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JPS62200998

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DESCRIPTION JPS62200998
[0001]
FIELD OF THE INVENTION The present invention relates to a parametric speaker used as a
directional speaker system for expanding sound only in a limited listening area. 2. Related Art In
the prior art, there has been an extremely strong demand for making the directivity of sound
sharp and audible only in a limited range, such as the explanation of exhibits at exhibitions.
Although horn speakers have been conventionally used for such applications, there was a
drawback that the horn length of one aperture would be extremely large in order to make the
directivity sharp to a low frequency like the voice band using the horn speakers . On the other
hand, in recent years, a speaker (parametric speaker) using non-linearity of air to ultrasonic
waves has attracted attention because it can realize directivity much sharper than before. First, a
conventional parametric speaker will be described with reference to FIG. 1Q. In FIG. 10, reference
numeral 1 denotes an ultrasonic wave generator, which is driven at a high frequency modulated
by an audio signal (usually 25 to 80 are used). Next, the driver will be described. 7 is an audio
signal source, 8 is a high frequency transmitter, 9 is a modulator, and the high frequency signal
modulated by the audio signal is amplified by the power amplifier 10 to drive the ultrasonic wave
generator 1 Do. When the modulated ultrasonic waves are emitted from the ultrasonic generator
1 into the air at a finite amplitude level, the nonlinearity of the air is caused by the non-linearity
of the air, so that the original sound signal having sharp directivity is generated in the air. Here,
the modulated ultrasonic wave emitted into the air is referred to as a primary wave, and the
sound signal generated by its non-linear interaction 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 high primary wave sound pressure of about 130 to 150 dB is required
to obtain a practical secondary wave sound pressure. Therefore, when the parametric speaker is
put into practical use, in order to protect the listener from strong ultrasonic waves, the space
(parametric array) where the secondary wave is generated from the primary wave is sealed, or
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between the ultrasonic wave generator and the listener An acoustic filter 4 for blocking
ultrasonic waves is provided (e.g., JP-A-58-119293). However, when the parametric array is
sealed, the sharp directivity characteristic of the parametric speaker is lost. For example, as
shown in FIG. 11, attach an iron vibrator 11 with a diameter of 18 cm and a length of 1 m to an
ultrasonic generator 1 with a diameter of 16? and attenuate the primary wave (40 kHz) by
about a o dB When the acoustic filter 4 for attenuating the wave (1 k) h by only about 4 dB was
installed, the directivity of 1 k Hx became the characteristic shown by the broken line a in FIG.
The directivity characteristic is measured by moving the microphone 6 placed at a position of 1
m on the axis from the acoustic filter 4 in the X direction. -It can be seen that the directivity is
significantly wider than the characteristics when the iron pipe shown by the solid line in FIG. 2 is
not installed. Next, although only the acoustic filter is installed instead of sealing with an iron
pipe, although the directivity of the ultrasonic wave is sharp, as shown by the solid line a in FIG.
Since it is about 120 dB and an extremely large acoustic filter is required to block this, it is not
practical. When the acoustic filter 4 is small, a primary wave of about 120 dB directly hits the
listener. This level is not always safe for the human body. As described above in detail, in the
prior art, it is difficult to reduce the level of the primary wave received by the listener to a
sufficiently safe level (1 ood B or less) without losing the directivity. there were. An object of the
present invention is to provide a parametric speaker capable of attenuating the sound pressure
level of the primary wave without impairing the directivity of the secondary wave. Means for
Solving the Problems In order to solve the above problems, the parametric speaker according to
the present invention comprises an ultrasonic generator, and a secondary wave having a large
transmission loss of the primary wave emitted from the ultrasonic generator. The transmission
loss of the container is composed of a sealing container made of such a material as to be small.
Operation According to the present invention, according to the above-described configuration,
first, a strong modulated ultrasonic wave is emitted from the ultrasonic generator into the air to
reproduce the secondary wave in the air. A distance of at least a few meters is usually required
for the generation of a sufficient secondary wave, but in practice it is necessary to use a
container with an appropriately sized space on the front of the ultrasonic generator, due to
installation space constraints etc. It seals it and prevents the primary wave from leaking outside.
However, usually in the vicinity of the sound axis of the closed container, the level of the primary
wave is still relatively high, and a laminated structure in which thin plastic films and foams are
alternately stacked is often used for the closed container. . Here, if a material that shuts off the
primary wave and hardly blocks the secondary wave is used as the characteristic of the closed
container, the primary wave can be shut off without affecting the directivity characteristics of the
secondary wave. Next, this point will be described in more detail. In the parametric speaker, the
secondary wave is generated as a result of waveform distortion caused by nonlinear interaction
of the primary wave, and hence the secondary wave is generated in the absence of a strong
primary wave. Does not occur.
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For example, suppose that the secondary wave is eliminated only in a circular space which is a
broken circuit surrounded by a broken line a in FIG. Assuming that a part of the space is sealed
in a rectangular cross-sectional shape so as to cut a circular swelling portion like a broken line by
an ideal sealed container that completely shuts off the primary wave, the secondary wave The
space generated is limited within this enclosed space. However, if the size of the enclosed space
covers most of the secondary wave generation area, there is almost no change in the energy of
the secondary wave generated regardless of the presence or absence of the enclosed space. If
there is no attenuation with respect to the frequency, there is no change in the sound pressure
level of the secondary wave at each point in the space outside the container, and as a result,
there is no change in directivity either. Next, assuming that the closed container blocks the
secondary wave, the secondary wave does not appear outside the container, but among the
closed containers, only the listening area near the sound axis is secondary as shown in FIG. It is
an acoustic filter that transmits waves, and the side wall is such as to block secondary waves. In
this case, there is no change in the energy of the secondary wave generated in the container, but
the secondary wave generated is repeatedly reflected on the wall of the container because the
closed container has sound insulation with respect to the secondary wave. It is radiated to space
through the filter. For this reason, as in the case of using an iron pipe for the side wall, the
sharpness of directivity, which is a feature of the parametric speaker, is lost. According to the
present invention, the primary wave is not affected on the directivity characteristics of the
secondary wave by using a material that blocks the primary wave on the side wall of the closed
container but has almost no transmission loss for the secondary wave. It can be shielded. An
embodiment of the present invention will be described with reference to the drawings. FIG. 1
shows the configuration of a parametric speaker in a first embodiment of the present invention.
The basic configuration of the drive unit is the same as that of the conventional example shown
in FIG. In FIG. 1, the ultrasonic wave generator 1 is an array of 190 piezoelectric ceramic
ultrasonic transducers arranged in a substantially circular shape, having a diameter of about 16
crn and a drive frequency of 40%. A perforated metal pipe 2 having a diameter of about 18 crn
and a length of 1 rrL is attached as a side wall to this, and further, a sealing side wall 3 formed of
various sealing materials is wound around the outside. Further, at the end of the pipe, an acoustic
filter 4 in which five layers of urethane foam having a thickness of 16 ?m and a polyethylene
film having a thickness of 20 ?m are alternately laminated is installed. Next, directivity
characteristics of the parametric speaker having the above configuration will be described. The
directivity characteristic is measured by moving the microphone 6 provided at 1rrLO from the
end of the pipe on the sound axis horizontally in the direction perpendicular to the sound axis (in
the X direction).
An example of the result is shown in FIG. Placing the acoustic filter 4 on the sound axis
corresponds to shortening of the parametric array, which causes a decrease in the secondary
acoustic pressure level and impairs the sharpness of the directional characteristics as compared
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to the case where the acoustic filter 4 is not placed. This can be solved in principle by increasing
the installation position of the filter. There is almost no change in directivity due to the
installation of the punching metal pipe 2, and the difference in directivity in FIG. 2 can be
regarded as the difference in the sealing material. That is, in FIG. 2, b indicates the directivity
when using a material in which urethane foam and a polyethylene film are stacked, and the
directivity is sharper than the directivity when using only punching metal, rather than the
directivity C. On the other hand, when the 100 ?m film of polyvinyl chloride is used, the
directivity characteristic d is clearly impaired in directivity. Therefore, in order to study the
material of the side wall of the closed container, the measurement results of the sound
transmission loss of various materials are shown in FIG. The numbers in 0 indicate the thickness
of the material, and the unit is (?m). In general, the larger the transmission loss of the primary
wave, the larger the transmission loss of the secondary wave tends to be, but the transmission
loss of the primary wave is more pronounced for a single layer film or a laminate of these than
urethane foam. It can be seen that a large secondary wave transmission loss can be realized.
Detailed characteristics of the directivity of the secondary wave and the secondary wave
transmission loss of these materials are shown in FIG. In FIG. 3, the directional characteristic is
indicated by the attenuation of the sound pressure level at the position of X = 1771. As a result,
there is almost no change in directivity when the secondary wave attenuation is 1 dB or less, and
it may be sharper, but the directivity is rapidly lost if the amount of reduction in resources
exceeds 2 dB. In either case, the primary wave, which was about 140 dB at the maximum when
the sealed container is not provided, attenuates to about 100 dB as shown in b of FIG. Therefore,
it can be said that the secondary wave transmission loss of the side wall needs to be at most 3 dB
or less. As described above, according to the present embodiment, the level of the primary wave
can be significantly attenuated without impairing the directivity characteristic, and the safety of
the listener can be secured. In this embodiment, since the listening point is set to 1 m from the
acoustic filter, the primary wave transmission loss of the acoustic filter is larger than that of the
side wall, but the listening point is far and the primary wave is sufficiently attenuated. If this is
recognized, the same side wall may be used. Next, a second embodiment of the present invention
will be described with reference to FIG.
In FIG. 4, 1 is a 60 О 25 m ultrasonic generator made using an ultrasonic transducer of the same
configuration as that of the first embodiment, and 2 to 4 are respectively the same as those of
the first embodiment. And the acoustic filter 4 is a side wall for sealing. It is provided on the
extension of the tip of the punching metal 2. Reference numeral 6 denotes a reflector for
reflecting secondary waves generated in the container in the direction of the acoustic filter 4. The
parametric speaker often was restricted by the installation location in order to require a
considerably large space for the generation of the secondary wave, but the use of the reflector
plate 6 dramatically increases the degree of freedom associated with the installation As shown in
FIG. 5, the parametric speaker 24 can be installed so that the listener 23 can hear even on a
general floor where the height from the ceiling 21 to the floor 22 is low. Next, a third
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embodiment of the present invention will be described with reference to FIG. In FIG. 6, the basic
structure is the same as that of the first embodiment except that the cross section of the side wall
increases with distance. Since the sound generally spreads with distance, the shape of the
parametric array also spreads with distance as shown by the broken line in FIG. However, in the
first embodiment, the acoustic film 4 attenuates the primary wave by about 40 dB, but the
material of the closed vessel does not attenuate 15 to 2 odB1. Therefore, the sound pressure
level of the primary wave measured on the X-axis becomes as shown in the characteristic of FIG.
7, and a swell occurs in the vicinity of the characteristic indicated by the arrow A. This is because
in the portion B of the closed container, the blocking performance of the primary wave is
insufficient. Therefore, in the present embodiment, the sound pressure level of the primary wave
is as shown by the characteristic C in FIG. 7 because the cross-sectional area of the side wall is
made thicker with distance and the acoustic filter is enlarged corresponding to the opening of the
side wall. As a result, the primary wave can be attenuated as a whole, and the safety can be
improved. It is to be noted that instead of changing the cross sectional area of the side wall, the
material is changed by changing the material near the end B of the cylindrical side wall as shown
in FIG. You can get the effect of In the above embodiment, although the secondary wave
transmission loss has been described by taking one for example, in the case of a parametric
speaker, in particular, the sound pressure level of 1 kHz or less and the directivity become a
problem in principle. In the above, the above conditions should be satisfied. Also, if the diameter
of the closed container is considerably larger than that of the ultrasonic wave generator, or if the
diameter of the closed container is larger than the length, it is pointed even if the secondary
wave transmission loss of the closed container is somewhat large (It is natural that the influence
is small, but the size of the speaker is always required to be miniaturized. Therefore, in most
cases, the diameter of the closed container should be almost equal to the diameter of the
ultrasonic wave generator. It can be said that the present invention is extremely effective because
Further, the side wall may be formed of a thin film having a thickness of 100 ?m or less or a
plurality of thin films laminated via an air layer, and the air layer may be formed using a porous
body having a small secondary wave transmission loss. Good. As described above, according to
the present invention, the ultrasonic wave generator, the acoustic filter, and the sealing side wall
for blocking the primary wave and transmitting the secondary wave are received without
impairing the directivity of the secondary wave. It is possible to attenuate the sound pressure
level of the primary wave received by the listener, and further increase the cross-sectional area of
the sealing side wall with distance, or transmit part of the material of the sealing side wall to the
primary wave. By changing to a lossy material, the primary acoustic pressure level can be further
reduced.
[0002]
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Brief description of the drawings
[0003]
FIG. 1 is a block diagram of a parametric speaker according to a first embodiment of the present
invention, FIG. 2 is a characteristic diagram showing a difference in secondary wave directivity
characteristics due to differences in side walls, and FIG. 3 is a secondary wave of material of side
walls Characteristic diagram showing the relationship between the transmission loss and the
directivity of the secondary wave, FIG. 4 is a block diagram of the second embodiment of the
present invention, and FIG. 5 is the case where the parametric speaker of the second embodiment
is mounted on the ceiling FIG. 6 is a block diagram of the third embodiment of the present
invention, FIG. 7 is a characteristic chart showing the relationship between the primary wave
transmission loss of the sidewall material and the directivity of the primary wave, and FIG. Fig. 9
is a characteristic diagram showing the relationship between the transmission loss of the primary
wave and the secondary wave of the sealing material for various side walls, and Fig. 10 is a
conventional parametric speaker. Fig. 11 is a block diagram of a parametric speaker using iron
eve on the side wall, Fig. 12 Primary wave directivity characteristic diagram of FIG. 13 is a block
diagram showing the relationship between the space to be sealed with the parametric array.
1 ...... ultrasonic generator, 2 ...... punching metal pipe, 3 ...... sealing side wall, 4 ...... acoustic filter,
6 ..... и Microphone, 6 ииииии Reflector. Name of agent Attorney Nakao Toshio and 1 other person
Figure 1 Figure 2-t, o-a, s l) a, st, Q distance from the axis of T (? 2 Figure 3; ? ?? ?? I;; S 2
X X 5 & t j E ? ? ')) 1 1 ((X-1 m) (dB) Fig. 4 Fig. 7 Fig. 8 -? i 2 ? ? ?? (fKHv) (d-?) ? 12
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