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JPS5848597

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DESCRIPTION JPS5848597
[0001]
The present invention relates to an MFB (motional feedback) speaker configured to detect
vibrational acceleration of a vibration system as an electrical signal and to control the detected
signal by negative feedback to the speaker vibration input. The objective is to detect the vibration
acceleration of the vibration system in a noncontact manner optically and control the detection
signal by negatively feeding it back to the speaker vibration input to reduce distortion and
reduce distortion without causing a drop in efficiency. An object of the present invention is to
provide an MFB speaker capable of expanding a reproduction band in a sound range. Generally,
in an electrodynamic speaker, when the input is large in the low frequency range where the
vibration amplitude is large, the voice coil is partially deviated from the magnetic gap to cause
non-linearity in the driving force. In addition, even at the supporting mechanism of the vibration
system such as an edge and a damper, non-linearity occurs at the time of large amplitude. Then,
due to these non-linearities, harmonic distortion occurs in the sound radiated from the speaker.
In addition, the lower limit reproduction limit of the speaker is determined by the lowest
resonance frequency f0 in consideration of the size of the cabinet. The MFB speaker is conceived
to reduce the harmonic distortion and the bass reproduction limit in the dynamical type speaker
by 3 and 1, and detects the vibration acceleration of the imaging system as an electric signal. The
detection signal is negatively fed back to the speaker drive input. A conventional MFB speaker
has a voice coil bobbin wound with a detection coil independent of a drive coil as a method of
obtaining a high speed signal. This is because the detection coil is in the magnetic gap, an
electromotive force Bflv is generated in proportion to the vibration velocity V of the vibration
system, B; the magnetic flux density of the magnetic gap, 2; the effective length of the detection
coil) To obtain an acceleration signal. In addition, there is one in which an acceleration pickup
using piezoelectric ceramics is joined to a vibration system, and an output proportional to
acceleration is directly obtained as a negative feedback signal. However, since the conventional
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MFB speaker directly adds a detection coil or an acceleration pickup to the vibration system, the
weight of the imaging and moving system increases, and there is a problem that the conversion
efficiency of the speaker is lowered. The present invention solves the above-mentioned
conventional drawbacks, and is characterized in that means for optically detecting the vibration
acceleration of the vibration system without contact is provided. As a result, it is possible to
reduce the increase in the weight of the vibration system as much as possible and to reduce the
conversion efficiency of the speaker (it is possible to reduce the harmonic distortion and expand
the reproduction band in the low tone range). Have the advantage of
Hereinafter, the MFB Mebyca of the present invention will be described with reference to the
drawings of one embodiment. 1 and 2 show an embodiment of the MFB speaker according to the
present invention, in which 1 is a voice coil bobbin, 2 is a winding coil coil, 1 is a winding voice
coil, 3 is a diaphragm, 4 is a diaphragm. The edge S6 is a damper, and 6 is a dust cap, which
constitute the vibration system of these hes- sackers. Further, 7 is a frame, 8 is an upper yoke, 9
is a lower yoke including a center pole 9 ', and 10 is a macnet. These are the outer peripheral
surface of the center double ball 9' and the inner periphery of the upper yoke 8. A magnetic
circuit having a predetermined magnetic gap 11 with the surface is configured. Also, 12 is a
vertically elongated photodiode disposed on the upper surface of the center pole 9 'so as to face
the inner circumferential surface of the upper 6-6 voice coil povin 1; 13 is the photodiode 12 A
light emitting diode disposed at the opposing portion of the voice coil bobbin 1, 14 is a speed
calculation circuit to which a signal drawn out from the lead wire 12 'of the photodiode 12 is
input, and these are oscillation methods of the oscillation system It constitutes a means for
detecting the test speed optically and in a noncontact manner. 16, a subtractor for subtracting
the electric signal from the input terminal 16 and the electric signal from the detection means;
17, an amplifier for amplifying the electric signal from the subtracter 16; the output signal of the
amplifier 17 is an input lead It is adapted to be applied to the voice twill 2 through 18. Further, a
voltage from a voltage source 20 is applied to the light emitting diode 13 constituting the
detecting means via a lead wire 19. Here, as shown in FIG. 3, the photodiode 12 constituting the
detection means has a light receiving portion 21 which is long on the front surface, and has three
terminal pins 22 and 23.24 on its back surface, as shown in FIG. Among them, RB is a surface
resistance, P is a current source, D is an ideal diode, CT is a junction capacitance, and R8h is a
parallel resistance. The photodiode 12 detects the center position of the light spot from the light
emitting diode 13 which strikes the light receiving portion 21. The response speed is also very
fast as several microseconds, and it is sufficient for detecting the acceleration of the bass
speaker. Further, as shown in FIG. 6, the acceleration calculation circuit 14 constituting the
detection means receives the differential amplifier 26 to which one output of the photodiode 12
is input, and the other output of the photodiode 12 as shown in FIG. A differential amplifier 26, a
voltage-current converter 27.28 for current-converting an output voltage from the differentialamplifier 25.26, and an adder for adding an output current from the voltage-current converter
27.28 29, a subtractor 3o for subtracting the output current from the voltage-current converter
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27.28, a subtractor 31 for subtracting the output signals from the adder 2e and the subtracter
30, and A buffer circuit 32 for amplifying the output signal from the calculator 32 and a twostage differentiating circuit 33.34 for differentiating the output signal from the buffer circuit 77132 are provided, and the output end 36 corresponds to an acceleration. It is adapted to output
the electrical signal.
A predetermined current is applied from the current source 36 to the center terminal bin 23 of
the -l self photodiode 12. The detection means performs one-dimensional position detection with
the photodiode 12 and can pass through the arithmetic circuit 14 to obtain the acceleration of
the vibration system as an electric signal. That is, the displacement electrical signal by the
vertical vibration of the light emitting diode 13 provided on the bobbin 1 is obtained at the first
stage of the arithmetic circuit 14, and the displacement electrical signal is passed through the
differential circuit 33.34 of two stages to accelerate the vibration system. To obtain an electrical
signal corresponding to The MFB speed is realized by negatively feeding back the electric signal
from the detection means to the input signal. The sound pressure frequency characteristics of the
MFB speaker configured as described above are measured with a standard IS box, and the result
is shown by curve A in FIG. In FIG. 6, the curve B shows the sound pressure frequency
characteristics of the electrodynamic speaker without the detection means. As apparent from FIG.
6, the MFB speaker of this embodiment is effective for expanding the reproduction band in the
low band 0. Further, the equalizer amplifier is connected to the driving amplifier to expand the
reproduction band in the low band. However, simply expanding the reproduction band using an
equalizer amplifier has a drawback that harmonic distortion increases. However, as is clear from
FIG. 7, the reproduction band in the bass range can be expanded in the MFB speaker of this
embodiment without increasing the harmonic distortion. In FIG. 7, A is the sound pressure
frequency characteristic curve of the MFB speaker of this embodiment, B is the second harmonic
distortion, C is the third harmonic distortion, and B 'is an electrokinetic connected with an
equalizer amplifier. Harmonic distortion of the second type speaker, C 'shows the respective
characteristic curves of the third harmonic distortion as well, and it is about 1 odB harmonic if it
is intended to expand the reproduction band in the bass region with the MFB speaker of this
embodiment Wave distortion can be reduced. As described above, according to the present
invention, a means for optically detecting the vibrational acceleration of the vibration system
without contact is provided, and the electric signal from the detection means is negatively fed
back to the input signal. Wave distortion can be reduced and the reproduction band in the bass
region can be expanded. In addition, since the detection means has an optical system and only
provides a lightweight light source in the vibration system, the weight of the vibration system
itself is less likely to be boosted, which lowers the conversion efficiency of the speaker. It has the
advantage of eliminating the need to
[0002]
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Brief description of the drawings
[0003]
1 is a cross-sectional view showing an embodiment of the MFB speaker of the present invention,
FIG. 2 is a cross-sectional view of the main part of the speaker, FIG. 3 shows a photodiode used
for the speaker, A is a front view, B Is a plan view, FIG. 4 is an equivalent circuit diagram of the
photodiode, FIG. 6 is a block diagram of an acceleration detection means of the same speaker,
FIG. 6 is a comparison characteristic curve of sound pressure frequency of the same speaker and
the conventional speaker, FIG. FIG. 7 is a comparison characteristic curve diagram of harmonic
distortion of the same speaker and the conventional speaker.
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