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The present invention relates to a speaker using non-linearity of air to ultrasonic waves, so-called
parametric speaker. Conventional Technology In the conventional loudspeaker system, the
demand for making the directivity of sound sharp like a spotlight and telling only a certain range
of people without being affected by the ambient noise is an exhibition 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 it is long to obtain sharp directivity in the low frequency range like
voice. There was a drawback that the two diameters would be extremely large. On the other
hand, in recent years, a speaker utilizing non-linearity 1'4 of air with respect to ultrasonic waves,
a so-called parametric speaker, has attracted attention because it can obtain directivity much
sharper than before (for example, JP-A-58-119293). First, a conventional parameter link speaker
will be described. FIG. 6 shows the configuration of a conventional parametric speaker. In FIG. 6,
1 is an ultrasonic transducer, and 2 is an ultrasonic generator (hereinafter referred to as a
speaker) configured 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 a carrier wave of 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 above configuration, an ultrasonic wave having a spectrum as shown on the right
side of FIG. 7 is emitted to the air from the speaker. When the ultrasonic wave has a large
amplitude and is regarded as a finite amplitude level, a non-linear interaction occurs between the
carrier wave and the upper and lower sideband waves in the air, and a modulated wave having
sharp directivity is generated. Here, the modulated ultrasonic wave emitted from the speaker 2 is
referred to as a primary wave, and the original audio signal (modulated wave) generated as a
result of nonlinear interaction of the primary wave is referred to as a secondary wave. By the
way, in the parametric speaker, since the conversion efficiency from the primary wave to the
secondary wave is low, a strong open sound ?U is required to generate a secondary wave of
practical level. In particular, the sound pressure level of the secondary wave in the low range is
proportional to the square of the secondary wave frequency, and the sound pressure level in the
low range is insufficient, which is a problem with the sound quality. In terms of point, it is known
that it is better to lower the carrier frequency. An example of calculating the relationship
between the sound pressure level of the primary wave and the sound pressure level of the
secondary wave with the carrier wave frequency as a parameter is shown in FIG. 8 (Japanese
Acoustical Society Journal 41, 6 (1985), 378).
Although it is meaningless to reduce the frequency of the carrier wave to audio frequency for the
purpose as a speaker, for example, according to FIG. 8, in order to obtain a secondary wave
sound pressure level of 110 dB, the carrier frequency is It can be seen that while 40 kHz requires
a primary sound pressure level of about 145 dB, 20 kHz requires only 135 dB. No, recently an
attempt to improve conversion efficiency by lowering the carrier wave frequency has been
shown (Japanese Acoustical Society Conference Proceedings 61-64 Spring 1-4-18 P, 31ts).
Problems to be solved by the invention However, if the frequency of the carrier wave is lowered,
if the lower sideband of the primary wave is applied to the audio as shown in FIG. The problem is
that the band of the signal wave must be narrowed so that it does not cover the audio frequency,
and the frequency of the carrier wave can be lowered to no more than 25 kHz at most, as the
sound of the wave becomes inaudible. I had a point. SUMMARY OF THE INVENTION In view of
the above problems, the present invention provides a parametric speaker capable of reproducing
a wide-band signal and having an improved low-pass conversion efficiency. Means for Solving the
Problem In order to solve the above-mentioned problems, the parametric speaker of the present
invention is characterized in that a first pass filter and a low pass filter are used to modulate a
first carrier wave with a signal wave passed through. A second modulator for modulating a
second carrier that is at least 20 kllx higher, for example, by at least 20 kllx than the first carrier,
with a modulator and the original speech signal wave from a low pass filter unfiltered speech
signal source; And an ultrasonic generator driven by the combined output of the two modulators.
Operation The present invention divides the voice signal wave into two parts according to the
above configuration, one passes through the low pass filter, and the other part passes through
the first one. It is input to the second modulator. The signals modulated by the respective
modulators drive the speakers through the summer. Since the signal wave input to the first
modulator is only low frequency, the sideband wave does not cover the audio frequency and the
primary wave can not be heard. In addition, since the two carrier frequencies are separated by
20 klh or more, the secondary waves generated by the non-linear interaction between the
carriers can not be heard. Then, since the low frequency band signal with low conversion
efficiency is modulated by two carrier waves, and one of the low frequency frequency bands with
high conversion efficiency is used as the carrier wave, the secondary wave sound pressure in the
low frequency band is greatly improved. An embodiment of the present invention will be
described with reference to the drawings.
FIG. 1 shows the configuration of a parametric speaker according to an embodiment of the
present invention. In the loudspeaker of this embodiment, the ultrasonic transducers having the
structure shown in FIG. 2 are arranged in the form of 1OO dots and connected in parallel. In FIG.
2, reference numeral 9 denotes a piezoelectric ceramic plate, which constitutes a bimorph by
laminating two polarization directions opposite to each other. 1 o is a resonator adhesively fixed
to the center of the bimorph, 11 is a terminal plate, and 12 is a coupling axis. The characteristics
of this ultrasonic transducer can be changed by changing the shape and material of the bimorph
or resonator. FIG. 3 shows sound pressure frequency characteristics of a commercially available
26 kl (+ ultrasonic transducer. When the diameter of the ultrasonic transducer is increased and
the material of the resonator is changed from aluminum to mica reinforced resin, the
characteristics shown in FIG. 3 a are as follows: 1st peak from 25 kHz to 22 kHz And the second
peak was doubled from 36 kll + to 46 kHz. Therefore, the first one. Oscillation frequency f1 of
the second carrier oscillators 5a, 5b. Let f2 be 22 kHz and 4 es klb respectively. The cutoff
frequency of the low pass filter 13 is 1.5 kflz. Reference numeral 2 denotes an ultrasonic wave
generator that constitutes a speaker. 3 is an audio signal source, 6 is a power amplifier, and these
are the same as the configuration of the conventional example. In this embodiment, with the
configuration described above, the low frequency band of 1.5 kHz or less uses both 22 kHz and
46 kHz as a carrier wave and 1.5 kl (a high frequency band of z or more has 45 kt (z alone as a
carrier wave amplitude Modulate and input to the speaker. As a result, a primary wave having a
spectrum as shown in FIG. 4 is emitted from the speaker 2 into the air. Then, non-linear
interaction occurs in the air, and the signal wave is regenerated. The frequency characteristics of
the secondary wave become as shown in FIG. FIG. 6 c, each c. This is the characteristic when
modulation is applied only to the second modulator. As apparent from this, according to the
present embodiment, the sound pressure level in the low range can be improved by 1 odB or
more as compared with the conventional case where the carrier frequency is around 40 kHz
(corresponding to the characteristic C). it can. In the present embodiment, an ultrasonic
transducer exhibiting a bimodal frequency characteristic using a piezoelectric ceramic is used as
a speaker, but the ultrasonic transducer to be used is not limited to this, and it is possible to use a
wider band. It goes without saying that electrodynamic speakers etc. may be used. In that case, it
is also possible to modulate at more frequencies than two frequencies.
Furthermore, if only the upper sideband is used as single sideband modulation (SSB) instead of
the double sideband amplitude modulation (DSB) shown in this embodiment as a modulation
scheme for the first carrier (lower one). The carrier frequency can be lowered to the audible limit
of 20 kHz. As described above, according to the Paran of the present invention according to the
present invention, according to the IJ Sock-Peker, it is driven by two kinds of carrier waves of fl
and f2 (fl ?f2), and fl is modulated by a low pass filtered signal wave. As a result, (1) the
conversion efficiency in the low band is greatly improved and moreover, wide band reproduction
becomes possible, and (2) the sideband of the primary wave does not reach the audio frequency,
so that noiseless reproduction is possible. You can get any excellent effect.
Brief description of the drawings
FIG. 1 is a block diagram showing the configuration of a parametric speaker according to an
embodiment of the present invention, FIG. 2 is a structural view of an ultrasonic transducer, and
FIG. 3 is the sound of an ultrasonic transducer used in the prior art and this embodiment.
Pressure level frequency characteristic diagram, FIG. 4 is a characteristic diagram showing the
frequency spectrum of the primary wave and the secondary wave in the present invention, FIG. 5
is a sound pressure level frequency characteristic diagram of the secondary wave in one
embodiment of the present invention, Fig. 6 is a block diagram showing the configuration of a
conventional parametric speaker, Fig. 7 is a characteristic diagram showing the frequency
spectrum of the primary wave and secondary wave of the conventional parametric speaker, and
Fig. 8 is a primary wave and secondary wave. FIG. 9 is a characteristic diagram showing the
relationship between the sound pressure level, and FIG. 9 is a characteristic diagram showing the
frequency spectrum of the primary wave when the carrier frequency is low.
1 иииииии Ultrasonic transducer, 2 ииииииии Ultrasonic generator (speaker), 3 ииииии Audio signal source,
4.4a, 4b ииииииииииии , 5, 5a, esb ииииииии Carrier wave oscillator, 6 иииииии Power amplifier, 7 и и и и Acoustic
filter, 8 и и и и и и и и и и и и и и и и и и и и и и ... piezoceramic, 10 ..... resonator, 11 ..... terminal plate, 12 ......
coupling shaft, 13 ... ... low-pass filter, 14 ... ... Adder. Name of agent Attorney Nakao Toshio Other
1 Natsuka ? ? ? (9p) 4 (> '1 arc rumor X the number of shoulder plates Fig. 8 amount / next
Touhou 2 / to the number 9 shoulder plates 1st order Jdv ball bell (dB)
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