JPH0654437B2 - Acoustic lens - Google Patents

Description

TECHNICAL FIELD The present invention relates to an acoustic lens, and more particularly to an acoustic lens that converges sound waves in a predetermined frequency band.

[Background Art] In recent years, traffic noise generated when a vehicle passes along railways and roads has become a problem, and measures such as installing a sound barrier have been taken, but sound waves are diffracted. Not enough soundproofing effect. Further, with the mechanization of office processing, that is, the so-called OA, there are more opportunities to use devices such as computers and word processors in offices, and dot impact printers are frequently used as output devices. Such a dot impact printer produces a large amount of noise during operation, and since it is impossible to prevent the sound waves from being diffracted by a partition wall or the like, deterioration of the office environment due to noise is a problem. Furthermore, the noise of the air-conditioning system fan that blows off the wind, the noise of the motor, etc. are also sources of noise, and the air-conditioning system cannot block the air inlet and outlet and surround it with a wall.
Sound insulation cannot be performed. As described above, the conventional sound insulation devices have not sufficiently considered the sound wave diffraction, and in that sense, the sound insulation effect is not sufficient.

On the other hand, a device for converging a sound wave while transmitting the sound wave has been considered. That is, as shown in FIG. 14, a combination of a plurality of inclined plates 2 and a combination of a plurality of obstacles 3 as shown in FIG. 15 are provided. Provide multiple routes inside,
The sound waves are converged by using the difference in the path length. In this device, since the path length is related to the converged wavelength, there is a problem that the converging frequency band is narrow, and the size of the sound wave in the low frequency range is large as a whole. In addition, there is a problem that it cannot be applied to a sound source that requires a large aperture ratio (for example, when air permeability such as air conditioning is required).

[Object of the Invention] The present invention has been made in view of the above points, and its main purpose is to focus a sound wave only at a specific point so that a sound is generated only at a specific location. Another object of the present invention is to provide an acoustic lens that enables hearing and avoids propagation of sound waves to unnecessary places. Another object is to provide an acoustic lens that can have a relatively large aperture ratio. Especially.

DISCLOSURE OF THE INVENTION (Structure) In an acoustic lens according to the present invention, a straight tubular hollow tube formed of a material having viscoelasticity is used as a control element for controlling a phase velocity of a sound wave, and a plurality of control elements are provided. Are arranged such that their axial directions are substantially parallel to each other, and other control elements are arranged on a concentric circle centered on one control element in the plane orthogonal to the axial direction of the control element, and The closer the control element is, the smaller the phase velocity of the passing sound wave is set. With this configuration, a relatively large aperture ratio is formed, and the sound wave is converged to prevent the sound wave from propagating to unnecessary places. .

(Basic Principle) First, the basic principle of the present invention will be described. In the explanation below,
As shown in FIG. 8, a straight tubular hollow cylindrical body is used as the control element 1, but the control element 1 is not limited to the cylindrical body, and a tube having another cross-sectional shape may be used. Such control element 1
When the sound wave passes through the inside, the tube wall of the control element 1 vibrates as shown by the arrow in FIG. However, the unit length portion of the hollow body whose both ends are open can be considered as an equivalent electric circuit as shown in FIG. That is, the acoustic characteristics of the unit length of the control element 1 are determined by the inertance La of the gas in the tube, the acoustic capacitance Ca of the gas in the tube, the inertance Lw of the tube wall, the compliance Cw of the tube wall, and the conductance Gw of the tube wall. It is. The inner radius of the pipe is r, the wall thickness of the pipe is t, the inner surface area per unit length of the pipe is S, and the mass per unit length of the pipe is m (=
2πrρwt: ρw is the density of the tube wall), Young's modulus of the tube wall is E,
Κ is the proportional constant of the tube wall conductance, ρ is the density of the gas in the tube, K is the bulk modulus of the gas in the tube, and ω is the angular frequency of the sound wave.
(= 2πf: f is the frequency of the sound wave), each parameter is represented by the following equation.

La = ρ / S Ca = S / K Lw = m / 4π ² r ² Cw = 2πr ³ / Et Gw = κω Therefore, the resonance frequency fr of the control element 1 is expressed by the following equation.

fr = 1 / (2π (Lw · Cw) ^1/2 ) = r / (m · Cw) ^{1/2 If the} equivalent conductance Geq and the equivalent capacitance Ceq represented by the following equations are introduced into the above-mentioned equivalent electric circuit,
The equivalent electric circuit of FIG. 9 can be further replaced by the equivalent electric circuit of FIG.

If the phase constant β of the equivalent electric circuit of FIG. 10 is obtained, that is, the phase constant of the sound wave passing through the pipe is obtained, and the phase velocity Vp can be defined from the phase constant β.

As is clear from the above equation, the equivalent conductance G
Since eq and the equivalent capacitance Ceq have a singular point at the resonance frequency, the phase velocity Vp changes greatly near the resonance frequency, as shown in FIG.
Further, since the phase velocity Vp is obtained for the unit length, it is clear that it is a function of the length of the control element 1. Further, the phase constant β includes the tube wall density, Young's modulus, and inner diameter. Therefore, it can be seen that these functions are obtained. FIG. 12 shows the result of experimentally obtaining the frequency characteristic of the phase velocity for this control element 1. in this way,
It can be seen that the phase velocity changes greatly at a specific frequency. Here, as shown in FIG. 13, the measuring device includes a speaker 11 that sends out a sound wave from one end of the control element 1 into the control element 1, and a speaker 11 that is inserted into the control element 1 and located in one place inside the control element 1. A probe microphone 12 for picking up sound and a speaker 1
The signal generating circuit 13 and the amplifying circuit 14 for generating the electric signal input to the unit 1, the measuring amplifying circuit 15 for amplifying the sound picked up by the probe microphone 12, and the phase of the sound output from the measuring amplifying circuit 15 A phase meter 16 for detecting,
It is composed of an arithmetic control unit 17 which controls the frequency of the signal output from the signal generating circuit 13 and records the output of the phase meter 16 to calculate the phase velocity in the control element 1. Here, the arithmetic and control unit 17 is configured by using a microcomputer or the like. The control element 1 is a silicon having a Young's modulus of 7.0 × 10 ⁵ [N / m ² ] (measured by a complex elastic modulus measuring device, that is, a viscoelastic spectrometer) and a density of 1190 [kg / m ³ ]. An inner diameter of 25 mm and a tube wall thickness of 0.3 mm were used.

If the above-mentioned characteristics are utilized, a phase velocity inside the control element 1 and a phase velocity outside the control element 1 are different from each other. Therefore, the sound wave can be refracted by using this control element. In the following embodiments, the sound waves are focused by using a plurality of the cylindrical control elements 1 described above.

Example 1 As shown in FIGS. 1 and 2, a plurality of control elements 1
Are arranged with their axial directions parallel to each other, and one end surface of the control element 1 is arranged on the same plane A in a plane orthogonal to the axial direction of the control element 1. Each control element 1 is arranged on a plurality of concentric circles having different radii with one control element 1a as a center and another control element 1 in the plane orthogonal to the axial direction of the control element 1. The pipe lengths of the control elements 1 on each circumference are set equal, and the pipe length is set shorter as the distance from the control element 1a at the center increases. As described above, the control element 1 delays the phase velocity inside the pipe as compared with the phase velocity outside the pipe at the resonance frequency or lower, so that the longer the pipe length, the larger the phase delay. As shown in FIG. In addition, the wavefront Wsi which was orthogonal to the axial direction of the control element 1 before being introduced into the control element 1 converges on the extension line of the central axis of the control element 1a which becomes the center after passing through the control element 1. . That is, the wavefront W of the sound wave that has passed through each control element 1
so will be inclined with respect to the central axis as shown in FIG. Here, the control element 1 is formed of the material used in the experiment, the tube length of the control element 1 is 200 mm, and the adjacent control element 1 is
The distance between them is 5 mm, and the pipe length difference between adjacent control elements 1 is 50 mm.
, The phase velocity in the tube of the sound wave of 500 Hz is 1
It is 00m / s, and the converging point of the sound wave is the center of the control element 1a.
It became a point 35 mm in front of.

(Embodiment 2) In the present embodiment, the control elements 1 having different tube wall densities or Young's moduli are arranged in a plane orthogonal to the axial direction of the control elements 1 with a plurality of concentric circles having different radii about one control element 1a. By arranging other control elements 1 on the circumference,
The sound waves are focused as in the first embodiment. That is, as shown in FIG. 3, the axial length of the control element 1 is set to the same length, and only the density or Young's modulus of the tube wall is changed. For example, when the Young's modulus is fixed and only the density is changed, the density is set smaller as the distance from the central control element 1a increases.
On the contrary, if the Young's modulus is changed with the density kept constant,
The Young's modulus is set larger as the distance from the central control element 1a increases. Here, the control elements 1 on the same circumference have the same density or Young's modulus. The phase velocity in the control element 1 becomes smaller as the tube wall density becomes higher as shown in FIG. 4, and becomes smaller as the Young's modulus becomes smaller as shown in FIG. Therefore, in the above-described configuration, the wavefront that was orthogonal to the axial direction of the control element 1 before being incident on the control element 1 converges on the central axis of the control element 1a that becomes the center after passing through the control element 1. To do. Here, the control element 1 used in the measurement of FIGS. 4 and 5 has a wall thickness of 0.16 mm and an inner diameter of 20 mm, and in FIG. 4, the Young's modulus is 4.6 ×. 10 ⁶ N /
m ² , the density of the material of the tube wall in FIG. 5 is 1110 kg / m
^{It was 3} .

As described above, the density and Young's modulus of the material of the tube wall of the control element 1 are changed, and the control element 1 is the same as in the first embodiment.
By changing the length of, the sound waves can be more efficiently focused.

(Example 3) In this example, the phase velocity in the control element 1 is controlled by using the control elements 1 having different inner diameters. That is, the larger the inner diameter of the control element 1, the smaller the phase velocity. Therefore, as shown in FIG. 6, the other control elements 1 are arranged on a plurality of concentric circles having different radii about one control element 1a.
Are arranged, and the control element 1 having a smaller inner diameter is arranged as the distance from the central control element 1a increases, so that the sound wave is converged on the extension line of the central axis of the central control element 1a. When the phase velocity is measured by actually changing the inner diameter of the control element 1, the result shown in FIG. 7 is obtained. Here, the wall thickness of the tube wall was 0.16 mm, the Young's modulus was 4.6 × 10 ⁶ N / m ² , and the density was 1110 kg / m ³ .

It goes without saying that the sound waves can be more efficiently converged by combining the present embodiment with the configurations of the first and second embodiments.

In each of the above examples, since the phase velocity is controlled by targeting the frequency equal to or lower than the resonance frequency at which the phase velocity changes rapidly, if the resonance frequency is set to a high frequency, the converging frequency band can be widened. You can In addition, even in the region where the frequency is higher than the resonance frequency, there is a region where the phase velocity becomes small corresponding to the Young's modulus and density of the pipe wall or the inner diameter of the pipe in the high frequency region. If so, the sound wave can be refracted outside the low frequency region.

[Advantages of the Invention] As described above, the present invention uses a straight tubular hollow tube thinly formed of a material having viscoelasticity as a control element for controlling the phase velocity of a sound wave, and the plurality of control elements are mutually controlled. The control elements are arranged so that their axial directions are substantially parallel to each other, and other control elements are arranged on a concentric circle centered on one control element in a plane orthogonal to the axial direction of the control element, and the control element is close to the center. Since the phase velocity of the passing sound wave is set smaller, the sound wave can be converged, and as a result, the sound wave can be prevented from propagating in an unnecessary direction and can be propagated only in the necessary direction. Has the advantage that Further, since the control element is made of a material having viscoelasticity and formed in a thin straight tube shape, it has an advantage of good air permeability.

[Brief description of drawings]

1 is a schematic configuration diagram of Embodiment 1 of the present invention, FIG. 2 is a side view of the same as above, FIG. 3 is a schematic configuration diagram of Embodiment 2 of the present invention, and FIG. 4 and FIG. Explanatory diagram, FIG. 6 is a schematic configuration diagram of Embodiment 3 of the present invention, FIG. 7 is an operational explanatory diagram of the same,
FIG. 8 is a perspective view showing a control element used in the present invention, FIGS. 9 and 10 are equivalent electric circuit diagrams of the control element, FIGS. 11 and 12 are operational explanatory diagrams of the same, and FIG. 13 is the same. FIG. 14 is a configuration diagram showing a measuring device of a control element used in FIG. 14, FIG. 14 is a perspective view showing a conventional propagation direction control device, and FIG. 15 is an operation explanatory diagram of the same. Reference numeral 1 is a control element, and 1a is a central control element.

Claims (5)

[Claims] 1. A straight tubular hollow tube formed of a thin material having viscoelasticity is used as a control element for controlling the phase velocity of a sound wave, and the plurality of control elements are arranged so that their axial directions are substantially parallel to each other. And the other control elements are arranged on a concentric circle centered on one control element in the plane orthogonal to the axial direction of the control element, and the closer the control element is to the center, the smaller the phase velocity of the passing sound wave. An acoustic lens characterized by being configured. 2. The length of each control element in the axial direction is gradually shortened as the distance from the central control element in the plane orthogonal to the axial direction of the control element is increased, and the axis of each control element on the same circumference. The acoustic lens according to claim 1, wherein the lengths in the directions are set equal to each other. 3. The inner diameters of the respective control elements are gradually reduced with increasing distance from the central control element in the plane orthogonal to the axial direction of the control elements, and the inner diameters of the respective control elements on the same circumference are set equal. The acoustic lens according to claim 1 or 2, wherein the acoustic lens comprises: 4. The density of the material forming each control element is gradually reduced with increasing distance from the central control element in a plane orthogonal to the axial direction of the control element, and each control element on the same circumference is the same. The acoustic lens according to claim 1, 2, or 3, wherein the acoustic lens is formed of a material. 5. The Young's modulus of the material forming each control element is gradually increased with increasing distance from the central control element in the plane orthogonal to the axial direction of the control element, and the control elements on the same circumference are the same. Claims 1, 2, 3, or 4 characterized by being formed of a material
The acoustic lens according to item.