a magnetic angle. The magnetic flux vector generated by the displacement detecting magnet 211 acting on the displacement sensor 3 changes due to the displacement of the displacement detecting magnet 211 accompanying the vertical displacement of the voice coil bobbin 204, so this magnetic angle is a value according to the vertical displacement of the voice coil bobbin 204, and therefore the vertical position of the vibration system of the speaker 2.

[0025] Returning to FIG. 1, the displacement detection unit 46 of the signal processing device 4 calculates the vertical displacement position z_VC of the vibration system of the speaker 2 from the magnetic angle detected by the displacement sensor 3 and outputs it to the control unit 47. The control unit 47 controls the amplification factor A1 of the first amplification factor adjustment unit 41 in accordance with the displacement position z_VC of the vibration system of the speaker 2 detected by the displacement detection unit 46, and also controls the amplification factor A2 of the second amplification factor adjustment unit 42 in accordance with the displacement position z_VC. Furthermore, the control unit 47 predicts the occurrence of excessive amplitude in the vibration system of the speaker 2 from the displacement position z_VC of the vibration system of the speaker 2 detected by the displacement detection unit 46 and the amplitude of vibration of the vibration system of the speaker 2 indicated by the displacement position z_VC, and if the occurrence of excessive amplitude is predicted, controls the execution of the excessive amplitude protection operation of the excessive amplitude protection unit 45. Here, excessive amplitude of the vibration system of the speaker 2 refers to an amplitude large enough to cause a mechanical malfunction such as bottoming out, in which the voice coil bobbin 204 hits the yoke 201.

[0026] Furthermore, the control unit 47 relays the displacement position z_VC of the vibration system of the speaker 2 detected by the displacement detection unit 46 to the first distortion suppression unit 43 , the second distortion suppression unit 44 and the over-amplitude protection unit 45 . Next, the first amplification factor adjustment unit 41 amplifies the audio signal input from the sound source device 1 by the amplification factor A1 set by the control unit 47 and outputs the amplified signal to the first distortion suppression unit 43. However, when the amplification factor A1 is 0, the first amplification factor adjustment unit 41 may achieve amplification by the amplification factor A1 by setting the output to 0. The first distortion suppression unit 43 applies a transfer function set therein to the audio signal input from the first amplification factor adjustment unit 41 and outputs the result to the overamplitude protection unit 45. The first distortion suppression unit 43 also performs an operation of updating the transfer function set therein to a transfer function that corrects the audio signal so that the response of the displacement position z_VC notified by the control unit 47 to the audio signal input from the first amplification factor adjustment unit 41 becomes a distortion-free response. However, the first distortion suppression unit 43 may also perform an operation of setting the output to 0 when the amplification factor A1 is 0.

[0027] The overamplitude protection unit 45 normally transparently outputs the audio signal input from the first distortion suppression unit 43 directly to the first amplifier 5, but performs the following overamplitude protection operation while the control unit 47 is controlling the execution of the overamplitude protection operation. That is, as the overamplitude protection operation, the overamplitude protection unit 45 performs a braking operation by, for example, generating a brake signal that applies a driving force to the first voice coil 2051 in a direction opposite to the displacement direction indicated by the displacement position z_VC notified by the control unit 47, and outputting the brake signal to the first amplifier 5 instead of the audio signal. Alternatively, as the overamplitude protection operation, the overamplitude protection unit 45 performs an amplitude suppression operation by, for example, attenuating the audio signal input from the first distortion suppression unit 43 and outputting the attenuated audio signal to the first amplifier 5.

[0028] The first amplifier 5 amplifies the audio signal or brake signal input from the over-amplitude protection unit 45 by a preset fixed amplification factor and outputs the amplified signal to the first voice coil 2051 of the speaker 2 . Next, the second amplification factor adjustment unit 42 amplifies the audio signal input from the sound source device 1 by the amplification factor A2 set by the control unit 47 and outputs the amplified signal to the second distortion suppression unit 44. However, when the amplification factor A2 is 0, the second amplification factor adjustment unit 42 may achieve amplification by the amplification factor A2 by setting the output to 0. The second distortion suppression unit 44 applies a transfer function set therein to the audio signal input from the second amplification factor adjustment unit 42 and outputs the result to the second amplifier 6. The second distortion suppression unit 44 also performs an operation of updating the transfer function set therein to a transfer function that corrects the audio signal so that the response of the displacement position z_VC notified by the control unit 47 to the audio signal input from the second amplification factor adjustment unit 42 becomes a distortion-free response. However, the second distortion suppression unit 44 may also perform an operation of setting the output to 0 when the amplification factor A2 is 0.

[0029] The second amplifier 6 amplifies the audio signal input from the second distortion suppression unit 44 with the same amplification factor as the first amplifier 5 and outputs the amplified signal to the second voice coil 2052 of the speaker 2 . The control of the amplification factor A1 of the first amplification factor adjustment unit 41 and the amplification factor A2 of the second amplification factor adjustment unit 42 according to the displacement position z_VC of the vibration system of the speaker 2 performed by the control unit 47 will be described below. Now, as shown in FIG. 3a1, assuming that the displacement position z_VC is represented using the vertical coordinate axis z of the speaker 2, when the central positions in the height direction (vertical direction) of the first voice coil 2051 and the second voice coil 2052 coincide at the central position in the height direction of the gap, z_VC = 0. When the voice coil bobbin is displaced upward from the position in FIG. 3a1 and the lower end of the second voice coil 2052 reaches the upper end of the gap as shown in FIG. 3a2, z_VC = Uz > 0. When the voice coil bobbin is displaced downward from the position in FIG. 3a1 and the upper end of the second voice coil 2052 reaches the lower end of the gap as shown in FIG. 3a3, z_VC = Lz < 0. As such, the control unit 47 controls the amplification factor A1 of the first amplification factor adjustment unit 41 and the amplification factor A2 of the second amplification factor adjustment unit 42 as shown in FIGS. 3b1 and 3b2, for example.

[0030] That is, within the range of Uz ≧ z_VC ≧ Lz, in the range where the second voice coil 2052 is at a position where it can act properly with the magnetic flux of the gap, the amplification factor A1 is set to 0, and the amplification factor A2 is set to a predetermined amplification factor A2st. And in the range where the amplification factor A2 is not set to the amplification factor A2st, the amplification factor A2 is set to 0, and the amplification factor A1 is set to a predetermined amplification factor A1st. Here, the range where the amplification factor A1 is set to the amplification factor A1st includes the range of z_VC < Lz where the second voice coil 2052 is at a position outside the gap and the range of z_VC > Uz. Note that the range where the second voice coil 2052 is at a position where it can act properly with the magnetic flux of the gap may be the range of Uz ≧ z_VC ≧ Lz where the second voice coil 2052 is not completely outside the gap.

[0031] Here, A1st is the amplification factor at which the driving force required for the desired vibration can be realized by driving only the first voice coil 2051 with the audio signal amplified by A1st, and A2st is the amplification factor at which the driving force required for the desired vibration can be realized by driving only the second voice coil 2052 with the audio signal amplified by A2st when the second voice coil 2052 can properly interact with the magnetic flux in the gap. When the second voice coil 2052 can properly interact with the magnetic flux in the gap, the driving force for the same input is greater for the second voice coil 2052, which has a smaller height, than for the first voice coil 2051, as shown in the figure. <A2st<A1stとなる。 Therefore, when the audio signal amplified by A2st is given to the second amplifier 6 to drive only the second voice coil 2052, the amplitude of the audio signal output from the second amplifier 6 and the power consumption of the second amplifier 6 are smaller than the amplitude of the audio signal output from the first amplifier 5 and the power consumption of the first amplifier 5 when the audio signal amplified by A1st is given to the first amplifier 5 to drive only the first voice coil 2051. Furthermore, the power consumption of the first amplifier 5 when the amplification factor A1 is set to 0 and the power consumption of the second amplifier 5 when the amplification factor A2 is set to 0 are also sufficiently small because the outputs of the first amplifier 5 and the second amplifier 5 are also 0.

[0032] Therefore, by controlling the amplification factor A1 of the first amplification factor adjustment unit 41 and the amplification factor A2 of the second amplification factor adjustment unit 42 in this manner, the power consumption of the first amplifier 5 and the second amplifier 6 when driving only the second voice coil 2052 can be kept lower than the power consumption of the first amplifier 5 and the second amplifier 6 when driving only the first voice coil 2051, and the driving force required for the required vibration can be secured even when driving only the first voice coil 2051.

[0033] In addition, the second voice coil 2052 is driven by the second amplifier 6, which has a lower power supply voltage (rated output / maximum output voltage) than the first amplifier 5 used to drive the first voice coil 2051. This also helps to keep power consumption low when driving only the second voice coil 2052. Furthermore, even if the second voice coil 2052 moves out of the gap and can no longer interact with the magnetic flux propagating through the gap, the first voice coil 2051, which has a wider axial range than the second voice coil 2052, is still able to interact with the magnetic flux propagating through the gap, and therefore, by driving this first voice coil 2051, it is possible to obtain the driving force required for proper playback of audio signals and for preventing malfunctions.

[0034] When controlling the amplification factor A1 of the first amplification factor adjustment unit 41 and the amplification factor A2 of the second amplification factor adjustment unit 42 as shown in Figures 3b1 and b2, the control unit 47 controls the operation of the first distortion suppression unit 43 and the second distortion suppression unit 44 so that the transfer function set in the first distortion suppression unit 43 is updated only during the period when the amplification factor A2 is 0, and the transfer function set in the second distortion suppression unit 44 is updated only during the period when the amplification factor A1 is 0.

[0035] Here, the control of the amplification factor A1 of the first amplification factor adjuster 41 and the amplification factor A2 of the second amplification factor adjuster 42 may be performed as shown in FIGS. 3c1 and 3c2. The control in Figs. 3c1 and 3c2 is the same as the control in Figs. 3b1 and 3b2, except that the amplification factor A1 is gradually changed between 0 and A1st and the amplification factor A2 is gradually changed between 0 and A2st. In addition, even when controlling the amplification factor A1 of the first amplification factor adjustment unit 41 and the amplification factor A2 of the second amplification factor adjustment unit 42 as shown in Figures 3c1 and c2, the control unit 47 controls the operation of the first distortion suppression unit 43 and the second distortion suppression unit 44 so that the transfer function set in the first distortion suppression unit 43 is updated only during the period when the amplification factor A2 is 0, and the transfer function set in the second distortion suppression unit 44 is updated only during the period when the amplification factor A1 is 0.

[0036] Here, the control of the amplification factor A1 of the first amplification factor adjuster 41 and the amplification factor A2 of the second amplification factor adjuster 42 may be performed as shown in FIGS. 3d1 and 3d2. 3d1 and d2, in the control of Figures 3c1 and 3c2, A2st of the amplification factor A2 is made smaller than that of Figure 3c2, and while the amplification factor A2 is A2st, the amplification factor A1 is set to A1sp. A1st > A1sp > 0, and A1sp is an amplification factor that can realize the driving force of the vibration system required for the desired vibration by driving the second voice coil 2052 with the amplification factor A2st and driving the first voice coil 2051 with the amplification factor A1sp.

[0037] 3d1 and 3d2, the control unit 47 controls the first distortion suppression unit 43 so that the transfer function set in the first distortion suppression unit 43 is updated only during the period when the amplification factor A2 is 0. In this case, the transfer function of the second distortion suppression unit 44 is not updated, and a predetermined transfer function is fixedly used as the transfer function of the second distortion suppression unit 44. Alternatively, the second distortion suppression unit 44 may not be provided, and the audio signal may be output directly from the second amplification suppression unit 42 to the second amplifier 6. [Explanation of symbols]

[0038] 1...sound source device, 2...speaker, 3...displacement sensor, 4...signal processing device, 5...first amplifier, 6...second amplifier, 41...first amplification factor adjustment unit, 42...second amplification factor adjustment unit, 43...first distortion suppression unit, 44...second distortion suppression unit, 45...overamplitude protection unit, 46...displacement detection unit, 47...control unit, 201...yoke, 202...magnet, 203...top plate, 204...voice coil bobbin, 206...frame, 207...damper, 208...diaphragm, 209...edge, 210...dust cap, 211...displacement detection magnet, 220...magnetic circuit, 2011...center pole, 2051...first voice coil, 2052...second voice coil.

Claims

1. An audio system including a speaker and a driver that drives the speaker with an audio signal, a displacement detection means for detecting a displacement of the vibration system of the speaker in the axial direction of the speaker; The speaker includes: a first voice coil and a second voice coil disposed in a gap through which magnetic flux propagates in a radial direction of the speaker; a size of the axial range of the first voice coil in the speaker axial direction is larger than a size of the axial range of the gap, and a size of the front axial range of the second voice coil is smaller than a size of the axial range of the first voice coil; the axial extent of the second voice coil falls within the axial extent of the first voice coil; The drive unit is a first driving means for applying an audio signal amplified by a set first amplification factor to the first voice coil; a second driving means for applying an audio signal amplified by a set second amplification factor to the second voice coil; an amplification factor setting means for setting the second amplification factor to 0 and the first amplification factor to a predetermined first standard amplification factor when the second voice coil is located within an ineffective range where the second voice coil cannot interact with magnetic flux propagating through the gap, based on the axial position of the second voice coil represented by the displacement detected by the displacement detection means; and for setting the first amplification factor to 0 and the second standard amplification factor to a predetermined second standard amplification factor when the second voice coil is located within an effective range where the second voice coil can properly interact with magnetic flux propagating through the gap.

2. 2. The acoustic system of claim 1, An acoustic system characterized in that the ineffective range is a range in the axial direction in which the position of the second voice coil is outside the range of the gap, and the effective range is a range in the axial direction in which the position of the second voice coil is not outside the range of the gap.

3. 2. The acoustic system of claim 1, An acoustic system characterized in that, when no audio signal is applied to either the first voice coil or the second voice coil, the gap and the midpoint of the axial range of the first voice coil and the second voice coil are equal in the axial direction.

4. 2. The acoustic system of claim 1, the first standard amplification factor is greater than the second standard amplification factor; the first driving means has a first amplifier that outputs an audio signal to be applied to the first voice coil; the second driving means has a second amplifier that outputs an audio signal to be applied to the second voice coil, An acoustic system, wherein a power supply voltage of the second amplifier is lower than a power supply voltage of the first amplifier.

5. An audio system including a speaker and a driver that drives the speaker with an audio signal, a displacement detection means for detecting a displacement of the vibration system of the speaker in the axial direction of the speaker; The speaker includes: a first voice coil and a second voice coil disposed in a gap through which magnetic flux propagates in a radial direction of the speaker; a size of the axial range of the first voice coil in the speaker axial direction is larger than a size of the axial range of the gap, and a size of the front axial range of the second voice coil is smaller than a size of the axial range of the first voice coil; the axial extent of the second voice coil falls within the axial extent of the first voice coil; The drive unit is a first driving means for applying an audio signal amplified by a set first amplification factor to the first voice coil; a second driving means for applying an audio signal amplified by a set second amplification factor to the second voice coil; an amplification factor setting means for setting, based on the axial position of the second voice coil represented by the displacement detected by the displacement detection means, when the second voice coil is in a position where it cannot interact with magnetic flux propagating through the gap, a smaller amplification factor as the second amplification factor than when it is in a position where it can interact with magnetic flux, and when the second voice coil is in a position where it cannot interact with magnetic flux propagating through the gap, a larger amplification factor as the first amplification factor than when it is in a position where it can interact with magnetic flux.

6. 6. An acoustic system according to claim 1, 2, 3, 4 or 5, An acoustic system characterized in that the driving unit has a malfunction suppression means that manipulates the audio signal that drives the first voice coil based on the displacement detected by the displacement detection means to prevent malfunction of the speaker.

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