JPS6155696A - Sound wave absorbing body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field to Which the Invention Pertains] The present invention relates to a sound wave absorber for absorbing unnecessary sound waves propagating in a pipe in which an ultrasonic flowmeter is installed, for example.

[Prior art and its problems]

As shown in FIG. 6, the ultrasonic flowmeter is composed of a pair of ultrasonic transducers 2a and 2b arranged outside a pipe 1 that guides fluid with their positions shifted from each other in the direction of movement of the fluid. The time for the ultrasonic waves emitted from the first transducer 2a to reach the second transducer 2b, and the time for the ultrasonic waves emitted from the second transducer 2b to reach the first transducer 2a. The velocity of the fluid in the pipe 1 and the corresponding flow rate are determined based on the difference between However, when this ultrasonic flowmeter is applied to a small-diameter pipe (100 mm or less in diameter), a reflected wave propagates through the pipe 1 with multiple reflections, and these multiple reflected waves are combined with the sound waves that directly propagate within the fluid. Since the transducer receives the signal at the same time, there is a drawback that the signal cannot be determined with high accuracy. Therefore, this unnecessary multiple reflected wave propagating inside the pipe 1 is reduced, and the sound wave directly propagating inside the fluid has a high S.
/N ratio, it is necessary to be able to receive the signals at the transducers 2a and 2b and measure with high accuracy.
It is desirable that the surface of the pipe 10 around the pipes 1 and 2b has an acoustic impedance Z2 that corresponds to sound absorption and has a high sound absorption coefficient, and generally, as shown in FIG. Rubber (/Kotoba/Recon rubber) 4 with low impedance and high absorption rate and metal particles (for example, tungsten) 5 with a higher acoustic impedance than the pipe 1 uniformly mixed into the rubber 3. Can be configured.

Acoustic impedance Z is expressed by the following equation.

Z=ρ・C・・・・・・(1) Here, ρ is the density of the medium, and C is the sound speed of the medium.

However, this sound wave absorber 3, which is made by uniformly mixing metal particles 5 into rubber 4, has poor sound absorption characteristics compared to rubber, as shown in FIG. It has the disadvantage of being necessary. In addition, in FIG. 7, 6 is an unnecessary sound wave propagating inside the pipe 1.

[Purpose of the invention]

An object of the present invention is to provide a thin sound wave absorber that can match the acoustic impedance of the medium through which the sound waves to be absorbed propagate and has good sound absorption characteristics.

[Key points of the invention]

According to the invention, this object consists of a sound absorber having an acoustic impedance lower than the acoustic impedance of the medium through which the sound waves are propagating, and particles having an acoustic impedance higher than said medium mixed into this sound absorber; An acoustic impedance bonding layer in which the particles are mixed at a mixing ratio such that the acoustic impedance thereof substantially matches the acoustic impedance of the medium is formed on the side in contact with the acoustic impedance matching layer. This is achieved by a sound absorber characterized by a reduced mixing ratio of particles.

[Embodiments of the invention]

Next, the present invention will be explained in detail based on the embodiment shown in FIG.

In particular, the sound wave absorber 13 according to the present invention makes the acoustic impedance of the side in contact with the medium (piping) 1 through which the fll sound waves propagate substantially equal to the acoustic impedance of the medium 1 . Specifically, the former is about 0.8 to 1.5 times the latter.

+21 Considering that the side far from the medium (piping) 1 through which the sound waves propagate has a good sound absorption coefficient, it is composed of a mixture of rubber 14 and metal particles 15 whose mixing ratio decreases in the sound wave propagation direction. .

Now, as an example, pipe 1 is made of iron, rubber 14 is made of 7-recon rubber,
To explain the case where the metal particles 15 are tungsten, the acoustic customization of these materials is as shown in the following table.

Table 1 = Acoustic characteristics of iron, silicone rubber, and tungsten When the particle size of metal particles 15 is sufficiently smaller than one wavelength of a sound wave,
Sound wave absorber 1 in contact with piping 1 considering the above conditions (11)
Acoustic impedance matching% of 3 rubber at 161C
The mixing ratio X of the metal particles 15 and 15 is determined by the following formula.

< I X l l' 2C2+ X l' 5C3
= ρ + CI (21 Here, ρI is the density of pipe 1, ρ2
is the density of the rubber 14, ρ3 is the density of the metal particles 15, cl is the sound speed of the pipe 1, C2 is the sound speed of the rubber 14, sC3 is the sound speed of the metal particles 15.

When the values in Table 1 above are applied to this equation (2), the mixing ratio of the metal particles 15 becomes 224%. On the other hand, the sound velocity C of this sound wave absorber 13 is expressed by the following equation.

C= (IX) C2+ XC3-□□ f41
Applying the values in Table 1 above to this formula (4), we get
The sound speed C of the sound wave absorber 13 is 2193 m/+.

If the frequency of a sound wave is I MHz, the wavelength of a sound wave is 219
However, the particle size of the metal particles 15 must be sufficiently smaller than this wavelength, for example, about 50 μm.

Furthermore, in order to satisfy the above condition (2), the sound wave absorber 1
The mixing ratio X of the metal particles 15 of No. 3 is continuously lowered in the propagation direction of the sound wave as shown in FIG. It is formed of only rubber 14. The acoustic impedance of the sound wave absorber 13 in the sound wave propagation direction changes continuously as shown in FIG. 3, and the sound absorption characteristics correspond to the sound absorption characteristics of the rubber 14 as shown in FIG. As a result, the sound waves 6 propagating through the pipe 1 are sufficiently absorbed and attenuated within a short distance reaching the interface between the sound wave absorber 13 and the air 17. Note that the acoustic impedance matching layer 16 has an acoustic impedance of 0.8 to 1.5 of the acoustic impedance of the pipe 1, and can be acoustically well matched to the pipe 1.

Further, as shown in FIG. 2, the sound wave absorber 13 in which the mixing ratio of the metal particles 15 is changed is, for example, as shown in FIG. The metal particles 15 should be stirred with a stirrer 19 so that the mixing ratio of the metal particles 15 is uniform (see Fig. 5a), Fig. 5b), and the metal particles 1
Manufactured by curing the rubber 14 at a constant temperature of 4X times between curing salinity and curing 11.5 taking into account settling time so that the mixing ratio is as shown in Fig. 2 (m5 Fig. C). can.

〔Effect of the invention〕

According to the sound wave absorber based on the present invention, since the acoustic impedance matching layer is formed on the side in contact with the medium (piping) through which the sound waves are propagating, most of the sound waves can be guided to the sound wave absorber. Since the acoustic impedance continuously decreases in the direction of sound wave propagation and the sound absorption effect increases in the direction of sound wave propagation, there is no reflection of sound waves due to differences in acoustic impedance, and sound waves can be absorbed over a short distance. Therefore, a thin sound wave absorber with good sound absorption properties can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a sound wave absorber according to the present invention, and FIGS. 2 and 3 are diagrams showing changes in the mixing ratio of metal particles and acoustic impedance in the sound wave propagation direction of the sound wave absorber in FIG. 1, respectively. , FIG. 4 is a diagram showing the sound absorption characteristics of the sound wave absorber in FIG. 1, FIG. 5 is an explanatory diagram showing the manufacturing process of the sound wave absorber based on the present invention, and FIG. A perspective view of a pipe in which a wave flow meter is installed and a sound wave absorber is provided around it, Figure 7 is a sectional view of the conventional sound wave absorber in Figure 6, and Figure 8 is a cross-sectional view of the conventional sound wave absorber in Figure 7. FIG. 3 is a diagram showing the sound absorption characteristics of 1... Piping (medium through which sound waves propagate), 2a, 2b
...Ultrasonic transducer, 3.13...Sound wave absorber, 4
．． 14... Rubber (a plaster), 5.15... Metal particles,
16...Acoustic impedance matching layer. (6118) Agent Patent Attorney Tomimura b Ko 2 Tsushima 3 Ward Ko 4 Fig. 5 Ward (a, ) (b) (c)

Claims (1)

[Claims] 1) A sound absorber having an acoustic impedance lower than the acoustic impedance of a medium through which sound waves are propagating, and particles having an acoustic impedance higher than the medium mixed in the sound absorber, An acoustic impedance matching layer in which the particles are mixed at a mixing ratio such that the acoustic impedance thereof substantially matches the acoustic impedance of the medium is formed on the contacting side, and the acoustic impedance matching layer is formed continuously in the sound wave propagation direction from this acoustic impedance matching layer. A sound wave absorber characterized by a reduced mixing ratio of particles. 2) The sound absorber according to claim 1, wherein the sound absorber is rubber, preferably silicone rubber, and the particles are metal particles, preferably tungsten particles. 3) The sound wave absorber according to claim 1 or 2, wherein the side opposite to the acoustic impedance matching layer does not contain any particles. 4) In the sound wave absorber according to any one of claims 1 to 3, metal particles are mixed into molten rubber and stirred, and the curing temperature and curing are determined in consideration of the settling time of the metal particles. A sound wave absorber characterized by being made of rubber that hardens over time.