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JP2004064373

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DESCRIPTION JP2004064373
An acoustic reproducing apparatus for reproducing a highly directional audible sound by using
an ultrasonic wave solves the drawback that the distance from the ultrasonic wave generating
element to the reproduction of audibleness is too long. SOLUTION: An ultrasonic signal is
amplitude-modulated with an audio signal and emitted from a plurality of ultrasonic generating
elements arranged in a disk shape, and superdirectivity is realized in a desired direction using a
self-demodulation effect by nonlinearity of a medium. In the sound reproducing method, the first
distance L is set to a distance L perpendicular to the disk such that the angle ψ with respect to
the direction horizontal to the disk is ψ = arctan (2a / L) / 2 with respect to the disk The
ultrasonic wave is reflected by the reflecting plate to the plane including the disc, and the second
and third reflecting boards are provided on both the plane including the disc and the plane of the
distance L in the direction in which the first reflecting plate is parallel. The reflected ultrasonic
waves are multiply reflected, and the multiply reflected ultrasonic waves are reflected in a
desired direction by the fourth variable angle reflector to reproduce the sound in the desired
direction. [Selected figure] Figure 1
Sound reproduction device
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
acoustic reproducing apparatus that emits sound only in a desired direction, and in particular,
the property that an audible wave is demodulated in the process of propagation from an
ultrasonic wave amplitude-modulated by an audible sound. The present invention relates to a
sound reproducing apparatus which is miniaturized by forming a distance until it is sufficiently
demodulated by multiple reflection. [0002] In recent years, a video conference system (desktop
conference system) using a personal computer is becoming widespread. These conference
systems used regular speakers to play the other party's voice. [0003] However, ordinary
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speakers generally have a wide directivity, and it is possible to hear the other party's voice
(reproduced sound) not only in the person using the TV conference system but also in the
surroundings thereof. There was a problem that it was heard as an unnecessary noise around.
One possible way to solve this problem is to use a parametric speaker in which the ultrasonic
signal is amplitude-modulated with the reproduced sound signal and then emitted. The principle
of the parametric speaker and its superdirectivity will be briefly described below. FIG. 4 shows
the electrical circuit configuration of the parametric speaker. The ultrasonic signal generation
means 11 generates an ultrasonic signal having a frequency of, for example, about 40 kHz. The
ultrasonic signal is input to the amplitude modulation means 13 and amplitude-modulated by the
audio signal input from the audio signal input means 12 in the amplitude modulation means 13.
The amplitude-modulated ultrasonic signal is amplified by the amplification means 14, and the
amplified output drives the ultrasonic wave generator 15. The ultrasonic wave generator 15 has,
for example, a plurality of ultrasonic wave generating elements 15-1, 15-2, 15-3. .. 15-n can be
driven in parallel to generate strong ultrasonic waves. The plurality of ultrasonic wave generating
elements 15-1 to 15-n are mounted close to each other on the substantially circular printed
wiring board 16 as shown in FIG. 5, and all of them are connected in parallel by the printed
wiring formed on the printed wiring board 16 Be done. With such a structure, the parametric
speaker emits a strong ultrasonic wave (about 100 dB or more) and generates an audible sound
by using air non-linearity. Modulating a sinusoidal ultrasonic wave (sin (ωct): carrier) with an
audio wave s (t) (Eq. (1)) and radiating it into the air, the non-linear term of air is proportional to
the square of Eq. (1) The audible wave s (t) is demodulated as in equation (2).
(1 + s (t)) sin (ωct) (1) (1 + s (t)) <2> sin <2> (ωct) = (1 + s (t)) <2> (1-cos (2ωct)) / 2 = 1/2 + s (t)
+ s <2> (t) / 2- (1 + s (t)) <2> cos (2ωct) / 2 (2) The intensity of this audio wave s (t) gradually
increases with distance, When the ultrasonic generating element for emitting ultrasonic waves
has a disk shape, assuming that the radius is a, it increases to the Rayleigh length R defined by
the following equation (3), and then it is said to diffuse on the sphere and attenuate the audio
wave. (Y. Toshikura "Basics of nonlinear acoustics" Aichi Publishing (1996) p. 201). R = πa <2> /
λ (λ = f / c. F is the carrier frequency, c is the speed of sound (approximately 340 [m / s])) (3)
Also, Kamakura et al. It has been shown that if the distance of R / (2α) is taken with respect to
the absorption coefficient α for the case, it will be sufficiently increased and saturated
(Kamakura et al., “Study on practical use of parametric speaker” Journal of the Acoustical
Society of Japan Vol. 41 No. 6 ( 1985) pp. 378-385). However, in the case of a = 0.1 [m], f =
40,000 [Hz], R ≒ 1.2 [m], and furthermore, when the medium is air, α = 0.14, so the optimum
listening distance There is a problem that R / (2α) = 4.3 m, which is very large compared to a
normal speaker. In the present invention, an ultrasonic signal generating means, an audible
signal input means, and an audible signal inputted by the above-mentioned audible signal means
are generated by the ultrasonic signal generating means. Acoustic reproducing apparatus in
which a plurality of ultrasonic wave generating elements are disposed in a disk shape with an
amplitude modulation means, an amplification means for amplifying an ultrasonic signal
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amplitude-modulated by the amplitude modulation means, and a plurality of ultrasonic wave
generation elements for emitting the output of the amplification means as ultrasonic waves. The
distance L in the vertical direction to the ultrasonic wave generating element disposed in the disk
shape makes an angle に 対 し て with respect to a plane parallel to the disk, and ψ = arctan (2a
/ L) / 2 with respect to the radius a of the disk The first reflecting means is provided at such an
angle, and the second and third reflecting means and the variable angle are both on the plane
including the disc and on the plane parallel to the plane of the disc at a distance L from the plane
of the disc. Acoustic reproduction provided with a fourth reflection means Suggest a raw device.
The present invention further proposes an acoustic reproduction device in which an angle 同等
equivalent to that of the first reflector is added to both the second reflector and the third
reflector.
Operation According to the present invention, when the first reflector is given an angle ψ (=
arctan (2a / L) / 2) as shown in FIG. 1, light is incident on the perpendicular of the first reflector.
The ultrasonic wave is incident at the angle 、, and is reflected at the angle 鏡面 at specular
reflection, so it is incident on the second reflecting plate at the angle 2ψ and the reflection angle
is also 2ψ. The second reflector and the third reflector are parallel and reflect multiple times as
desired. Finally, the light is reflected in a desired direction by the fourth variable angle reflector.
By reflecting the light waves a desired number of times by the second and third reflecting plates,
the propagation distance of the sound wave can be increased even at a short distance. As a result,
the apparent distance from the sound emitting surface of the ultrasonic wave generating element
to the optimum listening position can be shortened. In particular, when both the second and third
reflectors have an angle ψ equal to that of the first reflector, the reflection angles of ultrasonic
waves at the second and third reflectors are acute angles. As a result, the third reflector can be
shortened. As a result, the apparent distance to the optimum listening position can be further
shortened. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a
sound reproducing apparatus according to the present invention. In the figure, 15 shows the
ultrasonic wave generator shown and demonstrated in FIG.4 and FIG.5. The ultrasonic wave
generator 15 has all the ultrasonic wave generating elements 15-1, 15-2, 15-3... 15-n mounted
on the printed wiring board 16 in a disk shape as described in FIG. The ultrasonic wave
generating elements 15-1, 15-2, 15-3... 15-n are connected in parallel and driven by the drive
circuit shown in FIG. In the present invention, the first reflection plate 21 is provided at a
distance L from the sound emitting surface of the ultrasonic wave generator 15. The first
reflection plate 21 has an angle に 対 し て with respect to a plane parallel to the sound emitting
surface of the ultrasonic wave generator 15. The angle と な る = arctan (2a / L) / 2 with respect
to the radius a of the disk of the ultrasonic wave generator 15 as the angle ψ. By causing the
first reflection plate 21 to have an angle = (= arctan (2a / L) / 2), ultrasonic waves are incident on
the perpendicular A of the first reflection plate 21 at an incident angle ψ. Do. When an
ultrasonic wave is specularly reflected by the first reflection plate 21, the ultrasonic wave is
reflected at an angle 入射, so that it is incident on the second reflection plate 22 at an angle 2 角
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度, and the reflection angle of the second reflection plate 22 also becomes 2ψ. The second
reflecting plate 22 and the third reflecting plate 23 are parallel, and perform multiple reflection
by a desired number of times M. If the number of reflections n is R / α (R is the Rayleigh length
(R = fπa <2> / C; f is the frequency of ultrasonic waves: C is the speed of sound) α: the
absorption coefficient of the medium) R / α From = R / α to n = (R / α-L) / x, and from the
figure x = L / cos 2 ψ, substituting the above ψ and rearranging gives the following equation (4).
n = (R / (αL) -1) cos (arctan (2a / L)) (4) L = 0.4 [m], 2a = 0.05 [m] as a practical size, of
ultrasonic waves Assuming that the frequency f = 40,000 [Hz] and the absorption coefficient of
air as a medium α = 0.14, the number of reflections n becomes n か ら 3 from the equation (4).
By reflecting the ultrasonic wave in an arbitrary direction by the fourth reflection plate 24 at the
fourth reflection, sound can be reproduced in an arbitrary direction. An example of the
dimensions L, W, and D of each part will be described with reference to FIG. The dimension of the
first reflection plate 21 is preferably about three times the size of the disc formed by the
ultrasonic wave generator 15. Therefore, assuming that the radius of the disc is a, the size of the
first reflection plate 21 is about 6a × 6a. Good. Assuming that L = 0.4 [m], 2a = 0.05 [m], W = 6a
= 0.15 [m], and the dimension D is that the length of the third reflection plate 23 is 6a + 6a. In
total, 18a = 6a × 3 = 0.15 × 3 = 0.45 [m]. These reflecting plates 21, 22, 23 can be made of
metal, glass, plastic or the like, and their surface is smooth and can be a surface on which
ultrasonic waves can be specularly reflected. FIG. 3 shows an embodiment of the sound
reproducing apparatus proposed in claim 2 of the present invention. In this embodiment, a case
is shown in which the second reflecting plate 22 and the third reflecting plate 23 have the same
angle as that of the first reflecting plate 21. In the case of this structure, the first reflecting plate
21, the second reflecting plate 22 and the third reflecting plate 23 are aligned in parallel with
each other, whereby the second reflecting plate 22 and the third reflecting plate 23 are arranged.
The reflection angle of the ultrasonic wave in the reflection plate 23 of the second embodiment
coincides with the reflection angle .PHI. In the first reflection plate 21 and is 1/2 of the reflection
angle 2.PHI. In the embodiment shown in FIG. The plate 23 can be arranged directly above the
second reflector 22. That is, the dimension corresponding to the dimension D shown in FIG. 2 is
12a in this embodiment. As a result, it is possible to obtain an effect that the apparent distance to
the optimum listening position can be further shortened. As described above, according to the
present invention, by providing the first to fourth reflecting plates 21, 22, 23, 24, the distance
required to demodulate an audible wave is reflected. Because of the configuration to be covered,
it is possible to obtain an excellent effect that is compact and capable of generating an audible
wave in a desired direction. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view for
explaining an embodiment of the present invention.
FIG. 2 is a perspective view for explaining an example of dimensions of each part of the
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embodiment shown in FIG. 1; FIG. 3 is a side view for explaining a modified embodiment of the
present invention. FIG. 4 is a block diagram for explaining an acoustic reproduction device using
ultrasonic waves. FIG. 5 is a plan view for explaining an example of an ultrasonic wave generator
used in the sound reproduction apparatus shown in FIG. 4; Description of the code 15 ultrasonic
wave generator 23 third reflector 21 first reflector 24 fourth reflector 22 second reflector
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