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FIELD OF THE INVENTION The present invention relates to a speaker using an ultrasonic parm)
IJ hook fruit, and in particular to a parametric speaker having a directivity that is most suitable
for explanation of exhibits at exhibitions and guidance broadcasting at station homes. It is a
thing. Due to the construction of the prior art and its problems, in the case where sharp
directivity is required for a loudspeaker without a loudspeaker, a loudspeaker is mainly used.
However, the directivity of the horn speaker strongly depends on the length and aperture of the
horn, and there is a disadvantage that the horn must be extremely large, in particular, in order to
obtain sharp directivity in the low frequency range. On the other hand, in recent years, a speaker
using a parametric effect, which is one of the nonlinear interaction of ultrasonic waves, is
attracting attention because it can obtain directivity much sharper than the linear region. First, a
conventional example of a speaker using a parametric effect (hereinafter referred to as a
parametric speaker) will be described with reference to FIG. In FIG. 1, reference numeral 1
denotes an ultrasonic transducer using a ceramic voltage transducer having a bimorph structure.
The diameter is 11.5ffff, the center frequency is 140KHz, and the efficiency is 113dB at 1m on
an axis at 10V input. The transducers 1 are arranged in a honeycomb form and arrayed to
constitute a sound source 2 for parametric speakers. The signal from the audio signal source 3 is
AM-modulated by the modulator 4 and input to the sound source 2 through the power amplifier
5. The frequency of the carrier wave (hereinafter referred to as the primary wave) is 40 KHz. The
primary wave and the sideband wave emitted from the sound source 2 interfere due to the nonlinearity of air, and a modulated wave (hereinafter referred to as a secondary wave) having a
sharp directivity in the air is generated. By the way, since the parametric speaker is a system
which tries to obtain an audible sound which is a secondary wave from a primary wave by the
non-M oil property of a medium (for example, air), its conversion efficiency is extremely low. For
example, in order to obtain a secondary wave sound pressure of about 90 dB which is a practical
level, a strong primary wave sound pressure of 140 dB or more is required. Direct exposure of
the human body to such strong ultrasonic waves is known to cause adverse effects such as
hearing loss, dizziness, etc. MAY 1974 P, 124) Therefore, when actually listening to a parametric
speaker, it is possible between the speaker and the person It is essential to provide an acoustic
filter that attenuates only the primary wave that is an ultrasonic wave and does not affect the
sound pressure level or directivity of the secondary wave. 1 ° Conventionally, cloths and felts
are used as acoustic filters. Some material properties, such as glass wool, absorb and damp
specific bands, and some attempt to attenuate structurally specific frequencies, such as hollow
silencers, but 4QKH2 The design of cavity type valve vibrators is difficult at such high
frequencies, and the sound absorbing material is made for the purpose of attenuation of audible
sound, which makes it unsuitable as an acoustic filter for use in parametric speakers. It had an
intermittent point.
OBJECTS OF THE INVENTION 1) The present invention solves the above-mentioned interval m,
effectively attenuates the primary wave, and uses an acoustic filter that hardly affects the sound
pressure and directivity of the secondary wave. An object of the present invention is to provide a
safe and directional directional parametric speaker. The present invention comprises an
ultrasonic source and an acoustic filter provided at a predetermined distance in front of the
ultrasonic source, the acoustic filter comprising a plurality of paper or plastic films with a
predetermined spacing. It has a stacked structure, which attenuates only the ultrasonic noise,
does not attenuate the audible sound, and does not affect the directivity characteristics.
Description of the Embodiment FIG. 2 shows the configuration of the parameter IJ '7' force
according to the embodiment of the present invention. In FIG. 2, 21 is a sound source consisting
of arrayed ultrasonic transducers, 22 is a 18 μm thick polyethylene film (hereinafter referred to
as a film) and has three layers of thickness 1 c 1 +117) shee ) 23 The separated acoustic
filter 24 is configured, and the distance between the sound source 21 and the sound / resonance
filter 24 is 1.6 m. A microphone 26 is provided at a distance of 1 m from the acoustic filter 22. In
the above configuration, the directional characteristics of the primary wave and the secondary
wave whose sound pressure levels were measured by moving the microphone 25 parallel to the
acoustic filter 24 along the X axis are shown in FIGS. 3 and 4 respectively. . In FIG. 3 and FIG. 4, a
person shows the directional characteristics when the acoustic filter 24 is not used, and B shows
the directional characteristics when the acoustic filter 24 is used. The horizontal axis indicates
the movement distance when the sound wave central axis X-x 'of the sound source 21 is zero. As
shown in FIGS. 3 and 4, when the acoustic filter 24 is used, the sound pressure level of the
primary wave is attenuated by 25 dB or more compared to when it is not used, while the
secondary wave (1 KHz) is about 2 dp There is no change in the directivity characteristics. By the
way, an acoustic filter used for a parametric speaker can not obtain a sufficient secondary wave
sound pressure unless it is installed at a distance of about 1 m or less from the sound source. It
needs a size to say. In this case, it is difficult to keep the pui film 22 as described above at a
constant interval, and the intertwined portion is inevitably attached to each other. However,
when it is stuck, it is the same as when one thick film 22 is worn, and the attenuation of the
secondary wave becomes large.
On the other hand, the film 22 can be prevented from sticking when the film 22 is applied under
strong tension, but the film 22 vibrates just like this skin at the frequency where standing waves
occur, and the sound pressure frequency and characteristics are comb-like sharp Not only does
this cause irregularities, but the sound quality deteriorates, the film 22 attenuates the secondary
wave in order to reflect the sound well. That is, it is better not to apply tension to the film 22 at
all. Then, as shown in FIG. 5 as a 2nd Example, the 2nd spacer 26 which cut ¦ disconnected the
flexible polyurethane foam in the grid ¦ lattice form was inserted between the films 22, and the
acoustic filter 27 was comprised. The material of the grid-like spacer 26 may be wood, hard
plastic or the like, but the hard material reflects ultrasonic waves and disturbs the secondary
wave sound source, so the material of the spacer 26 is a material with little reflection and good
sound absorption. It is desirable to have one. Further, it is desirable not to adhesively fix the gridlike spacer 26 to the film 22. As a result, the interval of 22 is kept constant, and the performance
as the acoustic filter 27 is not degraded at all. In the present embodiment, the case where the
film 22 is formed into three layers has been described, but it goes without saying that multiple
layers may be used, and the film material may be another plastic film or paper as well. The effect
of As described above, according to the present invention, the following effects can be obtained.
(1) By adopting an acoustic filter that lowers the primary wave to a level safe for the human body
and has almost no effect on the sound pressure level and directivity of the secondary wave It is
possible to eliminate the influence on the human body by the strong ultrasonic waves, which is a
conventional defect, without losing the sexuality. (2) By inserting a lattice-like spacer between the
films, the installation direction can be arbitrarily selected without a decrease in filter
performance due to the films sticking together. (3) To prevent the reduction of the sound
pressure level of the secondary wave and the disturbance of the directional characteristics by
using a lightweight material such as soft urethane foam that transmits the secondary wave
without reflection as the lattice-like spacer Can.
Brief description of the drawings
1 shows the principle of the parametric speaker, FIG. 2 shows the arrangement of the sound
source of the parametric speaker and the acoustic filter in the first embodiment of the present
invention, and FIG. 3 shows the presence or absence of the acoustic filter. FIG. 4 is a diagram
showing the sound pressure characteristics of the secondary wave, and FIG. 5 is a diagram
showing the structure of the acoustic filter in the second embodiment.
1 ······· Ultrasonic transducer, 2.21 ··· ··· Sound FA, 22 ··· Film · · · · · · · · · · · · · · · · · · · · · · · · Acoustic
filter, 25 ······ Microphone, 26 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · microphones, 26 · · · · · · · · · · · ·
spacer shaped lattice. Name of Agent Attorney Nakao Toshio 1 person Figure 1 Figure 2 X Figure
3 Figure 4 Life Range C town ry retreat (town