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FIELD OF THE INVENTION The present invention relates to acoustic wave receivers used in
liquids. [Prior Art] Conventionally, a solid-state element such as a piezoelectric element has been
used as a receiver of ultrasonic waves in liquid, and for example, a resonant type is used for
receiving a continuous wave of a single frequency or a long pulse wave. Wavers use-any. These
elements have a sufficiently high sound pressure and are suitable for receiving a sound wave
having a short wavelength, but if one frequency is lowered, the displacement of the elements can
not be taken large and the sensitivity is lowered. [Problems to be Solved by the Invention] In
recent years, a method using low frequency sound waves has been developed for transmitting
and receiving sound waves in the ocean or water, but in the conventional solid-state element, the
displacement of the receiving surface is limited Low frequency sound waves have the
disadvantage of being unable to increase the reception sensitivity 1 degree. In addition, there is
also a problem in that the matching between the medium and the acoustic impedance of the
wave receiving element is poor and the sensitivity is lowered. The object of the present invention
is to eliminate such drawbacks and to provide a receiver having good delivery performance for
low frequency sound waves. [Means for Solving the Problems] The present invention relates to a
receiver for receiving ultrasonic waves in a liquid, comprising: a nonmagnetic cylinder for
causing sound waves to enter from the liquid through the opening; and a magnetic fluid held in
the cylinder A receiver comprising: a magnet for aligning the magnetization of dispersed
ferromagnetic fine particles in a magnetic fluid in parallel with the cylinder; and a detection coil
for detecting a change in magnetic flux generated in the magnetic fluid. [Operation] The
operation of the present invention will be described below with reference to the drawings. FIG. 3
is a schematic view showing the operation of the present invention. As shown in the
magnetization curve of FIG. 3 (a), the magnetization of the magnetic fluid is saturated under a
magnetic field of more than -chamber. In the state where the magnetic field H in a fixed direction
is applied, the ferromagnetic fine particles P in the magnetic fluid align and align the magnetic
moment in one direction as shown in FIG. At this time, the sound wave incident from the liquid 8
side to the magnetic fluid 3 whose one side is closed as shown in FIG. And, for sound waves of a
specific wavelength, resonance occurs at a frequency determined by the cylinder length,
viscosity, and density. At this time, the ferromagnetic fine particles P in the fluid vibrate at the
same frequency due to the standing waves present in the magnetic fluid 3. The displacement d of
the particle is shown in FIG. Since the dispersed fine particles in the magnetic fluid 3 have a
single magnetic domain structure, an induced current is extracted through the amplifier 7 to the
coil 2 circumscribed to the cylinder 4 accommodating the magnetic fluid 3 along with the change
in magnetic flux with the same frequency as the sound wave. . Incidentally, since the magnetic
fluid can be greatly deformed, even if low frequency sound waves are arranged to generate a
standing wave by changing the above parameters, sufficient reception sensitivity can be
Further, as the magnetic fluid, one in which ferromagnetic fine particles are dispersed in a
solvent insoluble in a liquid to be used with little ultrasonic loss can be used. The present
invention will be described in detail with reference to the drawings. FIG. 1 is a longitudinal
sectional view showing an embodiment of the present invention. The magnetic fluid 3 is filled in
the cylinder 4 which is open at one end, and the holding of the magnetic fluid 3 and the magnet
1 for aligning the magnetization parallel to the axis of the cylinder around the cylinder 4 are
included in the magnetic fluid A detection coil 2 for detecting a change in magnetic flux of
ferromagnetic particles is provided. The magnetic fluid 3 is in contact with the liquid 8 at the
interface 6 of the opening of the cylinder 4, and the horn 5 is attached to the opening of the
cylinder 4. 7はアンプである。 The induced current obtained in the detection coil 2 is amplified
by the amplifier 7 and extracted as a detection signal. The sound waves propagated in the liquid
8 can easily enter the magnetic fluid 3 because the acoustic impedances of the magnetic fluid 3
and the liquid 8 are almost equal. The sound wave reflected by the end face of the cylinder 4
resonates with the cylinder 4 at a specific wavelength to generate a standing wave, and the
magnetic flux change accompanying the vibration of the ferromagnetic fine particle at that time
is detected by the detection coil 2 Ru. As shown in FIG. 2 (b), in FIG. 2 (a), an electromagnet is
used for the magnet 1 and a magnetic field H is applied in the direction shown in the figure.
Shows the frequency characteristics of the output voltage of the coil when a sound wave is made
to change. The oscillator 10 was excited by the oscillator 9 and its vibration was transmitted to
the water 8. As apparent from FIG. 2 (b), it can be seen that the output has a peak at the
resonance frequency. Although this example shows an example of the output at the first order
resonance, the same output can be obtained for higher order resonances. And, by changing the
cylinder length, it is possible to receive the sound wave of the target frequency. Also, the
frequency of the sound wave can be selectively received by making the cylinder length variable
and changing it continuously. [Effects of the Invention] As described above, according to the
present invention, the use of a magnetic fluid as a vibrator for receiving waves has the effect of
being able to receive low-frequency sound waves with high sensitivity.
Brief description of the drawings
FIG. 1 is a cross-sectional view showing an embodiment of the present invention, and FIGS. 2 (a)
and 2 (b) are the same schematics showing the embodiment <(a) is a frequency dependent
waveform diagram of detection coil output, b) is a structural view of the embodiment, FIG. 3 (a),
(b) and (c) are schematic diagrams showing the operation of the present invention, and (a) is a
diagram showing a magnetization curve of magnetic fluid, (b) Is a schematic view showing the
orientation of particles, and (c) is a schematic view showing a standing wave generated in a
magnetic fluid in a cylinder.
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