Acoustic transmission member and method of designing same

Abstract

The present technology provides an acoustic transmission member that includes a core layer layered between two skin layers, the skin layers being formed of a material having a higher elastic modulus than a material of the core layer. A thickness ds of the skin layers is formed such that Equation (1) is satisfied, where f is the center frequency of the transmission frequency band, cw is the acoustic velocity in the medium surrounding the acoustic transmission member, is the wavelength corresponding to the center frequency in the medium, Cds is the longitudinal wave acoustic velocity in the skin layer, and n is a discretionary natural number. $\begin{array}{cc}\mathrm{ds}=\frac{n}{2}\frac{\mathrm{Cds}}{\mathrm{cw}}=\frac{n}{2}\frac{\mathrm{Cds}}{f}& \left(1\right)\end{array}$

Claims

1. An acoustic transmission member with a planar shape having a predetermined acoustic frequency band as a transmission frequency band, the acoustic transmission member comprising: a core layer layered between two skin layers, wherein the skin layers are formed of a material having a higher elastic modulus than a material of the core layer; a thickness ds of the skin layers satisfies Equation (1), where f is a center frequency of the transmission frequency band, cw is an acoustic velocity in a medium surrounding the acoustic transmission member, is a wavelength corresponding to the center frequency in the medium, Cds is a longitudinal wave acoustic velocity in the skin layers, and n is a discretionary natural number; $\begin{array}{cc}\mathrm{ds}=\frac{n}{2}\frac{\mathrm{Cds}}{\mathrm{cw}}=\frac{n}{2}\frac{\mathrm{Cds}}{f}.& \left(1\right)\end{array}$

2. The acoustic transmission member according to claim 1, wherein the skin layer is formed of a material having the elastic modulus from 100 MPa to 350 GPa.

3. The acoustic transmission member according to claim 1, wherein the skin layer is formed by including any one of fiber reinforced plastic, plastic, and metal.

4. The acoustic transmission member according to claim 1, wherein the core layer is formed of a viscoelastic member or plastic.

5. The acoustic transmission member according to claim 4, wherein the core layer is formed to have a density and elastic modulus being at predetermined values by injecting glass microballoons into the viscoelastic member or the plastic.

6. The acoustic transmission member according to claim 1, wherein a difference between an acoustic impedance of a material forming the core layer is within a range of 50% of an acoustic impedance of the medium.

7. The acoustic transmission member according to claim 1, wherein the medium is a liquid, and a material forming the core layer is a liquid having an acoustic impedance within a range of 50% of an acoustic impedance of the liquid.

8. The acoustic transmission member according to claim 1, wherein a thickness dc of the core layer and the wavelength satisfy 0.0675dc/5.4.

9. The acoustic transmission member according to claim 1, wherein a protective layer is formed on a surface of the skin layer.

10. A method of designing an acoustic transmission member, the acoustic transmission member having a planar shape, including a core layer layered between two skin layers, and having a first acoustic frequency band as a transmission frequency band and a second acoustic frequency band as a shielded frequency band, the method comprising: a skin layer thickness determining step of determining a thickness ds of the skin layers using Equation (1); a transmission loss measuring step of measuring or calculating transmission losses of each layer formed by layering a core layer, having varied thickness, successively on the skin layers with a thickness determined in the skin layer thickness determining step; and a core layer thickness determining step of determining a thickness of a core layer with a maximal value of transmission loss, among the transmission losses measured or calculated, being closest to a center frequency of a shielded frequency band as a thickness of the core layer of the acoustic transmission member, and forming the acoustic transmission member including the skin layers with the skin layer thickness and the core layer with the core layer thickness, where in Equation (1), f is a center frequency of the transmission frequency band, cw is an acoustic velocity in a medium surrounding the acoustic transmission member, is a wavelength corresponding to the center frequency in the medium, Cds is a longitudinal wave acoustic velocity in the skin layers, and n is a discretionary natural number; $\begin{array}{cc}\mathrm{ds}=\frac{n}{2}\frac{\mathrm{Cds}}{\mathrm{cw}}=\frac{n}{2}\frac{\mathrm{Cds}}{f}.& \left(1\right)\end{array}$

11. A method of designing an acoustic transmission member, the acoustic transmission member having a planar shape, including a core layer layered between two skin layers, having a predetermined acoustic frequency band as a transmission frequency band, and a predetermined required strength and rigidity, the method comprising: a skin layer thickness determining step of determining a thickness ds of the skin layers using Equation (1); a core layer thickness determining step of calculating a thickness of the core layer having strength satisfying the required strength and rigidity when layered between the skin layers with a thickness determined in the skin layer thickness determining step, and an acoustic transmission member forming step of forming the acoustic transmission member including the skin layers with the skin layer thickness and the core layer with the core layer thickness, where in Equation (1), f is a center frequency of the transmission frequency band, cw is an acoustic velocity in a medium surrounding the acoustic transmission member, is a wavelength corresponding to the center frequency in the medium, Cds is a longitudinal wave acoustic velocity in the skin layers, and n is a discretionary natural number; $\begin{array}{cc}\mathrm{ds}=\frac{n}{2}\frac{\mathrm{Cds}}{\mathrm{cw}}=\frac{n}{2}\frac{\mathrm{Cds}}{f}.& \left(1\right)\end{array}$

12. The method of designing an acoustic transmission member according to claim 11, wherein a thickness dc of the core layer and the wavelength satisfy 0.0675dc/5.4.

13. The method of designing an acoustic transmission member according to claim 11, wherein the skin layers are formed of a material having a higher elastic modulus than a material of the core layer.

14. The method of designing an acoustic transmission member according to claim 10, wherein a thickness dc of the core layer and the wavelength satisfy 0.0675dc/5.4.

15. The method of designing an acoustic transmission member according to claim 10, wherein the skin layers are formed of a material having a higher elastic modulus than a material of the core layer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view illustrating a configuration of an acoustic transmission member 10 according to an embodiment.

(2) FIG. 2 is a table listing material constants of each material for forming skin layers 102A, 102B and a core layer 104.

(3) FIG. 3 is a graph showing frequency characteristics of the longitudinal wave acoustic velocity in a carbon fiber reinforced plastic (CFRP).

(4) FIG. 4 is a graph showing transmission loss characteristics of a single layer plate of the CFRP for acoustic waves of 50 kHz.

(5) FIG. 5 is a graph showing transmission loss characteristics of acoustic transmission members 10 with a skin layer thickness of 18 mm.

(6) FIG. 6 is a graph showing transmission loss characteristics of a single layer plate of the CFRP for acoustic waves of 500 kHz.

(7) FIG. 7 is a graph showing transmission loss characteristics of acoustic transmission members 10 with a skin layer thickness of 3 mm.

(8) FIG. 8 is a graph showing transmission loss characteristics of acoustic transmission members 10 with a skin layer thickness of 2.5 mm.

(9) FIG. 9 is a graph showing transmission loss characteristics of acoustic transmission members 10 with a skin layer thickness of 2 mm.

(10) FIG. 10 is a graph showing transmission loss characteristics of acoustic transmission members 10 with a skin layer thickness of 1.5 mm.

(11) FIG. 11 is a graph showing transmission loss characteristics of acoustic transmission members 10 with a skin layer thickness of 1 mm.

(12) FIG. 12 is a table listing the transmission loss at 500 kHz of the acoustic transmission members 10 with a skin layer thickness of 2.5 mm and 3.0 mm.

(13) FIG. 13 is a graph showing transmission characteristics of the acoustic transmission members 10 with good transmission characteristics at 500 kHz extracted from FIG. 12.

(14) FIG. 14 is an explanatory diagram illustrating a configuration of a marine sonar device 20 installed with an acoustic transmission member.

(15) FIG. 15 is an explanatory drawing illustrating a structure of an acoustic transmission member according to U.S. Pat. No. 4,997,705.

(16) FIG. 16 is a graph showing transmission characteristics of the acoustic transmission member illustrated in FIG. 15.

(17) FIG. 17 is an explanatory drawing illustrating a structure of an acoustic transmission member according to Japan Patent No. 2015-098086.

(18) FIG. 18 is a graph showing transmission characteristics of the acoustic transmission member illustrated in FIG. 17.

DETAILED DESCRIPTION

(19) Acoustic transmission members according to preferred embodiments of the present technology are described in detail below with reference to the accompanying drawings.

(20) FIG. 14 is an explanatory diagram illustrating a configuration of a marine sonar device 20 installed with an acoustic transmission member. The acoustic transmission member according to the present technology is used in order to form an acoustic transmission region in the sonar device 20, for example.

(21) The sonar device 20 is disposed at a leading end of a sonar device support platform (not illustrated) provided to the body of a vessel.

(22) The sonar device 20 includes a transducer 24 and an acoustic window 26.

(23) The transducer 24 is mounted on a frame 28 made of fiber reinforced plastics (FRP) supported in an inside portion surrounded by the acoustic window 26, and is disposed towards the front direction of the vessel.

(24) The acoustic window 26 is configured to protect the transducer 24 and is provided so as to cover the transducer 24, with water filling the inside portion surrounded by the acoustic window 26.

(25) The acoustic window 26 has a cylindrical portion 26A, and a curved surface portion 26B that is formed in a curved shape that protrudes toward the front from the tip of the cylindrical portion 26A.

(26) The frame 28 on which the transducer 24 is mounted is supported in the inside portion surrounded by the cylindrical portion 26A.

(27) The acoustic window 26 has the acoustic transmission region that enables acoustic transmission, and an acoustic insulation region that blocks sound.

(28) The curved surface portion 26B of the acoustic window 26 that constitutes the acoustic transmission region is made of the acoustic transmission member according to the present technology.

(29) The cylindrical portion 26A of the acoustic window 26 constituting the acoustic insulation region is constituted of an acoustic insulating material that blocks sound (this includes reducing oscillation noise). As such an acoustic insulating material, various known materials can be used such as a viscoelastic member (air bubbles dispersion type acoustic insulating material) such as rubber or polyurethane with air bubbles (vinyl-based microballoons) mixed in, or a material in which thin rubber plates sandwiching a thin rubber plate with many small holes provided therethrough (air hole-type acoustic insulating material) on both surfaces.

(30) In the present embodiment, the acoustic transmission region is the entirety of the curved surface portion 26B excluding edge portions of the curved surface portion 26B that are located near the cylindrical portion 26A, and a range illustrated in FIG. 14 is the range in which the transducer 24 searches for obstacles in the front such as whales and driftwood.

(31) Next, the acoustic transmission member constituting the acoustic transmission region will be described.

(32) FIG. 1 is a cross-sectional view illustrating a configuration of an acoustic transmission member 10 according to an embodiment.

(33) In the description below, for ease of explanation, the acoustic transmission member 10 is illustrated as having a planar shape, but the acoustic transmission member 10 can be molded in any shape depending on the application.

(34) The acoustic transmission member 10 has a planar shape and acoustic transmission properties. The acoustic transmission member 10 has a predetermined acoustic frequency band as a transmission frequency band, with f defined as the center frequency of the transmission frequency band.

(35) The acoustic transmission member 10 includes a core layer 104 layered between two skin layers 102A, 102B. More specifically, the skin layer 102A, the core layer 104, and the skin layer 102B are layered in the described order from the upper side of FIG. 1.

(36) Protective layers may further be provided on the surfaces of the skin layers 102A, 102B, that is, the surfaces of the two skin layers 102A, 102B opposite to the surfaces bonding the core layer 104. Such protective layers are provided in order to protect the sonar device 20 from external loads or to evade from marine organisms, for example.

(37) Additionally, a protective layer may be provided on only on surface of one of the skin layers 102A, 102B.

(38) Next, the materials of the respective layers will be described.

(39) The skin layers 102A, 102B are formed of a material with a higher elastic modulus than the core layer 104.

(40) The skin layers 102A, 102B are formed of a material with an elastic modulus from 100 MPa to 350 GPa, for example. This is because the material having an elastic modulus of less than 100 MPa makes it difficult to maintain the layers as structural members, and the material having an elastic modulus of greater than 350 GPa has too high of an acoustic impedance.

(41) The skin layers 102A, 102B can specifically be formed using any of fiber reinforced plastics (FRP, glass fiber reinforced plastics (GFRP), carbon fiber reinforced plastics (CFRP), or the like), plastic, or metal (copper, titanium, or the like).

(42) Also, the core layer 104 is formed of a viscoelastic member such as rubber or polyurethane, or plastic, for example.

(43) The core layer 104 may be formed to have a density and elastic modulus being at desired values (predetermined values) by injecting glass microballoons into the above-described viscoelastic member or plastic.

(44) It is preferable that the difference in acoustic impedance between the material forming the core layer 104 and the medium surrounding the acoustic transmission member 10 (fresh water or seawater in the present embodiment) be within a range of 50%.

(45) Specifically, the acoustic impedance is calculated as the product c of a material density and a longitudinal wave acoustic velocity c. In other words, the acoustic impedance of the material forming the core layer 104 is calculated as a product 1c1 of a density 1 of the material forming the core layer 104 and the longitudinal wave acoustic velocity c1 in the material forming the core layer 104. The acoustic impedance of the medium surrounding the acoustic transmission member 10 is calculated as a product 2c2 of a density 2 of the medium surrounding the acoustic transmission member 10 and the longitudinal wave acoustic velocity c2 in the same medium.

(46) It is preferable that the density and elastic modulus of the core layer 104 be adjusted such that the difference between the acoustic impedance 1c1 of the material forming the core layer 104 and the acoustic impedance 2c2 of the medium surrounding the acoustic transmission member 10 is within a range of 50%.

(47) This is because when the difference between the acoustic impedance 1c1 of the material forming the core layer 104 and the acoustic impedance 2c2 of the medium surrounding the acoustic transmission member 10 is too great, the boundary becomes highly reflective, reducing the acoustic transmission properties.

(48) In the present embodiment, the medium surrounding the acoustic transmission member 10 is a liquid such as fresh water or seawater. Accordingly, the material constituting the core layer 104 may be a liquid with a difference in acoustic impedance within range of 50% of the acoustic impedance of the surrounding fluid, specifically, the same type of liquid as the surrounding fluid.

(49) In other words, fresh water, sea water, or the like being the surrounding fluid may be inserted between the skin layers 102A, 102B to form the core layer 104. Note that in a configuration in which the surrounding fluid is sea water, fresh water is preferably used to reduce the chances of corrosion and the like.

(50) Next, the thicknesses of the respective layers will be described.

(51) A thickness ds of the skin layers 102A, 102B is formed such that Equation (1) below is satisfied, where f is the center frequency of the transmission frequency band of the acoustic transmission member 10, cw is the acoustic velocity in the medium surrounding the acoustic transmission member 10, is the wavelength corresponding to the center frequency fin the medium, Cds is the longitudinal wave acoustic velocity in the skin layer, and n is a discretionary natural number.

(52) $\begin{array}{cc}\mathrm{ds}=\frac{n}{2}\frac{\mathrm{Cds}}{\mathrm{cw}}=\frac{n}{2}\frac{\mathrm{Cds}}{f}& \left(1\right)\end{array}$

(53) The skin layers 102A, 102B that satisfy Equation (1) above take a half-wavelength structure with respect to the center frequency to minimize acoustic transmission loss. By selecting such a thickness, the acoustic transmission member 10 can be formed with good transmission characteristics with respect to the transmission frequency band irrespective of the thickness of the core layer 104.

(54) The thickness dc of the core layer 104 is preferably set in relation to the wavelength corresponding to the center frequency f in the medium to satisfy 0.05dc/5.4.

(55) In varying the thickness dc of the core layer 104 as described below, the present inventors determined that by satisfying the range described above allows good acoustic transmission characteristics to be achieved.

EXAMPLES

(56) The characteristics of acoustic transmission members according to the present technology will be described in detail below.

(57) In the examples below, CFRP (prepreg C3WPW/F-6986) was used for the skin layers 102A, 102B, and natural rubber F-9099 was used for the core layer 104. The materials and constants of the materials are listed in FIG. 2.

(58) Additionally, the longitudinal wave acoustic velocity in the CFRP, i.e., the skin layers 102A, 102B, is shown in FIG. 3. The vertical axis indicates longitudinal wave acoustic velocity in the CFRP, and the horizontal axis indicates frequency (kHz). The acoustic velocity in the CFRP is assumed to have the frequency characteristics. Specifically, it is assumed to be 1830 m/s at 50 kHz and 2900 m/s at 500 kHz, for example.

(59) First, the acoustic transmission member 10 with a center frequency of 50 kHz will be discussed.

(60) FIG. 4 is a graph showing the transmission loss (IL) of a single layer plate of the CFRP (skin layers 102A, 102B) for acoustic waves of 50 kHz.

(61) In the graph of FIG. 4, the vertical axis indicates transmission loss (dB) and the horizontal axis indicates thickness of the CFRP plate (mm).

(62) As illustrated in FIG. 4, the CFRP plate has a thickness whereby transmission loss periodically decreases. The thickness to obtain a minimal value is the skin layer thickness calculated from Equation (1) above. For example, when n=1 in Equation (1) above, the minimal value corresponds to at or near a CFRP plate thickness of 18 mm, and when n=2, the minimal value corresponds to at or near a CFRP plate thickness of 36 mm.

(63) FIG. 5 is a graph showing the transmission loss (IL) of the acoustic transmission members 10 of examples with various core layer thicknesses and a fixed skin layer thickness.

(64) FIG. 5 shows the transmission loss (IL) characteristics calculated with the thickness of the skin layers 102A, 102B set to 18 mm corresponding to n=1 and the thickness of the core layer 104 ranging from 2 mm to 20 mm in 2 mm increments.

(65) In the graph of FIG. 5, the vertical axis indicates transmission loss (dB), and the horizontal axis indicates frequency (kHz). In other words, each of the acoustic transmission members 10 has properties by which sounds in a frequency band with a small transmission loss are transmitted, and sounds in a frequency band with a large transmission loss cut-off.

(66) As seen from FIG. 5, the acoustic transmission members 10 in which the thickness of the skin layers 102A, 102B was 18 mm achieve good transmission characteristics at or near 50 kHz irrespective of the thickness of the core layer 104.

(67) Also, as seen from the characteristics curve, the transmission loss is high each on the low frequency side and the high frequency side of the center frequency, i.e., 50 kHz. This has the effect of shielding acoustic waves in bands outside of the center frequency.

(68) Next, the acoustic transmission member 10 with a center frequency of 500 kHz will be discussed.

(69) FIG. 6 is a graph showing the transmission loss (IL) of a single layer plate of the CFRP (skin layers 102A, 102B) with respect to acoustic waves of 500 kHz.

(70) In the graph of FIG. 6, the vertical axis indicates transmission loss (dB) and the horizontal axis indicates thickness of the CFRP plate (mm).

(71) When using 500 kHz, the thickness of the CFRP plate to obtain a minimal value for transmission loss is 2.9 mm (n=1), 5.8 mm (n=2), and so on. This value is in accordance with Equation (1) above.

(72) FIG. 7 is a graph showing the transmission loss characteristics of the acoustic transmission members 10 of examples with various core layer thicknesses and the skin layer thickness fixed at 3 mm. FIG. 8 is a graph showing the transmission loss characteristics of the acoustic transmission members 10 of examples with various core layer thicknesses and the skin layer thickness fixed at 2.5 mm.

(73) Note that in FIGS. 7 and 8, the thickness of the core layer 104 ranges from 0.5 mm to 10 mm in 1 mm increments.

(74) As described above, the thickness of the CFRP plate to obtain a minimal value for transmission loss with the acoustic waves at 500 kHz is 2.9 mm. Thus, the trough of the minimal value is not on 500 kHz in both FIG. 7 (3 mm) and FIG. 8 (2.5 mm). However, when looking also at FIGS. 9 to 11, it can be seen that as the thickness of the CFRP plate decreases, the trough of the minimal value moves toward higher frequencies, validifying Equation (1). Specifically, in FIG. 7 (3 mm), the trough of the minimal value is in a section very close to 500 kHz.

(75) Solving Equation (1) above with an acoustic velocity approaching 2900 m/s at a longitudinal wave acoustic velocity of 500 kHz in the CFRP plate, the center frequency for the 3 mm-CFRP plate thickness of FIG. 7 is 483 kHz and the center frequency for the 2.5 mm-CFRP plate thickness of FIG. 8 is 580 kHz. The trough of the minimal value in these graphs are roughly in the same position. Thus, it can be assumed that by setting the CFRP plate thickness to approximately 2.9 mm, an acoustic transmission member with the desired 500 kHz as the center frequency can be obtained.

(76) Note that should the specifications of the acoustic transmission member 10 such as required dimensions and plate material dictate that the CFRP plate thickness cannot be a value in accordance with Equation (1) above, the thickness of the core layer 104 can be adjusted to obtain sufficient transmission performance at the center frequency.

(77) For example, a CFRP plate with a thickness of 3 mm like that of FIG. 7 or a CFRP plate with a thickness of 2.5 mm can be used to form an acoustic transmission member 10 with a center frequency of 500 kHz.

(78) FIG. 12 is a table listing the transmission loss (unit: dB) at 500 kHz of the acoustic transmission members 10 formed using a CFRP plate with a thickness of 2.5 mm and a CFRP plate with a thickness of 3.0 mm.

(79) From FIG. 12, the following six configurations resulting in a transmission loss of 2 dB or less is extracted: (skin layer thickness/core layer thickness) 2.5 mm/1.0 mm, 3.0 mm/0.5 mm, 3.0 mm/1.0 mm, 3.0 mm/2.0 mm, 3.0 mm/3.0 mm, 3.0 mm/4.0 mm.

(80) FIG. 13 is a graph showing transmission characteristics of the acoustic transmission members 10 with good transmission characteristics at 500 kHz extracted from FIG. 12.

(81) In particular, the combination of skin layer thickness 3.0 mm/core layer 0.5 mm is the best of this group. With this combination, the transmission loss at from 450 kHz to 540 kHz is 2 dB or less, and the high transmission bandwidth with a transmission loss of 2 dB or less is 18%.

(82) In this way, values near but outside of the value calculated using Equation (1) for the skin layer thickness can be used in combination with adjusting the core layer thickness to form an acoustic transmission member 10 having sufficiently high transmission performance.

(83) FIGS. 9 to 11 are graphs showing the transmission loss of the acoustic transmission members 10 of examples with various core layer thicknesses and a fixed skin layer of 2 mm for FIG. 9, a fixed skin layer of 1.5 mm for FIG. 10, and a fixed skin layer of 1 mm for FIG. 11.

(84) Note that in these drawings, the thickness of the core layer 104 ranges from 0.5 mm to 10 mm in 1 mm increments.

(85) In FIG. 9, the trough of the minimal value is at or near 800 kHz. In FIGS. 10 and 11, the frequency at which the trough of the minimal value (transmission window) is formed is 1000 kHz or less, making the trough of the minimal value not seen in the drawings.

(86) A Study of Core Layer Thickness

(87) In FIG. 5, the skin layer thickness is fixed at 18 mm and the thickness dc of the core layer 104 ranges from 2 mm to 20 mm. Additionally, when the acoustic velocity in the surrounding medium (water in the present embodiment) is 1481 m/s, the wavelength corresponding to a center frequency of 50 kHz is 29.6 mm.

(88) Thus, the range of dc/ in FIG. 5 is from 0.0675 (2 mm) to 0.675 (20 mm).

(89) In FIG. 9, the skin layer thickness is fixed at 2 mm and the thickness dc of the core layer 104 ranges from 0.5 mm to 10 mm. Additionally, when the acoustic velocity in the surrounding medium (water in the present embodiment) is 1481 m/s, the wavelength corresponding to a center frequency of 800 kHz is 1.85 mm.

(90) Thus, the range of dc/ in FIG. 9 is from 0.27 (0.5 mm) to 5.4 (10 mm).

(91) From this, it can be seen that for the acoustic transmission member 10 according to an embodiment, by satisfying 0.0675dc/5.4, where dc is the thickness of the core layer 104 and is the wavelength corresponding to the center frequency f, an acoustic transmission member 10 having good transmission performance at the center frequency f can be formed.

(92) Method of Designing Each Layer Thickness

(93) In this way, in the acoustic transmission member 10, by adjusting the thickness of the skin layers 102A, 102B, a transmission frequency band with a center frequency of a discretionary frequency can be formed. Thus, the thickness of the skin layers 102A, 102B needs to merely be determined in accordance with the transmission frequency band required for the acoustic transmission member 10.

(94) Additionally, the thickness of the core layer 104 may be determined as follows, for example.

(95) (1) Determine the thickness of the core layer 104 in accordance with the strength and rigidity required for the acoustic transmission member 10.

(96) As described above, the thickness of the core layer 104 can be set to a discretionary thickness. Thus, the thickness of the core layer 104 can be set to a discretionary thickness in accordance with the strength and rigidity required for the acoustic transmission member 10.

(97) For example, when the required strength and rigidity is relatively high, the core layer 104 can be made relatively thick to ensure the required strength and rigidity. Also, when the required strength and rigidity is relatively low, the core layer 104 can be made relatively thin to achieve a reduction in weight and cost.

(98) In other words, designing an acoustic transmission member having a predetermined transmission frequency band and required strength and rigidity includes a skin layer thickness determining step of determining the thickness ds of the skin layers 102A, 102B using Equation (1) above and a core layer thickness determining step of calculating the thickness of the core layer 104 having strength satisfying the required strength and rigidity described above when layered between the skin layers 102A, 102B with the thickness determined in the skin layer thickness determining step. An acoustic transmission member having desired transmission performance and required strength and rigidity can be designed with this method.

(99) (2) Determine the thickness of the core layer 104 in accordance with the cut-off frequency band.

(100) As illustrated in FIG. 5 and the like, the acoustic transmission member 10 of the present embodiment has a consistently small transmission loss at or near the center frequency. However, in frequency bands outside of the center frequency, the transmission loss performance varies greatly depending on the thickness of the core layer 104. In other words, the maximal value of the transmission loss varies with the thickness of the core layer 104.

(101) Thus, by selecting a core layer thickness with the maximal value of transmission loss located at or near the desired cut-off frequency band, an acoustic transmission member 10 having desired transmission performance including cut-off performance can be formed.

(102) In other words, designing an acoustic transmission member having a predetermined transmission frequency band and shielded frequency band includes a skin layer thickness determining step of determining the thickness ds of the skin layers 102A, 102B using Equation (1) above, a transmission loss measuring step of measuring or calculating the transmission loss of an each layer formed by layering the core layer 104 of varying thickness successively between the skin layers 102A, 102B with the thickness determined in the skin layer thickness determining step, and a core layer thickness determining step of determining the thickness of the core layer 104 with the maximal value of transmission loss, among the transmission losses measured or calculated in a previous step, being located closest to the center frequency of the shielded frequency band as a thickness of the core layer 104 of the acoustic transmission member 10. An acoustic transmission member having desired transmission performance and cut-off performance can be designed with this method.

(103) In reality, these two methods are combined to design the acoustic transmission member. In other words, the strength and rigidity and the cut-off frequency band required for the acoustic transmission member are considered and a trade-off is made in determining the final design values.

(104) As described above, the acoustic transmission member 10 according to an embodiment includes the skin layers 102A, 102B with a half-wavelength structure, allowing the acoustic transmission member 10 to be formed with good transmission characteristics at a center frequency irrespective of the thickness of the core layer 104 and is advantageous in that the degree of freedom in designing the acoustic transmission member 10 is improved.

(105) Additionally, the acoustic transmission member 10 is advantageous in that a certain wide transmission band can be ensured at a center frequency, and is adaptable to temperature change in the surrounding medium due to temperature change.

(106) Furthermore, the acoustic transmission member 10 has the function of shielding acoustic waves outside of the center frequency. This is advantageous in that noise resistance is improved.