JP2009171043A - Speaker driving method and apparatus thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain large output sound pressure even when the vibration amplitude of a diaphragm is small. <P>SOLUTION: A sound signal to be reproduced is input from an input terminal 10. A high frequency signal with fixed amplitude is output at a fixed frequency from a high frequency signal source 12. The frequency of the high frequency signal is higher than an audible frequency and is set to 10 to 1,000 times e.g. as much as the upper limit frequency of the sound signal to be reproduced. An amplitude modulation circuit 14 modulates the amplitude of the high frequency signal by the sound signal. The amplitude-modulated high frequency signal has a waveform to be deflected only in the same polarity direction according to the polarity of a sound signal waveform. The amplitude-modulated high frequency signal is amplified by an amplifier 16 to drive a speaker 18. The speaker 18 is a speaker of a type that the displacement amount of the diaphragm follows an application voltage and is composed of an electrostatic speaker, a piezoelectric speaker, or the like. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a loudspeaker driving method and a driving apparatus in which a displacement amount of a diaphragm follows an applied voltage such as an electrostatic loudspeaker, a piezoelectric loudspeaker, etc. As compared with the above, the output sound pressure of the sound signal can be obtained larger with respect to the same vibration amplitude of the diaphragm.

  A conventional speaker driving method is to directly drive a speaker with an audio signal to be uttered. According to this driving method, the volume of sound (sound pressure) emitted from the speaker is determined by the volume of air that the diaphragm moves. That is, in order to produce a loud sound, it is necessary to increase the volume of air that the diaphragm moves. Here, since the volume of air moved by the diaphragm is given by multiplying the area of the diaphragm and the vibration amplitude, it is necessary to increase the area of the diaphragm or increase the vibration amplitude in order to produce a loud sound. However, since it is difficult to increase the vibration amplitude, a diaphragm having a large area (that is, a speaker having a large diameter) is required as a result.

Japanese Patent No. 3000982

  The present invention has been made in view of the above points. Compared to a conventional driving method in which a speaker is directly driven by an audio signal to be uttered, the output sound pressure of the audio signal is equal to the vibration amplitude of the diaphragm. It is an object of the present invention to provide a driving method and a driving apparatus for a loudspeaker that can obtain a larger value.

  The speaker driving method of the present invention uses a speaker of a type in which the amount of displacement of the diaphragm follows the applied voltage, and drives the speaker with a high frequency signal higher than the audible frequency, which is amplitude-modulated with an audio signal. Compared to the case where the sound signal is driven by the same sound signal that is not amplitude-modulated, the output sound pressure of the sound signal can be obtained with respect to the same vibration amplitude of the diaphragm. The high-frequency signal is a signal that swings only in the same polarity direction as the polarity of the audio signal waveform.

  According to the present invention, the speaker is driven with a high-frequency signal higher than the audible frequency, which is amplitude-modulated with an audio signal, as will be described later, compared to a case where the speaker is directly driven with an audio signal to be uttered, A louder sound can be generated for the same vibration amplitude of the diaphragm. Moreover, since the high frequency signal for driving the speaker is a signal having a frequency higher than the audible frequency, only the sound of the audio signal can be heard by the human ear.

  In the case of a dynamic speaker, even if a high frequency signal is applied, it does not vibrate according to its voltage waveform, so the effect of the present invention cannot be expected. Therefore, the present invention can be suitably applied to an electrostatic type or piezoelectric type speaker in which the displacement amount of the diaphragm follows the applied voltage. A parametric array speaker is conventionally known as a speaker that radiates an ultrasonic wave by modulating the amplitude with an audio signal (for example, the one described in Patent Document 1). This is a superdirectivity using the principle of the parametric array. Which is different from that according to the present invention.

  In the present invention, the frequency of the high-frequency signal is preferably 10 to 1000 times, more preferably 100 to 1000 times the upper limit frequency of the audio signal to be reproduced.

  The speaker driving apparatus of the present invention is characterized by executing the speaker driving method of the present invention. For example, the speaker driving device can include an input terminal for inputting an audio signal, and an amplitude modulation circuit for outputting a signal obtained by amplitude-modulating a high-frequency signal with the audio signal. The speaker driving device can also be configured as a device for driving a speaker by receiving a high-frequency signal that has already been amplitude-modulated with an audio signal.

  An embodiment of a speaker driving apparatus according to the present invention is shown in FIG. Here, a system using analog signal processing is shown, but it can also be configured as a system using digital signal processing. An audio signal to be reproduced is input from the input terminal 10. The high frequency signal source 12 outputs a high frequency signal having a constant frequency and a constant amplitude. The frequency of the high-frequency signal can be set, for example, 10 to 1000 times, more preferably 100 to 1000 times the upper limit frequency of the audio signal to be reproduced. For example, if the upper limit frequency of the audio signal to be reproduced is 20 kHz, the frequency of the high frequency signal can be set to 200 kHz to 20 MHz, more preferably 2 MHz to 20 MHz. The amplitude modulation circuit 14 modulates the amplitude of a high frequency signal with an audio signal. The amplitude-modulated high frequency signal is amplified by an amplifier 16 and drives one speaker (speaker unit) 18. The speaker 18 is a speaker of a type in which the displacement amount of the diaphragm follows the applied voltage, and is configured by an electrostatic speaker, a piezoelectric speaker, or the like.

  FIG. 2 shows operation waveforms of the speaker driving device of FIG. In this case, the loudspeaker is driven by a high frequency signal that swings only in the same polarity direction as the polarity of the audio signal waveform. (A) is an audio | voice signal, (b) is a high frequency signal. The amplitude modulation circuit 14 modulates the high frequency signal with an audio signal and outputs the amplitude-modulated high frequency signal shown in (c). The amplitude-modulated high-frequency signal has a waveform that swings only in the same polarity direction according to the polarity of the audio signal waveform. This waveform can be obtained by extracting only the component having the same polarity as the audio signal waveform from the high-frequency signal that is symmetrically swung in both polar directions obtained by normal amplitude modulation. When the loudspeaker 18 is driven with this amplitude-modulated high-frequency signal, the high-frequency component is much higher than the audible frequency, so that it cannot be heard by the human ear, and only the audio signal component that is the envelope component can be heard.

（但し、Δは高周波信号による駆動周期（＝２π／ω₀）、Ａは振動板の振動振幅）

振動板の変位ｙ(t)によって生じる音圧ｐ(t)は、ｙ(t)の時間に関する二回微分に比例するので、

（但し、Ｂは比例定数）

と表される。このｐ(t)に含まれるω成分が音声信号成分による音圧の振幅である。なおこの式は正確にはデルタ関数の項が存在するが、このデルタ関数の項による成分は高周波であり音圧には変換されないのでここではこのデルタ関数を無視している。 Here, the sound pressure of the audio signal component when the speaker 18 is driven by the amplitude-modulated high-frequency signal of FIG. For simplicity, the case of a single tone will be described. Assume that the angular frequency of the audio signal is ω and the angular frequency of the high-frequency signal is ω ₀ (ω ₀ >> ω). At this time, the displacement y (t) of the diaphragm of the speaker 18 from the origin in the direction perpendicular to the surface is driven so as to satisfy the following equation according to time.

(Where Δ is the driving period (= 2π / ω ₀ ) by the high-frequency signal, and A is the vibration amplitude of the diaphragm)

Since the sound pressure p (t) generated by the displacement y (t) of the diaphragm is proportional to the second derivative with respect to time of y (t),

(B is a proportional constant)

It is expressed. The ω component included in p (t) is the amplitude of the sound pressure due to the audio signal component. Although this equation has a term of the delta function precisely, the component by the term of the delta function is a high frequency and is not converted into sound pressure, so this delta function is ignored here.

Then, the magnitude of the ω component (the amplitude of the sound pressure due to the sound signal component) included in p (t) is next determined. If the integration range is now L, the sinωt component p _s (ω) of p (t) is

Next, the cosωt component p _c (ω) of p (t) is

In summary,

Next, Gs (ωΔ) and Gc (ωΔ) of “Expression 5” are obtained.

Therefore, p _s (ω) and p _c (ω) of “Formula 5” are

Next, Fs (ω) and Fc (ω) of “Expression 5” are obtained.

を利用すると、

Where the integral formula

If you use

Also,

Therefore p _s of "Formula 7" _{(ω), p c (ω} ) , using "Equation 11", Fc obtained by "Formula 10" (omega), Fs and (omega), respectively,

Thus, the sin ωt component p _s (ω) and the cos ωt component p _c (ω) of p (t) were obtained. According to this, since ω ₀ >> ω, the main component is p _s (ω). When the loudspeaker 18 is driven by the amplitude-modulated high frequency signal of FIG. 2C, the sound pressure amplitude by the sound signal component is ABω ₀ ² / π. Since the amplitude of the sound pressure by the conventional driving method directly driven by the audio signal is ABω ² , when the vibration amplitude A of the diaphragm is the same, the method of driving by the amplitude-modulated high frequency signal of FIG. Compared with the method of driving directly with an audio signal, (ABω ₀ ² / π) / ABω ² = ω ₀ ² / πω ² times larger sound pressure can be obtained. In other words, the vibration amplitude of the diaphragm necessary for producing the same sound pressure is πω ² / ω ₀ ² times. Therefore, according to the method of driving the speaker 18 with the amplitude-modulated high frequency signal of FIG. 2C, the vibration amplitude A of the diaphragm of the speaker 18 remains small by increasing the angular frequency ω ₀ of the high frequency signal. Can obtain a large sound pressure. Therefore, in the case where the same sound pressure as that of the conventional driving method that directly drives with an audio signal is obtained, the diaphragm area can be made smaller and the speaker can be made smaller than the conventional driving method.

It is a block diagram which shows embodiment of the speaker drive device of this invention. It is an operation | movement waveform diagram of the speaker drive device of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Audio | voice signal input terminal, 12 ... High frequency signal source, 14 ... Amplitude modulation circuit, 18 ... Speaker of the form which the displacement amount of a diaphragm follows the applied voltage, such as an electrostatic type speaker and a piezoelectric type speaker

Claims (5)

Use a speaker of the type that the displacement of the diaphragm follows the applied voltage,
By driving the loudspeaker with an audio signal that is amplitude-modulated and having a higher frequency than the audible frequency,
Compared to the case where the speaker is driven by the same audio signal that is not amplitude-modulated, the output sound pressure of the audio signal can be obtained larger with respect to the same vibration amplitude of the diaphragm,
A speaker driving method in which the amplitude-modulated high-frequency signal is a signal that swings only in the same polarity direction as the polarity of an audio signal waveform.   The speaker driving method according to claim 1, wherein the speaker is an electrostatic speaker or a piezoelectric speaker.   The speaker driving method according to claim 1 or 2, wherein a frequency of the high-frequency signal is 10 to 1000 times, more preferably 100 to 1000 times, an upper limit frequency of an audio signal to be reproduced.   A speaker driving apparatus that executes the speaker driving method according to claim 1. An apparatus for driving a speaker of a type in which the amount of displacement of the diaphragm follows the applied voltage,
An input terminal for inputting audio signals;
An amplitude modulation circuit that outputs a signal obtained by amplitude-modulating a high-frequency signal with the audio signal;
By driving the speaker with a high-frequency signal amplitude-modulated with the sound signal, the sound signal is compared with the same vibration amplitude of the diaphragm as compared with the case where the speaker is driven with the same sound signal without amplitude modulation. A device that can obtain a larger output sound pressure,
A speaker driving device, wherein the amplitude-modulated high-frequency signal is a signal that swings only in the same polarity direction as the polarity of an audio signal waveform.

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