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JP2005057687

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DESCRIPTION JP2005057687
PROBLEM TO BE SOLVED: To provide an electrostatic transducer capable of emitting an acoustic
signal of high sound pressure level over a wide frequency band with a low DC bias voltage and an
alternating signal voltage. SOLUTION: An insulating base 10 having an opening at the top and
being U-shaped and cylindrical in cross section, and a fixed electrode 12 formed of a conductive
material on the upper surface of the bottom of the insulating base 10 are fixed. A movable
electrode 14 formed of a conductive material at the upper end portion of the insulating base
material 10 facing the electrode 12 is provided, and an alternating current signal in the
ultrasonic frequency band is applied between the fixed electrode 12 and the movable electrode
14 An electrostatic transducer for generating a sound wave of constant high sound pressure over
a frequency band, wherein the fixed electrode 12 or the movable electrode 14 is made to
increase the surface area of at least one of the fixed electrode 12 and the movable electrode 14.
Form. [Selected figure] Figure 1
Electrostatic transducer and ultrasonic speaker using the same
[0001]
The present invention relates to an electrostatic transducer that generates an acoustic signal of
constant high sound pressure over a wide frequency band (ultrasonic frequency band), and an
ultrasonic speaker using the same.
[0002]
As a conventional ultrasonic transducer of this type, one that generates an acoustic signal
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(ultrasound) of high sound pressure level over a wide frequency band is known (see, for example,
Patent Document 1).
This ultrasonic transducer has different depths of resonance at a predetermined frequency by
providing a conductive spacer, a back plate consisting of at least one electrode, and a dielectric
spacer between the film and the pack plate. Resonant cavities are formed so that individual
resonance characteristics radiated from the plurality of resonant cavities can cover a desired
frequency range.
[0003]
Further, as a conventional electrostatic transducer, there is one shown in FIG. This electrostatic
transducer is known as a wide frequency band ultrasonic transducer capable of generating an
acoustic signal of high sound pressure level over a wide frequency band. In FIG. 6, the
electrostatic transducer 1 has an opening at the top, an insulating base 10 which is U-shaped in
cross section and cylindrically formed of an insulating material, and a conductive material on the
upper surface of the bottom of the insulating base 10. And the movable electrode 14 formed of a
conductive material on the upper end portion of the insulating base material 10 with the spacer
16 interposed therebetween. Reference numeral 20 denotes a movable electrode lead-out
member which forms the lead portion of the movable electrode 14 and crimps and fixes the
movable electrode 14 to the insulating base 10, and 12 A denotes a lead-out portion of the fixed
electrode 12.
[0004]
In the above configuration, a DC bias voltage of several tens to several hundreds of volts is
applied between the movable electrode 14 and the fixed electrode 12 by the DC bias power
supply 30 to suck the movable electrode 14 in advance to the fixed electrode 12 side. The
electrostatic transducer 1 is driven by superimposing an alternating current signal having a peak
value slightly lower than the voltage level of the DC bias voltage on the DC bias voltage by using
the position of the attracted movable electrode 14 as an operating point. Japanese Patent LaidOpen No. 2000-50392
[0005]
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2
However, in the ultrasonic transducer shown in Patent Document 1, there is a problem that
frequency characteristics having flat sound pressure levels can not be obtained over a wide
frequency band. Further, in the electrostatic transducer shown in FIG. 6, the DC bias voltage and
the AC signal applied between the movable electrode 14 and the fixed electrode 12 are high
voltages, and while the risk is increased, the size of the device is increased. , There is a problem
that it leads to high voltage and high cost.
[0006]
The present invention has been made in view of such circumstances, and provides an
electrostatic transducer capable of emitting an acoustic signal of high sound pressure level over a
wide frequency band with low DC bias voltage and AC signal voltage, and It aims at providing the
used ultrasonic speaker.
[0007]
In order to achieve the above object, the invention according to claim 1 (electrostatic transducer)
is an insulating base having an opening at the top and being U-shaped and cylindrical in cross
section, and the insulating base A fixed electrode formed of a conductive material on the upper
surface of the bottom of the base, and a movable electrode formed of a conductive material on
the upper end portion of the insulating base opposite to the fixed electrode, between the fixed
electrode and the movable electrode An electrostatic transducer that generates an acoustic wave
of constant high sound pressure over a wide frequency band by applying an alternating current
signal in an ultrasonic frequency band to the surface, and the surface area of at least one of the
fixed electrode and the movable electrode The fixed electrode or the movable electrode is formed
to increase the
[0008]
The invention according to claim 2 (electrostatic transducer) is an insulating base having an
opening at the top and being U-shaped in cross section and cylindrically formed, a conductive
material on the upper surface of the bottom of the insulating base A movable electrode formed
of: a vibrating film formed of a non-conductive material on the upper end portion of the
insulating base material facing the fixed electrode; and a movable electrode formed of a metal
film formed on one surface of the vibrating film It is an electrostatic transducer which generates
a sound wave of constant high sound pressure over a wide frequency band by applying an
alternating current signal of an ultrasonic frequency band between the fixed electrode and the
movable electrode, comprising: the vibrating film surface The movable electrode is formed by
forming asperities on the surface and forming a metal film on the surface of the vibrating film
including the asperities.
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[0009]
The invention according to claim 3 (electrostatic transducer) is an insulating base having an
opening at the top and being U-shaped in cross section and cylindrically formed, a conductive
material on the upper surface of the bottom of the insulating base A movable electrode formed
of: a vibrating film formed of a non-conductive material on the upper end portion of the
insulating base material facing the fixed electrode; and a movable electrode formed of a metal
film formed on one surface of the vibrating film An electrostatic transducer that generates an
acoustic wave of constant high sound pressure over a wide frequency band by applying an
alternating current signal in an ultrasonic frequency band between the fixed electrode and the
movable electrode, Irregularities are formed on the surface facing the movable electrode.
[0010]
The invention according to claim 4 (electrostatic transducer) is an insulating base having an
opening at the top and being U-shaped in cross section and cylindrically formed, a conductive
material on the upper surface of the bottom of the insulating base A movable electrode formed
of: a vibrating film formed of a non-conductive material on the upper end portion of the
insulating base material facing the fixed electrode; and a movable electrode formed of a metal
film formed on one surface of the vibrating film It is an electrostatic transducer which generates
a sound wave of constant high sound pressure over a wide frequency band by applying an
alternating current signal of an ultrasonic frequency band between the fixed electrode and the
movable electrode, comprising: the vibrating film surface The movable electrode is formed by
forming asperities on the surface and forming a metal film on the surface of the vibrating film
including the asperities, and asperities are formed on the surface of the fixed electrode facing the
movable electrode. .
[0011]
The invention according to claim 5 (ultrasonic speaker) includes the electrostatic transducer
according to any one of claims 1 to 4.
[0012]
According to the invention as set forth in claims 1 to 4, the surface area of the counter electrode
of the electrostatic transducer can be increased, and therefore, the levels of the DC bias voltage
and the alternating signal voltage for driving the electrostatic transducer. Can be lowered.
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That is, it is possible to emit an acoustic signal of high sound pressure over a wide frequency
band with low DC bias voltage and AC signal voltage.
[0013]
Further, according to the invention as set forth in claim 5, since an electrostatic transducer
capable of emitting an acoustic signal of high sound pressure over a wide frequency band with
low DC bias voltage and AC signal voltage is used, the ultrasonic speaker Can be downsized, the
voltage can be reduced, and the cost can be reduced.
The effect is obtained.
[0014]
Hereinafter, embodiments of the present invention will be described in detail with reference to
the drawings.
FIG. 1 shows the configuration of the main part of an electrostatic transducer according to an
embodiment of the present invention.
The overall configuration is basically the same as that of the conventional electrostatic
transducer shown in FIG. 6, but the electrostatic transducer according to this embodiment has
the surface area of at least one of the fixed electrode and the movable electrode. It is
characterized in that a fixed electrode and a movable electrode are formed so as to increase.
In FIG. 1, the same parts as those of the conventional apparatus shown in FIG. 6 are denoted by
the same reference numerals.
[0015]
In FIG. 1, the electrostatic transducer 1 has an opening at the top, a U-shaped cross section Ushaped insulating base 10 and a conductive material on the upper surface of the bottom of the
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insulating base 10, for example, A fixed electrode 12 formed of a metallic bulk material, a
vibrating membrane 140 formed of a non-conductive material on the upper end of the insulating
base 10 with a spacer 16 facing the fixed electrode 12, and the vibrating membrane 140 And a
movable electrode 14 formed of a metal film 142 formed on one side of the substrate.
[0016]
Further, asperities are formed on the surface of the fixed electrode 12 facing the vibrating film
142.
The movable electrode 14 and the fixed electrode 12 are closely fixed by the movable electrode
lead-out member 20 and the insulating base 10.
Power feeding to the metal film 142 in the movable electrode 14 is performed via the movable
electrode extraction member 20. As described in FIG. 6, the drive voltage is applied between the
metal film 142 of the movable electrode 14 and the fixed electrode 12 in a state in which an AC
signal is superimposed on a DC bias voltage.
[0017]
In the electrostatic transducer 1 configured as described above, when a DC bias voltage is applied
between the fixed electrode 12 and the metal film 142 of the movable electrode 14, the movable
electrode 14 is attracted to the fixed electrode 12 and displaced. It stabilizes at this position. The
position of the movable electrode 14 is taken as an operating point. In this state, when an
alternating current signal in the ultrasonic frequency band is applied, the movable electrode 14
vibrates around the operating point to generate a vibration with less distortion.
[0018]
Here, since unevenness is formed on the surface of the fixed electrode 12 facing the vibrating
film 140, the surface area can be increased as compared to the case where the surface of the
fixed electrode 12 facing the vibrating film 140 is flat. As a result, since the capacitance formed
between the fixed electrode 12 and the movable electrode 14 can be increased, the drive voltage
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of the electrostatic transducer can be reduced. In the electrostatic transducer according to the
present embodiment, a constant high-pitched sound over a wide frequency band is applied by
applying an alternating current signal with a low DC bias voltage and a low ultrasonic frequency
band between the fixed electrode 10 and the movable electrode 14 as compared with the
conventional. A pressure sound wave can be generated.
[0019]
Next, an example configured to increase the surface area of the movable electrode 14 compared
to the conventional electrostatic transducer will be described with reference to FIGS. 2 and 3. In
FIG. 2, the vibrating film 140A in the movable electrode 14 has a plurality of irregularities
formed on one surface of an insulating material such as PET (polyethylene terephthalate resin).
The thickness of the vibrating film 140A is preferably 10 μm or less in view of its vibration
characteristics, but the unevenness formed on the surface of the vibrating film 140A is several
μm deep. Aluminum is vapor-deposited on the surface on which the unevenness is formed to
form a metal film 142A to be an electrode. The thickness of the metal film 142A formed by this
vapor deposition is several thousand Å.
[0020]
According to the movable electrode 14 configured as described above, the surface area of the
metal film 142A opposed to the fixed electrode 12 can be increased as compared with the
movable electrode in the conventional electrostatic transducer shown in FIG. By increasing the
surface area of the metal film 142A of the movable electrode, the electrostatic capacitance C
formed between the fixed electrode 12 and the movable electrode 14 is increased. Therefore, if
the DC bias voltage is the same, the electrostatic capacitance C is increased. The charge Q
accumulated in the
[0021]
The fact that the charge Q accumulated in the capacitance C is large means that the attraction
force acting on the metal film 142A of the movable electrode 14 is larger than Coulomb's law,
and as a result, the amplitude of the movable electrode 14 can be increased. The sound pressure
level of the sound wave (acoustic signal) emitted from the electrostatic transducer 1 is increased.
On the other hand, if the sound pressure level is not increased, that is, the attraction force is not
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increased, the DC bias voltage and the AC signal voltage as drive signals applied between the
fixed electrode 12 and the movable electrode 14 are lowered. Electrostatic transducers can be
operated.
[0022]
Next, an example of the configuration of the fixed electrode is shown in FIG. In the configuration
example shown in FIG. 4, the fixed electrode 12A uses a metal bulk material of a plane circular
shape. Irregularities of several micrometers to several tens of micrometers in depth are formed
on the surface of the fixed electrode 12A formed of a metal bulk material by chemical treatment
(for example, corrosion treatment). By configuring in this manner, the surface area of the surface
facing the movable electrode is increased compared to the fixed electrode in the conventional
electrostatic transducer, which was simply a flat surface, so the movable electrode described with
reference to FIG. The same effect as obtained for
[0023]
Although the fixed electrode 12A shown in FIG. 4 has been described as a planar circular metal
bulk material, since the fixed electrode is a surface facing the movable electrode, the surface of
the fixed electrode facing the movable electrode is formed of a conductive material. However, the
entire fixed electrode does not necessarily have to be formed of a conductive material. In
addition, the fixed electrode may be formed with a nonconductive film that protects the surface
of the layer formed of the conductive material.
[0024]
In the electrostatic transducer according to the present embodiment, the fixed electrode and the
movable electrode are formed to increase the surface area of at least one of the fixed electrode
and the movable electrode. However, the fixed electrode and the movable electrode are provided.
If the area of both opposing electrode surfaces is increased by the above-described method, the
driving voltage of the electrostatic transducer is further reduced compared to the case where the
surface area of one of the fixed electrode and the movable electrode is increased. be able to.
[0025]
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Next, the configuration of an ultrasonic speaker using the electrostatic transducer according to
the embodiment of the present invention described above is shown in FIG.
In the figure, the ultrasonic speaker 100 has the audio frequency wave oscillation source 110,
the carrier wave oscillation source 112, the reproduction range setting unit 114, the
reproduction range control processing unit 116, the modulator 118, the power amplifier 120,
and And an electro-conductive transducer 122.
[0026]
The audio frequency wave oscillation source 110 has a function of generating an audio
frequency band signal wave. The carrier wave oscillation source 112 also has a function of
generating a carrier wave of a frequency in the ultrasonic frequency band. The reproduction
range setting unit 114 includes operation means such as operation keys and a knob, and the user
can input data for specifying a reproduction range of the reproduction signal, and the data is
input. And the function of setting and holding the data. The setting of the reproduction range of
the reproduction signal is performed by specifying the distance that the reproduction signal
reaches from the sound wave emission surface of the electrostatic transducer 122 in the
radiation axis direction.
[0027]
The reproduction range control processing unit 116 refers to the setting contents of the
reproduction range setting unit 114, and changes the frequency of the carrier wave generated by
the carrier wave oscillation source 112 so as to become the set reproduction range. It has a
function to control 112. For example, when the distance corresponding to the carrier wave
frequency of 50 kHz is set as internal information of the reproduction range setting unit 114, the
carrier wave oscillation source 112 is controlled to oscillate at 50 kHz.
[0028]
The reproduction range control processing unit 116 stores in advance a table indicating the
relationship between the distance that the reproduction signal reaches in the radiation axis
direction from the sound wave emission surface of the electrostatic transducer 122 that defines
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the reproduction range and the frequency of the carrier wave. A storage unit 1160 is provided.
The data of this table can be obtained by actually measuring the relationship between the
frequency of the carrier wave and the reach distance of the reproduction signal.
[0029]
When data indicating the reproduction range is set in the reproduction range setting unit 114,
the frequency of the carrier wave to be changed is determined by referring to the above table,
and the carrier wave oscillation source 112 is set to generate the carrier wave of the frequency.
Control. The storage unit may be provided in the reproduction range setting unit 114.
[0030]
The carrier wave oscillation source 112 generates a carrier wave of the frequency instructed by
the reproduction range control processing unit 116 and supplies the carrier wave to the
modulator 118. The modulator 118 has a function of AM-modulating a carrier wave supplied
from the carrier wave oscillation source 112 with a signal wave output from the audio frequency
wave oscillation source 110 and outputting a modulation signal to the power amplifier 120.
[0031]
The electrostatic transducer 122 is driven by a modulation signal output from the modulator 118
through the power amplifier 120, converts the modulation signal into a sound wave of a finite
amplitude level and emits it into the medium, and outputs an audio frequency band signal. It has
a function of reproducing sound (reproduction signal). The electrostatic transducer 122 is, for
example, an electrostatic transducer that can emit an acoustic signal (ultrasound) in a wide
frequency band.
[0032]
The operation of the ultrasonic speaker according to the embodiment of the present invention
configured as described above will be described. When the reproduction range setting unit 114
sets the reproduction range of the reproduction signal (the distance at which the reproduction
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signal reaches from the sound wave emission surface of the electrostatic transducer 122 in the
radiation axis direction), the reproduction range control processing unit 116 A table showing the
relationship between the distance that the reproduction signal reaches from the sound wave
emitting surface of the ultrasonic transducer 22 stored in the built-in storage unit 1160 in the
radiation axis direction and the frequency of the carrier wave by taking in setting data of the
setting unit 114 Referring to the frequency of the carrier wave to be generated by the carrier
wave oscillation source 12, the carrier wave oscillation source 112 is controlled to generate the
carrier wave of the frequency.
[0033]
As a result, the carrier wave oscillation source 112 generates a carrier wave of a frequency
corresponding to the reach distance of the reproduction signal set by the reproduction range
setting unit 114, and outputs the carrier wave to the modulator 118. On the other hand, a signal
wave of an audible frequency band is input to the modulator 118 from the audible frequency
wave oscillation source 110. The modulator 118 AM modulates the carrier wave input from the
carrier wave source 114 with the low frequency signal input from the audio frequency wave
oscillation source 110, and outputs the modulated signal to the power amplifier 120.
[0034]
The modulation signal amplified by the power amplifier 120 is applied between a movable
electrode (not shown) of the electrostatic transducer 122 and a fixed electrode, and the
modulation signal is converted into a sound wave (acoustic signal) of a finite amplitude level. ,
Emitted to the medium (in the air). Here, to briefly describe the non-linear effect of the medium
(air), in the propagation of the ultrasonic wave radiated into the medium (in the air) by the
ultrasonic transducer, the speed of sound is high in the high sound pressure part along with the
propagation. The speed of sound becomes slower in areas where sound pressure is low. It is
known that this results in distortion of the waveform.
[0035]
When the signal (carrier wave) of the ultrasonic wave band to be emitted is modulated (AM
modulated) with the signal of the audio frequency band, the signal wave of the audio frequency
band used at the time of modulation is It is also known to be formed in the form of demodulation.
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At this time, the spread of the reproduction signal is in the form of a beam due to the
characteristics of the ultrasonic waves, and the sound is reproduced only in a specific direction
which is completely different from that of a normal speaker.
[0036]
The ultrasonic speaker according to the embodiment of the present invention uses an
electrostatic transducer that can emit an acoustic signal of high sound pressure over a wide
frequency band with a low DC bias voltage and an alternating signal voltage. The speaker can be
miniaturized, the voltage can be reduced, and the cost can be reduced.
[0037]
BRIEF DESCRIPTION OF THE DRAWINGS The figure which shows the structure of the principal
part of the electrostatic transducer which concerns on embodiment of this invention.
FIG. 2 is a view showing the configuration of a movable electrode in the electrostatic transducer
according to the embodiment of the present invention shown in FIG. 1. The figure which shows
an example of a structure of the movable electrode in the conventional electrostatic transducer.
FIG. 2 is a view showing an example of the configuration of a fixed electrode in the electrostatic
transducer according to the embodiment of the present invention shown in FIG. 1. FIG. 1 is a
block diagram showing an electrical configuration of an ultrasonic speaker according to an
embodiment of the present invention. The figure which shows the whole structure of the
conventional electrostatic transducer.
Explanation of sign
[0038]
DESCRIPTION OF SYMBOLS 1 ... Electrostatic type transducer, 10 ... Insulating base material, 12
... Fixed electrode, 14 ... Movable electrode, 16 ... Spacer, 20 ... Movable electrode taking-out
member, 30 ... DC bias power supply, 40 ... Signal source, 100 ... Ultrasonic speaker 110: audio
frequency wave oscillation source, 112: carrier wave oscillation source, 114: reproduction range
setting unit, 116: reproduction range control processing unit, 118: modulator, 120: power
amplifier, 122: electrostatic transducer, 140, 140A ... vibrating membrane, 142, 142A ... metal
film
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