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JP2005150797

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DESCRIPTION JP2005150797
The present invention relates to a thermally induced acoustic radiation element such that a
sound wave emitted from a thermally induced acoustic wave radiation element includes an
acoustic wave component having the same frequency as the fundamental frequency of an
alternating current component of current flowing to the thermally induced acoustic wave
radiation element. To provide a driving method for A driving method of a thermally induced
acoustic wave emitting element in which a tungsten layer 3 and a pair of metal electrodes 4 are
formed on a porous silicon layer 2 formed on one surface of a silicon single crystal substrate 1 In
the method, an alternating current and a direct current are applied to the tungsten layer 3
through the metal electrode 4 so as to emit an acoustic wave having a sound component having
the same frequency as the fundamental frequency of the alternating current. A method of driving
a thermally induced acoustic wave emitting element is provided. [Selected figure] Figure 1
Method of driving thermally induced acoustic radiation element
[0001]
The present invention relates to a method of driving a thermally induced acoustic wave emitting
element.
[0002]
Heretofore, as described in Non-Patent Document 1 below, the emission of sound waves using a
thermally induced acoustic wave emitting element is known.
[0003]
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The thermally induced acoustic wave emitting element causes an alternating current to flow
through the electric resistor in a state of being in contact with a gas (in many cases, air), and
generation of Joule heat by energization causes a periodic temperature change in the electric
resistor. It is an element that causes a gas in contact with an electrical resistor to periodically
change its density, thereby emitting a compressional wave of the gas, that is, an acoustic wave
that travels through the gas.
[0004]
Such a heat-induced sound wave emitting element is used as an ultrasonic wave emitting element
as described in Non-Patent Document 1 below.
[0005]
H.Shinoda,et al.,“Thermally induced
ultrasonic emission from porous silicon”,
Nature,Vol.400,No.6747,pp.853−855,26 August
1999.
[0006]
However, when the heat-induced sound wave emitting element is viewed as a signal conversion
means for converting a current signal into a sound wave signal, its fidelity is low.
That is, when the alternating current flowing through the thermally induced acoustic wave
emitting element is viewed as an alternating current signal, the acoustic wave component having
the same frequency as the fundamental frequency is not included in the acoustic wave emitted
from the element.
It will be as follows if this is explained in more detail.
[0007]
An alternating current flowing through the element is I0 sin (ωt).
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Here, I0 (where I0> 0) is the amplitude of the current, ω is the angular frequency or 2πf (f is the
frequency), and t is time.
In this case, f is the fundamental frequency of this alternating current.
Assuming that the resistance value of the electric resistor of the element is R, the amount of heat
generated per unit time by this resistor is R (I0 sin (ωt)) <2> = RI0 <2> (1-cos (2ωt)) / 2 = RI0
<2> / 2− (RI0 <2> / 2) cos (2ωt), and in the right side of the above equation, a term that
oscillates at twice the fundamental frequency f as an AC component ((RI0 < 2> / 2) only cos
(2ωt)), not including the term oscillating at the fundamental frequency f. Therefore, a component
having the fundamental frequency f is not included in the sound wave induced by such heat
generation.
[0008]
Fig. 4 shows the current waveform ("input") when the thermally induced acoustic wave emitting
element is operated by a conventional driving method, that is, a method of applying an
alternating current (in this case, a sinusoidal current) to the electric resistor of the element. ), The
temperature change waveform of the electric resistor (indicated by “heat change”), and the
sound pressure waveform of the emitted sound wave (indicated by “output”). In this case, as
described above, sound waves having the same frequency as the fundamental frequency of the
alternating current are not emitted.
[0009]
The main object of the present invention is to solve the above-mentioned problem of low fidelity
when the heat-induced acoustic wave emitting element is viewed as a signal conversion means,
and the acoustic wave emitted from the heat-induced acoustic wave emitting element is thermally
induced It is an object of the present invention to provide a method of driving a thermally
induced acoustic wave emitting element that includes an acoustic wave component having the
same frequency as the fundamental frequency of the AC component of the current supplied to
the acoustic wave emitting element.
[0010]
In order to solve the above-mentioned problems, in the present invention, according to the first
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aspect of the present invention, in the method of driving a thermally induced acoustic wave
radiation element, a direct current bias current is superimposed on an alternating current, and
the thermally induced acoustic wave radiation element The method for driving a thermally
induced acoustic wave emitting element is characterized in that the acoustic wave having the
acoustic wave component having the same frequency as the fundamental frequency of the
alternating current is emitted from the thermally induced acoustic wave emitting element.
[0011]
In the present invention, as described in claim 2, in the method of driving a thermally induced
acoustic wave radiation element, an alternating current electrical signal is superimposed on a
direct current bias to form a bias added electrical signal, and the bias added electrical signal is
proportional. The heat-induced acoustic wave emitting element to emit a sound wave having an
acoustic wave component having the same frequency as the fundamental frequency of the
alternating current signal by causing a current to flow through the thermally induced sound
wave-emitting element A method of driving a sound wave emitting element is configured.
[0012]
Further, in the present invention, according to the third aspect of the present invention, in the
method of driving a thermally induced acoustic wave radiation element, a bias added electric
signal which does not take a negative value by superimposing a positive value bias on an
alternating current electrical signal. Where n is a number equal to or greater than 1/2 and equal
to or less than 1, and the same as the fundamental frequency of the alternating current electrical
signal by flowing a current proportional to the n-th power of the bias added electrical signal to
the thermally induced acoustic radiation element. A method of driving a thermally induced
acoustic wave emitting element is characterized in that an acoustic wave having an acoustic wave
component of frequency is emitted from the thermally induced acoustic wave emitting element.
[0013]
According to the practice of the present invention, a thermally induced acoustic wave emitting
element includes an acoustic wave component having the same frequency as the fundamental
frequency of the alternating current component of the current supplied to the thermally induced
acoustic wave emitting element. It becomes possible to provide a method of driving a sound wave
emitting element.
[0014]
In the present invention, the acoustic wave component having the same frequency as the
fundamental frequency of the alternating current is emitted from the thermally induced acoustic
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radiation element by superposing the direct current bias current on the alternating current and
passing it through the thermally induced acoustic radiation element. To be included in
[0015]
The alternating current is I 0 sin (ωt) as described above, and the direct current bias current is
IB.
Here, the direction of the current is appropriately set to IB> 0.
In this case, ω is the fundamental angular frequency of this alternating current, and f = ω / (2π)
is the fundamental frequency of this alternating current.
Assuming that the resistance value of the electric resistor of the element is R, the amount of heat
generated per unit time by this resistor is R (I0 sin (ωt) + IB) <2> = R (I0 <2> sin (ωt) < 2> +
2I0IB sin (ωt) + IB <2>) = RI0 <2> / 2 + RIB <2>-(RI0 <2> / 2) cos (2ωt) + 2RI0 IB sin (ωt), and
the fundamental frequency f is the rightmost side of the above equation. The oscillating term
(2RI 0 IB sin (ωt)) appears, so that the acoustic wave emitted from the thermally induced
acoustic wave emitting element also includes an acoustic wave component having the same
frequency as the fundamental frequency f.
[0016]
The above equation also includes a term that oscillates at twice the fundamental frequency f
((RI0 <2> / 2) cos (2ωt)).
Amplitude of sound wave having the same frequency as the fundamental frequency f generated
due to the term (2RI 0 IB sin (ωt)) oscillating at the fundamental frequency f, and a term ((RI 0
<2> / 2) oscillating at twice the fundamental frequency f The amplitude of a sound wave having a
frequency twice as high as the fundamental frequency f generated due to cos (2ωt)) is
considered to be proportional to 2RI0IB and RI0 <2> / 2, respectively.
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Since the strength of the sound wave is proportional to the square of the amplitude, the ratio of
the strength of the sound wave having twice the fundamental frequency f to the strength of the
sound wave having the same frequency as the fundamental frequency f is (RI0 <2> / 2) <2> /
(2RI0IB) <2> = (I0 / IB) / 16.
For example, if I0 = IB, the value of this ratio is 1/16, which indicates that good fidelity is
obtained. If the value of IB is larger than this, the value of this ratio becomes smaller than 1/16,
and the fidelity is further improved. However, if IB is larger than I0, the power consumed
ineffectively increases. Specifically, IB is set to 1 to 3 times I0, or 1.5 to 3 times. Is good.
[0017]
Hereinafter, embodiments of the present invention will be described in detail using the drawings.
[0018]
First Embodiment FIG. 1 shows an example of a thermally induced acoustic wave radiation
element.
(A) of a figure is a front sectional view, (b) is a top view. In the figure, reference numeral 1
denotes a silicon single crystal substrate, and a porous silicon layer 2 is formed on one surface of
the silicon single crystal substrate 1. A tungsten layer 3 is formed on the surface of the porous
silicon layer 2, and a pair of metal electrodes 4 for energizing the tungsten layer 3 is formed. The
portion (5 mm × 5 mm) of the tungsten layer 3 located between the pair of metal electrodes 4
functions as an electrical resistor of the thermally induced acoustic wave emitting element.
[0019]
2 operates according to the driving method of the present invention, that is, a method in which a
DC bias current is applied to the tungsten layer 3 serving as an electric resistor of the element by
causing a DC bias current to flow. Shows the current waveform (indicated by "input"), the
temperature change waveform of the electric resistor (indicated by "thermal change"), and the
sound pressure waveform of the emitted sound wave (indicated by "output"). . In this case,
assuming that the alternating current is a sine wave current and bias currents of 0.5 times, 1.0
times, and 1.5 times the amplitude thereof are superimposed, respectively, (a), (b), (c) Shown in).
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As already described using the equation, it can be seen that as the bias current value increases,
the proportion of sound components having the same frequency as the fundamental frequency f
of the alternating current increases. In addition, since the follow-up speed of the temperature
change of the electric resistor following the change of the heat generation of the electric resistor
is sufficiently fast, the temperature change of the electric resistor draws the temperature change
waveform as proportional to the change of the heat generation of the electric resistor. The sound
pressure waveform of the emitted sound wave is drawn based on the measurement results.
[0020]
As described above, according to the present embodiment, the acoustic wave emitted from the
thermally induced acoustic wave emitting element has an acoustic wave component having the
same frequency as the fundamental frequency of the AC component of the current flowed to the
thermally induced acoustic wave emitting element. It is possible to provide a method of driving a
thermally induced acoustic wave emitting element, which includes the method.
[0021]
In the present embodiment, a direct current bias current is superimposed on an alternating
current, and the current flows in the heat-induced acoustic wave emitting element. However, the
current flowing in the thermally induced acoustic wave emitting element may be obtained by
another method.
That is, an AC electrical signal (AC current signal or AC voltage signal) is superimposed on a DC
bias (bias current in the case of a current signal, bias voltage in the case of a voltage signal) to
obtain a bias added electrical signal. The same effect as in the present embodiment can be
obtained by creating a current proportional to the additional electrical signal and passing the
current to the heat-induced acoustic wave emitting element, and the acoustic wave emitted from
the thermally induced acoustic wave emitting element is It is possible to provide a method of
driving a thermally induced acoustic wave emitting element that includes an acoustic wave
component having the same frequency as the fundamental frequency of the AC component of the
current supplied to the thermally induced acoustic wave emitting element.
[0022]
Second Embodiment A method of further improving the fidelity of signal conversion by the
thermally induced acoustic wave radiation element than the first embodiment described above
will be described below.
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[0023]
In this method, a positive value bias is superimposed on an alternating current electrical signal to
form a bias-added electrical signal that does not take a negative value, and a current proportional
to the square root of this bias-added electrical signal is generated to thermally induce that
current. Flow to the acoustic wave emitting element.
This method will be described below, taking the case where the alternating current electrical
signal is a sine wave signal as an example.
[0024]
An electrical signal to be converted into a sound pressure signal is S0 sin (ωt). S0 may be a
current value or a voltage value, and is assumed to be a positive value in any case. A bias SB
satisfying SB ≧ S0 is superimposed on this electric signal to make a bias added electric signal S0
sin (ωt) + SB. This bias added electrical signal does not take a negative value because SB ≧ S0. A
current proportional to the square root of this bias-added electrical signal, that is, (S0 sin
(.omega.t) + SB) <1/2> is generated, and the current is allowed to flow through the thermally
induced acoustic radiation element.
[0025]
Since the generation rate of Joule heat in the electrical resistor of the thermally induced acoustic
wave radiation element is proportional to the square of the current flowing through the electrical
resistor, the generation rate of Joule heat by the above current is ((S0 sin (ωt) + SB) < 1/2>) <2>
= S0 sin (ωt) + SB, ie, proportional to the bias-added electric signal, and emitted from the
thermally induced acoustic radiation element in proportion to the change in the generation rate
of this Joule heat The sound wave consists only of components proportional to the original
electrical signal S0 sin (ωt). That is, the fidelity in this case is 100%. In this case, SB may be equal
to S0, and if it is larger than that, the power consumed ineffectively increases.
[0026]
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Fig. 3 shows the current waveform (indicated by "input") flowing through the thermally induced
acoustic wave radiating element, the temperature change waveform of the electric resistor
(indicated by "thermal change"), and the sound wave emitted when S0 = SB. Shows the sound
pressure waveform (indicated by "output").
[0027]
It is apparent from the above description that even when the original electrical signal contains a
plurality of frequency components, the method of the present embodiment can convert the
original electrical signal into sound waves with extremely high fidelity.
In this case, the value of SB may be selected so that the value of the bias-added electrical signal
obtained by superimposing the bias on the AC electrical signal does not become negative. When
the thermally induced acoustic wave emitting element is driven by this method, the thermally
induced acoustic wave emitting element exhibits a function as a high fidelity acoustic wave
output element.
[0028]
As described above, according to the present embodiment, the acoustic wave emitted from the
thermally induced acoustic wave emitting element has an acoustic wave component having the
same frequency as the fundamental frequency of the AC component of the current flowed to the
thermally induced acoustic wave emitting element. It is possible to provide a method of driving a
thermally induced acoustic wave emitting element, which includes the method.
[0029]
In the present embodiment, a current proportional to the square root of the bias-added electrical
signal is generated, and the current is supplied to the thermally induced acoustic wave emitting
element. Generally, an index n satisfying 1/2 ≦ n ≦ 1 is satisfied. By using a current that is
proportional to the n-th power of the bias-added electrical signal and passing the current through
the thermally induced acoustic wave emitting element, the acoustic wave emitted from the
thermally induced acoustic wave emitting element is thermally induced type It is possible to
provide a method of driving a thermally induced acoustic wave emitting element that includes an
acoustic wave component having the same frequency as the fundamental frequency of the AC
component of the current supplied to the acoustic wave emitting element.
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The case of n = 1 corresponds to the case of a part of the first embodiment (when the direction of
the current supplied to the heat-induced acoustic wave emitting element is not reversed), and it is
true that n becomes smaller than 1 and approaches 1⁄2. The degree is improved, and an ideal
condition is realized when n is equal to 1/2, and very high fidelity is obtained. That is, even when
such an index n is used, a sound wave component having the same frequency as the fundamental
frequency of the AC component of the current flowed to the heat-induced sound wave emitting
element is emitted from the heat-induced sound wave emitting element It is possible to provide a
method of driving a thermally induced acoustic wave emitting element, which has a high fidelity
of signal conversion.
[0030]
It is a figure explaining an example of a heat induction type sound wave radiation element, (a) is
a front sectional view, (b) is a top view. FIG. 2 is a diagram for explaining the first embodiment.
FIG. 7 is a diagram for explaining a second embodiment. It is a figure explaining the conventional
method of driving a thermally induced sound wave emitting element.
Explanation of sign
[0031]
1 silicon single crystal substrate 2 porous silicon layer 3 tungsten layer 4 metal electrode
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