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DESCRIPTION JP2002369290

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DESCRIPTION JP2002369290
[0001]
FIELD OF THE INVENTION The present invention relates to a piezoelectric speaker system.
[0002]
2. Description of the Related Art Some piezoelectric elements have frequency characteristics
capable of reproducing a sound wave having an ultrahigh frequency of 100 kHz or more.
Therefore, the piezoelectric element may be used as an audio speaker for a tweeter or a super
tweeter which has a high sound range. In addition, since a speaker using a piezoelectric element
is small and light, it is also used as a built-in speaker for a telephone receiver, a portable
information terminal, a mobile phone, and the like. For example, in a mobile phone, the melody
sound and the ringing tone reproduced by the internal sound source LSI are emitted from the
piezoelectric speaker.
[0003]
Such a piezoelectric speaker is configured, for example, by sticking a piezoelectric element on a
diaphragm made of resin, metal or the like. When current flows through the piezoelectric
element, the piezoelectric element is deformed according to the current, the diaphragm on which
the piezoelectric element is attached is driven, and sound (sound wave) is emitted.
Conventionally, a signal including an ultrahigh frequency component is input as it is to a
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piezoelectric element, and as described above, it has been used as a speaker capable of
reproducing a sound wave of ultrahigh frequency.
[0004]
The piezoelectric element is also a crystal of a dielectric, and when viewed as an electric circuit, is
equivalent to a capacitor. Therefore, the piezoelectric element has frequency characteristics
similar to that of the capacitor with respect to its impedance. In other words, the impedance of
the piezoelectric element decreases as the frequency becomes higher. In other words, the current
flowing through the piezoelectric element is more likely to flow as the frequency becomes higher.
Therefore, if a signal having many high frequency components is supplied to the piezoelectric
speaker built in the portable information terminal, the current flowing through the piezoelectric
element increases, and the power consumption of the piezoelectric speaker increases. As a result,
the battery of the portable information terminal is quickly exhausted.
[0005]
In addition, in a portable information terminal, when using a glass plate as a diaphragm of a
speaker by directly attaching a piezoelectric element to a glass plate forming a liquid crystal
panel for display, the size and weight of the portable information terminal should be reduced.
Can. However, when a signal having many high frequency components is supplied to the
piezoelectric element, the glass plate may be driven more strongly than usual and may be
broken. Therefore, in order to attach the piezoelectric element to the glass plate, it is necessary to
adjust the material and thickness of the glass plate to increase the strength, which also
contributes to hindering the reduction in size and weight of portable information terminals and
the like. It was
[0006]
The present invention has been made in view of the above circumstances, and an object thereof
is to provide a piezoelectric speaker system capable of reducing power consumption. Another
object of the present invention is to provide a piezoelectric speaker system capable of preventing
the glass plate from being damaged by vibration.
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[0007]
In order to achieve the above object, a piezoelectric speaker system according to a first aspect of
the present invention comprises a glass plate forming a part of a display panel, and an electrode,
A piezoelectric element that vibrates in response to an electric signal supplied to an electrode
and transmits the vibration to the glass plate, and reduces the signal level of a component of a
predetermined frequency or more of the electric signal emitted from the signal source, And a low
pass filter for supplying the electrode of the piezoelectric element.
[0008]
The low pass filter may be configured to attenuate the signal level of the frequency component of
10 kHz or more at a predetermined rate.
[0009]
The piezoelectric element decreases its impedance by 6 dB each time the frequency of the
supplied signal increases by one octave, and the low pass filter generates a signal level equal to
or higher than the cutoff frequency, and the frequency is one octave from the cutoff frequency It
may be configured to attenuate at a rate of 6 dB or more for each increase.
[0010]
The display panel comprises two plates of which at least one is made of glass, and a display
member sandwiched between the two plates, and the two plates each have a display surface and
a back surface. The piezoelectric element may be directly attached to a plate that forms the back
surface of the display panel.
[0011]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A piezoelectric speaker system
according to an embodiment of the present invention will be described below with reference to
the drawings.
[0012]
FIG. 1 is a three-sided view showing the configuration of a portable information terminal 100
adopting this piezoelectric speaker system.
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FIG. 1A is a front view showing a surface on which a liquid crystal panel 110 is disposed as a
display unit of the portable information terminal 100. FIG.
As shown in FIG. 1A, a piezoelectric element 120 is disposed behind the liquid crystal panel 110.
FIGS. 1B and 1C respectively show a bottom view and a left side view when the portable
information terminal 100 is viewed from the lower side and the left side toward the front.
As shown in FIG. 1 (b), the liquid crystal panel 110 is disposed on the housing 140 of the
portable information terminal via the spacer 130.
[0013]
The liquid crystal panel 110 includes two glass plates, liquid crystal molecules disposed between
the two glass plates, and a gap material for maintaining a gap between the glass plates, and
displays various information.
The two glass plates respectively constitute a display surface and a substrate (the back surface of
the liquid crystal panel 110).
[0014]
The piezoelectric element 120 is made of, for example, barium titanate or a zircon-lead titanatebased compound, is processed into a plate shape, and includes an electrode. As shown in FIG. 1B,
the piezoelectric element 120 is disposed on the back of the glass substrate on the back surface
of the liquid crystal panel 110 and is fixed by an adhesive. Therefore, when the piezoelectric
element 120 is driven in response to the input signal supplied to the electrode, the glass plate of
the liquid crystal panel 110 vibrates and a sound (sound wave) is generated. That is, the
piezoelectric element 120 and the glass plate of the liquid crystal panel 110 constitute a
diaphragm of a speaker.
[0015]
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The piezoelectric element 120 is equivalent to a capacitor when viewed as an electric circuit.
Therefore, the impedance of the piezoelectric element 120 decreases as the value of the
frequency of the input signal increases. For example, when the capacitance of the piezoelectric
element 120 is 0.8 μF, its impedance is about 200 Ω at 1 kHz, about 20 Ω at 10 kHz, and about
10 Ω at 20 kHz, and the frequency of the piezoelectric element 120 increases by one octave. The
impedance decreases by 6 dB (-6 dB / oct). Therefore, the current flowing through the
piezoelectric element 120 is more likely to flow as the frequency is higher.
[0016]
The spacer 130 has such a strength that it does not significantly dent when pushed by hand, and
is made of, for example, hard rubber. The spacer 130 is disposed between the liquid crystal panel
110 and the housing 140 to make the glass plate of the liquid crystal panel 110 easy to vibrate.
[0017]
This piezoelectric speaker system can emit a predetermined sound by the drive circuit shown in
FIG. The drive circuit is composed of a signal source 150, a low pass filter 160, and a power
amplifier 170, as shown.
[0018]
The signal source 150 includes, for example, a sound source LSI that reproduces a
predetermined melody sound, a control unit that controls the sound source LSI, and the like, and
outputs a predetermined sound signal and the like.
[0019]
The low-pass filter 160 has a cutoff frequency of 10 kHz as shown in FIG. 3 and attenuates signal
components higher than the cutoff frequency at a rate of 18 dB each time the frequency
increases by one octave (−18 dB / oct).
[0020]
The power amplifier 170 amplifies and outputs the output signal of the low pass filter 160.
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[0021]
In this piezoelectric speaker system, predetermined sound is reproduced as follows.
The signal source 150 outputs a predetermined audio signal S1.
The output signal S1 of the signal source 150 is supplied to the low pass filter 160.
The low pass filter 160 cuts the frequency component of 10 kHz or more of the supplied signal
S1 at 18 dB / oct, and outputs a signal S2. The power amplifier 170 amplifies the signal level of
the output signal S 2 of the low pass filter 160 and supplies the amplified signal S 3 to the
piezoelectric element 120. The piezoelectric element 120 is driven according to the amplification
signal S3, and the glass plate of the liquid crystal panel 110 vibrates to reproduce a
predetermined sound.
[0022]
As described above, the signal level of the frequency component of 10 kHz or more is cut at 18
dB / oct by the operation of the low pass filter 160. Therefore, when the amplification signal S3
is 20 kHz, for example, There is only about one eighth of the voltage compared to the case
without. Furthermore, the ultrasonic component of 20 kHz or more is cut more than that.
[0023]
On the other hand, since the frequency characteristic of the impedance of the piezoelectric
element 120 is -6 dB / oct, if the same voltage is applied to the piezoelectric element 120, a
current flows in the piezoelectric element 120 as the frequency of the voltage is higher. It will be
easier.
[0024]
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However, the frequency characteristic (−18 dB / oct) of the cutoff by the low pass filter 160 has
a sufficiently large inclination compared to the frequency characteristic (−6 dB / oct) of the
impedance of the piezoelectric element 120.
For this reason, the current flowing through the piezoelectric element 120 is reduced as it
becomes a high frequency component.
[0025]
Therefore, in this piezoelectric speaker system, power consumption associated with sound
emission can be reduced. Further, since the signal level of the ultra high frequency is reduced,
the vibration of the glass plate is reduced, and it is possible to prevent the phenomenon such as
breakage of the glass plate in advance.
[0026]
In the above embodiment, the cutoff frequency of the low pass filter 160 is set to 10 kHz.
However, the range of human audio frequency is about 20 Hz to 20 kHz, and the cutoff
frequency value is limited to 10 kHz, especially if mid-range component frequencies of about
several hundred Hz to several kHz, which are said to be sensitive, remain It is not something to
be done. The smaller this value is, the easier it is to reduce the signal level of high frequency, and
the reduction effect of power consumption becomes larger.
[0027]
In the above embodiment, the low pass filter 160 has a frequency characteristic that attenuates
the signal level at a rate of 18 dB / oct from the cutoff frequency as in the slope shown in FIG. It
is not something to be done. The value is preferably 6 dB / oct or more so that the frequency
characteristic of the impedance of the piezoelectric element 120 can be offset.
[0028]
Further, in the above embodiment, the drive circuit of the piezoelectric element 120 is configured
of the signal source 150, the low pass filter 160, and the power amplifier 170, but the circuit
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configuration is not limited to this. .
[0029]
Moreover, in the above-mentioned embodiment, although the piezoelectric element 120 shall
adhere to the glass plate of the liquid crystal panel 110 with an adhesive agent, the adhering
method is not limited to this.
For example, any method such as fixing by a thermal or chemical method may be used.
Moreover, although the piezoelectric element 120 was demonstrated as what is stuck on a glass
plate directly, it is not restricted to this. For example, the vibration of the piezoelectric element
120 may be indirectly transmitted to the glass plate by sandwiching another member made of an
elastic body, metal or the like between the piezoelectric element 120 and the glass plate.
[0030]
Further, in the above-described embodiment, the piezoelectric element 120 is fixed to the glass
plate of the liquid crystal panel 110. However, for example, the piezoelectric element 120 is
configured to be attached to the glass plate constituting a PDP (plasma display panel) May be
[0031]
In this case, the piezoelectric speaker system is incorporated in an electronic device having a
larger size than a portable information terminal, for example, a wall-mounted television.
The PDP has a structure in which a noble gas is sealed between two glass plates. The
piezoelectric element 120 is fixed to the back surface of the glass substrate of the PDP.
Therefore, when the piezoelectric element 120 is driven by the drive circuit, the glass plate
forming the PDP vibrates and a sound (sound wave) is generated. That is, the piezoelectric
element 120 and the glass plate of the PDP constitute a diaphragm of a speaker. As described
above, when the PDP is configured of the glass plate to which the piezoelectric element is
attached, the size of the diaphragm is larger than that of the liquid crystal panel, so that the
sound quality may be improved.
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[0032]
Moreover, in the above-mentioned embodiment, although the two boards which form liquid
crystal panel 110 and PDP shall be both glass boards, it is not limited to this, At least one is glass.
I hope there is. Therefore, one sheet may be made of glass and the other may be made of plastic,
for example.
[0033]
Further, in the above-described embodiment, the signal source 150 outputs the predetermined
signal by the sound source LSI and its control unit, but the present invention is not limited to
this. For example, a predetermined sound may be emitted based on a signal output from a CD
(compact disc) or the like. Similarly, a DVD (Digital Versatile Disc) or an SACD (Super Audio CD)
may be used as a sound source. Audio standards such as DVD and SACD have sampling
frequencies that are much higher than those of CDs, and the output signal contains high
frequency components that exceed the human audible limit frequency (about 20 kHz). Therefore,
in this case, a more remarkable effect may appear than in the case of CD.
[0034]
As described above, according to the present invention, the power consumption of the speaker
can be reduced as compared with the prior art. Moreover, according to this invention, it can
prevent that a glass plate is damaged by vibration.
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