JPH0755599Y2 - Underwater sound absorption wedge - Google Patents

Description

[Detailed Description of the Invention] [Industrial field of application] The present invention relates to an underwater sound absorbing wedge used in an anechoic water tank for underwater sound experiments, a pressurized anechoic water tank for underwater sound experiments, a cavitation water tank, a non-reflective water tank, etc. .

[Conventional technology]

Conventionally, in water tanks and the like, a solid sound absorbing wedge formed into a truncated pyramid wedge shape has been used.

That is, as shown in FIGS. 4 and 5, two solid sound absorbing wedges 1A and 1B having a truncated pyramid shape are set as one set, and the respective sets are alternately arranged with their directions changed by 90 °. Base plate 2 with a large cross section
It is fixed to. When constructing an anechoic water tank, the base plate 2 to which the above-mentioned sound absorbing wedge is attached is installed on the inner wall of the water tank.

[Problems to be solved by the device]

FIG. 6 is an actual measurement example of the sound pressure reflectance R on the plane of the tip of the sound absorbing wedge when the aggregate of the pyramidal sound absorbing wedges made of wood is installed in the water tank.

As can be seen from the figure, the reflectance R increases at 1.5 kHz or less, and the sound absorption characteristics in the low frequency range sharply deteriorate.
When the wavelength of the sound wave is λ and the length of the wedge is h, this region corresponds to a region of approximately h / λ <0.5.

In the conventional sound absorbing wedge, as shown in FIG. 6, in the low frequency region where the wavelength is long, the reflectance is large and the sound absorbing property is reduced, and the frequency is limited to a frequency that can be anechoicized for a given wedge length. Occurs.

The lower limit frequency of anechoicization is fc, the wavelength of the corresponding sound wave is λc,
When the wedge length is h and the sound velocity of the medium is c, fc can be estimated by the following equation.

h / λc = hfc / c≈0.5 fc≈0.5c / h Therefore, if the lower limit frequency fc of anechoicization is lowered, it is necessary to lengthen the wedge length h, and if the wedge length h is lengthened, the water tank of the given size will be obtained. Then the effective measurement volume becomes smaller.

In addition, if an attempt is made to secure an effective measurement volume, the size of the water tank will be increased, and the construction cost will increase.

The present invention is intended to solve the problems of the conventional underwater sound absorbing wedge.

[Means for Solving the Problems]

According to the present invention, in a sound absorbing wedge formed in a solid wedge shape, a plurality of resonance absorbing cavities having different resonance frequencies are provided in the sound absorbing wedge and communicate with the outside.

[Action]

The resonance absorbing cavity that communicates with the outside absorbs a sound wave having a frequency equal to its resonance frequency. In the present invention, a plurality of resonance absorption cavities are provided, and the resonance frequency of these resonance absorption cavities is set to a low frequency region in which the sound absorption of the sound absorption wedges is reduced, so that the wedge-shaped underwater sound absorption wedges cannot be anechoicized. Sound waves are absorbed.

Further, the weight of the underwater sound absorbing wedge is reduced by providing the resonance absorbing cavity.

〔Example〕

An embodiment of the present invention will be described with reference to FIGS. 1 and 2.

The sound absorbing wedge 1 has a truncated pyramid wedge shape similar to the conventional sound absorbing wedge shown in FIGS. 4 and 5, and is divided into two equal parts in the longitudinal direction as shown in FIG. It consists of individual parts 1a, 1b. Helmholtz resonators 11a-11c in the portion 1a,
Helmholtz resonators 11d to 11f are provided in the portion 1b. Each resonator is provided with a cavity 12 having a circular cross section and a communication hole 13 having a circular cross section for communicating the cavity 12 to the outside. This cavity 12
And the communication hole 13 have a circular cross section, and as shown in FIG.
From the joint surface 30 of each part 1a, 1b, a large-diameter cavity 12 and the same cavity 12
It is formed by processing a small-diameter communication hole 13 that is open to the outside and communicates with the outside. As described above, the sound absorbing wedge 1 is formed by joining the portions 1a and 1b each provided with the plurality of Helmholtz resonators each including the cavity 12 and the communication hole 13 at the joint surface 30. Cavities 12a, 1 of the divided parts 1a, 1b
In 2b ..., the arrangement of cavities is set so that they do not communicate with each other.

The resonance frequency fo of the Helmholtz resonator is given by the following equation.

C: Sound velocity of medium S: Cross-sectional area of communication hole l: Length of communication hole V: Volume of cavity The same Helmholtz resonator is remarkable for sound waves propagating in the external fluid near its resonance frequency. Bring a good absorption capacity.

In this embodiment, the resonance frequencies of the plurality of Helmholtz resonators 11a to 11f are set to some frequencies within the frequency range of f <0.5c / h that cannot be absorbed by the solid sound absorbing wedge.
The cavity volume V of the Helmholtz resonators 11a to 11f, the cross-sectional area S of the communication hole, and the length l thereof are selected so that fo matches.
Helmholtz resonators 11a to 11f having different resonance frequencies fo
To form.

In the present embodiment configured as described above, a plurality of Helmholtz resonators 11a to 11f provided in the sound absorbing wedge 1 absorb some of the frequencies in the band that cannot be absorbed by the wedge-shaped sound absorbing wedge 1.
The sound absorption capability of the sound absorption wedge 1 can be expanded to a low frequency by absorbing by the sound absorption, and the anechoicization lower limit frequency can be further lowered.

Further, by providing the Helmholtz resonators 11a to 11f in the sound absorbing wedge 1, the weight of the sound absorbing wedge 1 can be reduced and its attachment can be facilitated.

Another embodiment of the present invention is shown in FIG.

In this embodiment, in the embodiment shown in FIG. 1 and FIG. 2, the closing plates 20a and 20b of the cavity 12 are attached to the entire surfaces of the divided portions 1a and 1b on the side of the joint surface 30, and the closing members are closed. The parts 1a and 1b are integrated by joining the plates 20a and 20b.

This embodiment can also achieve the same actions and effects as the embodiment shown in FIGS. 1 and 2.

In each of the above embodiments, the cavity and the communication hole have a circular cross section, but an appropriate cross sectional shape can be adopted.

[Effect of device]

The present invention reduces the lower limit of the frequency of sound waves that can be absorbed by providing a plurality of resonance absorption cavities with different resonance frequencies, which communicate with the outside, inside the underwater sound absorption wedge formed into a solid wedge shape. It is possible to lower the anechoicization lower limit frequency.

Further, by providing the underwater sound absorbing wedge with the cavity for resonance absorption, the weight of the sound absorbing wedge can be reduced and the mounting thereof can be facilitated.

[Brief description of drawings]

1 is a vertical sectional view of an embodiment of the present invention, FIG. 2 is a sectional view taken along the line II-II of FIG. 1, FIG. 3 is a vertical sectional view of another embodiment of the present invention, and FIG. A plan view of a conventional underwater sound absorption wedge, FIG.
FIG. 6 is a cross-sectional view taken along the line VV in the figure, and FIG. 6 is a graph showing the sound absorption characteristics of a conventional sound absorption wedge. 1 ... Sound absorption wedge, 1a, 1b ... Divided part of sound absorption wedge, 11a
~ 11f ... Helmholtz resonator, 12 ... cavity, 13 ... communication hole, 20a, 20b ... closing plate, 30 ... joining surface.

Claims (1)

[Scope of utility model registration request] 1. An underwater sound absorbing wedge formed into a solid wedge shape, wherein a plurality of resonant absorbing cavities having different resonance frequencies are provided in the sound absorbing wedge and communicate with the outside.