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JPS61118096

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DESCRIPTION JPS61118096
[0001]
TECHNICAL FIELD The present invention relates to a speaker (parametric speaker) using nonlinearity of air to ultrasonic waves. Conventional technology In the conventional loudspeaker
system, the demand for making the directivity of sound sharp like a spotlight and to be heard
only by a certain range of people without being influenced by ambient noise is a trade show etc.
When we wanted to give a separate explanation for each individual exhibit, there were strong
ones in applications such as guidance broadcasting at the station home. Horn speakers have been
mainly used for such applications, but the directivity of a horn speaker strongly depends on its
length and aperture, and in particular K that obtains sharp directivity in the low range like voice
is long. There is a drawback that the diameter and the diameter are both very large K. On the
other hand, in recent years, speakers using the non-linearity of air with respect to ultrasonic
waves (hereinafter referred to as parametric speakers) are attracting attention because they can
obtain directivity much sharper than before. First, a conventional parametric speaker will be
described. FIG. 5 shows the structure of a conventional parametric speaker. In FIG. 5, 1 is an
ultrasonic transducer, and 2 is a speaker formed by arranging them in a honeycomb shape. An
audio signal source 3 inputs this output to the modulator 4. Here, the audio signal (modulated
wave) is amplitude-modulated with the carrier wave output from the carrier wave oscillator 6 as
40 KHz, and is input to the speaker 2 through the power amplifier 6. An acoustic filter 7 is
provided between the speaker 2 and the listener 8 for absorbing ultrasonic waves. In the
parametric speaker described above, the carrier wave of the amplitude modulated ultrasonic
wave emitted from the speaker 2 and the upper and lower sideband waves cause non-linear
interaction in the air, and a modulated wave having sharp directivity is generated. Here, the
amplitude modulated ultrasonic wave emitted from the speaker 2 is referred to as a primary
wave, and an original voice signal (modulated wave) generated as a result of nonlinear
interaction of the primary wave is referred to as a secondary wave. Then, since the conversion
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efficiency from the primary wave to the secondary wave is extremely low (1% or less), strong
ultrasonic waves are required to generate a secondary wave of practical level. For example, in
order to obtain a secondary sound pressure of 5 odB, a primary sound pressure of 140 dB or
more is required. If the listener is directly exposed to such strong ultrasound, it is possible that
adverse effects such as hearing loss occur. Therefore, the installation of the acoustic filter 7
shown in FIG. 6 is indispensable for practical use of the parametric speaker.
Problems to be Solved by the Invention However, the following problems remain in the above
configuration. (1) Even if the level of the ultrasonic wave is sufficiently lowered under the
acoustic filter 7, the ultrasonic wave from the speaker 2 is directly received where it deviates
from the acoustic filter 7. Although ultrasonic waves have sharp directivity, this level is not
always safe. 0 (2) When using a parametric speaker indoors, the ultrasonic waves are reflected to
the surrounding wall etc. An area reaching a strong ultrasonic wave level may appear 0 (鴻 When
raising the ultrasonic wave absorption coefficient of the acoustic filter 7 to lower the ultrasonic
wave level, the audible sound level also decreases. (→ → Even when the level of the signal wave
is small (eg, silent), the energy utilization factor is poor because the ultrasonic wave level is
constant. Furthermore, when silent, the subharmonic generated from the ultrasonic transducer is
heard as noise. SUMMARY OF THE INVENTION In view of the above problems, the present
invention provides a parametric speaker in which the average level of ultrasonic waves is
significantly reduced by controlling the level of ultrasonic waves according to the level of
modulated waves. Means for Solving the Problems In order to solve the above-mentioned
problems, a parametric speaker according to the present invention comprises an ultrasonic
transducer and a modulator for driving the ultrasonic transducer with an audio signal and
modulated ultrasonic waves; And a controller for controlling the level of the ultrasonic wave
according to the level of the audio signal. Operation The present invention controls the level of
the carrier wave in the ultrasonic region input to the modulator according to the above
configuration by the level of the modulation wave which is the audio frequency, and lowers the
level of the carrier wave when the modulation wave level is small. When the wave level is high,
the carrier level is also raised. As a result, the degree of modulation is kept approximately
constant. The signal in the ultrasonic region modulated by the modulator is emitted to the air as
ultrasonic waves input to the ultrasonic transducer. Then, due to the non-linearity of air, the
carrier wave and the sideband interfere with each other, and an original sound signal (modulated
wave) is generated in the air. In the conventional parametric speaker, the level of the ultrasonic
wave is set based on the time when the level of the modulated wave is maximum, so the level of
the ultrasonic wave remains strong even when the level of the modulated wave is zero, that is,
silent state . However, according to the present invention, the level of the ultrasonic waves
changes in accordance with the level of the modulated wave, so that the average level of the
ultrasonic waves radiated into the air can be significantly reduced. Embodiment An embodiment
of the present invention will be described with reference to the drawings.
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In FIG. 1, reference numeral 9 denotes a controller for controlling the signal level of the carrier
wave, and the other configuration is the same as that of the conventional example, and hence the
description thereof is omitted. The controller 9 can be realized by the configuration shown in FIG.
The center is the voltage controlled amplifier 1o. The modulation wave is rectified by the rectifier
circuit 11 and the signal amplified by the amplifier 12 is added to the control signal. If necessary,
an appropriate DC bias may be superimposed and added. The output of the carrier wave
oscillator 6 is input to the voltage control type amplifier 1 o, amplified according to the control
signal level <<, and then output to the modulator 4 via the logarithmic / linear conversion
amplifier 13. Next, the operation of the controller 9 will be described. The output of the audio
signal source 3 is input to the modulator 4 and used as a control signal of the controller 9. The
signal from the carrier oscillator 6 is controlled by the level of the control signal and input to the
modulator 4. A characteristic diagram of the control signal voltage vc and the output voltage /
input voltage (V0 / V) of the controller 9 is shown in FIG. At this time, the modulation degree can
be made constant (0, 3). As a result, the average level of ultrasound decreased by about 20 dB as
compared to the conventional one. In each of FIGS. 4 to 6, the dotted line A shows the
conventional technology, and the solid line B shows the time change of the audio signal level, the
modulation factor and the ultrasonic wave level of this embodiment. Conventionally, since the
output of ultrasonic waves was set so that the modulation degree was 0.3 at the peak of the
audio signal, the level of the ultrasonic waves was always constant at 140 dB. However, in the
present embodiment, since the output of ultrasonic waves is controlled so that the modulation
degree is always 1 / C0, 3, the level of the ultrasonic waves can be reduced by about 20 dB on
average. Has the following effects by providing a controller that controls the level of the carrier
wave (ultrasound) according to the level of the modulation wave (audio frequency). (1) Since the
average level of ultrasonic waves radiated into the air can be significantly reduced, the acoustic
filter, which has conventionally been essential for short-distance listening, can be miniaturized or
eliminated. Even when an acoustic filter is provided, the amount of attenuation of ultrasonic
waves by the filter can be reduced as compared with the prior art, so the structure is simplified
and the reduction of the audible sound level due to the acoustic filter can be prevented. ('4 When
the level of the modulation wave is small or zero, the output of the ultrasonic wave also becomes
small, so that the drawback that the subharmonic generated from the ultrasonic transducer as
before is heard as noise is eliminated . (萄 The energy saving and the life of the amplifier are
extended because the average load on the power amplifier is reduced.
[0002]
Brief description of the drawings
[0003]
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FIG. 1 is a block diagram of a parametric speaker according to an embodiment of the present
invention, FIG. 2 is a block diagram of the controller, FIG. 3 is a diagram showing voltage
attenuation characteristics of the controller, and FIGS. FIG. 7 is a diagram showing the
configuration of the conventional example, showing the time change of the audio signal level, the
modulation degree, and the ultrasonic wave level in the prior art and the present embodiment.
DESCRIPTION OF REFERENCE NUMERALS 1 ultrasonic transducer 2 speaker 3 audio signal
source 4 modulator 5 carrier oscillator , 6 ... Power amplifier, 7 ... Acoustic filter, 8 ... Listener, 9 ...
Controller. Name of Agent Attorney Nakao Toshio and 1 other person Figure 1 [2 Figure Q Figure
3 $ 4 FP (f! ('N Vc (v) FIG. 4 7% 1- FIG. 6 JP-A
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