JP2005150797A - Method of driving thermal induction type sound wave radiating element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of driving a thermal induction type sound wave radiating element in which a sound wave radiated from the thermal induction type sound wave radiating element contains sound wave components having the same frequency as a fundamental frequency of an alternative current of a current to be allowed to flow to the thermal induction type sound wave radiating element. <P>SOLUTION: In the method of driving a thermal induction type sound wave radiating element in which a tungsten layer 3 and a pair of metallic electrodes 4 are formed on a porous silicon layer 2 formed on a silicon single crystal substrate 1, AC and DC currents are allowed to flow to the tungsten layer 3 via the metallic layer 4 so that both the currents are superimposed, thereby irradiating a sound wave having the sound wave components having the same frequency as the fundamental frequency of the AC current, so that the method of driving a thermal induction type sound wave radiating element is configured. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for driving a thermally induced acoustic wave radiating element.

  Conventionally, as described in Non-Patent Document 1 below, radiation of sound waves using a heat-induced sound wave emitting element is known.

  A heat-induced acoustic wave radiating element applies an alternating current to an electric resistor in contact with a gas (in many cases, air), and generates a periodic temperature change in the electric resistor by generating Joule heat by energization. This is an element that causes a periodic density change in the gas in contact with the electric resistor, thereby emitting a dense wave of the gas, that is, a sound wave transmitted in the gas.

  Such a heat-induced acoustic radiation element is used as an ultrasonic radiation element as described in Non-Patent Document 1 below.

H. Shinoda, et al. , “Thermally Induced Ultrasonic Emission from Porous Silicon”, Nature, Vol. 400, no. 6747, pp. 853-855, 26 August 1999.

  However, the fidelity of the heat-induced acoustic wave radiating element is low when viewed as signal conversion means for converting a current signal into a sonic signal. That is, when an alternating current flowing through the heat-induced acoustic wave radiating element is viewed as an alternating current signal, a sonic component having the same frequency as the fundamental frequency is not included in the sonic wave emitted from the element. This will be described in more detail as follows.

The alternating current that flows through the element is defined as I ₀ sin (ωt). Here, I ₀ (I ₀ > 0) is an amplitude of current, ω is an angular frequency, that is, 2πf (f is a frequency), and t is time. In this case, f is the fundamental frequency of this alternating current. If 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 (I ₀ sin (ωt)) ² = RI ₀ ² (1-cos (2ωt)) / 2
^{_{= RI 0 2/2 - (}} RI 0 2/2) cos (2ωt)

Next, during the rightmost side of the above equation, as the AC component, contains only the term which oscillates at twice the fundamental frequency ^{_{f ((RI 0 2/2}} ) cos (2ωt)), term which oscillates at the fundamental frequency f Is not included. Therefore, the component having the fundamental frequency f is not included in the sound wave induced by such heat generation.

  FIG. 4 shows a current waveform (“input”) when a heat-induced acoustic wave radiating element is operated by a conventional driving method, that is, a method in which an alternating current (in this case, a sine wave current) is passed through the electric resistor of the element. ), A temperature change waveform of the electric resistor (displayed as “thermal change”), and a sound pressure waveform of the radiated sound wave (displayed as “output”). In this case, as already described, sound waves having the same frequency as the fundamental frequency of the alternating current are not emitted.

  The main object of the present invention is to solve the above-mentioned problem of low fidelity when the heat-induced acoustic radiation element is viewed as a signal conversion means, and the sound wave emitted from the thermally-induced acoustic radiation element is thermally induced. An object of the present invention is to provide a method for driving a heat-induced acoustic wave radiating element that includes a sonic wave component having the same frequency as the fundamental frequency of the alternating current component of the current flowing through the sonic wave radiating element.

In order to solve the above problems, in the present invention, as described in claim 1,
In the method for driving a heat-induced acoustic wave radiating element, a sound wave having a sound wave component having the same frequency as the fundamental frequency of the alternating current is obtained by superimposing a direct current bias current on an alternating current and passing the current through the thermally induced acoustic wave radiating element. A driving method for a heat-induced acoustic wave radiating element, characterized by radiating from a heat-induced acoustic wave radiating element, is configured.

In the present invention, as described in claim 2,
In the method for driving a thermally induced acoustic radiation element, a direct current bias is superimposed on an alternating current electrical signal to form a biased electrical signal, and a current proportional to the biased electrical signal is caused to flow through the thermally induced acoustic radiation element. A driving method for a heat-induced acoustic wave radiating element, characterized in that a sound wave having a sound wave component having the same frequency as the fundamental frequency of an AC electrical signal is emitted from the heat-induced acoustic wave radiating element.

In the present invention, as described in claim 3,
In the driving method of the heat-induced acoustic wave radiating element, a positive bias is superimposed on the AC electrical signal to obtain a biased electrical signal that does not take a negative value, and n is a number of 1/2 or more and 1 or less, By causing a current proportional to the nth power of the biased electrical signal to flow to the heat-induced sound wave emitting element, a sound wave having a sound wave component having the same frequency as the fundamental frequency of the AC electric signal is generated from the heat-induced sound wave emitting element. A driving method of a heat-induced acoustic wave radiating element, characterized by radiating, is configured.

  By implementing the present invention, the sound wave radiated from the thermally induced sound wave radiating element includes a sound wave component having the same frequency as the fundamental frequency of the alternating current component of the current flowing through the heat induced sound wave radiating element. It is possible to provide a method for driving a sound wave radiating element.

  In the present invention, a sound wave component having the same frequency as the fundamental frequency of the alternating current is radiated from the heat-induced sound radiation element by superimposing a direct current bias current on the alternating current and flowing it through the heat-induced sound radiation element. To be included.

An alternating current, the _Like, and I 0 sin (ωt), the DC bias current and _{I B.} Here, I _B > 0 is set appropriately for the direction of the current. 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. If 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 (I ₀ sin (ωt) + I _B ) ^{2
_{= R (I 0 2 sin (}} ωt) 2 + 2I 0 I B sin (ωt) + I B 2)
^{_{= RI 0 2/2 + RI}} B 2 - (RI 0 2/2) cos (2ωt) + 2RI 0 I B sin (ωt)

Next, appeared term which oscillates at the fundamental frequency f to the rightmost side of the above equation _{(2RI 0 I B sin (ωt} )) is, therefore, to sound waves radiated from the heat-induced wave emitting element of the same frequency as the fundamental frequency f The sound wave component is included.

Term which oscillates at twice the fundamental frequency f in the above equation ^{_{((RI 0 2/2)}} cos (2ωt)) are also included. Term that vibration and amplitude of the sound wave with the same frequency as the fundamental frequency f generated due to the term _{(2RI 0 I B sin (ωt} )) which oscillates at the fundamental frequency f, at twice the fundamental frequency f ((RI ₀ the ^2/2) cos (amplitude of sound waves having a frequency twice the fundamental frequency f generated due to 2.omega.t)), respectively, believed to be proportional to the _2RI 0 _{I B} and _RI ^{0 2/2.} Since the intensity of a sound wave is proportional to the square of the amplitude, the ratio of the intensity of a sound wave having a frequency twice the fundamental frequency f to the intensity of a sound wave having the same frequency as the fundamental frequency f is

_{^{(RI 0 2/2) 2}} / (2RI 0 I B) 2 = (I 0 / I B) / 16

It becomes. For example, if I ₀ = I _B, the value of this ratio is found to be 1/16, good fidelity is obtained. If this increasing the value of I _B than, the value of this ratio is smaller than 1/16, more fidelity is improved. However, if I _B is made larger than I ₀ , the power consumed ineffectively increases. Specifically, I _B is 1 to 3 times, or 1.5 to 3 times that of I _0. The following is recommended.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 1 shows an example of a thermally induced acoustic wave radiating 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. A portion (5 mm × 5 mm) of the tungsten layer 3 between the pair of metal electrodes 4 functions as an electric resistor of the heat-induced acoustic wave radiating element.

  FIG. 2 shows the operation of the thermally induced acoustic wave radiating element shown in FIG. 1 according to the driving method of the present invention, that is, the method in which a direct current bias current is superimposed on the tungsten layer 3 serving as an electric resistor of the element. Current waveform (displayed as “input”), temperature change waveform of electrical resistor (displayed as “thermal change”), and sound pressure waveform of emitted sound wave (displayed as “output”) . In this case, the alternating current is a sine wave current, and the bias currents of 0.5 times, 1.0 times, and 1.5 times the amplitude are overlapped, respectively (a), (b), (c) ). As already explained using the equation, it can be seen that the proportion of the sound wave component having the same frequency as the fundamental frequency f of the alternating current increases as the bias current value increases. In addition, since the follow-up speed of the temperature change of the electric resistor that follows the change of the heat generation of the electric resistor is sufficiently high, a temperature change waveform is drawn assuming that the temperature change of the electric resistor is proportional to the change of the heat generation of the electric resistor. The sound pressure waveform of the radiated sound wave is drawn based on the actual measurement result.

  As described above, according to the present embodiment, the sound wave radiated from the heat-induced sound wave radiating element has a sound wave component having the same frequency as the fundamental frequency of the alternating current component of the current flowing through the heat-induced sound wave radiating element. It is possible to provide a method for driving a heat-induced acoustic wave radiating element as described above.

  In the present embodiment, a direct current bias current is superimposed on an alternating current to flow through the heat-induced acoustic radiation element. However, a current that flows through the heat-induced acoustic radiation element may be obtained by another method. That is, a DC bias (bias current in the case of a current signal, bias voltage in the case of a voltage signal) is superimposed on an AC electric signal (AC current signal or AC voltage signal) to obtain a bias-added electric signal. Even if a current proportional to the additional electrical signal is created and the current is passed through the heat-induced acoustic radiation element, the same effect as in the present embodiment can be obtained, and the sound wave emitted from the heat-induced acoustic radiation element It is possible to provide a method for driving a heat-induced sound wave emitting element that includes a sound wave component having the same frequency as the fundamental frequency of the alternating current component of the current flowing through the heat-induced sound wave emitting element.

(Embodiment 2)
A method for further improving the fidelity of signal conversion by the heat-induced acoustic wave radiating element as compared with the first embodiment will be described below.

  In this method, a positive bias is superimposed on the AC electrical signal to produce a biased electrical signal that does not take a negative value, and a current proportional to the square root of this biased electrical signal is created and the current is thermally induced. Flow through the acoustic wave emitting element. This method will be described below by taking as an example the case where the AC electrical signal is a sine wave signal.

An electric signal to be converted into a sound pressure signal is defined as S ₀ sin (ωt). S ₀ may be a current value or a voltage value, and is assumed to be a positive value in any case. This electrical signal, superimposed bias _{S B} that satisfy the _{S B} ≧ _{S 0,} the bias added electrical signal _{S 0 sin (ωt) + S} B. This biased electrical signal does not take a negative value because S _B ≧ S ₀ . A current proportional to the square root of the biased electric signal, that is, (S ₀ sin (ωt) + S _B ) ^1/2 is created, and the current is passed through the heat-induced acoustic wave radiating element.

Since the generation rate of Joule heat in the electrical resistor of the heat-induced acoustic radiation element is proportional to the square of the current flowing through the electrical resistor, the generation rate of Joule heat due to the current is

((S ₀ sin (ωt) + S _B ) ^1/2 ) ² = S ₀ sin (ωt) + S _B ,

That is, it is proportional to the bias-added electric signal, and the sound wave radiated from the heat-induced sound wave radiating element is proportional to the original electric signal S ₀ sin (ωt) in proportion to the change in the generation rate of Joule heat. It consists only of ingredients. That is, the fidelity in this case is 100%. In this case, it is sufficient that S _B is equal to S _0, and if it is larger than that, the power consumed ineffectively increases.

3, in the case of the S 0 ₌ S _B, (indicated by "Input") current flowing through the heat-induced sound wave emitting element waveform (indicated by "thermal change") temperature change waveform of the electric resistor and the emitted The sound pressure waveform of the sound wave (indicated by “output”).

It is clear from the above description that the original electrical signal can be converted into a sound wave with extremely high fidelity by the method of the present embodiment even when the original electrical signal includes a plurality of frequency components. In this case, it may be selected values of S _B so that the value of the bias-added electrical signal formed by superimposing a bias to the alternating electrical signal is not negative. When a heat-induced acoustic wave radiating element is driven by this method, the thermally-induced acoustic wave radiating element exhibits a function as a high-fidelity sound wave output element.

  As described above, according to the present embodiment, the sound wave radiated from the heat-induced sound wave radiating element has a sound wave component having the same frequency as the fundamental frequency of the alternating current component of the current flowing through the heat-induced sound wave radiating element. It is possible to provide a method for driving a heat-induced acoustic wave radiating element as described above.

  In the present embodiment, a current proportional to the square root of the bias-added electric signal is generated and the current is passed through the heat-induced acoustic wave radiating element. In general, an index n satisfying 1/2 ≦ n ≦ 1 is satisfied. Is used to generate a current proportional to the nth power of the bias-added electrical signal, and the current is passed through the heat-induced acoustic radiation element, so that the sound wave radiated from the heat-induced acoustic radiation element is also thermally induced. It is possible to provide a method for driving a thermally induced acoustic radiation element that includes a acoustic wave component having the same frequency as the fundamental frequency of the alternating current component of the current flowing through the acoustic radiation element. The case of n = 1 corresponds to a part of the first embodiment (the case where the direction of the current flowing through the heat-induced acoustic wave radiating element is not reversed), and faithful as n becomes smaller than 1 and approaches 1/2. The degree is improved and an ideal state is realized when n is equal to 1/2, resulting in very high fidelity. That is, even when such an index n is used, a sound wave component in which the sound wave radiated from the heat-induced sound wave radiating element has the same frequency as the fundamental frequency of the AC component of the current flowing through the heat-induced sound wave radiating element. It is possible to provide a method for driving a heat-induced acoustic wave radiating element including the above, and the fidelity of signal conversion is high.

It is a figure explaining an example of a heat induction type sound wave emitting element, (a) is a front sectional view and (b) is a top view. FIG. 5 is a diagram illustrating Embodiment 1; FIG. 5 is a diagram illustrating Embodiment 2; It is a figure explaining the conventional method of driving a heat induction type sound wave emitting element.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Silicon single crystal substrate, 2 ... Porous silicon layer, 3 ... Tungsten layer, 4 ... Metal electrode.

Claims (3)

  In the method for driving a heat-induced acoustic wave radiating element, a sound wave having a sound wave component having the same frequency as the fundamental frequency of the alternating current is obtained by superimposing a direct current bias current on an alternating current and passing the current through the thermally induced acoustic wave radiating element. A method of driving a heat-induced acoustic wave radiating element, wherein the heat-induced acoustic wave radiating element emits light.   In the method for driving a thermally induced acoustic radiation element, a direct current bias is superimposed on an alternating current electrical signal to form a biased electrical signal, and a current proportional to the biased electrical signal is caused to flow through the thermally induced acoustic radiation element. A method for driving a heat-induced acoustic wave radiating element, characterized in that a sound wave having a sound wave component having the same frequency as the fundamental frequency of an AC electrical signal is radiated from the heat-induced sound wave radiating element.   In the driving method of the heat-induced acoustic wave radiating element, a positive bias is superimposed on the AC electrical signal to obtain a biased electrical signal that does not take a negative value, and n is a number of 1/2 or more and 1 or less, By causing a current proportional to the nth power of the biased electrical signal to flow to the heat-induced sound wave emitting element, a sound wave having a sound wave component having the same frequency as the fundamental frequency of the AC electric signal is generated from the heat-induced sound wave emitting element. A method for driving a heat-induced acoustic wave radiating element, characterized by causing radiation.

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