The present invention relates to a sound-proofing component for a rigid structural member, in particular of a vehicle.
In a vehicle, it is desirable to sound-proof the passenger compartment. To this end, there is used an “insulation” which prevents medium- and high-frequency sound waves from entering the sound-proofed space substantially by reflecting the waves towards the sources of noise or outside the sound-proofed space, and “acoustic absorption” which dissipates the energy of the sound waves (at medium and high frequencies) in an absorbent material.
It is relatively simple to obtain good sound-proofing at high frequencies using simple means but, in the range of medium frequencies (in particular between 400 and 1000 Hz), the problem of sound-proofing is all the more acute because the human ear is very sensitive in this frequency range.
For this sound-proofing, a mass/spring system is already used and substantially provides insulation, in particular in the region of the bulkhead which separates the passenger space from the engine compartment of a vehicle. A layer of a material which has a spring-like effect, such as a foam which may optionally have viscoelastic properties, carries a layer of a material of the “heavy mass” type, that is to say, the layer constituting the mass of the mass/spring system. Such a mass/spring system is known for the good insulation which it provides. Document GB-2 163 388 describes such a mass/spring system whose heavy mass portion comprises two layers.
The problem presented by this system is that, owing to its low level of absorption, the layer which forms the heavy mass must have a significant mass surface density in the order of from 3 to 7 kg/m2. Furthermore, such a system is not very effective in the medium frequency range.
Since it is desirable to make vehicles lighter in order to reduce consumption, pollution, etcetera, document WO 98/18 657 has proposed a system which has two layers with differing levels of resistance to air passage. In this system, a porous decoupling layer is in contact with the structural support member, such as a metal sheet, preferably with partial interposition of air, and it is covered by another porous layer 22. In one example, one layer is a compressed phenolic felt and the other a flexible non-compressed felt. The action of this system is a result of its dual-permeability, that is to say, the difference in resistance to air passage between the porous layers. The advantage of this system is that it is light, but the disadvantage thereof is that it provides practically no insulation.
Document WO 03/069 596 describes a complex sound-proofing system which comprises two groups of layers: one group of layers comprises a layer of the heavy mass type which is associated with a porous layer which forms a spring. This group constitutes a spring/mass system of the conventional type but the mass of the heavy layer and the thickness of the layer which forms a spring are reduced, for example, by from a third to a half, compared with the conventional system. The other group of layers comprises a porous layer of the acoustic spring type and an external layer which has a high level of resistance to air passage. This external layer may be a felt which has a level of resistance to air passage which is from 3 to 20 times greater than that of the other porous layer.
This document WO 03/069 596 thus describes an improved system which combines the mass/spring system with the system having a sound-proofing complex which has layers which have differing levels of resistance to air passage and provides excellent sound-proofing results with a reduced weight.
An improvement of the sound-proofing assembly described in this document WO 03/069 596 has also been envisaged, that is to say, which comprises superimposed layers and which comprises a first group of layers which have a good level of resistance to air passage and a second group of layers which has a mass/spring function; according to this improvement, the first group of layers is produced so that it comprises a layer of a foam with open cells with a high level of porosity, a high level of complexity and good resistance to the passage of air, this layer having, owing to its high level of complexity, excellent sound absorption properties at medium and high frequencies. The complexity, which corresponds to the inclination of the line illustrating the variation of the square of the refractive index for the acoustic wavelength used as a function of the inverse of the square root of the frequency is connected to the shape of the circulation paths in a porous material. The foams with open cells which are commonly used have a complexity of between 1 and 1.35 and a high level of complexity, preferably in the order of from 1.5 to 2.5.
In this manner, according to this improvement, the “sound-proofing” complex of the document WO 03/069 596 is replaced by a single layer of a material with a high level of complexity.
It has now been realised according to the invention that, when the sound-proofing system described in the above-mentioned document WO 03/069 596 or the above-mentioned improvement thereof was separated into two portions which are placed at one side and the other of the associated rigid structural member, it was possible to obtain, with identical constituent elements, either much better sound-proofing, or, with practically identical sound-proofing, a reduction in the weight of the component of at least 25% compared with the conventional mass/spring system.
Document WO 99/58371 describes a sound-proofing component which includes a structural member and which is constituted by a rigid protective plate which is mounted at the outer side of the structural member and so as to be remote therefrom, and an optional acoustic insulator which is mounted between the structural member and the protective plate.
Document WO 01/92086 describes an embodiment of a floor sound-proofing component whose main element is a “thick” structural member, that is to say, an element which is constituted by a core in the form of a honeycomb surrounded by two rigid layers. This core in the form of a honeycomb forms a thick layer of air.
More precisely, according to the invention, the portion which forms the sound-proofing complex which has layers whose levels of resistance to air passage are different is arranged at the side of the structural element which is directed towards the source of noise, and the mass/spring system is arranged at the side to be sound-proofed, for example, at the side of the passenger compartment of a motor vehicle.
On the bulkhead which separates the engine compartment from the passenger space, a single layer of foam or felt is sometimes arranged and is occasionally provided with a decorative element. This layer, which is incorporated for reasons of protection and aesthetics, has no sound-proofing function. This is particularly a result of the fact that this layer covers only a portion of the surface of the bulkhead, so that the portions of bulkhead which are not covered form a short-circuit or preferred acoustic path which cancels out the sound-proofing effect which such a layer could have.
The invention eliminates this effect involving an acoustic short-circuit since the opposing surfaces of the structural member which are covered by the mass/spring system on the one hand and the sound-proofing complex or acoustic absorption system on the other hand are practically opposite each other and of the same size.
More precisely, the invention relates to a sound-proofing component for a rigid structural member, of the type comprising a mass/spring system which comprises at least one layer of heavy mass and a layer which forms a spring, and an acoustic absorption system having two layers which have differing levels of resistance to air passage; according to the invention, the structural member is a metal sheet, the mass/spring system is arranged at one side of the rigid structural member, the acoustic absorption system is arranged at the other side of the rigid structural member, and the opposing surfaces of the structural member which are covered by the mass/spring system on the one hand and the acoustic absorption system on the other hand are opposite each other and of the same size.
Preferably, a first of the two layers of the acoustic absorption system has an adjustable level of resistance to air passage. Preferably, this first layer is formed by a non-woven material. The non-woven material is advantageously based on unsaturated polyester combined with cellulose or “Nylon”.
Preferably, the second of the two layers is porous and flexible. Preferably, the second of the two layers is formed by a material selected from a felt or a foam.
In one embodiment, the acoustic absorption system comprises a layer of foam which has a high level of complexity.
Preferably, the mass/spring system comprises a layer which forms a spring and which is adjacent to the rigid structural member. For example, the layer which forms a spring is a foam.
In a particularly efficient embodiment, the component further comprises, at the side of the mass/spring system opposite the rigid structural member, a sound-proofing complex which is formed either by two layers which have different levels of resistance to air passage, or a layer of foam with a high level of complexity.
In embodiments, the rigid structural member is a metal sheet which separates the passenger compartment of a vehicle from a space selected from an engine compartment, a wheel housing and an exhaust device.
Other features and advantages of the invention will be better understood from a reading of the following description with reference to the appended drawings, in which:
FIG. 1 is a conventional system of the type described in document WO 03/069 596;
FIG. 2 is a sound-proofing system according to the invention;
FIG. 3 comprises an improved system in accordance with document WO 03/069 596 compared with the system illustrated in FIG. 2; and
FIG. 4 is an improved system in accordance with the improvement of document WO 03/069 596 compared with the system illustrated in FIG. 2.
FIG. 1 is a schematic section of the structure of a sound-proofing component according to document WO 03/069 596 associated with a rigid structural member 10, for example, a metal sheet.
The component comprises a mass/spring system 12 which comprises a layer 14 which forms a spring, for example, a felt or a foam, and a layer 16 of heavy mass, for example, based on an elastomer which is charged with a mineral material.
The component also comprises a system or complex 18 for sound-proofing by means of acoustic absorption which comprises a layer 20, for example, of felt or foam, which has a pre-determined resistance to air passage, and a porous layer 22 which has a high level of resistance to air passage, this resistance preferably being adjustable. This outer layer 22 is advantageously formed by a non-woven fabric. Although it is not illustrated, a carpet may be arranged on the layer 22.
The sound-proofing component illustrated in FIG. 1 provides sound-proofing characteristics which are equivalent to those of a component of the mass/spring system, but with a weight reduction of up to 50% in some cases. It is therefore very advantageous to use such a component for the low weight thereof.
It has now been realised according to the invention that, when the component described in document WO 03/069 596 was used, if the mass/spring system 12 was placed at one side of the structural member 10 and the complex 18 for sound-proofing by means of absorption at the other side of this structural member 10, even better sound-proofing properties were obtained.
FIG. 2 illustrates such a system. The same references as in FIG. 1 have been used to refer to elements which are similar.
When the system illustrated in FIG. 2 is produced, that is to say, in which the mass/spring system 12 is positioned towards the passenger compartment of a vehicle and the sound-proofing complex 18 is positioned towards the engine compartment, the acoustic insulation increases from 10 to 35 dB between 1000 and 8000 Hz compared with the system described in the above-mentioned document WO 98/18 657.
Since it is desirable to reduce the weight of the components of motor vehicles, this property, for similar acoustic performance levels, corresponds to a reduction in weight of the sound-proofing component in the order of 25% compared with the conventional mass/spring system.
FIG. 3 illustrates a variant of the component of FIG. 2 providing an even greater level of sound-proofing. In this component, a complex 18′ for sound-proofing by means of absorption which is similar to the complex 18 but which may be thinner, is arranged inside so as to co-operate with the mass/spring system 12. Such a system allows a reduction in weight of up to 35% compared with the conventional mass/spring system.
The natures and the characteristics of the different layers used may be identical to those which are described in the above-mentioned document WO 03/069 596.
FIG. 4 illustrates another variant of the component of FIG. 2 which also provides even greater sound-proofing and which corresponds to the improvement already described for the component of the above-mentioned document WO 03/069 596. The references which are identical to those of FIG. 3 refer to elements which are similar.
In this component, the complex 18′ for sound-proofing by means of absorption is replaced by a single layer 18″ with a high level of complexity, for example, a layer which is formed by a polyurethane foam having a complexity close to 2, and which has a mass surface density of 0.6 kg/m2 and a thickness of 10 mm.
According to another improvement compared with the documents which have been described above, it is particularly advantageous to use, for the layers 22, 22′, light thermo-formable non-woven materials, for example, based on unsaturated polyester associated with cellulose or “Nylon” (“Ahlstrom” or “3M”) which may provide levels of resistance to air passage of from 300 to 2500 N.s/m3 for a mass surface density of from only 20 to 200 g/m2. Such non-woven fabrics readily allow an adjustable resistance to be obtained with respect to air passage.
It should be noted that the component illustrated in FIG. 2 is more effective at levels approaching medium frequencies, whilst the component illustrated in FIG. 3 is more effective at levels approaching high frequencies.
The layer most remote from the structural member, towards the interior of the vehicle, may constitute a decorative layer or carry such a layer.
Owing to a layer of the acoustic absorption system which has an adjustable level of resistance to air passage and which is advantageously formed by a non-woven material, associated with a flexible porous layer, providing an effect of differing resistance to air passage, the acoustic properties may be optimised, in particular in accordance with the acoustic characteristics of each of the two sides of the structural member: nature of the source of noise (engine noise, travel noise, exhaust noise, etcetera..), properties of the receiving cavity (passenger compartment, etcetera . . . ), etc.
For constituent elements which are identical to those of a sound-proofing component according to document WO 98/18 657, a sound-proofing component according to the invention provides an increase of from 10 to 35 dB for the insulation.
Compared with the conventional mass/spring system and with practically identical acoustic performance levels, the weight gain may reach 25% in the case of FIG. 2 and 35% in the case of FIG. 3, in accordance with the shape of the emission and receiving cavities.
There are numerous portions of the vehicle to which such components are well suited in addition to the partition which separates the engine compartment from the passenger compartment. It is possible to mention in particular the wheel housings, floors and the components close to the exhaust.
In this manner, according to the invention, the sound-proofing complex, which functions substantially by means of acoustic absorption, is positioned at the side of the source of noise; it thus ensures that the sound-proofing component is adapted to the emission spectrum of the source of noise (for example, the engine or the movement of the tyres), whilst the other portion which is constituted by a simple or improved mass/spring system in accordance with the above-mentioned document WO 03/069 596 or the improvement thereof (FIGS. 3 and 4) substantially provides the insulation of the passenger compartment. Consequently, this mass/spring system which is simple or improved, may be practically the same for the various sources of noise, that is to say, regardless of the location thereof in the passenger space (front bulkhead, floor or wheel housing).