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DESCRIPTION JP2010074488

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DESCRIPTION JP2010074488
An object of the present invention is to provide an audio reproduction apparatus capable of
demodulating and reproducing an audio band signal with a stable sound pressure in a wide
frequency band. In order to achieve the above object, according to the present invention, in a
sound reproducing apparatus, a sound emitting unit includes a plurality of resonance modes at
which vibration displacement is maximal at different frequencies and a frequency at which the
plurality of resonance modes are excited. And a part of a frequency band capable of exciting the
mode-coupled vibration as a carrier frequency. Thus, by using the mode coupling frequency with
a low rate of change of vibration displacement with respect to the frequency as the carrier signal,
the audio band signal output from the audio band signal source is demodulated with a stable
sound pressure in a wide frequency band, It can be made renewable. [Selected figure] Figure 7
Sound reproduction device
[0001]
The present invention relates to an acoustic reproduction apparatus having high directivity,
capable of reproducing an acoustic wave in an audible band in a specific spatial range by
modulating and emitting a signal in an audible band using a signal in an ultrasonic band as a
carrier wave.
[0002]
A typical sound reproducing apparatus can emit an acoustic wave in the audible band directly
into a medium such as air through a diaphragm, and can propagate an audible wave in a
relatively wide range by the diffraction effect.
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[0003]
On the other hand, in order to selectively propagate the sound wave of the audible band only in a
specific spatial range, a sound reproducing device having high directivity has been put to
practical use.
[0004]
This sound reproducing apparatus is generally called a superdirective speaker or parametric
speaker, and modulates an audio band signal as a carrier wave with an ultrasonic band signal and
further amplifies it at a specific magnification, and then this modulated signal Is input to a sound
output unit including an ultrasonic transducer or the like, and is emitted as a sound wave in the
ultrasonic band into a medium such as air.
[0005]
Then, the sound wave emitted from the sound emitting part propagates the medium with high
directivity due to the propagation characteristic of the ultrasonic wave which is the carrier wave.
Furthermore, while the sound wave in the ultrasonic band propagates through the medium, the
nonlinearity of the medium causes the amplitude of the sound wave in the audible band to
increase cumulatively, and the sound wave in the ultrasonic band is attenuated by absorption by
the medium and spherical diffusion. Do.
As a result, the sound wave in the audible band modulated to the ultrasonic band selfdemodulates to the sound wave in the audible band due to the non-linearity of the medium, and
the sound wave in the audible band can be regenerated only in a limited narrow space area.
[0006]
That is, the superdirective speaker uses the non-linearity of the medium through which the sound
wave propagates and the high directivity of the ultrasonic wave.
For example, if a superdirective speaker is used as a speaker for explaining a museum or
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museum display, sound waves in the audible band can be transmitted only to persons present
within a specific space range.
[0007]
Non-Patent Document 1 is known as prior art document information related to the abovedescribed sound reproducing apparatus.
Tsuneo Tanaka, Mikio Iwasa, Yoichi Kimura "On Practical Application of Parametric
Loudspeaker" Material of the Japan Society of Acoustics, Japan, US 84-61, 1984 (Page 1-Page 2,
Fig. 1, Fig. 2)
[0008]
The above-mentioned sound reproducing apparatus is configured to increase the sound pressure
of the sound wave in the audible band reproduced with the lowest possible input electric field, by
setting the frequency near the resonance frequency which is the frequency to excite the
resonance mode of the ultrasonic transducer made of piezoelectric or the like. , As the carrier
frequency. In the vicinity of this resonance frequency, the mechanical quality factor Qm (a
constant indicating the sharpness of mechanical vibration displacement in the vicinity of the
resonance frequency when the piezoelectric material or the like causes resonance vibration) is
high, and the maximum for the applied AC electric field Vibration displacement can be obtained.
[0009]
However, structural conditions such as the shape, size and support / fixing method of the
piezoelectric body and other components, and material characteristic conditions such as the
piezoelectric constant and elastic constant in the process of polarization and firing when the
piezoelectric body is ceramic. Thus, the resonance frequency of the ultrasonic transducer varies
among individuals. Also, since the mechanical quality factor Qm is also affected by the
temperature change of the ultrasonic transducer itself and the load fluctuation due to the
medium such as air, even if an electric field of the same frequency and the same amplitude is
applied to a plurality of ultrasonic transducers. Because the vibration amplitudes of the
ultrasonic transducers are individually different, there is a problem that the desired sound
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pressure can not be obtained depending on the frequency band of the audible band signal when
the audible band signal is demodulated and regenerated. The
[0010]
Therefore, an object of the present invention is to provide an audio reproduction device capable
of reproducing and reproducing an audio band signal with a stable sound pressure in a wide
frequency band.
[0011]
In order to achieve this object, the present invention comprises an audio band signal source for
generating an audio band signal, a carrier oscillator for generating a carrier wave, a modulator
for modulating the audio band signal with the carrier wave, and the modulation. And at least a
sound emitting unit for outputting a reproduced sound, the sound emitting unit includes a
plurality of resonance modes in which vibration displacement is maximal at different frequencies,
and the plurality of resonance modes. It consists of an ultrasonic transducer which can excite
mode-coupled vibration between the frequencies to be excited, and a part of a frequency band
capable of exciting the mode-coupled vibration is a frequency of the carrier wave.
[0012]
As described above, in the sound reproducing apparatus according to the present invention, the
ultrasonic transducers that constitute the sound emitting unit mutually affect each other in at
least two or more resonance modes and in the vibration amplitude and the vibration direction
between these resonance modes. Since a part of the frequency band that has mode coupling and
can excite this mode coupled vibration is used as the carrier frequency, ultrasonic vibration is
caused by the manufacturing process of the ultrasonic transducer, load fluctuation during
operation, etc. Even if the resonant frequency of the resonator fluctuates or fluctuates, the
vibration amplitude fluctuation of the ultrasonic transducer is small and stable within the
frequency range in which the mode coupled vibration can be excited. When the sound wave in
the audible band is self-demodulated, a wide band and stable sound pressure can be realized.
[0013]
First Embodiment The configuration of a sound reproduction device 1 according to the present
embodiment will be described below with reference to the drawings.
[0014]
FIG. 1 is a block diagram for explaining a drive unit of the sound reproduction apparatus 1 of the
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present invention.
[0015]
The audible band signal (approximately 20 Hz to 20 kHz as a frequency) generated by the
audible band signal source 2 and the carrier wave (ultrasound of approximately 20 kHz or more)
generated by the carrier oscillator 3 are input to the modulator 4 to produce an audible band
signal. Is modulated by the carrier wave.
The modulated signal is amplified by the power amplifier 5 and input to the sound emitting unit
6.
[0016]
Then, the signal from the modulator 4 input to the sound emitting unit 6 is emitted as an
ultrasonic wave to a medium such as air, and after propagating a certain distance, the sound
wave in the ultrasonic band as a carrier wave is attenuated. Due to the non-linearity of the
medium, sound waves in the audible band self demodulate.
[0017]
As described above, in the sound reproducing apparatus 1 according to the present embodiment,
the sound wave in the audible band can be reproduced only in a very narrow space range by
using the ultrasonic wave having high directivity as the carrier wave. ing.
[0018]
Next, the ultrasonic transducer 7 constituting the sound emitting unit 6 will be described with
reference to FIG.
FIG. 2 is a cross-sectional view of the ultrasonic transducer 7.
[0019]
The ultrasonic transducer 7 vibrates the piezoelectric body 8 by receiving a signal from the
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modulator 4 and emits a sound wave to a medium such as air.
[0020]
The piezoelectric body 8 is a cylindrical piezoelectric ceramic made of PCM (for example,
PbTiO3-ZrTiO3-Pb (Mg1 / 2Nb1 / 2) TiO3 etc.) ceramic, and as shown in FIG. 2, the thickness
direction of the acoustic matching layer 9 It is located approximately at the center of one side of
the
Assuming that the thickness of the piezoelectric body 8 is L and the diameter is D, the
dimensional ratio L / D is about 0.7 and is polarized in the thickness L direction.
Here, the piezoelectric body 8 is a PCM-based ceramic, but in addition to this, a piezoelectric
ceramic such as a PZT (PbTiO3-ZrTiO3) -based or barium titanate (BaTiO3), a piezoelectric single
crystal, or the like may be used.
[0021]
In the vicinity of the peripheral portion of the acoustic matching layer 9, a cylindrical case 10 is
fixed so as to surround the piezoelectric body 8, thereby protecting the piezoelectric body 8 from
the outside.
In the present embodiment, the case 10 is made of aluminum.
[0022]
Furthermore, a terminal block 11 is provided at the opening of the case 10 (the inner side near
the end opposite to the connecting portion of the acoustic matching layer 9).
The terminal block 11 and the piezoelectric body 8 are provided with a fixed gap so as not to be
in contact with each other due to external impact, vibration of the piezoelectric body 8 or the
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like.
Furthermore, two rod-like terminals 12 are provided on the terminal block 11, and these
terminals 12 are electrically connected to the electrodes of the piezoelectric body 8 through the
lead wires 13 respectively.
That is, an alternating electric field can be applied to the piezoelectric body 8 through the
terminal 12.
[0023]
In the ultrasonic vibrator 7 having such a configuration, when an AC electric field of a specific
frequency is applied to electrodes provided on both main surfaces of the piezoelectric body 8,
elastic vibration determined by the material constant, shape, size, etc. of the piezoelectric body 8
Can be excited. A sound wave generated by this elastic vibration is emitted to a medium such as
air via the acoustic matching layer 9 and is propagated in a specific direction (upward direction
in FIG. 2).
[0024]
Here, the acoustic matching layer 9 is for matching the acoustic impedance between the
piezoelectric body 8 and a medium such as air, and attenuates the sound wave due to reflection
or the like at the interface due to the difference in acoustic impedance between the piezoelectric
body and the medium. To reduce the
[0025]
In the present embodiment, the audio band signal source 2, the carrier wave oscillator 3, the
modulator 4, and the power amplifier 5 described above are configured by only one set.
[0026]
Next, the method of determining the carrier frequency, which is the point of the present
invention, will be described in detail.
[0027]
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FIG. 3 is a diagram showing an example of the frequency characteristic of admittance and the
frequency characteristic of vibration displacement in the thickness direction in the conventional
piezoelectric body.
[0028]
Generally, a piezoelectric body has a plurality of resonance modes different in vibration direction
and vibration mode (vibration mode) depending on the shape (dimension ratio), the direction of
polarization (c axis in the case of single crystal) and the direction of alternating electric field
applied. Can be excited.
[0029]
FIG. 3 shows a cylindrical piezoelectric body, and when the thickness is L and the diameter is D,
the frequency of the admittance and the vibration displacement in the thickness direction when
the dimensional ratio L / D is 2.5 or more. It is the figure which showed an example of the
characteristic.
The piezoelectric body in the figure is one in which an alternating electric field is applied in the
thickness direction to the piezoelectric ceramic polarized in the thickness direction.
[0030]
When the frequency of the AC electric field applied to the piezoelectric material is changed from
the low frequency side to the high frequency side, the vibration displacement ξL1 in the
thickness direction is obtained near the frequency fL1 at which the admittance Y is maximal as
shown in FIG. The first resonance mode in which x is maximal occurs.
The resonance mode at this frequency fL1 is called thickness direction longitudinal vibration.
[0031]
Furthermore, as the frequency is increased, a second resonance mode in which the radial
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vibration displacement is maximal occurs in the vicinity of the frequency fD1 where the
admittance Y is maximal next.
The resonance mode at this frequency fD1 is called radial spread vibration.
The radial displacement of this radial spreading vibration is not shown in FIG.
[0032]
As shown in the figure, since the piezoelectric body is also an elastic body, vibrational
displacement occurs in the radial direction and also in the thickness direction due to Poisson
coupling.
However, the vibration displacement in the thickness direction in the vicinity of the frequency
fD1 is much smaller than the vibration displacement 1L1 in the vicinity of the frequency fL1
because the thickness L of the cylinder is larger than the diameter D.
[0033]
Except near the frequency fL1 and the frequency fD1, the vibration displacement in the thickness
direction of the piezoelectric body rapidly decreases and can hardly be obtained. Similarly,
vibration displacement in the radial direction is also reduced and can hardly be obtained except
in the vicinity of the frequency fL1 and the frequency fD1. That is, at frequencies other than the
frequencies fL1 and fD1, the piezoelectric body hardly vibrates in the thickness direction or in
the radial direction. This means that the two resonance modes, ie, the thickness direction
longitudinal vibration and the radial expansion vibration, do not affect each other, and vibrate
independently in the vicinity of each resonance frequency.
[0034]
Thus, in the cylindrical piezoelectric body, either one of the thickness L and the diameter D is
increased (generally, a cylindrical shape in which the thickness L is 2.5 times or more the
diameter D or a diameter D By making the disk shape 15 times larger than L, each resonance
mode vibrates independently without affecting each other, and the mechanical quality factor Qm
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of each resonance mode becomes high.
[0035]
On the other hand, in the ultrasonic transducer 7 of the sound reproducing device 1 according to
the present embodiment, the cylindrical piezoelectric body 8 having a dimensional ratio L / D of
thickness L to diameter D of about 0.7 is used. .
By using the piezoelectric body 8 having such a dimensional ratio, the mode-coupled vibration is
excited at a frequency between the resonance frequencies for exciting the two resonance modes
of the thickness direction longitudinal vibration and the radial expansion vibration. While being
able to obtain a certain amount or more of the vibration displacement ξ L in the thickness
direction, it is possible to excite the vibration displacement 少 な い L with less change with
respect to the frequency fluctuation in the piezoelectric body 8. In this embodiment, a part of the
frequency band that can excite the mode-coupled vibration is used as the frequency band of the
carrier wave.
[0036]
FIG. 4 shows an example of the numerical results of the frequency characteristics of the
admittance Y of the piezoelectric body 8 and the vibration displacement ξ L in the thickness
direction according to the present embodiment using the finite element method.
[0037]
As shown in FIG. 4, the piezoelectric body 8 excites a high resonance mode with a mechanical
quality factor Qm at two resonance frequencies, the frequency fm1 and the frequency fm2,
respectively.
Furthermore, between the frequency fm1 and the frequency fm2, the mode-coupled vibration is
excited, and the absolute value of the vibration displacement ξL in the thickness direction is
smaller than that in the vicinity of the two frequencies fm1 and fm2. A frequency band with a
small amount of change with respect to frequency fluctuation can be obtained. In particular, in
the vicinity of the frequency fLm where the vibration displacement in the thickness direction
becomes the minimum value ξLm, it is possible to obtain a flat region in which the amount of
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change in the vibration displacement ξL is the smallest with respect to the frequency
fluctuation.
[0038]
By using the frequency region based on the frequency f Lm at which the vibration displacement
ξ L in the thickness direction is minimized as the frequency of the carrier wave by exciting the
mode-coupled vibration described above as the frequency of the carrier wave, the piezoelectric
body 8 is Even if the resonance frequencies of the thickness direction longitudinal vibration and
the radial expansion vibration respectively fluctuate, the vibration amplitude fluctuation of the
ultrasonic transducer is small and stable within the frequency range in which the mode coupled
vibration can be excited. As a result, when the audio band signal is self-demodulated, a wide band
and stable sound pressure can be realized.
[0039]
The details of the fact that stable sound pressure can be obtained when the audio band signal is
self-demodulated will be described below.
[0040]
As shown in FIG. 5, assuming that the amplitude of the electric field applied to the ultrasonic
transducer 7 is fixed and the frequency is a constant frequency band fm1 ± Δf centered on the
resonance frequency fm1, in the vicinity of the resonance frequency fm1, Since the mechanical
quality factor Qm of the resonant mode is high, the vibration displacement of the ultrasonic
transducer 7 is large, and the sound wave emitted from the ultrasonic transducer 7 can also
obtain high sound pressure.
However, at a frequency apart from the resonance frequency fm1 by the frequency fluctuation
range Δf, the vibration displacement of the ultrasonic transducer 7 becomes smaller than in the
vicinity of the resonance frequency fm1.
[0041]
As described above, when the ultrasonic transducer 7 is excited with a signal obtained by
modulating a wide-band audible band signal with the resonance frequency fm1 as the frequency
of the carrier wave, the change of the vibration displacement of the ultrasonic transducer 7
within the frequency range of the applied electric field Because the amount is large, the sound
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pressure fluctuation with respect to the frequency of the sound wave emitted from the ultrasonic
transducer becomes large, and the sound wave of the demodulated audio band also has a large
amplitude fluctuation due to the frequency, and it is very important to obtain stable sound
pressure It becomes difficult.
[0042]
Therefore, as in the sound reproduction device 1 according to the present embodiment, a part of
the frequency band capable of exciting mode-coupled vibration with a relatively small amount of
change in the vibration displacement ξ L with respect to frequency fluctuation is used as the
carrier frequency. Thus, it becomes possible to reproduce signals in the audible band with a wide
band and stable sound pressure.
[0043]
Furthermore, by using the piezoelectric body 8 of the present embodiment, it is possible to
obtain the sound reproduction device 1 capable of exhibiting stable performance against stress
received from the surroundings due to disturbance such as temperature change and vibration.
The details will be described below.
[0044]
FIG. 6 shows only the frequency characteristics of the vibration displacement ξ L in FIG. 5, and
the horizontal axis and the vertical axis indicate the minimum value ξ Lm of the vibration
displacement in the frequency band capable of exciting the mode-coupled vibration and Each is
normalized based on the frequency fLm.
The solid line shows the frequency characteristics when the piezoelectric body 8 is unloaded
without disturbance and the dotted line shows the frequency characteristics when the
piezoelectric body 8 is externally stressed.
[0045]
It can be seen that the mechanical quality factor Qm of the resonant mode fluctuates and the
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vibration displacement ξL largely changes depending on the presence or absence of stress near
the respective resonant frequencies, frequencies fm1 and fm2, which excite the first and second
resonant modes. .
[0046]
As an example, in the case of the first resonance mode (thickness direction longitudinal vibration:
resonance frequency fm1), the mechanical quality factor Qm becomes low when stress due to
disturbance or the like is applied, and the vibration displacement ξ L is in the case of no load. It
decreases to about 1/5.
On the other hand, in the vicinity of the frequency fLm which is the frequency of the carrier wave
used in the present embodiment, the vibration displacement ξL hardly decreases even when the
same stress is applied.
[0047]
That is, FIG. 6 shows that the susceptibility to the vibration displacement of the ultrasonic
transducer 7 due to the load fluctuation from the outside is different depending on the frequency
of the AC electric field applied to the ultrasonic transducer 7.
In particular, in a frequency band in which mode-coupled vibration can be excited, it is
understood that load fluctuation hardly affects the vibration displacement.
[0048]
Therefore, in the present embodiment, by using a part of the frequency band capable of exciting
the mode-coupled vibration as the frequency of the carrier wave, stress on the piezoelectric body
8 is caused by disturbance such as temperature change, vibration, and support fixed condition.
Even when is added, the change of the vibration displacement ξ L is small, and as a result, it is
possible to obtain the sound reproduction apparatus 1 capable of reproducing sound waves in a
wide band and stable sound pressure audible band.
[0049]
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The frequency of the carrier wave is preferably selected based on the frequency at which the
vibration displacement ξ L of the ultrasonic transducer 7 is minimized, in the frequency band in
which the mode-coupled vibration can be excited.
[0050]
This is apparent from FIG. 7 and from FIG. 4 to FIG. 6 so far, in the vicinity of the frequency fLm
at which the vibration displacement ξL becomes the minimum value ξLm, the variation of the
vibration displacement ξL with respect to the frequency fluctuation becomes small, and the
frequency characteristic Is flat.
By using a constant frequency band fLm ± Δf centered on the frequency fLm as the carrier
wave frequency, the sound pressure of sound waves in the audible band to be reproduced can be
further stabilized and the frequency band can be expanded.
[0051]
Finally, a method of designing the dimensional ratio L / D of the thickness L to the diameter D of
the cylindrical piezoelectric body 8 will be described.
[0052]
FIG. 8 shows that in the piezoelectric body 8 formed by using the PCM-based ceramic, the
resonance frequency fm1 of longitudinal vibration in the thickness direction and the resonance
frequency fm2 of radial expansion vibration can be excited between these two resonance modes.
It is the figure which showed the result calculated | required by changing the dimension ratio L /
D of the piezoelectric material 8, and changing the largest displacement (zeta) Lm in the modecoupled vibration by the numerical calculation by a finite element method.
[0053]
The horizontal axis is the normalized ratio of the dimensional ratio L / D of the piezoelectric body
8, and the vertical axis is the left axis of the frequency normalized based on the frequency fLm
when the dimensional ratio L / D is 1. Similarly, the vibration displacement normalized based on
the vibration displacement ξLm in the thickness direction when the dimensional ratio L / D is 1
is shown on the right axis.
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The frequency fm1 is a solid line, the frequency fm2 is an alternate long and short dash line, and
the vibration displacement ξLm is a dotted line.
[0054]
From FIG. 8, the vibration displacement ξLm in the mode-coupled vibration increases with the
increase of the dimensional ratio L / D of the piezoelectric body 8 and is about 1.7 times as large
as when the dimensional ratio L / D is 1 near 0.7. It can be seen that it takes a maximum and
then declines.
Therefore, in the present embodiment, the dimensional ratio L / D is set to 0.7 at which the
vibration displacement ξLm is maximum.
[0055]
The dimensional ratio L / D of the piezoelectric body 8 is not limited to 0.7, and the range of ±
0.3 with the vibration displacement ξLm having a maximum value of 0.7, ie, the dimensional
ratio L It is sufficient if / D is a value of 0.4 or more and 1.0 or less.
This means that if the dimension ratio L / D is a value of 0.4 or more and 1.0 or less, the
piezoelectric body 8 vibrates efficiently with respect to the applied AC electric field, and a sound
wave is emitted from the ultrasonic transducer 7 It is because it is possible to output the sound
wave of an audible zone efficiently as a sound reproduction apparatus.
[0056]
On the other hand, when the dimension ratio L / D of the piezoelectric body 8 is less than 0.4 or
over 1.0, the vibration loss of the piezoelectric body 8 becomes large, so the vibration amplitude
with respect to the applied AC electric field As the acoustic wave emitted from the ultrasonic
transducer 7 becomes smaller, heat generation due to vibration loss may adversely affect the
material characteristics of the piezoelectric body 8 and degrade the operation reliability of the
ultrasonic transducer 7 It is not preferable because it becomes expensive.
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[0057]
Although the above is an example in which the piezoelectric body 8 is formed by using the PCM
based ceramic, the same numerical calculation and trial examination are conducted even when
the piezoelectric ceramic such as the PZT based ceramic or the piezoelectric single crystal is
different. Thus, the dimensional ratio L / D of the optimal cylindrical piezoelectric body 8 is
determined.
[0058]
Second Embodiment In the first embodiment, the sound output unit 6 is configured of one
ultrasonic transducer, but in the second embodiment, the sound output unit is configured of a
plurality of ultrasonic transducers 7. An example will be described below.
[0059]
As shown in FIG. 9, the sound output unit 14 in the present embodiment is configured by
arranging a plurality of ultrasonic transducers 7 in a planar manner.
[0060]
FIG. 10 shows the frequency characteristics of the admittance of each of the piezoelectric bodies
8 constituting the three ultrasonic transducers 7 among the ultrasonic transducers 7 constituting
the sound emitting unit 14 of FIG. The admittance Y1 and the vibration displacement ξ L1, the
admittance Y2 and the vibration displacement ξ L2, and the admittance Y3 and the vibration
displacement ξ L3 respectively indicate the same admittance of the piezoelectric body 8 and the
frequency characteristics of the vibration displacement.
[0061]
As shown in FIG. 10, the admittance Y1, the admittance Y2, the admittance Y3, the vibration
displacement ξ L1, the vibration displacement ξ L2, and the vibration displacement ξ L3 of
the three piezoelectric bodies 8 do not have completely the same frequency characteristics, and a
shift occurs.
This is due to the dispersion of the manufacturing conditions, the material characteristics, and
the shape and dimensions of the piezoelectric body 8 when the piezoelectric body 8 is
manufactured.
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Furthermore, since the variation when assembling the ultrasonic transducer 7 by supporting and
fixing the piezoelectric body 8 is also affected, in the admittance of the plurality of ultrasonic
transducers 7 constituting the sound emitting portion 14 or the frequency characteristic of
vibration displacement, The resonant frequency at which the resonant mode can be excited also
varies.
When a plurality of ultrasonic transducers 7 having different resonance frequencies are used and
the frequency of the carrier wave is fixed near the frequency fm1 or near the frequency fm2,
radiation is generated from each of the ultrasonic transducers 7 The sound pressure level of the
sound waves varies, and as a result, stable sound pressure is not obtained when sound waves in
the audible band are demodulated.
[0062]
Therefore, in the present embodiment, as in the first embodiment, the frequency of the carrier
wave is not the resonant frequency for exciting the resonant mode, but a frequency band capable
of exciting mode-coupled vibration excited between the resonant modes. Use part of the
[0063]
The piezoelectric body 8 in the present embodiment is the same as the piezoelectric body 8 in
the first embodiment, and has a cylindrical shape with a dimensional ratio L / D of thickness L to
diameter D of 0.7. It is a body.
By setting it as such a dimensional ratio, as shown in FIG. 9, it is a part of the frequency band
which can comprise the sound emission part 14 by the some piezoelectric material 8, and can
excite the vibration mode-coupled to the piezoelectric material 8. In the case where the electric
field of the same amplitude and the same frequency is applied to each piezoelectric body 8 when
the frequency of the carrier wave is the carrier wave, the variation between the individual with
respect to the vibration displacement of each piezoelectric body 8 is small. The variation among
individuals is small also about the sound pressure of the sound wave emitted from.
As a result, sound waves in the audible band to be demodulated can be reproduced with high and
stable sound pressure.
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[0064]
The sound emitting unit 14 is an example in the case where there is an individual difference in
the resonance frequency of the piezoelectric body 8 constituting the ultrasonic transducer 7, but
in the case where the sound emitting unit 14 is formed of piezoelectric bodies 8 having the same
resonance frequency Even if it is effective.
That is, the frequency characteristic of the vibration amplitude of the ultrasonic transducer 14
may change due to the temperature change of the ultrasonic transducer 14 during operation and
the stress applied to the piezoelectric body 8 at the time of assembling the ultrasonic transducer
14 Also in such a case, the configuration of the present embodiment is applicable.
[0065]
In addition, although the sound reproduction device 1 according to the present embodiment in
FIG. 9 is illustrated as a configuration in which the ultrasonic transducers 7 in the sound emitting
unit 14 are densely arranged in a honeycomb shape, the arrangement method is limited thereto.
The same effect can be obtained as long as the sound wave emitted from the sound emitting unit
can be efficiently collected at a predetermined position instead of the one.
[0066]
In each embodiment of the present invention, the shape of the piezoelectric body 8 constituting
the ultrasonic transducer 7 is cylindrical, and the vibration excited by the piezoelectric body 8 is
resonant vibration and radial expansion of thickness direction longitudinal vibration. Although
the case has been described in which the mode resonance of the vibration and the resonant
vibration of the vibration is used, the present invention is limited to a specific shape and a
specific resonance mode with respect to the shape of the piezoelectric body and the vibration
mode excited on the piezoelectric body. is not.
For example, the same effect can be obtained also in the case where the piezoelectric body 8 is
shaped like a prism and resonance vibration in the thickness direction and the resonance
vibration in the diagonal direction or in the side direction is mode-coupled.
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[0067]
According to the present invention, by being able to emit sound with stable sound pressure with
respect to the frequency band of the emitting ultrasonic wave, the sound pressure of sound
waves in the audible band to be reproduced can be stabilized in a wide band, Modulates and
radiates to media such as air as ultrasound, and uses the nonlinearity of the media to selfdemodulate acoustic waves in the audio band, and also utilizes the high directivity of ultrasound
to a limited spatial range. It is useful as a sound reproduction device that reproduces sound
waves in the audible band only.
[0068]
Block diagram of an acoustic reproducing apparatus according to the first embodiment of the
present invention Sectional view of an ultrasonic transducer according to the first embodiment of
the present invention Figure showing frequency characteristics of admittance and vibration
displacement of a conventional piezoelectric body according to the present invention FIG. 6 is a
graph showing the frequency characteristics of the admittance and vibration displacement of the
piezoelectric body in FIG. 1 a graph showing that a specific frequency band centered on the
resonance frequency fm 1 is made the carrier frequency in the first embodiment of the present
invention 1 is a graph showing frequency characteristics of vibration displacement with respect
to mechanical quality factor Qm of piezoelectric body in 1 in Embodiment 1 of the present
invention, a specific frequency band centered on frequency fLm at which vibration displacement
takes local value ξLm is the frequency of carrier wave In the piezoelectric body according to the
first embodiment of the present invention, the frequency at which the admittance takes a
maximum value, and the thickness direction when the dimensional ratio is changed. The figure
which shows the relationship of the minimum value of vibration displacement of the front of the
sound emission part in Embodiment 2 of this invention The frequency characteristic of the
admittance of the piezoelectric material of three ultrasonic transducers and vibration
displacement in Embodiment 2 of this invention Figure showing
Explanation of sign
[0069]
DESCRIPTION OF SYMBOLS 1 sound reproduction apparatus 2 audible band signal source 3
carrier wave oscillator 4 modulator 5 power amplifier 6 sound emitting part 7 ultrasonic vibrator
8 piezoelectric material 9 acoustic matching layer 10 case 11 terminal block 12 terminal 13 lead
wire 14 sound emitting part
14-04-2019
19
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