Sound isolation plate structure

Abstract: A sound isolation plate structure comprises a sheet of perforated board in which a plurality of holes are formed, and cylindrical portions provided on one surface side of the perforated board to extend from peripheral portions of the holes like cylinders unilaterally, wherein pneumatic vibration systems are formed between the cylindrical portions. Accordingly, a resonance frequency of vibration systems is easily tuned by adjusting a projection amount of the cylindrical portions so as to mate with a frequency of the noise to be isolated.

Description: The present invention relates to a sound isolation plate according to the preamble part of the independent claim 1, for example, an under cover provided on the bottom of the engine room to reduce the noise on the outside of the car. As shown in FIG. 1, the under cover 3 is fitted on the bottom of the engine room 2 of the common car 1. This under cover 3 prevents an oil pan of the engine, etc. from directly striking against protruded things on the road. Also, the under cover 3 functions as a sound isolation wall to suppress an engine noise which is radiated from the engine room 2 to the outside of the car. The wider is a fitting area of the under cover 3 wider, the higher is a sound isolation effect achieved by the under cover 3. However, if the fitting area of the under cover 3 is increased, a sealing degree of the engine room 2 is enhanced by such under cover 3 and thus the air permeability of the engine room 2 is lowered. It is likely that an increase in an atmospheric temperature of the engine room 2 is caused. Therefore, in practice the air permeability must be assured on a preferential basis. As a result, a limitation is imposed on an improvement in the sound isolation effect by virtue of an increase of the fitting area of the under cover 3. GB 2 276 224 A1 disclosed a sound shielding plate for an underbody of an engine according to the preamble part of the independent claim 1. JP-A-07 064 564 discloses a sound insulating structure and sound insulating plate which comprises air mass and an air spring, wherein the air mass and the air spring form a resonance system being provided in through holes of the sound insulating structure and sound insulating plate. US-A-2 455 926 discloses an automotive body interior comprising rigid sound and heat insulating sheets which are attached to the automobile body inside the passenger's room. DE-A-1 180 155 discloses a sound insulating plate comprising two hard boards and a glass fiber mate covered by the two hard boards. It is an object of the present invention to improve a sound isolation plate structure as indicated above so as to improve both an air permeability and a sound isolation property. According to the present invention, the objective is solved by a sound isolation plate structure comprising a sheet of perforated board in which a plurality of holes are formed; cylindrical portions provided on one surface side of the perforated board to extend from peripheral portions of the holes; and pneumatic vibration systems formed between the cylindrical portions, wherein the holes of the perforated board are provided corresponding to the cylindrical portions, respectively, and each pneumatic vibration system has one longitudinal end closed by the perforated board. With the above configuration, because the pneumatic vibration systems formed between the plurality of cylindrical portions act as the dynamic damper against the transmitted wave through the holes, the anti-resonance is given to the vibration response of the transmitted wave. Therefore, the transmission loss is increased and thus a noise level is reduced. In addition, good air permeability is obtained because of the plurality of holes. Therefore, both the air permeability and the sound isolation property are improved. Besides, the frequency characteristic of transmission loss of the sound isolation plate is adjusted arbitrarily by changing the projection amount of the cylindrical portions, i.e., the effective hole length of the holes. Hence, the frequency characteristic of transmission loss of the sound isolation plate is tuned easily and properly so as to mate with all sound isolation needs by a simple structure of the sound isolation plate. Also, a thickness t of the perforated board and an effective hole length L of the cylindrical portion may be set such that the thickness t of the perforated board, the effective hole length L of the cylindrical portion, an acoustic velocity C, and a center frequency f1 in a frequency bandwidth of a noise to be isolated may satisfy a relationship L - t = C/4f1. Also, a thickness t of the perforated board and an effective hole length L of the cylindrical portion may be set such that the thickness t of the perforated board, the effective hole length L of the cylindrical portion, an acoustic velocity C, and a peak frequency f2 of a noise to be isolated satisfy a relationship L - t = C/4f2. With the above configuration, the noise having a frequency for purpose of shielding the noise is precisely isolated, so that a sound isolation effect is extremely improved. In addition, a porous material may be filled between the cylindrical portions. With the above configuration, the frequency characteristic of transmission loss of the sound isolation plate is more easily tuned by changing a filling density or a filling thickness of the porous material. Besides, a peak frequency of transmission loss which is changed according to a filling density and a filling thickness of the porous material may be set to substantially mate with the center frequency or the peak frequency in the frequency bandwidth of the noise to be isolated. With the above configuration, the noise having a frequency for purpose of shielding the noise is precisely isolated, so that a sound isolation effect is extremely improved. Further, one surface side of the perforated board from which the porous material is exposed may be arranged to direct toward a noise generating source side. With the above configuration, a noise absorbing effect is obtained by the porous material, and also the noise level of the sound isolation plate on the noise generating source side is reduced simultaneously. For this reason, the sound isolation effect is much more improved. Moreover, the sound isolation plate structure comprises another sheet of second perforated board in which a plurality of holes opening portions are formed and which is arranged at a predetermined interval such that the holes of the perforated board are opposed to the holes of the second perforated board, and the pneumatic vibration systems between the cylindrical portions are defined to open to an external space through the opening portions of the second perforated board. With the above configuration, since the pneumatic vibration systems formed between the plurality of cylindrical portions act as the dynamic damper against the transmitted wave through the continuous holes, the anti-resonance is given to the vibration response of the transmitted wave. Therefore, the transmission loss is increased and thus the noise level is reduced. In addition, good air permeability is obtained because of the plurality of continuous holes. Therefore, both the air permeability and the sound isolation property are improved. Besides, the frequency characteristic of transmission loss of the sound isolation plate is adjusted arbitrarily by changing any of a diameter of the opening portion of the air chamber, a rate of hole area of the opening portion, a depth of the opening portion, a total thickness of the sound isolation plate, and a height of the air chamber. Hence, the frequency characteristic of transmission loss of the sound isolation plate is tuned easily and properly so as to mate with all sound isolation needs. Moreover, a center frequency or a peak frequency of a frequency bandwidth of a noise to be isolated may be set to coincide substantially with a peak frequency of transmission loss which is changed according to the diameter of the opening portion of the second perforated board, the rate of hole area of the opening portion, the depth of the opening portion, the total thickness of the sound isolation plate structure, and the height of each air chambers formed between the cylindrical portions. With above configuration, the noise having a frequency for purpose of shielding the noise is precisely isolated, so that a sound isolation effect is extremely improved. In addition, a porous material may be filled into the cylindrical portions. With the above configuration, the frequency characteristic of transmission loss of the sound isolation plate is tuned by changing a filling density or a filling thickness of the porous material. Hence, tuning of the frequency characteristic is accomplished more easily. Moreover, the second perforated board, on which the opening portions to expose the porous material are provided, may be arranged so as to direct toward a noise generating source. With the above configuration, the noise absorbing effect is obtained by the porous material, and also the noise level of the sound isolation plate on the noise generating source side is reduced simultaneously. As a result, the sound isolation effect is much more improved. Further preferred embodiments of the present invention are laid down in the further subclaims. In the following, the present invention is explained in greater detail by means of several embodiments thereof in conjunction with the accompanying drawings, wherein: FIG. 1 is a bottom view showing a car having an under cover in the prior art; FIG.2 is a perspective view showing a sound isolation plate structure according to a first embodiment of the present invention; FIG. 3 is a sectional view showing a sectional shape of the sound isolation plate structure shown in FIG.2; FIG. 4 is a sectional view showing a sound isolation mechanism of the sound isolation plate structure shown in FIG.2; FIG. 5 is a graph showing a frequency characteristic of the sound isolation plate structure shown in FIG.2; FIG.6 is a characteristic view showing an example of noise frequency obtained by the sound isolation plate structure shown in FIG.2; FIG.7 is a perspective view showing a sound isolation plate structure according to a second embodiment of the present invention; FIG. 8 is a sectional view showing a sectional shape of the sound isolation plate structure shown in FIG.7 ; FIG. 9 is a graph showing a frequency characteristic of the sound isolation plate structure shown in FIG.7; FIG.10 is a perspective view showing a sound isolation plate structure according to a third embodiment of the present invention; FIG. 11 is a sectional view showing a sectional shape of the sound isolation plate structure shown in FIG.10; FIG. 12 is a sectional view showing a sound isolation mechanism of the sound isolation plate structure shown in FIG.10; FIG. 13 is a graph showing a frequency characteristic of the sound isolation plate structure shown in FIG.10; FIG.14 is a perspective view showing a sound isolation plate structure according to a fourth embodiment of the present invention; and FIG. 15 is a view showing a frequency characteristic of the sound isolation plate structure shown in FIG.14. First Embodiment As shown in FIGS. 2 and 3, a sound isolation plate 10 comprises a perforated flat board 11 which is made of resin and has a predetermined thickness 11. A plurality of holes 12 are provided in the perforated board 11. All holes 12 are formed as circular holes each having a diameter R1. Because of the holes 12, the perforated board 11 has a rate of hole area α1. Cylindrical portions 13 are provided as many as the holes 12 on one surface 11 a side of the perforated board 11. The cylindrical portions 13 are formed to extend, like cylinders, from peripheral areas of the holes 12 unilaterally respectively. All projection heights of the cylindrical portions 13 from the perforated board 11 are set to the same height (projection amount S1). Inner surfaces of the holes 12 and inner surfaces of the cylindrical portions 13 are connected to form coplanar circular surfaces respectively. Cylindrical internal spaces, which continue from one surface 11 a side of the perforated board 11 to the other surface 11 b thereof, are partitioned with the holes 12 and the cylindrical portions 13. FIG. 4 is a sectional view showing a sound isolation mechanism of the sound isolation plate 10 shown in FIG.2. Air layers included in the internal spaces, which are partitioned with the holes 12 and the cylindrical portions 13, serve as pneumatic masses 14 respectively. In addition, the other air layers each having a thickness S1 which is equal to the projection height of the cylindrical portion 13 are formed between neighboring cylindrical portions 13, 13. Since air has mass and acts as an elastic body, pneumatic vibration systems 17 are formed between the cylindrical portions 13, 13 respectively. Each of such pneumatic vibration systems 17 consists of a pneumatic mass 15 made up of an air layer between edge portions of the cylindrical portions 13, 13, and an air spring 16 made up of an air layer which is located inner (perforated flat board 11 side) than the edge portions of the cylindrical* portions 13, 13. These pneumatic vibration systems 17 act as dynamic dampers against the transmitted wave transmitted via the cylindrical portions 13 of the sound isolation plate 10 to thus provide anti-resonance to a vibration response of the transmitted wave. Hence, transmission loss is improved and a noise level is reduced. For this reason, if the sound isolation plate 10 constructed as above is employed as the under cover 3 which is provided on the bottom of the engine room 2 of the car 1 shown in FIG.1, good air permeability is accomplished via the holes 12, so that heat is discharged smoothly from the engine room to the outside. Further, because of the dynamic damper action of the pneumatic vibration systems 17 formed between the cylindrical portions 13, 13, the anti-resonance is given to the vibration response of the transmitted wave of the noise passing through the under cover 3 from the engine room 2 to thus increase the transmission loss. Therefore, a level of noise, which is radiated from the engine room 2 to the outside of the car 1, is reduced. As a result, the compatibility between an improvement in cooling capability and noise suppression in the engine room 2 is attained. In this case, the pneumatic vibration systems 17 formed between the cylindrical portions 13, 13 act as the dynamic dampers against the transmitted wave passing through these cylindrical portions 13, 13. Therefore, even if the cylindrical portions 13 are aligned toward the transmitted wave side on the outside of the car, for example, as shown in FIG. 4, or conversely the cylindrical portions 13 are aligned toward the incident wave side in the engine room 2, the similar sound isolation effect is achieved by the sound isolation plate 10. In the first embodiment, since an effective hole length L1 of the cylindrical portion 13 corresponds to a length of the internal space which is partitioned with the holes 12 and the cylindrical portions 13, such effective hole length L1 is given as L1 = t1 + S1 by use of the thickness t1 of the perforated board 11 and the projection amount S1 of the cylindrical portion 13. Among the projection amount S1 of the cylindrical portion 13, a resonance frequency f0 of the pneumatic-vibration system 17, and an acoustic velocity C, a relationship f0 = C X (2n-1)/4S1 is found. Where n is a natural number in Eq. (2). When n = 1, the lowest resonance frequency f0m out of the resonance frequencies f0 is detected. Then, f0m = C/4S1 is derived by substituting n = 1 and f0 = f0m into Eq.(2). Meanwhile, as shown in FIG. 6, for example, the engine noise of the car has a peak at a predetermined value (peak frequency f2). Therefore, it should be noted that, if the noise of the peak frequency f2 is isolated, most effective noise suppression is implemented. More particularly, a resonance frequency f0 (minimum resonance frequency f0m) should be put into coincidence with a peak frequency f2 of the noise to be isolated. In other words, by substituting f0m=f2 into Eq.(3), S1 = C/4f2 is derived. Then, if an acoustic velocity C and the peak frequency f2 are substituted into Eq. (4), the desired projection amount S1 is obtained. Then, substitution of Eq. (4) into Eq. (1) gives L1 - t1 = C/4f2 For example, as shown in FIG. 6, if the peak frequency f2 of the engine noise to be isolated is in the neighboring of 1700 Hz, the projection amount S1 of the cylindrical portions 13 is calculated as 50 mm by substituting the acoustic velocity C=340 m/sec and the peak frequency f2=1700 Hz into Eq. (4). In the event that the cylindrical portions 13 having the projection amount S1 = 50 mm calculated as above are formed in the perforated board 11 which has the diameter R1=10 mm of the holes 12, the rate of hole area α1 = 0.2, and the plate thickness t1 = 2 mm, transmission loss is shown by a curve LN1 in FIG. 5. That is, maximum transmission loss is detected around 1700 Hz, so that the noise level is extremely reduced. In this manner, the frequency characteristic of transmission loss of the sound isolation plate 10 is tuned readily and properly only by adjusting the projection amount S1 of the cylindrical portions 13 properly. As a result, effective noise suppression is achieved. In the structure shown in FIGS. 2 and 3, a curve LN2 in FIG. 5 shows a frequency characteristic of a comparative sample in which end portions of the cylindrical portions 13, 13 are closed. Various conditions for the comparative sample are set to be identical to those for the sound isolation plate 10 in the first embodiment, other than that the end portions of the cylindrical portions 13, 13 are closed. For instance, respective portions of the comparative sample are set to be identical in size to those of the sound isolation plate 10 in the first embodiment (R1=10 mm, α1 = 0.2, t1 = 2 mm, S1 = 50 mm). It would be understood, by comparing the curve LN1 with the curve LN2, that transmission loss of the sound isolation plate 10 in the first embodiment is extremely improved rather than the comparative sample. In the above example, the projection amount S1 of the cylindrical portions 13 has been set such that the peak frequency f0 of transmission loss coincides with the peak frequency f2 of the noise to be isolated. However, a center frequency f1 in the frequency bandwidth of the noise to be isolated may be used in place of the peak frequency f2. In this case, instead of above Eqs. (4) and (5), S1 = C/4f1 L1 - t1 = C/4f1 may be employed. Second Embodiment FIGS.7 and 8 show a sound isolation plate structure according to a second embodiment of the present invention. In the sound isolation plate 30 according to the second embodiment of the present invention, porous material 18 such as wool material, cloth, soft cellular resin, etc. is filled between the cylindrical portions 13, 13 of the perforated board 11 which is constructed in the same manner as the first embodiment (see FIGS. 2 and 3). The frequency characteristic of transmission loss of the sound isolation plate 30 is easily tuned by adjusting a filling factor p or a filling thickness W of such porous material 18. For instance, as shown by curves LN3, LN4, LN5 in FIG. 9, the frequency characteristic of transmission loss is tuned by changing the filling thickness W into 5 mm, 10 mm, 50 mm respectively. Hence, if the filling thickness W is adjusted such that the peak frequency f0 of resultant transmission loss is in agreement with the center frequency or the peak frequency in the frequency bandwidth of the noise to be isolated, the noise is precisely shielded. In addition, as evident from the characteristic view in FIG. 9, anti-resonant peaks in the vibration response of the transmitted wave, which is caused by the porous material 18 filled between the cylindrical portions 13, 13, are made relatively smooth. Further, if the sound isolation plate 30 is arranged such that one surface 11 a side of the perforated board 11 from which the porous material 18 is exposed is directed toward a noise generating source side (or, when the sound isolation plate 30 is employed as the under cover of the engine room, the sound isolation plate 30 is arranged such that the porous material 18 is directed toward the engine room side), a noise absorbing effect is also obtained by the porous material 18. As a result, a noise level of the noise generating source side (incident wave side) of the sound isolation plate 30 is simultaneously reduced, so that the sound isolation effect is much more improved as the overall sound isolation plate 30. Third Embodiment FIGS. 10 and 11 show a sound isolation plate structure according to a third embodiment of the present invention. The sound isolation plate 20 according to the third embodiment comprises two sheets of perforated boards 21, 23 made of resin. A plurality of holes 22, 24 of the same number are formed in the perforated boards 21, 23 respectively. All holes 22, 24 are formed as circular holes each having the same diameter R2. The perforated boards 21, 23 are positioned at a predetermined interval such that the holes 22, 24 are opposed to each other. Peripheral portions of the opposing holes 22, 24 are connected via cylindrical portions 25 which are formed like cylinders respectively. Inner surfaces of the holes 22, 24 and inner surfaces of the cylindrical portions 25 are connected to form coplanar circular surfaces respectively. Cylindrical continuous holes 25a which continue from one surface 21a side of one perforated board 21 to one surface 23a side of the other perforated board 23 are partitioned with the holes 22, 24 and the cylindrical portions 25. Air chambers 26 are formed between the neighboring cylindrical portions 25, 25. Circular opening portions 27 are formed on the other perforated board 23 to connect the air chambers 26 to the external space. Cylindrical portions 27a are projected from peripheral portions of the circular opening portions 27 into the air chambers 26 respectively. The circular opening portion 27 includes an inner surface of the cylindrical portion 27a and has a predetermined depth h. Because of the holes 22, 24, a rate of hole area of each of the perforated boards 21, 23 is set to a predetermined value α1. Because of the circular opening portions 27, a rate of hole area of the perforated board 23 is set to a predetermined value β (α1+ β < 1). FIG. 12 is a sectional view showing a sound isolation mechanism of the sound isolation plate 20. Continuous holes 25a, which are partitioned with the holes 22, 24 and the cylindrical portions 25, serve as pneumatic masses 28 respectively. Pneumatic vibration systems 29 are formed in the air chambers 26. Each of such pneumatic vibration systems 29 consists of a pneumatic mass 30 which is made up of an air layer in the circular opening portion 27, and an air spring 31 which is made up of an air layer which is located inner (the perforated boards 21 side) than the circular opening portion 27. Accordingly, in the third embodiment, these pneumatic vibration systems 29 act as dynamic dampers against the transmitted wave transmitted via the continuous holes 25a of the sound isolation plate 20 to thus provide anti-resonance to a vibration response of the transmitted wave, so that transmission loss is improved and also a noise level is reduced. As a result, if used as the under cover provided on the bottom of the engine room of the car, the sound isolation plate 20 produces a remarkable effect. While comparing with the above first embodiment, the third embodiment will be explained hereunder. In the third embodiment, a rate of hole area α2, diameters R2 of the holes 22, 24, thicknesses t2 of the perforated boards 21, 23, and an effective length (a total thickness of the sound isolation plate 20) L2 of the cylindrical portions 25 are set to be equal to those of the sound isolation plate 10 shown in FIGS. 1 and 2 ( α1 = α2, R1=R2, t1=t2, L1=L2). In this case, the peak frequency of transmission loss of the sound isolation plate 20 is indicted by a curve LN6 in FIG. 13, and such peak frequency of transmission loss is shifted to the lower frequency side than the peak frequency of transmission loss in the first embodiment indicated by the curve LN1. As a result, the third embodiment is effective for a case where the noise having the peak frequency on the lower frequency side is to be isolated. Moreover, the peak frequency of transmission loss is tuned by adjusting any of the diameter R3 of the circular opening portion 27, the rate of hole area β , the depth h of the circular opening portion 27, the total length L2, and the height S2 of the air chamber, all being constituent elements of the vibration systems 29. Therefore, the noise is precisely shielded by putting the peak frequency of transmission loss into substantial agreement with the center frequency or the peak frequency of the frequency bandwidth of the noise to be isolated. Fourth Embodiment In a sound isolation plate 40 according to a fourth embodiment of the present invention shown in FIG. 14, the porous material 18, as used in the second embodiment, is filled in the air chamber 26 of the sound isolation plate 20 according to the third embodiment shown in FIGS.10 and 11. Therefore, in the case of this fourth embodiment, a frequency characteristic of transmission loss is tuned arbitrarily by adjusting the filling density p of the porous material 18, in addition to the diameter R3 of the circular opening portion 27 of the vibration system 29 acting as the dynamic damper, the rate of hole area β, the depth h of the circular opening portion 27, the total length L2, and the height S2 of the air chamber. For instance, if the rate of hole area β of the circular opening portion 27 is changed into 0.04, 0.08, 0.4 respectively, the frequency characteristic of transmission loss is changed as indicated by curves LN7, LN8, LN9 in FIG. 15 respectively. If the peak frequency of transmission loss is brought into coincidence with the center frequency or the peak frequency of the frequency bandwidth of the noise to be isolated, the noise is precisely shielded. Like the case of the above second embodiment, anti-resonant peaks of the vibration response of the transmitted wave are also made smooth by filling the porous material 18 in the fourth embodiment. Further, if the sound isolation plate 40 is arranged such that the perforated board 23 on which the circular opening portions 27 are provided to expose the porous material 18 is directed toward the noise generating source side (or, such that the surface side from which the porous material 18 is exposed is directed toward the noise generating source), the noise absorbing effect is also obtained by the porous material 18. As a result, the noise level of the noise generating source side (incident wave side) of the sound isolation plate 40 is simultaneously reduced, so that the sound isolation effect is much more improved.

Claims: A sound isolation plate structure (10, 20, 30, 40) comprising: a sheet of perforated board (11, 21) in which a plurality of holes (12, 22) are formed, cylindrical portions (13, 25) provided on one surface side of the perforated board (11, 21) to extend from peripheral portions of the holes (12, 22), and pneumatic vibration systems (17) formed between the cylindrical portions (13, 25), characterized in that the holes (12, 22) of the perforated board (11, 21) are provided corresponding to the cylindrical portions (13, 25), respectively, and each pneumatic vibration system (17) has one longitudinal end closed by the perforated board (11, 21). A sound isolation plate structure (10, 20, 30, 40) according to claim 1, characterized in that a thickness t of the perforated board (11), an effective hole length L of the cylindrical portion (13), an acoustic velocity C, and a center of frequency f1 in a frequency bandwidth of a noise to be isolated are set to satisfy a relationship of L - t = C/4f1. A sound isolation plate structure (10, 20, 30, 40) according to claim 1, characterized in that a thickness t of the perforated board (11), an effective hole length L of the cylindrical portion (13), an acoustic velocity C, and a peak frequency f2 of a noise to be isolated are set to satisfy a relationship of L - t = C/4f2. A sound isolation plate structure (30) according to claim 1, characterized in that a porous material (18) is filled between the cylindrical portions (13). A sound isolation plate structure (30) according to claim 2, characterized in that a porous material (18) is filled between the cylindrical portions (13), and a peak frequency (f2) of transmission loss which is changed according to a filling density (p) and a filling thickness (W) of the porous material is set to coincide substantially with the center frequency (f1) in the frequency bandwidth of the noise to be isolated. A sound isolation plate structure (30) according to claim 3, characterized in that a porous material (18) is filled between the cylindrical portions (13), and the peak frequency (f2) of transmission loss which is changed according to a filling density (ρ) and a filling thickness (W) of the porous material (18) is set to coincide substantially with the peak frequency (f2) of the noise to be isolated. A sound isolation plate structure (20) according to claim 1, characterized in that the sound isolation plate structure (20) further comprises another sheet of second perforated board (23) in which a plurality of holes (24) and opening portions (27) are formed and which is arranged at a predetermined interval such that the holes (22) of the perforated board (21) are opposed to the holes (24) of the second perforated board (23), the pneumatic vibration systems (17) between the cylindrical portions (25) are defined to open to an external space through the opening portions (27) of the second perforated board (23). A sound isolation plate structure (20) according to claim 7, characterized in that a center frequency (f1) of a frequency bandwidth of a noise to be isolated is set to coincide substantially with a peak frequency (f2) of transmission loss which is changed according to a diameter (R1) of the opening portion (27) of the second perforated board (23), a rate of hole area (α1, β) of the opening portion (27), a depth (h) of the opening portion (27), a total thickness of the sound isolation plate structure (20), and a height of each of air chambers (26) formed between the cylindrical portions (25). A sound isolation plate structure (20) according to claim 7, characterized in that a peak frequency of a noise to be isolated is set to coincide substantially with a peak frequency (f2) of transmission loss which is changed according to a diameter of the opening portion (27) of the second perforated board, a rate of hole area of the opening portion (27), a depth of the opening portion (27), a total thickness of the sound isolation plate structure (20), and a height of each of air chambers (26) formed between the cylindrical portions (25). A sound isolation plate structure (40) according to claim 7, characterized in that a porous material (26) is filled between the cylindrical portions (25). A sound isolation plate structure (40) according to claim 10, characterized in that a center frequency (f1) of a frequency bandwidth of a noise to be isolated is set to coincide substantially with a peak frequency (f2) of transmission loss which is changed according to a filling density (ρ) of the porous material (26) filled between the cylindrical portions (25), a diameter (R1) of the opening portion (27) of the second perforated board (23), a rate of hole area (α1, β) of the opening portion (27), a depth of the opening portion (27), a total thickness of the sound isolation plate structure (40), and a height of each of air chambers (26) formed between the cylindrical portions (25). A sound isolation plate structure (40) according to claim 10, characterized in that a peak frequency of a noise to be isolated is set to coincide substantially with a peak frequency (f2) of transmission loss which is changed according to a filling density (p) of the porous material (26) filled between the cylindrical portions (25), a diameter (R1) of the opening portion (27) of the second perforated board (23), a rate of hole area (α1, β) of the opening portion (27), a depth of the opening portion (27), a total thickness of the sound isolation plate structure (40), and a height of each of air chambers (26) formed between the cylindrical portions (25). A sound isolation plate structure (40) according to claim 10, characterized in that the second perforated board (23) from which the porous material (18) is exposed to an external space through the opening portions (27) is arranged so as to direct toward a noise generating source. A sound isolation plate structure (20) according to claim 10, characterized in that the second perforated board (23), on which the opening portions (27) to expose the porous material (18) are provided, is arranged so as to direct toward a noise generating source.

Inventors: IWAO KEIJIRO, SHIMPO YUJI, IWAO, KEIJIRO, SHIMPO, YUJI
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1998
Main classification: B60R13
IPC codes: B60R13, B62D25, G10K11