GB2363364A

GB2363364A - Acoustically absorptive panel

Info

Publication number: GB2363364A
Application number: GB0014171A
Authority: GB
Inventors: Graham Tompson
Original assignee: Collins & Aikman Autom Syst
Current assignee: Collins & Aikman Autom Syst
Priority date: 2000-06-09
Filing date: 2000-06-09
Publication date: 2001-12-19
Also published as: GB0014171D0; JP2004516972A; KR20030022133A; AU2001268251A1; CA2412009A1; WO2001096151A1; EP1292466A1

Abstract

An acoustically absorptive panel 12 has a plurality of through-holes 14 formed therein and is intended to change the acoustic properties of an enclosure, for example a vehicle interior, by means of attaching the panel to an enclosure wall possibly in a spaced apart 22 manner. There may be a layer of acoustically absorptive material 16 covering the outer surface of the panel, and also a layer of acoustically absorptive/reflective material 18 on the inner surface. When used in a vehicle the panel may constitute an inside panel of a door, a vehicle seat back or an accessory mat. The through-holes 14 may have a diameter of between 3 and 8 millimetres and cover between 5 and 20 % of the surface area of the panel. There is also disclosed a method of changing the acoustic properties of an enclosure using such a panel.

Description

2363364 ABSORPTIVE AUTOMOBILE COVERINGS The present invention relates to a

method and apparatus for changing the acoustic ?D properties of the inside of an enclosure. The apparatus and method relate particularly 5 to body panels, instrument panels, seats, shelves, floor mats and pillars of the inside of vehicles.

It is known that sound can be absorbed using a Helmholtz resonator. A Helinholtz resonator is an air-filled cavity with an opening. The sound of a certain frequency 10 can be absorbed by the resonator. The frequency which is absorbed, the so-called resonant frequency, depends on the volume of the cavity and the mass of air which oscillates in the opening.

The inside of vehicles have previously been covered by panels to improve the visual 15 impact and comfort of the inside of the vehicle. Whilst much research effort has centred on producing car audio systems with greater power output and a flatter frequency response, little effort has centred on the frequency response of the interior surface of vehicle cabins.

20 The present invention provides an enclosure defined by enclosure walls, said enclosure further comprising a panel having a plurality of throughholes formed therein from an outer surface to and inner surface of said panel; said panel being attached to one of said enclosure walls in spaced apart relationship such that said outer surface faces away from said one of said enclosure walls.

The present invention further provides a member for covering inner surfaces and varying the acoustic properties of an enclosure, said member comprising: a panel having a plurality of through-holes formed therein from an outer surface intended for facing away from an enclosure wall to an inner surface of said panel; and a covering of acoustically absorptive material covering said outer surface of -2 said panel and said through-holes, The present invention further provides a method of changing the acoustic properties of an enclosure comprising the steps of 5 providing a panel having a plurality of through-holes formed therein from an outer surface to an inner surface; and attaching said panel to an inner wall of said enclosure such that said outer surface faces away from said inner wall.

10 In this way the acoustic properties of the inside of a vehicle can be changed. Because previously used aesthetic panels can be exchanged for the member the weight of the vehicle is not significantly increased and neither is the assembly time. Furthermore, the visual impact of the vehicle to the user need not be impaired.

15 When the member of the hereinafter described embodiments is used as a panel in the interior of a car, driver comfort is improved because the member absorbs more sound in the audible frequency range than panels of the prior art. Thus the level of engine

2:) noises and other road noises can be reduced inside the car by use of the member.

20 The member of the hereinafter illustrated embodiments can improve the acoustic performance of the interior of the car by providing panels with absorption coefficient versus frequency in the audible range flatter than for panels of the prior art thereby distorting the audio output of in-vehicle entertainment less. Furthermore, overall acoustic performance is improved because the member has a higher absorption across 25 the audible frequency range resulting in less reflection of sound.

The distribution and size of the holes may be varied to optimise the range of frequencies which are absorbed.

30 The present invention will be described by way of examples with reference to the accompanying drawings in which:

Figure 1 shows a schematic representation of the cross-section of a member in accordance with the present invention; Figure 2 shows a schematic illustration of a door panel member in accordance with the present invention; 5 Figure 3 shows a schematic illustration of an accessory mat member in accordance with the present invention; and Figure 4 shows a graph of experimental results showing the variation in absorption coefficient with varying frequency for a door panel in accordance with the present invention and for a standard door panel.

Figure 1 shows a member 10 according to the present invention. The member comprises a panel 12 with a plurality of through-holes 14 drilled therethrough from an outer surface 15 to an inner surface 17. The panel 12 may be of any material but injection moulded plastics materials and wood are particularly suitable.

The member may be attached in spaced apart relationship to an inner wall of an enclosure to change the acoustic properties of the interior of the enclosure. The space between the panel 12 and the inner wall 20 forms an empty cavity which can optionally be filled by an absorptive material such as open cell foams or mineral 20 wool. Typical enclosures which would benefit from the use of the member 10 are passenger enclosures in means for transport such as aeroplanes, road vehicles, trains, boats etc.

It has been found that the acoustic coefficient is increased if a covering 16 is placed 25 on the outer surface 15 of the panel 12 to cover the outer surface 15 of the panel and the plurality of through-holes 14. The covering 16 should be of an acoustically absorptive material such as a woven textile fabric and would be approximately 5 millimetres thick. The covering should be porous to the flow of air. A leather or PVC covering with through holes could also be used. Preferably the covering 16 30 should also be decorative. Typically the covering would be secured (for example by adhesion) only by the edges of its back onto the outer surface 15 of the panel 12.

The diameter of the holes and the number of holes per unit area may be varied. The frequencies which are absorbed by the panel 12 vary according to the diameter and spacing of the holes, the material of which the panel 12 is made and its thickness. Typically, the panel 12 is between 1 and 8 millimetres thick, the plurality of holes 14 5 have a diameter of between 3 and 8 millimetres and the hole surface area to panel surface area ratio should be between 0.05 and 0.2. Each of the plurality of through holes 14 need not have the same diameter.

The inner surface 17 of the panel 12, on the opposite side to the outer surface 15, can 10 optionally be covered by a layer 18 of either acoustically reflective or acoustically absorptive material. Examples of such materials are open cell foams, mineral wool, aluminum wool or glass fiber for instance and heavy layer or plastic sheet respectively. Such a layer can have any thickness. The advantage of this layer 18 is that noise from outside the interior of the vehicle may be reflected back outside or 15 may be absorbed. The layer 18 need not be in contact with the panel 12 but may be in spaced apart relation from it.

In the embodiment illustrated in Figure 1, the member 10 is attached to a door panel 20 of a vehicle via fasteners 22 such that said covering 16 faces away from said door 20 panel. As will be clear to the skilled person, the member 10 could be attached in several other ways to door panel 20.

The member 10 attached to a door panel as illustrated in Figure 2 is in accordance with the present invention. In this embodiment, which is the same as that shown in 25 Figure 1, the member 10 is arranged such that there are a plurality of through-holes 14 over the majority, but not all, of the door panel 20. In the illustrated embodiment certain areas 24 are reserved for attachments, such as door handles, ashtrays etc., to the door.

30 In the embodiment illustrated in Figure 3, a floor mat 40 for a vehicle is illustrated in which accessory mats 44 are placed on top of the floor mat 40 in the regions of high wear. The accessory mats 44 are comprised of members according to the present invention. Floor mats 44 are individually placed in the floor wells of the car to avoid excess wear of the floor covering 40. In the illustrated embodiment, the floor mats 44 are comprised of members according to the present invention and include a 5 plurality of through-holes 14 to adjust the acoustic response of the interior of the vehicle. The acoustic response can be varied by placing different accessory mats 44 in the vehicle with different hole densities, diameters and/or mat thicknesses.

The mechanism by which the present invention works is not fully understood though it is thought that the mechanism maybe similar to that of the Helmholtz Resonator.

The formula which governs the resonant frequency (fo) of a Helmholtz resonator is C S fo 2z /+A/) VTT(-f- where c is the speed of the sound waves, S is the surface area of the opening, V is the volume of the cavity, I is the length of the opening i. e. distance from the outside surface to the cavity and 41 is a constant characteristic of the opening.

20 Figure 4 is a graph showing the acoustic absorption of a typical side panel insulator C) (door) for the interior of a vehicle with varying frequency. The curves were measured by placing three different door panels in a reverberation room one after another and playing random white noise into the room and measuring the reverberation time. The bottom curve in the Figure is for a door panel of the prior 25 art, i.e. a panel (injection moulded plastic) covered by a non-woven fabric covering. The middle line is the response for the same panel and covering but with holes drilled in the panel, i.e. a member in accordance with the present invention. As can be seen, in a frequency range of between 400 and 2000 Hz, the side panel comprised of a member according to the present invention has a significantly improved 30 absorption coefficient over the prior art. It is this frequency, range which is of -6interest since that range of frequencies is within the sensitivity of the human ear.

As well as absorbing more sound than the prior art, the member gives a flatter frequency response, meaning that when sounds are reflected, a more constant fraction

5 of each frequency is reflected. This is important if the occupant of the vehicle is listening to music since a flatter frequency response of the panels will result in less distortion of the music because all frequencies will be reflected to the same extent.

The data values shown in Figure 4 are for a member with holes of 3.3 min diameter, 10 a panel thickness of 3 mm and a hole surface area to panel surface area ratio of 0.07. The panel is has a thickness of 5 mm.

The top line in Figure 4 is for a covering and panel as in the middle line, but with a foam backing layer attached to the panel on the side opposite the covering. As can 15 be seen, this absorptive layer further improves the absorbing properties of the door panel.

As will be appreciated by the skilled person, the member of the present invention can be used in any components in the interior of a car. The two examples given are for a t5 20 side panel insulator of a door and a floor mat of a car. However, the instrument panel, the back of the seats, shelves and pillars can all be made of a member according to the present invention whilst retaining their visual impact.

CY

Claims

I An enclosure defined by enclosure walls, said enclosure further comprising a panel having a plurality of through-holes formed therein from an outer surface to an inner surface of said panel; said panel being attached to one of said enclosure walls in spaced apart relationship such that said outer surface faces away from said one of said enclosure walls.

10
2. The enclosure of claim I wherein said panel further comprises a covering of acoustically absorptive material covering said outer surface of said panel and said through-holes.
3. The member of claim I or 2 further comprising an absorptive layer of 15 acoustically absorptive material covering said inner surface of said panel and said through-holes.
4. The enclosure of any one of claims I to 3 wherein said enclosure is a passenger space in a means of transport.
5. A member for covering inner surfaces and varying the acoustic properties of an enclosure, said member comprisinga panel having a plurality of through-holes formed therein from an outer surface intended for facing away from an enclosure wall to an inner surface of said panel-, and a covering of acoustically absorptive material covering said outer surface of said panel and said through-holes.
6. The member of claim 5 further comprising a layer of acoustically reflective material covering said inner surface of said panel and said through-holes.
7. The member of claim 5 further comprising an absorptive layer of acoustically absorptive material covering said inner surface of said panel and said through-holes.
8. The member of any one of claims 5 to 7 wherein said plurality of through5 holes formed in said panel have a diameter of between 3 and 8 millimetres.
9. The member of any one of claims 5 to 8 wherein the hole surface area to panel surface area ratio should be between 0,05 and 0.2 of said plurality of through holes formed in said panel.
10. The member of any one of claims 5 to 9 wherein said panel thickness is between 1 and 8 millimetres.
11. A covering for the interior of a vehicle comprising a member of any one of 15 claims 5 to 10.
12. An inside panel for a door of a vehicle comprising the member of any one of claims 5 to 10.

20
13. A vehicle scat wherein a back panel of said seat is comprised of a member of any one of claims 5 to 10.
14. An accessory mat comprising a member of any one of claims 5 to 10.
15. A method of changing the acoustic properties of an enclosure comprising the steps of.. providing a panel having a plurality of throughholes formed therein from an outer surface to an inner surface; and attaching said panel to an inner wall of said enclosure such that said 30 outer surface faces away from said inner wall.

-g-
16. The method of claim 15 further comprising the step of covering said panel and said through-holes on said outer surface with a covering of absorptive material;
17. The method of claim 15 or claim 16 wherein said panel and said covering of zn 5 absorptive material are the panel and covering of absorptive material of the member of any one of claims 5 to 12.
18. The enclosure of any one of claims 1 to 3 wherein said panel is the panel of the member of anyone of claims 5 to 12.
19. A member for covering surfaces and varying the acoustic properties of an enclosure substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15
20. A method of changing the acoustic properties of an enclosure substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.