WO2008141380A1

WO2008141380A1 - A sheet material for use in a multilayered acoustic shield

Info

Publication number: WO2008141380A1
Application number: PCT/AU2008/000707
Authority: WO
Inventors: Mark Borroni
Original assignee: Bellmax Acoustic Pty Ltd
Priority date: 2007-05-21
Filing date: 2008-05-20
Publication date: 2008-11-27

Abstract

A sheet material (1) for use as a layer in a multilayered acoustic shield. The sheet (1) includes a plurality of indentations, wherein the indentations each include an aperture (5) and side walls (7) which converge towards each indentations respective aperture (5). The apertures (5) are arranged to abut with an adjoining layer of an acoustic shield to thereby guide sound waves incident on the side walls (7) through the apertures (5) towards the adjoining layer. The sheet material (1) is particularly suited for use in an acoustic shield that can be mounted to a transmission tunnel of a motor vehicle.

Description

A SHEET MATERIAL FOR USE IN A MULTILAYERED ACOUSTIC SHIELD FIELD OF THE INVENTION

The present invention relates to a sheet material. More particularly, the present invention relates to a sheet material for use as a layer in a multilayered acoustic shield. As the sheet material is well suited for use in an acoustic shield that can be mounted to a panel, for example a transmission tunnel of a motor vehicle, it will be convenient to describe the invention in relation to that example application. It should however be understood that the invention is equally suitable for use as a layer in a multilayered acoustic shield for other applications besides vehicles.

BACKGROUND OF THE INVENTION

Acoustic shields are used to reduce the impact of undesirable sound, otherwise known as noise, on surrounding areas. Acoustic shields typically consist of one or more layers which interact with sound waves incident on the shield. In some instances acoustic shields are mounted on a surface to reduce the transmission of sound through the surface. For example, virtually all modern day motor vehicles have some form of acoustic shield lining the compartment areas of the motor vehicle, for example the luggage compartment, otherwise known as the vehicle's boot, and the passenger compartment, where the vehicle operator and/or passengers are located during transit. Whilst, the acoustic shielding is usually positioned internally of the compartment areas, it is however technically better to place the shielding externally, for example on the fire wall in the engine bay and/or underneath the vehicle's floorpan. Unfortunately, the engine bay and area underneath the floorpan of a vehicle are harsh environments for acoustic material as significant heat is generated by the engine, transmission and exhaust system in these areas. The exterior of an exhaust system which may include a muffler, catalytic convert and connecting pipes can typically reach temperatures of 500⁰C. Such extreme heat can result in the floorpan reaching temperatures in excess of 220⁰C. Further, these areas, particularly the underside of the floorpan, are usually subjected to dirt, grime, road debris, rocks and wet conditions. In the past, attempts have been made to protect acoustic shields in harsh environments by providing the shield with a protective outer layer made of sheet metal to protect underlying layers of the shield. A problem with this type of arrangement is that sound waves are not able to readily penetrate the outer layer and subsequently interact with the underlying layers. Instead, a significant proportion of sound waves incident on the outer layer are reflected away from the acoustic shield into the surrounding environment. This is particularly problematic as regulatory bodies are increasingly demanding a reduction in the emission of sound, for example by motor vehicles, into the surrounding environment. Attempts have been made to improve the acoustic performance of acoustic shields having a protective outer layer by providing a plurality of perforations in selected areas of the outer layer to increase the ability of sound waves to penetrate the layer and subsequently interact with the underlying layers. However, a problem with this approach is that the number and size of perforations needed to improve the performance of the shield undermines the outer layer's ability to protect the underlying layers from the harsh surrounding environment.

Accordingly, it would be desirable to provide a sheet material suitable for use in an acoustic shield which overcomes or ameliorates at least one of the abovementioned problems with the prior art.

Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other country on or before the priority date of the claims herein. SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a sheet material for use as a layer in a multilayered acoustic shield. The sheet includes a plurality of indentations wherein at least one of the indentations includes an aperture and sidewalls which converge towards the aperture. The aperture is arranged to abut with an adjoining layer of the acoustic shield to thereby guide sound waves incident on the sidewalls through the aperture towards the adjoining layer. In an embodiment of the present invention, the aperture is circular and the sidewalls provide a continuous curved surface extending around the aperture. Preferably, a multiple number of the indentations each include an aperture and sidewalls which converge towards their respective aperture. The sidewalls of adjoining indentations preferably converge towards one another with a cavity being formed between the sidewalls of adjoining indentations and the adjoining layer of the acoustic shield. An opening to each cavity is preferably provided at an intersection of the sidewalls of adjoining indentations to define each cavity as a Helmholtz resonator. In a particularly preferred embodiment, the sheet material is a support layer of a multilayered acoustic shield. The shield includes an outer portion and an inner portion. The outer portion includes the support layer and the inner portion includes a sound-absorbing layer. The outer portion may further include a metallic foil located between the support layer and the inner portion. In this regard, the metallic foil can be located between the support layer and the sound- absorbing layer. Further, the sound-absorbing layer is preferably made of a fibrous material.

In another embodiment, the inner portion may further include a backing layer made of a polymer film, paper or a thin layer of aluminium. The backing layer can be located next to the sound-absorbing layer such that the sound absorbing-layer is positioned between the backing layer and the outer portion.

The multilayered acoustic shield is fastenable to a panel such that the inner portion engages with the panel, to thereby reduce the transmission of sound waves through the panel. In use, the backing layer presses against the panel such that the sound-absorbing layer is preferably compressed between the backing layer and the metallic foil with the shield conforming with a curvature of the panel.

The present invention advantageously provides a sheet material which when used as an outer layer of a multilayered acoustic shield, is able to protect underlying layers from excessive heat, debris, water and the like whilst at the same time enhance the ability of sound waves to penetrate the layer and subsequently interact with the underlying layers. Further, the Helmholtz resonator created by each cavity formed between the sidewalls of adjoining indentations and the adjoining layer of the acoustic shield advantageously reduces mechanical and/or vibrational energy of the sound wave interacting with each cavity by converting the energy into thermal energy. The present invention also minimises the reflection of sound waves into the surrounding environment. BRIEF DESCRIPTION OF THE DRAWINGS

Further benefits and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention. The following description should not be considered as limiting any of the statements in the previous section. The preferred embodiments will be described with reference to the following figures in which:

Figure 1 is a perspective view of a first side of a sheet material, according to a first embodiment of the invention;

Figure 2 is a perspective view of a second side of the sheet material shown in Figure 1 ; Figure 3 is a cross-sectional view of the sheet material illustrated in Figure

1 taken along line A-A in Figure 1 ;

Figure 4 is a perspective view of a first side of a sheet of material according to a second embodiment of the invention;

Figure 5 is a perspective view of a second side of the sheet of material shown in Figure 4;

Figure 6 is a cross-sectional view of the sheet material illustrated in Figure 4 taken along line A-A in Figure 4;

Figure 7 is a perspective view of a first side of a sheet of material according to a third embodiment of the invention; Figure 8 is a cross-sectional view of the sheet material illustrated in Figure

7 taken along line A-A in Figure 7;

Figure 9 is a cross sectional view of a transmission tunnel in a vehicle with a multilayered acoustic shield attached thereto, the multilayered acoustic shield having a layer of sheet material in accordance with an embodiment of the invention;

Figure 10 is a cross sectional view of a multilayered acoustic shield illustrating the various components of the shield according to an embodiment of the invention; Figure 10a is a cross sectional view of a multilayered acoustic shield illustrating the various components of the shield according to another embodiment of the invention; and

Figure 11 is a graph of the absorption coefficient of various sample layers at a range of frequencies.

DESCRIPTION OF PREFERRED EMBODIMENT

With reference to the accompanying drawings, there is shown a sheet material 1 for use as the main structural layer of a multilayered acoustic shield. The sheet material 1 includes a plurality of indentations 3 with a multiple number of the indentations 3 each having an aperture 5 and sidewalls 7 which converge towards their respective aperture 5. The size of the apertures 5 and the depth of the indentations 3 have been magnified in the drawings, particularly in Figures 1 to 6, 10 and 10a, for clarity. The apertures 5 are preferably circular and have a diameter of between approximately 2.5 mm and 10 mm. The apertures 5 can however be of any other desired shape, for example square, triangular, etc. The sheet material 1 is preferably rigid and made of a metal, for example aluminium, with the thickness of the metal being between approximately 0.4 mm and 2 mm. The indentations 3 provide the sheet material 1 with improved strength and rigidity. First, second and third embodiments of the sheet material 1 are shown in

Figures 1 to 3, 4 to 6 and 7 & 8, respectively. Like reference numerals are used in the embodiments to identify the same features. The overall thickness of the sheet materials 1 shown in Figures 1 to 8 ranges between approximately 3mm and 5 mm, depending upon the depth of the indentations 3 and the thickness of the metal.

In the first and third embodiments of the sheet material 1 the sidewalls 7 of adjoining indentations 3 converge towards one another and are separated from each other by a surrounding region 6. The surrounding region 6 can be reduced in size by positioning the apertures 5 closer to one another. The surrounding region 6 can be planar, as shown in Figures 1 , 2 and 7, or alternatively be curved. In the sheet material shown in Figure 7, the apertures 5 preferably provide the sheet material 1 with an open area of approximately 25%. In the second embodiment of the sheet material 1 the sidewalls 7 of adjoining indentations 3 converge towards one another with a perforation 8 being provided at the intersection of the sidewalls 7 of adjoining indentations 3. As shown in Figures 10 and 1Oa₁ when the sheet material 1 is positioned such that the apertures 5 rest on a surface, for example an adjoining layer of an acoustic shield, a cavity 9 is formed between the sidewalls 7 of adjoining indentations 3 and the adjoining surface. The perforations 8 provide an opening to each cavity 9 to define each cavity as a Helmholtz resonator. The perforations 8 may range between approximately 0.1mm and 1.0mm in diameter and can be formed by piercing the sheet material 1 which can also provide each perforation 8 with a neck-like region. When a sound wave interacts with a perforation 8 the air in the neck-like region of the perforation 8 acts like a plug and is pushed into the cavity 9 by the sound wave. Pressure within the cavity 9 then acts like a spring and forces air out of the cavity 9. This causes a slight vacuum in the cavity 9 which results in air being forced back into the cavity 9. The net result of this back and forth motion is that some of the sound waves' energy will be absorbed.

In the accompanying drawings the apertures 5 are circular and the sidewalls 7 of the indentations 3 form a continuous curved surface around each aperture 5. In Figures 1 and 2 the sidewalls 7 also curve or flare outwardly from the apertures 5 by a considerable amount, in comparison with the sidewalls 7 in the sheet material 1 shown in Figures 4 and 5. The angle of the sidewalls 7 with respect to the aperture 5 and the degree of curvature away from the apertures 5 can be varied to suit each application. In the cross-sectional view of the sheet material 1 shown in Figures 3 and 6 the sidewalls 7 converge towards the aperture 5 and are generally parabolic with the overall shape of each indentation 3 generally resembling that of funnel, frustum or truncated cone. In the cross- sectional view of the sheet material 1 shown in Figure 8 the sidewalls 7 also converge towards the aperture 5 and are generally parabolic. However, in Figure 8 free ends 10 of the sidewalls 7 are substantially parallel to one another. The sidewalls 7 of the indentations 3 can be of any shape which is suitable for guiding sound waves incident on the sidewalls 7 towards and into the apertures 5. For example, the sidewalls 7 could be planar with the overall shape of each indentation 3 resembling a truncated pyramid. It is also possible that a combination of differently shaped indentations 3 could be provided over the sheet material 1.

As can be seen from Figures 3 and 6, in cross-section the sheet material 1 may generally be regarded as having the appearance of a sinusoidal wave with the apertures 5 being provided at the troughs and the sidewalls 7 terminating at the peaks.

The indentations 3 in the sheet material 1 shown in the accompanying drawings are arranged in columns and row. The indentations 3 may alternatively be non-uniformly distributed over the sheet material 1. Further, the indentations 3 may be grouped together and only provided over a specific section of the sheet material 1. In addition, the sidewalls 7 and apertures 5 in the indentations 3 are able to be formed in a single embossing process. With this process the apertures 5 can be created exactly at the trough of the indentations 3 rather than being randomly positioned as occurs when the apertures are created as a separate process.

In Figures 9, 10 and 10a of the accompanying drawings, there is shown a multilayered acoustic shield 13 which has an outer portion and an inner portion. In Figure 9, the multilayered acoustic shield 13 is shown mounted to the underside of a transmission tunnel 19 in a motor vehicle floorpan. In Figure 10, the multilayered acoustic shield 13 is illustrated prior to being suitably shaped and mounted to the transmission tunnel 19. The outer portion of the shield 13 includes a support layer 15 which consists of the sheet material 1. The support layer 15 is the main structural layer of the acoustic shield 13. The inner portion includes a sound-absorbing layer 17. The outer portion of the multilayered acoustic shield 13 further includes a metallic foil 21 which is located between the support layer 15 and the sound-absorbing layer 17. In Figures 10 and 10a, the apertures 5 are shown in abutment with the metallic foil 21. It is however possible to position the apertures 5 in close proximity to the metallic foil 21 rather than abutment. In either case sound waves incident on the sidewalls 7 of the indentations 3 in the support layer 15 are able to be guided through the apertures 5 by reflection off the sidewalls 7 such that sound waves are thereby directed to the metallic foil 21. The metallic foil 21 is preferably made of aluminium and can range between approximately 0.01 mm and 0.04 mm in thickness. In conjunction with the support layer 15, the metallic foil 21 adds an additional layer of protection to underlying layers of the multilayered acoustic shield 13. The metallic foil 21 may have a series of micro-perforations to enable sound to more readily penetrate the foil 21 and subsequently interact with the sound-absorbing layer 17. The sound-absorbing layer 17 can be made of a fibrous material, for example an acoustic grade polyester batt having polyester fibres with a fibre diameter of approximately 2 denier to 10 denier, depending upon the wavelengths and frequencies of sound which are required to be absorbed. The sound-absorbing layer 17 may also include a percentage of polyester co-extrusion melt fibre ranging from 5% to 10% to ensure that the surface of the sound-absorbing layer 17 facing the transmission tunnel 19 is relatively smooth with no loose fibres. The sound-absorbing layer 17 typically has a minimum surface density of 800 grams per metre square and a thickness of about 5 mm or greater. As the sound- absorbing layer 17 is porous, airborne sound waves can propagate into the material with the mechanical or vibrational energy of the sound wave being reduced by converting the energy into thermal energy to friction within the material. In Figure 10a, a multilayered acoustic shield 13 is illustrated prior to being suitably shaped and mounted to a transmission tunnel 19. The shield 13 shown in Figure 10a includes an outer portion and an inner portion. The outer portion of the shield 13 in this embodiment includes a support layer 15 which consists of the sheet material 1. The outer portion further includes a metallic foil 21. The inner portion includes a sound-absorbing layer 17 and a backing layer 14. The sound- absorbing layer 17 is located between the backing layer 14 and the metallic foil 21. The backing layer 14 is particularly useful for capturing loose fibres when the sound-absorbing layer 17 contains no polyester co-extrusion melt fibres. In addition, the backing layer 14 advantageously protects the sound-absorbing layer 17 and provides a relatively smooth surface which enables a plurality of acoustic shields 13 to be stacked one upon the other without the sound-absorbing layer 17 be snagged on the outer portion of an adjacent acoustic shield 13. The backing layer 14 may be a polymer, for example polypropylene or a film based material. The backing layer 14 may also be an aluminium film, or paper.

As shown in Figure 9, the multilayer acoustic shield 13 is fastened to the vehicle floorpan at a series of spaced locations across the width of the shield 13 by fasteners 23 which may be of any suitable form, for example rivets, studs, nuts, self-tapping screws or the like. Whilst three fasteners 23 are shown in Figure 9, the acoustic shield 13 can instead be secured with only two fasteners 23 at opposing sides of the transmission tunnel 19. The acoustic shield 13 can extend longitudinally along the transmission tunnel 19, directly above the transmission of the vehicle, and may have additional fasteners 23 at spaced locations along the length of the acoustic shield 13.

Preferably, the acoustic shield 13 has some degree of flexibility to enable the shield 13 to maintain conformance with the curvature of the underside of the transmission tunnel 19 and compress the sound-absorbing layer 17. If the acoustic shield 13 does not have a backing layer 14, the sound-absorbing layer 17 is compressed against the transmission tunnel 19, as shown in Figure 9. Alternatively, if the acoustic shield 13 includes a backing layer 14, the backing layer 14 presses against the transmission tunnel 19 and the sound-absorbing layer 17 is compressed between the backing layer 14 and the outer portion of the shield 13. By compressing the sound-absorbing layer 17, the shield 1 can be of a minimal thickness. Selected sections of the acoustic shield 13 can also be compressed to create strengthening ribs 12 which may extend longitudinally of the shield 1. Further, the compressibility of the sound-absorbing layer 17 enables the shield 13 to conform with the shape of any protrusions within the transmission tunnel 19, for example cables, ducts, wires 25 and the like.

The graph in Figure 11 shows the absorption coefficient of various samples at a range of frequencies. The first sample is a non woven polyester sound-absorbing layer having a surface density of 1200 grams per square metre. The second sample is a planar support layer in combination with the polyester sound-absorbing layer of sample one. The support layer consists of a sheet of aluminium with a thickness of 0.5mm. The sheet has a plurality of circular apertures each having a diameter of 3mm. The apertures provide the sheet with an open area of 33%. The third sample is a support layer comprising of the sheet material of the present invention in combination with the polyester sound- absorbing layer of sample one. The sheet material of the third sample is aluminium with a thickness of 0.5mm and has indentations which make the overall thickness of the support layer 3mm. Each indentation has sidewalls which converge towards a circular aperture of 3mm in diameter. Each indention has a diameter of 5mm which reduces to 3mm at the aperture. The graph in Figure 11 shows that a greater proportion of sound is absorbed by the third sample, i.e. the sheet material of the present invention in combination with the polyester sound- absorbing layer, particularly at frequencies below 1000Hz. The present invention advantageously provides a sheet material 1 which is particularly well suited for use as an outer, main structural layer of an acoustic shield. In this regard, the sidewalls 7 of the indentations 3 are able to increase the transmission of sound to the underlying layers of the acoustic shield by diverting and focusing sound waves incident of the sidewalls 7 towards the apertures 5 rather than reflecting sound waves of the sidewalls 7 into the surrounding environment. Accordingly the number and diameter of the apertures 5 can be reduced without a corresponding reduction in the acoustic performance of the shield 1 being made. Further, the shield 1 is able to retain its ability to adequately protect the underlying layers of the shield 1 from the surrounding environment.

As the present invention may be embodied in several forms without departing from the essential characteristics of the invention, it should be understood that the above described embodiments should not be considered to limit the present invention but rather should be construed broadly. Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention. Whilst the invention has been described in relation to its use in acoustic shields which abut with a panel of a vehicle it should not be considered as limiting the invention to only that example application. In this regard, the invention is also intended to be suitable for use in free-standing acoustic shields. Further, the invention is also intended to be suitable for use in acoustic shields for buildings, aeroplanes, vacuum cleaners, washing machines, industrial machinery, generators, refrigerators, photocopy machines and the like.

Claims

CLAIMS:

1. A sheet material for use as a layer in a multilayered acoustic shield, the sheet including a plurality of indentations, wherein at least one of the indentations includes an aperture and sidewalls which converge towards the aperture, and wherein the aperture is arranged to abut with an adjoining layer of the acoustic shield to thereby guide sound waves incident on the sidewalls through the aperture towards the adjoining layer.

2. A sheet material as claimed in claim 1 wherein the sidewalls provide a continuous curved surface extending around the aperture.

3. A sheet material as claimed in claim 1 or 2 wherein the aperture is circular.

4. A sheet material as claimed in any one of the preceding claims wherein a multiple number of the indentations each include an aperture and sidewalls which converge towards their respective aperture.

5. A sheet material as claimed in claim 4 wherein the sidewalls of adjoining indentations converge towards one another.

6. A sheet material as claimed in claim 5 wherein a cavity is formed between the sidewalls of adjoining indentations and the adjoining layer of the acoustic shield.

7. A sheet material as claimed in claim 6 wherein an opening to each cavity is provided at an intersection of the sidewalls of adjoining indentations to define each cavity as a Helmholtz resonator.

8. A sheet material as claimed in any one of the preceding claims wherein the sheet material is made of metal.

9. A sheet material as claimed in any one of the preceding claims wherein the sheet material is rigid.

10. A sheet material as claimed in any one of the preceding claims wherein the indentations are formed by embossing.

11. A multilayered acoustic shield, the shield including an outer portion and an inner portion, the outer portion including a support layer and the inner portion including a sound-absorbing layer, wherein the support layer is a sheet material as claimed in any one of the preceding claims.

12. A multilayered acoustic shield as claimed in claim 11 wherein the outer portion further includes a metallic foil located between the support layer and the inner portion.

13. A multilayered acoustic shield as claimed in claim 12 wherein the metallic foil is located between the support layer and the sound-absorbing layer.

14. A multilayered acoustic shield as claimed in any one of claims 11 to 13 wherein the sound-absorbing layer is made of a fibrous material.

15. A multilayered acoustic shield as claimed in any one of claims 11 to 14 wherein the inner portion further includes a backing layer, the sound-absorbing layer being located between the backing layer and the outer portion.

16. A multilayered acoustic shield as claimed in any one of claims 11 to 15 wherein the shield is fastenable to a panel such that the inner portion engages with the panel, to thereby reduce the transmission of sound waves through the panel.

17. A multilayered acoustic shield as claimed in claim 16 wherein in use the backing layer presses against the panel.

18. A multilayered acoustic shield as claimed in claim 17 wherein the sound- absorbing layer is compressed against the backing layer.

19. A multilayered acoustic shield as claimed in any one of claims 15 to 18 wherein the shield conforms with a curvature of the panel.

20. A multilayered acoustic shield as claimed in any one of claims 15 to 19 wherein the panel is a panel of a motor vehicle.

21. A multilayered acoustic shield as claimed in claim 20 wherein the panel is a transmission tunnel in a motor vehicle floorpan.

22. A multilayered acoustic shield as claimed in claim 21 wherein the shield is mounted to an underside of the transmission tunnel.

23. A sheet material substantially as hereinbefore described with reference to the accompanying drawings.

24. A multilayered acoustic shield substantially as hereinbefore described with reference to the accompanying drawings.