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JP2009171043

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DESCRIPTION JP2009171043
An object of the present invention is to obtain a large output sound pressure even if the vibration
amplitude of a diaphragm is small. An audio signal to be reproduced is input from an input
terminal. A high frequency signal having a constant frequency and a constant amplitude is output
from the high frequency signal source 12. The frequency of this high frequency signal is higher
than the audible frequency, and is set to, for example, 10 to 1000 times the upper limit
frequency of the audio signal to be reproduced. The amplitude modulation circuit 14 amplitudemodulates the high frequency signal with an audio signal. The amplitude-modulated highfrequency signal has a waveform that swings only in the same polarity direction according to the
polarity of the audio signal waveform. The amplitude-modulated high frequency signal is
amplified by the amplifier 16 to drive the speaker 18. The speaker 18 is a speaker of a type in
which the displacement amount of the diaphragm follows the applied voltage, and is formed of
an electrostatic speaker, a piezoelectric speaker, or the like. [Selected figure] Figure 1
Loudspeaker driving method and apparatus thereof
[0001]
The present invention relates to a method and apparatus for driving a speaker of a type in which
the displacement of the diaphragm follows an applied voltage, such as an electrostatic speaker or
a piezoelectric speaker, and a conventional driving method for directly driving a speaker by an
audio signal to be produced. As compared with the above, the output sound pressure of the
sound signal can be obtained larger for the same vibration amplitude of the diaphragm.
[0002]
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1
The conventional method of driving a loudspeaker is to drive the loudspeaker directly with an
audio signal to be produced.
According to this driving method, the magnitude (sound pressure) of the sound emitted from the
speaker is determined by the volume of air moved by the diaphragm. That is, to make a loud
sound, it is necessary to increase the volume of air moved by the diaphragm. 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 enlarge the area of the diaphragm or increase the vibration
amplitude in order to make a loud sound. However, since it is difficult to increase the vibration
amplitude, as a result, a diaphragm having a large area (that is, a speaker having a large
aperture) is required.
[0003]
Patent No. 3000982
[0004]
The present invention has been made in view of the above-described point, and compared to the
conventional drive method of driving a speaker directly with an audio signal to be produced, the
output sound pressure of the audio signal for the same vibration amplitude of the diaphragm. It
is an object of the present invention to provide a method and apparatus for driving a speaker in
which the
[0005]
The speaker driving method according to the present invention uses a speaker of a type in which
the displacement amount of the diaphragm follows the applied voltage, and drives the speaker
with a high frequency signal higher than an audio frequency which is amplitude modulated with
an audio signal. The output sound pressure of the audio signal can be obtained larger for the
same vibration amplitude of the diaphragm as compared with the case where the same audio
signal without amplitude modulation is driven. The high frequency signal is a signal which can be
oscillated only in the same polarity direction as that of the voice signal waveform.
[0006]
According to the present invention, by driving the speaker with a high frequency signal higher
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2
than an audible frequency, which is amplitude-modulated with the audio signal, as will be
described later, compared with the case where the speaker is driven directly with the audio
signal to be produced, 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 of a frequency
higher than the audible frequency, only the sound of the audio signal can be heard by the human
ear.
[0007]
In the case of a dynamic type speaker, even if a high frequency signal is applied, it does not
vibrate as its voltage waveform, so the effect of the present invention can not be expected.
Therefore, the present invention can be suitably applied to a speaker of an electrostatic type, a
piezoelectric type or the like in which the displacement amount of the diaphragm follows the
applied voltage.
In addition, a parametric array speaker is conventionally known as a speaker that amplitudemodulates and radiates an ultrasonic wave with an audio signal (for example, the one described
in Patent Document 1 above), but this is superdirectivity using the principle of parametric array
The speaker is different from that according to the present invention.
[0008]
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.
[0009]
The speaker driving apparatus of the present invention is characterized in that the speaker
driving method of the present invention is implemented.
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For example, this speaker drive device can be configured to 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. Moreover, this speaker
drive device can also be configured as a device that receives a high frequency signal that has
been amplitude-modulated as an audio signal to drive the speaker.
[0010]
An embodiment of a speaker drive device of the present invention is shown in FIG. Although a
system based on analog signal processing is shown here, it can also be configured as a system
based on digital signal processing. The input terminal 10 receives an audio signal to be
reproduced. A high frequency signal having a constant frequency and a constant amplitude is
output from the high frequency signal source 12. The frequency of the high frequency signal can
be set to, 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
amplitude-modulates the high frequency signal with an audio signal. The amplitude-modulated
high frequency signal is amplified by the amplifier 16 to drive 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 formed of an electrostatic speaker, a piezoelectric speaker, or the like.
[0011]
The operation ¦ movement waveform of the speaker drive device of FIG. 1 is shown in FIG. This is
intended to drive the speaker with a high frequency signal that oscillates 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 amplitude-modulates a high
frequency signal with an audio signal and outputs an 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 is obtained by extracting only the component of the same polarity as the audio signal
waveform out of the high frequency signal which is symmetrically oscillated in the bipolar
direction obtained by the usual amplitude modulation. When the speaker 18 is driven with this
amplitude-modulated high frequency signal, the high frequency component is much higher than
the audio frequency, so it can not be heard by the human ear, and only the audio signal
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component which is its envelope component can be heard.
[0012]
Here, the sound pressure of the audio signal component in the case of driving the speaker 18
with the amplitude-modulated high frequency signal of FIG. 2C is obtained. The case of a single
sound will be described for simplicity. Now, it is assumed that the angular frequency of the audio
signal is ω, and the angular frequency of the high frequency signal is ω0 (ω0 >> ω). At this
time, the displacement y (t) from the origin in the direction perpendicular to the surface of the
diaphragm of the speaker 18 is driven to be the following equation according to time. (Where Δ
is a drive cycle by a high frequency signal (= 2π / ω 0), A is a vibration amplitude of the
diaphragm) The sound pressure p (t) generated by the displacement y (t) of the diaphragm
relates to the time of y (t) Since it is proportional to the second derivative, (where B is a
proportional constant) is expressed. The ω component contained in p (t) is the amplitude of the
sound pressure due to the audio signal component. Although this equation correctly includes the
term of the delta function, the component according to the term of the delta function is high
frequency and is not converted into sound pressure, so the delta function is ignored here.
[0013]
Then, the magnitude of the ω component (amplitude of the sound pressure due to the audio
signal component) contained in p (t) is determined next. Now, assuming that the integration
range is L, the sin ωt component ps (ω) of p (t) is
[0014]
The cos ω t component pc (ω) of p (t) is then
[0015]
To summarize the above,
[0016]
Next, Gs (ωΔ) and Gc (ωΔ) of
Formula 5
are obtained.
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[0017]
Therefore, ps (ω) and pc (ω) of
Equation 5
are
[0018]
Next, Fs (ω) and Fc (ω) of
Formula 5
are obtained.
[0019]
If we use the integral formula here,
[0020]
また、
[0021]
Therefore, ps (ω) and pc (ω) of Formula 7 are respectively obtained using Fc (ω) and Fs
(ω) obtained by Formula 11 and Formula 10 , respectively.
[0022]
From the above, the sin ωt component ps (ω) of p (t) and the cos ωt component pc (ω) are
obtained.
According to this, since ω0 >> ω, the main component is ps (ω).
The amplitude of the sound pressure due to the audio signal component in the case of driving the
speaker 18 with the amplitude-modulated high frequency signal of FIG. 2C is ABω0 <2> / π.
Since the amplitude of the sound pressure according to the conventional driving method of direct
driving with an audio signal is AB.omega. <2>, when the vibration amplitude A of the diaphragm
is the same, driving is performed with the amplitude-modulated high frequency signal of FIG. In
this case, the sound pressure that is larger by (ABω0 <2> / π) / ABω <2> = ω0 <2> / πω <2>
times can be obtained as compared with the method of directly driving with an audio signal.
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Conversely speaking, the vibration amplitude of the diaphragm required to produce the same
sound pressure can be πω <2> / ω0 <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. Great sound
pressure can be obtained.
Therefore, in the case of obtaining the same sound pressure as that of the conventional drive
method of direct driving with an audio signal, it is possible to reduce the diaphragm area and
miniaturize the speaker as compared with the conventional drive method.
[0023]
It is a block diagram which shows embodiment of the speaker drive device of this invention.
It is an operation ¦ movement wave form diagram of the speaker drive device of FIG.
Explanation of sign
[0024]
10: Audio signal input terminal, 12: High frequency signal source, 14: Amplitude modulation
circuit, 18: Electrostatic speaker, piezoelectric speaker, etc. A speaker in which the displacement
of the diaphragm follows the applied voltage
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