WO2003100289A1

WO2003100289A1 - Vibration damping member, device and method

Info

Publication number: WO2003100289A1
Application number: PCT/US2003/013552
Authority: WO
Inventors: Tomoyuki Asano
Original assignee: 3M Innovative Properties Company
Priority date: 2002-05-21
Filing date: 2003-05-10
Publication date: 2003-12-04
Also published as: AU2003241336A1; JP4056296B2; JP2003336687A

Abstract

A vibration damping member (10) includes a first layer of viscoelastic material (12), a second layer (14) having a rigidity higher than the first layer, and an intermediate layer (16) having a rigidity higher than the first layer but lower than the second layer and interposed between the first layer and the second layer. In the intermediate layer (16), an annular peripheral portion extends outward from an outer peripheral edge of the second layer as a whole. The vibration damping member could be manufactured by blanking a laminated sheet to have a predetermined profile without cutting material forming the second layer (14).

Description

VIBRATION DAMPING MEMBER, DEVICE AND METHOD

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a vibration damping member having viscoelastic material and a method for manufacturing the same. Also, the present invention relates to a suspension device provided with the vibration damping member.

A vibration damping member having viscoelastic material has been used in various fields in the prior art. For example, in a disk drive unit used as an additional memory unit for an information processing apparatus, a head suspension elastically supporting a head writing/reading data to/from a disk-like memory medium is required to damp or restrict vibration of the head generated during the operation of the disk drive unit as soon as possible. Thus, in the prior art, a countermeasure is adopted in that a vibration damping member constituted by laminating a damping layer of metal or resin onto a restraint layer consisting of viscoelastic material is bonded to a flat spring of a head suspension while the damping layer is in contact with the latter (see, e.g., Japanese Unexamined Patent

Publication (Kokai) No. 11-66780).

According to the vibration damping member of such a laminated structure type, it has been known that a favorable vibration damping performance is exhibited when the restraint layer has the same rigidity as that of a substrate, the vibration of which is to be restricted (the flat spring in the above example). Therefore, the vibration damping member having a metallic restraint layer is generally used as the head suspension for the disc unit in the prior art because of its superior vibration damping performance. The vibration damping member of this type is generally manufactured by blanking via a press a large-sized laminated stock formed by coating one surface of a large-sized restraint sheet of metal or resin with a viscoelastic layer to have a predetermined profile.

Another vibration damping measure, adoptable in the head suspension of the disk drive unit, is also known, wherein a signal-transmittable circuit board is affixed to the head suspension, so that a substrate part of the circuit board serves as a damping layer and a circuit part thereof serves as a restraint layer (see, e.g., Japanese Patent No. 2633491). However, this countermeasure has some problems, which may deteriorate a practical use and a reliability in comparison with the exclusive vibration damping member as described above, such that the circuit may be peeled off or severed due to vibration applied to the circuit board, or that it is difficult to select a material or profile of the circuit, suitable for exerting desired damping effects, because of a circuit configuration determined to ensure a desired electrical conduction and a desired line layout.

When the vibration damping member having the metallic restraint layer described above is manufactured by the press, it is difficult to simultaneously blank a number of vibration damping members from the large-sized laminated stock since the rigidity of the restraint sheet is high, whereby the production efficiency becomes lower as well as the production cost rises. Also, upon the blanking, there may be some inconveniences such that a burr or slope on the peripheral edge of the restraint layer are formed by cutting the restraint sheet, that the restraint sheet is broken by the cutting, or that the viscoelastic layer protrudes from the periphery of the restraint layer, which may lower the yield. The burr or slope on the peripheral edge of the restraint layer not only adversely effects the vibration damping performance of the vibration damping member itself but also may disturb the operation of the disk drive unit when it is used as a head suspension in the disk drive unit. Further, the viscoelastic material protruded from the peripheral edge of the restraint layer may accelerate the adhesion of dust thereto which must be avoided particularly in the disk drive unit.

On the other hand, when the vibration damping member having the resinous restraint layer is manufactured by the press, it is possible to simultaneously blank a number of vibration damping members from a large-sized laminated stock effectively at a low cost because the rigidity of the restraint sheet is relatively low. Also, in the blanking, since the restraint sheet can be accurately cut at a high speed, it is possible to prevent as much as possible burr or slope from generating on the peripheral edge of the restraint layer as well as the viscoelastic material from protruding from the peripheral edge. In the vibration damping member having the resinous restraint layer, however, there may be a case in which the vibration damping performance could not be sufficiently exhibited in accordance with the uses as described above.

An object of the present invention is to provide a vibration damping member having viscoelastic material, capable of being effectively manufactured to have a stable quality at a low cost without lowering the vibration damping performance thereof, and a method for manufacturing such a vibration damping member. Another object of the present invention is to provide a suspension device having the vibration damping member capable of exhibiting a sufficient vibration damping performance and being manufactured to have a stable quality at a low cost.

To accomplish the above objects, the invention provides a vibration damping member having a laminated structure including a first layer made of a viscoelastic material and a second layer made of a metal, characterized in that the vibration damping member comprises an intermediate layer interposed between the first and second layers, the intermediate layer having a rigidity higher than that of the first layer but lower than that of the second layer; and that the intermediate layer is provided with a peripheral portion extending along an entire peripheral edge of the second layer and outward from the peripheral edge.

The invention also provides a vibration damping member as defined by claim 1, wherein the peripheral portion of the intermediate layer substantially covers the first layer. The invention also provides a vibration damping member as defined by claim 1 or 2, further comprising a release sheet layer provided on a side of the first layer opposite to the intermediate layer.

The invention also provides a method of manufacturing a vibration damping member including a viscoelastic material, characterized by the steps of providing a metal sheet; laminating a resinous layer on one surface of the metal sheet; locally removing the metal sheet to locally expose one surface of the resinous layer and to form a metallic region having a predetermined profile substantially encircled by an exposed area of the one surface of the resinous layer; laminating a viscoelastic material layer on a back surface of the resinous layer opposite to the one surface; and cutting the resinous layer and the viscoelastic material layer within the exposed area of the one surface of the resinous layer. The invention also provides a suspension device comprising a substrate and a vibration damping member attached to the substrate, characterized in that the vibration damping member comprises a first layer made of a viscoelastic material adapted to be attached to the substrate, a second layer made of a metal and an intermediate layer interposed between the first and second layers and having a rigidity higher than that of the first layer but lower than that of the second layer; and that the intermediate layer is provided with a peripheral portion extending along an entire peripheral edge of the second layer and outward from the peripheral edge. The present invention will be described in more detail below with reference to the preferred embodiments illustrated in the attached drawings wherein common reference numerals are used for denoting the corresponding constituent elements throughout all the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective view of a vibration damping member according to one embodiment of the present invention.

Fig. 2 is a plan view of the vibration damping member shown in Fig. 1. Fig. 3 is a plan view of a suspension device according to one embodiment of the present invention.

Fig. 4 is a side view of the suspension device shown in Fig. 3.

Fig. 5 is a schematic perspective view of a disk drive unit on which the suspension device shown in Fig. 3 is mounted. Fig. 6 shows illustrations of the steps (a) to (e) in a manufacturing process of a vibration damping member according to one embodiment of the present invention.

Fig. 7 is a perspective view of a modification of a vibration damping member.

Fig. 8 is a plan view of the vibration damping member shown in Fig. 7.

Fig. 9 is an illustration of the final step in a manufacturing process of a vibration damping member, according to another embodiment of the present invention.

Explanation of Reference Numerals

10 vibration damping member

12 first layer

14 second layer

16 intermediate layer

18 peripheral portion

19 slit

20 release sheet layer

22 suspension device

26 substrate

28 arm

50 laminated structure With reference to Figs. 1 and 2, a vibration damping member 10 is illustrated. The vibration damping member 10 is of a laminated structure including a first layer 12 made of a viscoelastic material, a second layer 14 made of a metal having a rigidity higher than that of the first layer 12, and an intermediate layer 16 interposed between the first layer 12 and the second layer 14 and having the rigidity higher than that of the first layer 12 but lower than that of the second layer 14.

The first layer 12 of the vibration damping member 10 is a damping layer to be bonded to a surface of a substrate (for example, a flat spring for a head suspension of a disk drive unit), of which the vibration is to be damped, and has a generally uniform thickness and a profile in accordance with the required vibration damping performance.

Material for the first layer 12 may be selected in accordance with material of the substrate to be vibration-damped from various viscoelastic materials of a resin type, a rubber type, an asphalt type or a metallic type. Particularly, it is advantageous to prepare the first layer 12 from material having a self- adhesiveness so that the vibration damping member 10 could be directly bonded to the substrate to be vibration-damped.

The second layer 14 is a restraint layer for constituting a so-called restraint- type vibration damping structure, and has a generally uniform thickness in accordance with the required vibration damping performance and a profile similar in shape but, preferable, slightly smaller in size than the first layer 12. Material for the second layer 14 may be selected, depending on the material of the substrate to be vibration-damped, from metal materials with various rigidities, such as a stainless steel, an aluminum alloy, a nickel alloy, a copper, a titanium, a iron, etc. Particularly, the second layer 14 preferably has the rigidity substantially the same as that of the substrate to be vibration-damped in view of ensuring the sufficient vibration damping performance. The intermediate layer 16 is a characteristic layer element of the vibration damping member 10, and has a generally uniform thickness and the same profile as that of the first layer 12. The intermediate layer 16 has a first surface 16a to be uniformly in contact with the first layer 12 as a whole, and a second surface 16b opposite to the first surface 16a and to be uniformly brought into contact with the second layer 14 in a predetermined central area thereof. Accordingly, in the intermediate layer 16, an annular peripheral portion 18 extended outward from the outer peripheral edge 14a is defined throughout an outer peripheral edge 14a of the second layer 14. The intermediate layer 16 covers substantially as a whole a surface 12a of the first layer 12 closer to the second layer 14 by itself including the peripheral portion 18. Material for the intermediate layer 16 may be selected from various materials, such as a metal, a thermoplastic resin or a hardened adhesive, which facilitates the expected operation and effect during a process for manufacturing the vibration damping member 10 as described later.

According to the vibration damping member 10 of the above-mentioned structure, it is possible to eliminate the cutting of second layer 14 when the vibration damping member is manufactured by a blanking process through a press from a large-sized laminated stock by forming the second layer 14 having the highest rigidity in the laminated structure smaller in profile size than the vibration damping member 10 to be blanked. Thereby, the blanking process of the vibration damping member 10 is substantially identical to that of the conventional vibration damping member having a resinous restraint layer. Accordingly, it is possible to assuredly avoid the problems of the lowering of the production efficiency or the generation of burr or slope in the peripheral edge portion of the restraint layer caused by the blanking of the metallic restraint sheet in the prior art blanking process of the vibration damping member. In addition, the required vibration damping performance can be easily ensured by optimizing material and or dimensions of the second layer 14 which is a restraint layer.

For the purpose of ensuring the above operation and effect, it is advantageous to manufacture the intermediate layer 16 from a resin in a similar manner to the resinous restraint layer used in the conventional vibration damping member. In such a case, the intermediate layer 16 not only functions as a supporting layer for directly supporting the second layer 14 which is a restraint layer but also exhibits the restraint performance to the first layer of viscoelastic material in cooperation with the second layer 14. Accordingly, the second layer 14 and the intermediate layer 16 may be regarded as a restraint sheet of a double-layered structure used for the vibration damping member 10.

Also, the intermediate layer 16 operates to protect by the peripheral portion 18 thereof an outer peripheral region of the first layer 12 extended outward from the outer peripheral edge 14a of the second layer 14. Further, if the intermediate layer 16 is made of a low-adhesive material, it is possible to avoid the adhesion of dust to the peripheral portion 18 exposed from the second layer 14. In this respect, when the vibration damping member 10 is provided to the user as a ready-made product, a release sheet layer 20 is preferably attached to aback surface 12b of the first layer 12.

The vibration damping member 10 of the above-mentioned structure may be attached to a surface of a substrate to be damped, made of metal, wood, concrete, plastic or others, so that the first layer 12 is directly adhered thereto by the adhesiveness thereof to exhibit the desired vibration damping performance to the substrate. Figs. 3 and 4 illustrate a suspension device 22 according to one embodiment of the present invention in which such a vibration damping member 10 is attached to a head suspension of a disk drive unit which is an additional memory unit for an information processing apparatus. Fig. 5 schematically illustrates a structure of a hard disk drive unit 24 widely used in a personal computer or the like provided with a disk drive unit having the suspension device 22.

The suspension device 22 includes a flat spring-like substrate 26, a vibration damping member 10 and an arm 28 for supporting the substrate 26 to be elastically deformable. The substrate 26 functions as a head suspension for elastically supporting a head 32 which operates to write/read data to/from a disk-like memory medium 30 rotating at a high speed. The substrate 26 has a generally trapezoidal profile, in an upper area including a shortest top side of which is provided the head 32 and a lower area including a longest side of which is fixedly connected to the arm 28. In the hard disk drive unit 24 illustrated, a plurality of suspension device 22 are integral with each other at proximal ends of the respective arms 28 to constitute an actuator 36 rotatable in a predetermined angular range about a pin 34. The actuator 36 supports a plurality of heads 32 carried by a plurality of suspension devices 22 so that the respective head 32 is opposite to a recording surface 30a of the disk-like memory medium 30 to carry out the tracking operation onto the recording surface 30a.

In the above-mentioned structure, the vibration damping member 10 in the respective suspension device 22 operates to damp and restrict the vibration of the respective head 32 generated during the operation of the hard disk drive unit 24 as soon as possible. If the intermediate layer 16 of the respective vibration damping member 10 is manufactured from a low adhesiveness material, the adhesion of dust to the peripheral portion 18 exposed outside the second layer 14 which must be absolutely inhibited is assuredly avoidable. In this regard, instead of or in addition to bonding the vibration damping member 10 onto the surface of the substrate 26 in the suspension device 22, the vibration damping member 10 may be bonded to a surface of the arm (that is, another substrate).

In the vibration damping member 10 applied to the suspension device 22, acrylic type viscoelastic material, silicone type viscoelastic material, rubber type viscoelastic material, polyolefine resin or others is favorably adopted as viscoelastic material forming the first layer 12. Particularly, the acrylic type viscoelastic material is advantageous because it is excellent in vibration damping characteristic as well as less in gas generation during the heating. A thickness of the first layer 12 may be variously selected in accordance with materials and desired vibration damping performances, for example, in a range from 1 to 150 μm. A stainless steel or an aluminum alloy may be suitably adopted as a material for the second layer 14. Particularly, if the substrate 26 of the suspension device 22 is formed from a stainless steel, the second layer 14 is also preferably formed from stainless steel. On the other hand, for the purpose of reducing the material cost, an aluminum alloy is advantageous. A thickness of the second layer 14 may be variously selectable in accordance with materials and desired vibration damping performances, for example, in a range from 1 to 100 μm.

The intermediate layer 16 of the vibration damping member 10 applied to the suspension device 22 may be formed from thermoplastic resin such as polyimide, polyethylene terephthalate or aramid, or from an adhesive in a hardened-state such as an epoxy-base or urethane-base adhesive. Particularly, polyimide is advantageously used, which has been widely used for the conventional resinous restraint layer. A thickness of the intermediate layer 16 may be selected, for example, in a range from 1 to 100 μm. If the thickness is less than 1 μm, the material for the intermediate layer 16 may be difficult to be handled in the process for manufacturing the vibration damping member 10 described later. On the other hand, if the thickness exceeds 100 μm, the rigidity of the material for the intermediate layer 16 becomes too high to cause the same inconvenience as when the metallic layer is cut.

One example of a method for producing the above-mentioned vibration damping member 10 will be described with reference to Fig. 6.

First, a metal sheet 38 is prepared, having a front surface 38a and a back surface 38b opposite to each other. The metal sheet 38 has a uniform thickness as a whole which is generally the same as that of the second layer 14 in the vibration damping member 10. Then, a resinous layer 40 is formed on the back surface 38b of the metal sheet 38 by, for example, a coating process (Fig. 6(a)). The resinous layer 40 is formed to have a uniform thickness and generally the same size as that of the intermediate layer 16 of the vibration damping member 10 so that a generally flat front surface 40a and a back surface 40b are imparted.

Then, unnecessary portions 42 of the metal sheet 38 are locally removed by, for example, an etching process. For this etching process, a process conventionally performed as a circuit forming process in the production of a printed circuit board may be adopted. Thereby, the front surface 40a of the resinous layer 40 is locally exposed, and a plurality of metallic regions 44 with predetermined profiles are formed, which are substantially encircled by the exposed area of the resinous surface 40a (Fig. 6(b)). The respective metallic region 44 thus formed constitutes the second layer 14 of the vibration damping member 10. On the other hand, a viscoelastic material layer 48 is formed on one surface of a separately prepared release sheet material 46 by, for example, a coating process

(Fig. 6(c)). The viscoelastic material layer 48 is formed to have a uniform thickness as a whole which is generally the same as that of the first layer 12 of the vibration damping member 10, whereby a generally flat front surface 48a and a back surface 48b are imparted. Next, the viscoelastic material layer 48 are laminated on the back surface of the resinous layer 40 carrying the plurality of metallic regions 44 so that the front surface 48a thereof is brought into tight contact with the latter to result in a large-sized laminated structure 50 (Fig. 6(d)). At this time, the viscoelastic material layer 49 is fixed to the resinous layer 40 by the adhesiveness of the viscoelastic material layer 48 of its own. Finally, the laminated structure 50 is supplied to a press to cut at once the resinous layer

40, the viscoelastic material layer 48 and the release sheet material 46 in the exposed area on the front surface 40a of the resinous layer 40 along the profile of the intermediate layer 16, the first layer 12 of the vibration damping member 10 and the release sheet layer 20. In this regard, unnecessary material generated by the cutting is discarded. In such a manner, the vibration damping member 10 having the peripheral portion 18 in the intermediate layer 16 is manufactured (Fig. 6(e)). In the method described above, since the metal sheet 38 having the highest rigidity in the laminated structure 50 is formed in advance to have the same profile as that of the second layer 16 in the vibration damping member 10 prior to the blanking process by the press, the cutting of the metal sheet 38 becomes unnecessary in the blanking process. A process for blanking the laminated structure 50 to have substantially the same profile as that of the vibration damping member 10 is substantially identical to a process for blanking the vibration damping member having the conventional resinous restraint layer. As a result, it is possible to assuredly avoid the problems expected when the metal sheet 38 is blanked, such as the generation of a burr or slope in the peripheral portion of the second layer, the protrusion of the first layer 12 from the peripheral portion of the second layer, and the breakage of the sheet 38.

Particularly, in the above-mentioned method, it is advantageous to simultaneously blank the plurality of vibration damping members 10 at once during the blanking process by the press. Also in such a case, the plurality of vibration damping members 10 could be effectively manufactured at low cost in a stable manner since the metal sheet 38 is not cut.

The vibration damping member 10 manufactured in this manner exhibits a favorable vibration damping performance required, for example, for the suspension device 22, due to the restraint function of the second layer having a high rigidity.

While the present invention has been described above with reference to the preferred embodiment, it should not be noted that the present invention is not limited to the illustrated embodiment but may include various changes and modifications made within a scope of claim for patent.

For example, as shown in Figs. 7 and 8, the second layer 14 in the vibration damping member 10 may be separated into a plurality of second layer portions 14' laid on the second surface 16b of the intermediate layer 16. In this arrangement, the shapes, layouts and numbers of the separated second layer portions 14' may be variously selected, depending on a damping performance required to the vibration damping member 10. For example, in the above-described application for the suspension device 22 shown in Fig. 3, the vibration damping member 10 is located on the substrate 26 with the upper and lower sides of the trapezoidal profile of the member 10 being oriented along a longitudinal direction of the substrate 26. In this arrangement, it has been found that the damping effect is improved when the second layer 14 has an elongated shape extending in the longitudinal direction of the substrate 26. Thus, as shown in Fig. 8, when the plural second layer portions 14' are formed to be elongated between the upper and lower sides of the trapezoidal profile of the vibration damping member 10, it is possible to improve the damping performance of the vibration damping member 10 in the suspension device 22. Also, in the manufacturing process of the vibration damping member 10, a separate resinous layer 40 may be formed in another process, and may be affixed to the back side 38b of the metal sheet 38 through an adhesive such as an epoxy-base or urethane-base adhesive, instead of the coating process of the resinous layer 40 onto the metal sheet 38. Alternatively, in place of the metal sheet 38, a metal layer having a desired thickness may be formed on the surface 40a of the resinous layer 40 through a deposition process such as a sputtering, or electroless plating and electroplating processes performed in this order. Further, instead of the coating process of the viscoelastic material layer 48 on the release sheet material 46, the viscoelastic material layer 48 may be directly coated on the back surface 40b of the resinous layer 40 through, e.g., a silk-screen printing process. In this case, the release sheet material 46 may be attached in a post process.

Also, in a final step in the manufacturing process of the vibration damping member 10, the release sheet material 46 in the laminated structure 50 may not be cut so as to provide slits 19, as shown in Fig. 9, in place of the full cut of the laminated structure into the separated independent vibration damping members 10. In this arrangement, the plural vibration damping members 10 are held in a strip form, mutually connected through the release sheet layers 20 thereof, and thus it is possible to store and transport the vibration damping members 10 in this strip form, as well as to improve an operational efficiency of mounting of the vibration damping member 10 due to the easy operation of peeling of the release sheet layers 20 from the respective damping members 10 in use. The vibration damping member according to the present invention may be used not only for the suspension device of the disk drive unit but also by adhering the same to a vibration source of various electronic equipments or audio equipments. Also, the method for manufacturing the vibration damping member according to the present invention may be applicable not only to the vibration damping member but also to other laminated products such as a heat-resistant film, a reinforcement tape or a decorative film. A laminated structure 50 having a basic structure shown in Fig. 6 was manufactured by laminating a viscoelastic material layer 48 of 25 μm formed by coating the viscoelastic material on a restraint layer material consisting of a copper sheet 38 of 30 μm thick and a polyimide layer 40 of 50 μm thick and drying the same at 100°C for 15 minutes, after which it is blanked into strips of 10 mm xlOO mm by the press to be vibration damping members 10. Nibration is applied to the vibration damping member 10 at a geometrical center of the first layer 12 or the second layer 14 and a vibration dissipation factor η in a secondary resonance frequency (in a range from 200 to 300 Hz) was measured. The dissipation factor η is calculated by the equation η= Kι/K₂ wherein a force f = Kι% at a maximum displacement χ and f = K₂% at a zero displacement are measured on a hysteresis curve of the vibration response system. As Comparative Examples 1 and 2, the dissipation factor η was measured in the same manner as above on a vibration damping member (1) having a restraint layer consisting solely of polyimide film of 50 μm and a vibration damping member (2) having a restraint layer consisting solely of stainless steel of 50 μm thick. Results thereof are shown in the following table.

Table 1

As is apparent from the above table, the vibration damping member according to the present invention exhibits the vibration damping performance at a level generally between those of the vibration damping member (1) having the conventional resinous restraint layer and the vibration damping member (2) having the conventional metallic restraint layer, which performance is more improved to be as good as the vibration damping member (2) as the temperature rises. As is apparent from the above description, according to the present invention, it is possible to efficiently manufacture a vibration damping member having viscoelastic material at a low cost while maintaining a stable quality without lowering the vibration damping performance thereof. Also, the inventive vibration damping member could exhibit an optimum vibration damping performance required for a substrate to be vibration-damped by suitably selecting kinds and profiles of the restraint layer.

Claims

1. A vibration damping member having a laminated structure including a first layer made of a viscoelastic material and a second layer made of a metal, characterized in that: said vibration damping member comprises an intermediate layer interposed between said first and second layers, said intermediate layer having a rigidity higher than that of said first layer but lower than that of said second layer; and that said intermediate layer is provided with a peripheral portion extending along an entire peripheral edge of said second layer and outward from said peripheral edge.

2. A vibration damping member as defined by claim 1, wherein said peripheral portion of said intermediate layer substantially covers said first layer.

3. A vibration damping member as defined by claim 1 or 2, further comprising a release sheet layer provided on a side of said first layer opposite to said intermediate layer.

4. A method of manufacturing a vibration damping member including a viscoelastic material, characterized by: providing a metal sheet; laminating a resinous layer on one surface of said metal sheet; locally removing said metal sheet to locally expose one surface of said resinous layer and to form a metallic region having a predetermined profile substantially encircled by an exposed area of said one surface of said resinous layer; laminating a viscoelastic material layer on a back surface of said resinous layer opposite to said one surface; and cutting said resinous layer and said viscoelastic material layer within said exposed area of said one surface of said resinous layer.

5. A suspension device comprising a substrate and a vibration damping member attached to said substrate, characterized in that: said vibration damping member comprises a first layer made of a viscoelastic material adapted to be attached to said substrate, a second layer made of a metal and an intermediate layer interposed between the first and second layers and having a rigidity higher than that of said first layer but lower than that of said second layer; and that said intermediate layer is provided with a peripheral portion extending along an entire peripheral edge of said second layer and outward from said peripheral edge.