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JPH03265399

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DESCRIPTION JPH03265399
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
electroacoustic transducer for generating an audible sound by a non-linear parametric action. 2.
Description of the Related Art Conventionally, an electrodynamic speaker has been widely
adopted as an electroacoustic transducer for generating an audible sound. In order to give strong
directivity to a conductive speaker, it is necessary to increase the aperture or to use a large horn,
which causes a problem of enlargement. In addition, there is a limit to the effect of enhancing
directivity even if the aperture or horn is increased. As an electroacoustic transducer which
solves the problem of directivity in such a speaker of a conductive type, a parametric speaker
which generates audible sound as a secondary sound wave by the interference of a plurality of
primary sound waves with rising frequency It has been known. The parametric speaker generates
a secondary sound wave of audible sound from the primary sound wave according to the
following principle. That is, as shown in FIG. 5, two kinds of ultrasonic waves (Fl, F2) whose
frequencies are enhanced as primary sound waves (solid line) from a sound source 1 such as a
speaker or an ultrasonic transducer are output. Then, in addition to the original primary sound
wave, a difference component (F1-F2) of the frequency of both primary sound waves and a sum
component (F1 to F2) are generated due to the interference of both primary sound waves (FIG. 6)
If the frequency difference between the primary sound waves is an audible frequency, a
secondary sound wave (broken line) of an audible sound will be generated as a difference
component. Such a phenomenon is referred to as non-linear parametric action, and the directivity
of the generated secondary sound wave shows a pattern (elliptical pattern in FIG. 5) close to the
directivity of the primary sound wave. By using high-quality ultrasonic waves, directivity can be
enhanced even for secondary sound waves that are audible sounds. [Practical Problem to be
Solved by the Invention] On the other hand, the parametric speaker has a small output energy
amount of the secondary sound wave and a low conversion efficiency with respect to the input
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energy amount of the primary sound wave. In order to obtain a secondary sound wave at a level,
it is necessary to increase the sound pressure level of the primary sound wave. That is, it
becomes necessary to increase the number of sound sources and the like, resulting in an increase
in cost and a problem of temperature rise due to heat generation of the sound sources. Moreover,
since the energy of the primary sound wave which is an ultrasonic wave is large, it is necessary
to consider the influence on the human body. The present invention is intended to solve the
above problems, and by increasing the conversion efficiency from primary sound waves to
secondary sound waves, the input energy can be reduced more than before, and the influence of
the primary sound waves on the human body It is an object of the present invention to provide
an electro-acoustic transducer in which the temperature rise of the sound source is suppressed.
According to the first aspect of the present invention, there is provided an electric source which
generates a primary sound wave including a plurality of frequency components and which
outputs a secondary sound wave which is an audible sound by the non-linear parametric action
of the primary sound wave. In the acoustic conversion device, a sound guide tube disposed to
guide a primary sound wave sent from a sound source in a predetermined direction, and a fluid
medium in the sound guide tube between either end of the sound guide tube And a fan to be
According to the second aspect of the invention, flow velocity control means is provided for
controlling the fan so as to adjust the flow velocity of the fluid medium in the sound introducing
pipe. According to the construction of the first aspect of the present invention, since the sound
guiding tube for guiding the primary sound wave is provided, the primary sound wave interferes
sufficiently in the sound guiding tube and the radiation of the primary sound wave in the
unnecessary direction Is prevented. In addition, the side ropes of the secondary sound waves
generated in the sound guide tube are also reduced, and the directivity of the secondary sound
waves is enhanced. Furthermore, since the fan is provided to flow the fluid medium in the sound
guiding tube, the fluid serving as the medium flows around the sound source to cool the sound
source. According to the second aspect of the present invention, since the flow velocity control
means for controlling the fan to adjust the flow of the fluid medium in the front pipe is provided,
the flow velocity of the medium is adjusted by adjusting the flow velocity of the medium. The
traveling speed can be controlled, and the group velocity of the secondary sound waves
generated as a beat component can be controlled. That is, by controlling the group velocity of the
secondary sound wave in the sound guiding tube, the rate at which the secondary sound wave is
generated from the primary sound wave causing interference through the medium is controlled,
and the flow velocity of the medium The sound pressure of the secondary sound wave can be
controlled by the adjustment of. Embodiment 1 As shown in FIG. 1, a sound source 1 consisting
of an ultrasonic transducer is disposed together with a driving circuit block 3 in one end of a
cylindrical sound guiding tube 2. The sound source 1 is usually composed of a plurality of
ultrasonic transducers. An intake box 4 is attached to the other end of the sound guiding tube 2.
In the air intake box 4, an acoustic filter 5 is attached to a portion facing the front surface of the
sound source 1, through which the primary sound wave that is an ultrasonic wave from the
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sound source 1 does not pass and the secondary sound wave that is an audible sound. An audible
sound is taken out of the intake box 4 to the outside. The other end of the sound guiding tube 2
is inserted at the end opposite to the acoustic filter 5 in the suction box 4, and air is drawn
between the outer circumferential surface of the sound guiding tube 2 and the inner
circumferential surface of the suction box 4. The fan 6 is disposed. Further, since the abovementioned one end of the sound guide tube 2 is open, the air introduced into the air intake box 4
by the fan 6 causes the sound guide tube 2 to move from the right to the left in FIG. It flows and
is discharged from the sound guiding tube 2.
Thus, in the sound guiding tube 2, an air flow is formed in the opposite direction to the direction
in which the primary sound wave travels. As shown in FIG. 2, the circuit block 3 comprises an
audible sound processing circuit 11 for performing processing such as amplification in a form
necessary for modulation when an audible sound signal is input, and a carrier signal of an
ultrasonic fundamental frequency F0. A carrier wave oscillation circuit 12 to output, a
modulation circuit 13 that performs amplitude modulation with an audible sound signal on a
carrier wave signal, and an amplification circuit 14 that amplifies the output of the modulation
circuit 13 are provided. If the output of this amplification circuit 14 is input to the sound source
1, as shown in FIG. 3, the primary sound wave is output from the sound source 1 as an ultrasonic
wave having double-sided waves corresponding to the audible sound signal centered on the
fundamental frequency F0. As a result of the interference of the primary sound waves, secondary
sound waves, which are audible sounds, are generated as beat components of double-sided
waves. Here, the interference of the primary sound wave is performed in the space in the sound
guiding tube 2. As described above, since the air as the medium flows in the direction opposite to
the traveling direction of the primary sound wave in the sound guide tube 2, the wavelength of
the primary sound wave is compressed and the wave number becomes large. It becomes easy to
interfere in the space of 2. That is, the generation of secondary sound waves in a limited space is
facilitated. Moreover, since the sound source 1 is disposed in the air flow, the sound source 1 can
be air cooled, and the heat generated when outputting an ultrasonic wave can be efficiently
discharged to the outside, and the conversion efficiency by the sound source 1 can be reduced. It
can be kept in good condition. Furthermore, if the speed of the air flow is controlled, the sound
pressure of the secondary sound wave generated in the sound guiding tube 2 changes, so the
sound pressure of the secondary sound wave transmitted to the outside through the acoustic
filter 5 It can be done by speed control. That is, by controlling the intake capability of the fan 6
or providing a damper in the air flow path, a flow velocity control means is configured, and the
sound pressure of the secondary sound wave can be controlled by controlling the flow velocity of
the air flow. In the sound guiding tube 2, side lobes of a secondary sound wave (indicated by a
broken line, the solid line is a primary sound wave) are generated, but since radiation to the
outside is blocked by the sound guiding tube 2, an acoustic filter The secondary sound waves
taken out through 5 become highly directional. Further, by disposing the acoustic filter 5 in the
vicinity of the outlet of the sound guide tube 2, it is possible to prevent the primary sound wave
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from leaking to the outside without interfering with the generation of the secondary sound wave
by the interference of the primary sound wave. . Embodiment 2 In the first embodiment, the air
taken into the sound guide tube 2 is exhausted to the outside from one end of the sound guide
tube 2. However, in the present embodiment, as shown in FIG. By circulating the air flow in 7, the
external radiation of the primary sound wave is suppressed to reduce the influence on the human
body.
Further, by making the case 7 into a sealed container, it is possible to enclose a fluid other than
air in the case 7. That is, the sound source 1, the sound guiding tube 2, the circuit block 3 and
the acoustic filter 5 are disposed in the sealed case 7, and the space between the outer peripheral
surface of the sound guiding tube 2 and the inner peripheral surface of the case 7 And the fan 6
for circulation is disposed, and as indicated by the arrow also here, the fan 6 is rotated so that
the flow of the medium is formed in the direction opposite to the traveling direction of the
primary sound wave. Therefore, the secondary sound waves are efficiently generated in the
limited space as in the first embodiment. Also, some mediums with low sound velocity may
generate secondary sound waves better than air, so if such a fluid medium is enclosed in the case
7, the generation of secondary sound waves will be more efficient. It will be done well, improving
the efficiency that has been considered a drawback of parametric speakers. In order to extract
the secondary sound wave from the housing 7, a vibrating film or the like may be provided at a
portion corresponding to the acoustic filter 5. As described above, according to the first aspect of
the present invention, since the sound guide tube for guiding the primary sound wave is
provided, the primary sound wave interferes sufficiently in the sound guide tube and the primary
sound wave is unnecessary. Radiation in any direction is prevented. In addition, the side lobes of
the secondary sound waves generated in the sound guide tube are also reduced, which is
advantageous in that the directivity of the secondary sound waves is enhanced. Furthermore,
since the fan is provided to flow the fluid medium in the sound guiding tube, the fluid serving as
the medium flows around the sound source to cool the sound source. According to the second
aspect of the present invention, since the flow velocity control means for controlling the fan to
control the flow velocity of the fluid wax in the sound guide tube is provided, the progress of the
primary sound wave is adjusted by adjusting the flow velocity of the medium. The velocity can be
controlled, and the group velocity of the secondary sound waves generated as a beat component
can be controlled. That is, by controlling the group velocity in the sound guiding tube, it is
possible to control the rate at which the secondary sound wave is generated from the primary
sound wave causing interference through the medium, and by adjusting the flow rate of the
medium 2 The sound pressure of the next sound wave can be controlled.
[0002]
Brief description of the drawings
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[0003]
1 is a schematic block diagram showing Example 1 of the present invention, FIG. 2 is a block
diagram showing a circuit block used in the above, FIG. 3 is an operation explanatory view of the
same, and FIG. 4 is Example 2 of the present invention. FIG. 5 is a block diagram of a parametric
speaker, and FIG. 6 is an operation explanatory diagram of the parametric speaker.
DESCRIPTION OF SYMBOLS 1 ... Sound source, 2 ... Sound conduction pipe, 3 ... Circuit block, 4 ...
Intake box, 5 ... Acoustic filter, 6 ... Fan, 7 ... Case.
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