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JP2008167252

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DESCRIPTION JP2008167252
The present invention provides a thermally excited sound wave generator capable of efficiently
generating a sound wave in an audio frequency band lower than that of the prior art. A thermally
conductive substrate (12), a thermally insulating layer (14) of a predetermined thickness formed
on one surface of the substrate, and a thermally conductive layer formed on the thermally
insulating layer and electrically conducted by an alternating signal current A heat excitation type
sound wave generator 10 having a heating element thin film 16 made of a resistor driven by the
following equation, and a Helmholtz resonator 18 is provided on the heating element thin film.
As a result, sound waves in an audio frequency band lower than conventional ones are generated
efficiently. [Selected figure] Figure 1
Thermal excitation type sound generator
[0001]
The present invention relates to, for example, a thermally excited sound wave generator applied
as a speaker such as a sound reproducer, and more specifically, heat is supplied to the air to
create air density and generate a sound wave, thereby generating a Helmholtz resonator. The
present invention relates to a thermally excited sound wave generator that generates sound
waves efficiently by combining the
[0002]
An electromagnetically driven sound wave generator is known as a conventional sound wave
generator such as a speaker.
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This sound wave generator generates a sound wave by vibrating the diaphragm integrated with
the voice coil under the Lorentz force of the magnetic field generated by the magnet, when
current flows in the voice coil due to the output voltage of the amplifier. It is widely used in
general, including acoustic applications. Such a conventional electromagnetic drive type sound
wave generator widely used generally has the following problems.
[0003]
<First Problem> This sound wave generator is a sound wave generator utilizing mechanical
vibration, and not only has a unique resonance frequency due to the mass of the vibrating body
and the spring, but also has a narrow frequency band, and the reproduction frequency There is a
problem that the characteristics are not smooth. <Second Problem> This sound wave generator is
susceptible to external vibration and external pressure fluctuation because it has a vibrating
body. Specifically, there has been a problem that when a fluctuation in external pressure such as
wind pressure is largely applied to the diaphragm, a back electromotive force is generated in the
voice coil, which may cause burnout due to heat generation.
[0004]
<Third Problem> This sound wave generator comprises a voice coil, a permanent magnet, a
diaphragm, an enclosure, a damper edge and the like, and there are limitations in the
configuration of each member in order to reduce the weight, size and thickness. there were.
Then, as a sound wave generator that solves the above-mentioned problems, a sound wave
generator operating on a new generation principle that does not involve mechanical vibration at
all has been proposed (Patent Document 1 and the like).
[0005]
This device is not susceptible to external vibration or fluctuations in external pressure, can stably
generate pressure waves such as ultrasonic waves in a wide frequency range, and is easy to
manufacture using integrated circuit technology. Sound wave generator. Specifically, as shown in
FIG. 5, the sound wave generator is provided on the substrate 2, the thermal insulation layer 4
provided on the substrate 2, and the thermal insulation layer 4 so as to be electrically It is
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composed of a driven heating element thin film 6. Then, by providing the heat insulating layer 4
such as a porous layer or a polymer layer having an extremely small heat conductivity, the heat
generated from the heat generating body thin film 6 causes a large temperature change of the air
layer on the surface of the heat generating body thin film. In this way, sound waves (ultrasound)
are generated. In this case, an alternating signal current is supplied from the signal source 8 to
the heating element thin film 6. Since this thermally excited sound wave generator does not
involve mechanical vibration as described above, it has a wide frequency band, is less susceptible
to the influence of the surrounding environment, and is relatively easy to be miniaturized and
arrayed. ing.
[0006]
Considering the generation principle of the acoustic wave generator by such thermal excitation,
the change in surface temperature when an alternating current is applied to the electrically
driven heating element thin film, that is, the change in solid surface temperature T (ω) Assuming
that the thermal conductivity of the insulating layer is α, the heat capacity per volume is C, and
the angular frequency is ω, energy per unit area q (ω) [W / cm <2>], Equation 1 below Given by
[0007]
[0008]
Also, the sound pressure generated at that time is given by Equation 2 below.
[0009]
[0010]
Here, (1-j) / √2 represents an applied alternating current, and A is a constant.
That is, by the signal current of the frequency f supplied from the signal source 8 generating the
signal current of the frequency of the sound wave, the heat generated from the heating element
thin film 6 changes the temperature of the air by heat exchange with the air as the surrounding
medium. Happens.
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This produces a compressional wave of air and generates a sound wave of frequency 2f.
[0011]
Here, according to Equation 2, the generated sound pressure is proportional to the energy input
/ output q (ω) per unit area, that is, the input power.
It can be seen that the smaller the thermal conductivity α of the thermal insulating layer 4 and
the thermal capacity C per volume, the larger.
Furthermore, the thermal contrast of the thermal insulation layer 4 and the substrate 2 plays an
important role. That is, the thickness of the heat insulating layer 4 having the thermal
conductivity α and the thermal capacity C per volume is L, and the thermally conductive
substrate 2 having the thermal conductivity α and the thermal capacity C per volume
sufficiently large is given below it. In some cases, when the thickness is set to a thickness L (heat
diffusion length of AC component) determined by Equation 3 below, the AC component of heat
generation is thermally insulated, and the heat of DC component generated due to the heat
capacity of the heat generating element thin film It can be efficiently released to the large
thermally conductive substrate 2.
[0012]
[0013]
Japanese Patent Application Publication No. 11-300274
[0014]
The above-described thermal excitation type acoustic wave generator is a porous layer or
polymer layer having a very low thermal conductivity between the substrate 2 and the heater
thin film 6 whose surface area is increased by forming the heater in a thin film shape. The
thermal insulation layer 4 is provided to thermally insulate the heat generating body thin film 6
from the substrate 2 so that the temperature change of the surface of the heat generating body
thin film 6 becomes large, and the sound wave generation efficiency is improved. Therefore, the
heat capacity of the heating element thin film 6 is extremely small, and it is effective as a sound
wave generator suitable for a high frequency range (ultrasonic range) of about 20 kHz to 100
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kHz.
[0015]
However, when trying to apply this high-speed response sound source as a speaker sound source
in the audio frequency band, it is difficult to generate sound waves with sufficient efficiency in
the low frequency band below 20 kHz, ie, the audio frequency band. , There was a problem.
The present invention has been made in view of the above-mentioned problems, and an object
thereof is to provide a thermal excitation type sound wave generator capable of efficiently
generating sound waves in an audio frequency band lower than that of the prior art.
[0016]
The invention according to claim 1 comprises a thermally conductive substrate, a thermally
insulating layer having a predetermined thickness formed on one surface of the substrate, and an
alternating signal current formed on the thermally insulating layer. A heat-excitation-type sound
wave generator having a heat-generating-element thin film formed of an electrically driven
resistor, the heat-excitation-type sound wave comprising a Helmholtz resonator on the heatgenerating-element thin film. It is a generator.
[0017]
According to the thermally excited sound wave generator of the present invention, it is possible
to efficiently generate a sound wave in an audio frequency band lower than that of the prior art,
for example, less than 20 kHz.
[0018]
Hereinafter, a preferred embodiment of a thermal excitation type sound wave generator
according to the present invention will be described in detail with reference to the attached
drawings.
FIG. 1 is a block diagram showing a heat-excitation type sound wave generator according to the
present invention, FIG. 2 is a view showing the relationship between signal current flowing in a
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heating element thin film and generated sound wave, FIG. It is explanatory drawing for
demonstrating the motion of an air column, and the principle of resonance.
[0019]
As shown in FIG. 1, the thermally excited sound wave generator 10 includes a thermally
conductive substrate 12 and a thermally insulating layer (heat insulating layer) having a
predetermined thickness formed on one surface of the substrate 12. 14, a heating element thin
film 16 formed on the thermal insulation layer 14 and made of a resistor electrically driven by an
alternating current signal current, and a Helmholtz resonator 18 formed on the heating element
thin film 16 Mainly composed.
[0020]
Then, at both ends of the heat generating body thin film 16, connection pads 20 made of, for
example, an aluminum material having high thermal conductivity electrically connected are
provided.
As a result, heat can be efficiently dissipated from the heater thin film 16.
A signal source 22 for generating a signal current (drive voltage waveform) is connected to the
connection pad 20 through a lead wire 24, and the heating element thin film 16 is heated by the
signal current having the predetermined waveform. It is supposed to get.
[0021]
Specifically, the substrate 12 is made of a material having a sufficiently large thermal
conductivity and heat capacity per volume and excellent heat dissipation performance, and a
single crystal silicon substrate or the like can be used, for example.
The heat insulating layer 14 formed on one surface of the substrate 12 uses a porous layer or a
polymer layer having extremely low thermal conductivity.
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The thickness L of the thermal insulation layer 14 is set to the thickness of the thermal diffusion
length of the predetermined AC component represented by the equation 3.
[0022]
The heating element thin film 16 formed on the heat insulating layer 14 generates Joule heat,
and is made of, for example, a metallic resistor. Specifically, PVD (chemical vapor deposition)
method such as CVD (chemical vapor deposition) method or sputtering method It can be formed
by a physical vapor deposition method, a vacuum evaporation method, or the like. The Helmholtz
resonator 18 is constituted by a resonance box 26 covering a space above the heating element
thin film 16 and a duct 28 formed at the ceiling of the resonance box 26 and having, for
example, a circular through hole. The resonance space 30 is formed in the resonance box 26 and
is communicated with the outside space through the duct 28.
[0023]
For example, a glass substrate can be used as the resonance box 26. The recess forming the
resonance space 30 and the through hole forming the duct 28 can be formed on the glass
substrate by performing microforming processing or etching processing for irradiating the fine
abrasive particles suspended in the liquid. . The resonance box 26 made of such a glass substrate
can be bonded onto the substrate 12 using anodic bonding.
[0024]
By setting the resonance frequency of the Helmholtz resonator 18 formed in this manner to be in
the vicinity of a desired frequency in the audio frequency band, it is possible to efficiently release
the compressional wave of air generated by the thermal excitation.
[0025]
Here, the internal volume of Helmholtz resonator 18 (volume in resonance box 26) is V0, the
length of duct 28 is l, the cross-sectional radius of cylindrical duct 28 is r (diameter is 2r), the
cross-sectional area of duct 28 is Assuming that the sound velocity of the gas in the duct 28 is S,
the resonance frequency FH of the Helmholtz resonator 18 is expressed by the following
Equation 4.
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[0026]
[0027]
The inner volume of the Helmholtz resonator 18 and the shape of the duct 28 are respectively
selected so that the resonance frequency FH is near a frequency that is a desired audio frequency
band.
Such a thermal excitation type sound wave generator 10 is characterized in that since there is
essentially no mechanical vibration, the frequency characteristic fluctuation due to resonance is
small and smooth characteristics can be obtained over a wide frequency band, but the present
invention By providing the Helmholtz resonator 18 according to the above, it is possible to
efficiently generate sound waves in an audio frequency band lower than that in the prior art.
[0028]
Also, in order to improve the sound pressure characteristics in the frequency region below the
Helmholtz resonance frequency of the Helmholtz resonator 18 described above, plural Helmholtz
resonators having different resonance frequencies are arrayed on the same substrate, as
described later. The frequency characteristics may be improved by
[0029]
Next, the thermal insulation layer 14 and the substrate 12 will be described in more detail. As
described above, the thickness of the thermal insulation layer 14 having the thermal conductivity
α and the thermal capacity C per volume is L, and the thermal conduction therebelow In the
case where the substrate 12 having a sufficiently high thermal conductivity per unit volume and
heat capacity is provided, setting the thickness L to the extent represented by Equation 3 (the
thermal diffusion length of the alternating current component) L makes it possible to generate
AC The components can be thermally insulated, and the heat of the direct current component
generated due to the heat capacity of the heating element thin film 16 can be efficiently
dissipated to the substrate 12 of large thermal conductivity.
In this case, if the thermal conductivity αS of the substrate 12, the thermal capacity CS per
volume of the substrate 12, the thermal conductivity αI of the thermal insulating layer 14, and
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the thermal capacity CI per volume of the thermal insulating layer 14, then the product of αS ·
CS and αI · CI A substrate material and a heat insulating layer material described in International
Publication No. WO 2004/077881 in which the product ratio of is 100: 1 or more and the
product of αS · CS is 1 × 10 <8> or more Is desirable from the efficiency of sound generation.
[0030]
Specifically, for example, when single crystal silicon is used as the material of the substrate 12,
polyimide, porous silicon, polystyrene foam, SiO 2 thin film, Si 3 N 4 thin film or the like can be
used as the thermal insulation layer 14 which is a heat insulation layer. .
These combinations are merely examples and can be selected as appropriate.
However, more preferably, it is preferable to select one that facilitates manufacturing processes
such as microfabrication / arraying processing. In addition to single crystal silicon, the material
of the substrate 12 may be polycrystalline silicon, ceramics such as copper, aluminum nitride, or
the like.
[0031]
When the heat insulation layer 14 is formed of a porous silicon layer, as described above, the
heat insulation layer 14 can be formed by anodizing the silicon surface in a fluoric acid solution.
At this time, desired porosity and depth (thickness) can be obtained by appropriately setting the
current density and the treatment time. The porous silicon layer is a porous material, and
exhibits a very small value both in thermal conductivity and heat capacity as compared to silicon.
[0032]
Specifically, the single crystal silicon has a porosity of about 70% with respect to the thermal
conductivity α = 168 W / m · K and the heat capacity C = 1.67 × 10 <6> J / m <3> · K. Silicon
has a thermal conductivity α = 0.012 W / m · K and a heat capacity C = 0.06 × 10 <6> J / m
<3> · K. αS · CS is 286 × 10 <6> αI · C1 is 0.26 × 10 <6>, the ratio of the product of αS · CS to
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the product of αI · CI is 1100: 1, and αS · CS The product is 1 × 10 <8> or more. Therefore, the
acoustic wave generation efficiency can be improved because the conditions described in the
above-mentioned international publication are satisfied.
[0033]
FIG. 2 is a view showing the relationship between the signal current flowing through the heating
element thin film 16 and the generated sound wave. As shown in FIG. 2, the heat (FIG. 2A)
generated from the heating element thin film 16 by the signal current of the frequency f (FIG.
2A) supplied from the signal source 22 (see FIG. 1) generating the sound wave frequency signal.
Heat exchange with air, which is the surrounding medium, causes temperature change of the air
(Fig. 2 (C)). This produces a compressional wave of air and generates a sound wave of frequency
2f (FIG. 2 (D)). Therefore, a sound wave having a frequency twice that of the signal current is
generated.
[0034]
Here, the principle of movement and resonance of the air column (air column) in the duct 28 of
the Helmholtz resonator 18 will be described with reference to FIG. FIG. 3 is a view showing the
principle of movement and resonance of the air column 28A in the duct 28 of the Helmholtz
resonator 18, FIG. 3 (A) shows the same view as FIG. 1, and FIG. 3 (B) is its equivalent Figure
shows. The Helmholtz resonator 18 formed of the resonance box 26 made of a glass substrate
has the duct 28 having the cross-sectional area S, radius r, and length l as described above, and
the duct 28 has the heating element thin film 16. It penetrates between the resonance space 30
which encloses the circumference of, and external space.
[0035]
The motion of the air column 28A in the duct 28 is due to the excited vibration X0 near the
heating element thin film of a system in which the mass m of the air column in the duct is
attached to the tip of the spring 32 having a spring constant k shown in FIG. It is equivalent to
exercise. Assuming that the density of the air is ρ, the air mass m of the air column 28A in the
duct 28 is expressed by the following Equation 5.
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[0036]
[0037]
Assuming that the outside air pressure is Po, and the specific heat ratio of the air γ 空 気 1.4, the
spring constant k is expressed by the following Equation 6.
[0038]
[0039]
That is, the motion of the air column 28A in the duct 28 of the Helmholtz resonator 18 performs
single vibration with the air mass m in equation 5 and the spring constant k in equation 6 at a
frequency f represented by equation 7 below. become.
[0040]
[0041]
Next, a modification of the heat excitation type sound wave generator of the present invention
will be described.
FIG. 4 is a schematic perspective view showing a modification of the heat excitation type sound
wave generator of the present invention.
Here, in order to compensate for the attenuation in the band below the resonance frequency FH
together with the efficiency improvement by the Helmholtz resonators, a plurality (large number)
of Helmholtz resonators 18 are provided on the substrate 12 made of one large silicon substrate.
The resonators 18 are set to different resonance frequencies.
As a result, sound waves can be generated efficiently over a wide range of the audio frequency
band.
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In this case, as a matter of course, the dimensions of the internal volume Vo of the Helmholtz
resonator, the cross-sectional area S of the duct, the radius r of the duct, and the length l of the
duct are individually changed to have variations in the resonance frequency.
[0042]
As described above, since the heat excitation type sound wave generator of the present invention
generates a sound wave without using mechanical vibration generating means such as the
conventional sound wave generator, the influence of the external vibration and the fluctuation of
the external pressure is generated. In addition, it is possible to make the frequency range of the
generation frequency of the sound wave wide.
Further, the Helmholtz resonator is used in the device of the present invention, so that it is
possible to output sound waves particularly in the audio frequency band efficiently.
[0043]
Furthermore, in the device of the present invention, utilization of integrated circuit technology is
easy, and for example, peripheral circuits of sound wave generating devices can also be formed
on a silicon substrate. Therefore, peripheral circuits are also formed on the same substrate to
integrate functions. It can be realized. A lightweight, compact and thin sound source, which can
not be achieved by the conventional electromagnetic drive type sound wave generator, can be
produced at low cost with an extremely simple configuration.
[0044]
It is a block diagram which shows the thermal excitation type sound wave generator which
concerns on this invention. It is a figure which shows the relationship between the signal current
which flows into a heat generating body thin film, and the sound wave which generate | occur |
produces. It is explanatory drawing for demonstrating the movement of the air column in the
duct of a Helmholtz resonator, and the principle of resonance. It is a schematic perspective view
which shows the modification of the heat excitation type sound wave generator of this invention.
It is a block diagram which shows the conventional heat excitation type sound wave generator.
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Explanation of sign
[0045]
DESCRIPTION OF SYMBOLS 10 ... Thermal excitation type sound wave generator, 12 ... Substrate,
14 ... Thermal insulation layer, 16 ... Heating body thin film, 18 ... Helmholtz resonator, 22 ...
Signal source, 26 ... Resonant box, 28 ... Duct, 28A ... Air column , 30 ... resonance space.
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