WO2021209618A1

WO2021209618A1 - A soundproofing system for a vehicle

Info

Publication number: WO2021209618A1
Application number: PCT/EP2021/059956
Authority: WO
Inventors: Mathieu GONTIER; Bruno Brasseur; Paul DE ROOVER; Peter De Wilde
Original assignee: Recticel
Priority date: 2020-04-16
Filing date: 2021-04-16
Publication date: 2021-10-21
Also published as: NL2025358B1; EP4135996A1

Abstract

A soundproofing system (2) for a vehicle in particular for soundproofing a passenger compartment thereof. The system comprises a stack of layers which include a first cover layer (11) which is substantially air-impermeable; a second cover layer (12), and a first (14) and a second sound absorbing layer (15) which are both arranged between the first cover layer (11) and the second cover layer (12). The first and the second cover layers have a low areal weight. The acoustic performance of the light weight soundproofing system is enhanced by providing an intermediate substantially air-impermeable barrier layer (16) in between the first (14) and the second sound absorbing layer (15) and by using a first sound absorbing layer (14) which has a high air flow resistance, namely an air flow resistance which is higher than 3000 N.s/m³.

Description

“A SOUNDPROOFING SYSTEM FOR A VEHICLE”

Field of the invention

The present invention relates to soundproofing systems, in particular for use in automotive applications. The soundproofing system of the present invention is for example applicable for soundproofing a passenger compartment of a vehicle by providing the soundproofing system in a side fender or in a wheel-arch of a vehicle.

Background A conventional soundproofing system usually makes use of one or more physical phenomena referred to as "stiffness controlled damping", substantially at low frequencies below 400 Hz, "insulating", substantially in the range of medium frequencies in the range of 400 Hz to 1000 Hz to high frequencies above 1000 Hz, and "sound absorption", also substantially in the range of medium and high frequencies. Within the meaning of the present invention, a soundproofing system ensures “insulation” when it prevents the entry of sound waves in the soundproofed space, essentially by reflecting waves toward the sources of noise or outside the soundproofed space. A soundproofing system operates by “sound absorption” when the energy from the sound waves is dissipated in a sound absorbing material.

One such soundproofing system is disclosed in patent publication EP1710126A1. The disclosed soundproofing system comprises a stack of layers comprising: - a first and second cover layer, wherein the cover layers have an areal weight of between 20 and 100 g/m²;

- a first sound absorbing layer which has a first airflow resistance between 250 and 2500 Ns/m³, and which has only a limited thickness of between 0.1 and 1.5 mm and which is arranged between the first cover layer and the second cover layer; and - a second sound absorbing layer arranged between the first sound absorbing layer and the second cover layer and having a thickness of between 2 and 30 mm, the first and the second sound absorbing layers having, when measured together, a combined airflow resistance in the range of 400 to 3000 Ns/m³.

In the sound proofing system of EP1710126A1 the sound absorption is increased by the presence of the thin airflow resistant (AFR) layer between the second sound absorbing layer and the first cover layer. An advantage of such a soundproofing system is that it is lightweight and that, due to the provision of the cover layers, it is impervious to contaminations, in particular to water. Unwanted penetration of the water into the sound absorbing layers can considerably change the properties of the sound absorption and thus needs to be avoided in order to create a durable soundproofing system.

A drawback of such a soundproofing system is however that, when compared for example with the insulating properties of a steel plate having for example a thickness of 1 mm, it has much smaller sound transmission loss values and therefore does not give entire satisfaction. In practice, steel plate cannot be used to close the side fender gap of a car and is also no longer used as wheel arch. Notwithstanding the quite good sound insulating properties of a steel plate, its very high areal weight is an important disadvantage, A 1 mm thick steel plate has for example an areal weight of about 7.85 kg/m².

US 2015/0034414 discloses a soundproofing system in a side fender or in a wheel-arch of a vehicle. In the example of the sound insulation material L the soundproofing system consists successively of a first breathable nonwoven fabric having a thickness of 0.55 mm, a first chip layer of EPDM rubber foam of 22.5 mm (1.25 kg/m²), a 80 μm non- breathable polyethylene membrane layer, a second chip layer having the same composition as the first chip layer and a second breathable non- woven fabric having a thickness of 0,55 mm. The chip layers have an air flow resistivity (air flow resistance divided by the layer thickness) of 2000 Ns/m⁴, or in other words an air flow resistance of 45 Ns/m³ (= 2000 Ns/m⁴ x 0.0225 m). By the presence of the intermediate polyethylene membrane the transmission loss obtained by the soundproofing system is improved. However, notwithstanding the relatively high areal weight of this soundproofing system (somewhat higher than 2.5 kg/m²), the obtained transmission losses are still relatively small, namely only about 10 dB at a frequency of 500 Hz increasing to about 30 dB at a frequency of 8000 Hz. A further increase of the transmission loss could be obtained by providing a second intermediate polyethylene membrane in the foam chip layer (sound insulation material M) but this increase was limited to only about 2 to 5 dB.

It is thus the aim of the present invention to provide a soundproofing system with improved overall acoustic performance especially per areal weight, i.e. a lightweight soundproofing system that provides a good soundproofing effect over a wide range of frequencies, in particular over the range of 500 to 5000 Hz,

Description of the invention

The present invention provides a soundproofing system for a vehicle according to the first claim. The soundproofing system comprises a stack of layers comprising:

- a first cover layer which has an areal weight of less than 500 g/m²;

- a second cover layer, which has an areal weight of less than 500 g/m²;

- a first sound absorbing layer which is arranged between the first cover layer and the second cover layer; and

- a second sound absorbing layer arranged between the first sound absorbing layer and the second cover layer, characterised in that the first cover layer is substantially air-impermeable, i.e. has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1.0 I/m².s, preferably less than 0.5 l/m².s, in that said first sound absorbing layer has a first airflow resistance, measured in accordance with ISO 9053-1 :2018, Part 1, which is higher than 3000 Ns/m³ but lower than 50000 Ns/m³, in that said stack of layers comprises an intermediate barrier layer which is substantially air- impermeable, i.e. has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1,0 I/m².s, preferably less than 0.5 l/m².s and in that the intermediate barrier layer is arranged between said first sound absorbing layer and said second sound absorbing layer.

The soundproofing system of the present invention has the advantage that a soundproofing system with better overall acoustic performance per areal weight can be obtained. The overall acoustic performance of the present soundproofing system is ensured by providing a modified absorber-barrier-absorber complex (further referred to as ABA) encapsulated between two cover layers. Conventional ABA-complexes which provide an overall good acoustic performance, such as the ABA- complex known from patent publication EP1899949A1, have the disadvantage of requiring a barrier layer having a high mass, as well as a high mass supporting member such as a metallic plate, in order to create a mass-spring-mass system. Even though the overall acoustic performance of such ABA-complexes is good, the acoustic performance per weight of the soundproofing system is poor. It has been found that by providing the first cover layer and the intermediate layer as substantially air-impermeable layers, and by increasing the airflow resistance of the first sound absorbing layer as described in the first claim of the present invention, the mass of the barrier layer can be reduced and the high mass supporting member can be omitted, whilst maintaining the overall good acoustic performance, thereby obtaining good overall acoustic performance per unit of areal weight.

Preferably, the intermediate barrier layer has an areal weight of less than 500 g/m², preferably of less than 400 g/m² and more preferably of less than 300 g/m². The intermediate barrier layer may be a relatively thin polymeric foil and may have an areal weight of for example only 25 g/m². the areal weight of the intermediate barrier layer is preferably higher than 20 g/m².

Due to the relatively low areal weight of the intermediate barrier layer, the combination thereof with the first sound absorbing layer is not a mass-spring system. It has however been found surprisingly that, due to the high air flow resistance of the first sound absorbing layer, good soundproofing properties could nevertheless be achieved notwithstanding the quite low areal weight of the system. Since no real mass layer is required, but only a substantially impermeable barrier layer, the weight of the soundproofing system can indeed be reduced substantially while keeping similar soundproofing properties.

According to an embodiment of the present invention, the first cover layer is interconnected to the second cover layer such as to form a bag enclosing the first sound absorbing layer, the intermediate barrier layer and the second sound absorbing layer. Providing the cover layers as a bag enables to easily support and manipulate the modified ABA-complex for example without requiring the application of an adhesive between the bag and the modified ABA-complex, or even without the need of an adhesive between the layers of the ABA complex. It has indeed been found that the absence of an adhesive between the cover layers and the sound absorbing layers improves the soundproofing properties of the system. The present embodiment furthermore has the advantage that the first and second sound absorbing layers are fully shielded from external contaminations thereby improving the durability of the soundproofing system i.e. the soundproofing system maintains good acoustic performance over a long period of time.

According to an embodiment of the present invention the cover layers are layers that are substantially impervious to water. The cover layers preferably substantially alleviate the penetration of water, and also of other contaminations, into the sound absorbing layers, thereby increasing the durability of the soundproofing system.

Within the present invention, the term “thickness direction” preferably refers to the direction in which the layers are stacked within the soundproofing system.

Within the present invention, the term “average thickness of a layer” is defined as the ratio of the volume of the layer to the surface of the layer disposed substantially perpendicular to the thickness direction.

Preferred embodiments regarding the first sound absorbing layer:

According to an embodiment of the present invention said first air flow resistance is higher than 4000 Ns/m³, preferably higher than 5000 Ns/m³ and more preferably higher than 6000 Ns/m³. According to a further embodiment of the present invention, said first air flow resistance is smaller than 40 000 Ns/m³, preferably smaller than 30 000 Ns/m³ and more preferably smaller than 25 000 Ns/m³. The present embodiment has the advantage that an improved overall acoustic performance is obtained.

According to an embodiment of the present invention, the first sound absorbing layer has an average thickness which is larger than 4 mm, preferably larger than 6 mm and more preferably larger than 8 mm. According to a further embodiment of the present invention, the first sound absorbing layer has an average thickness which is smaller than 30 mm, preferably smaller than 25 mm and more preferably smaller than 20 mm. The present embodiment has the advantage that the required air flow resistance of the first sound absorbing layer can be easily obtained using material properties as described below. Moreover good soundproofing properties can be obtained with relatively limited total thicknesses of the soundproofing system.

According to an embodiment of the present invention, the first sound absorbing layer comprises a foam layer, in particular a polymeric foam layer and/or a fibrous layer such as a woven or a non-woven, for example a felt. The present embodiments have the advantage of efficiently absorbing sound energy.

According to an embodiment of the present invention, the first sound absorbing layer has a density of between 20 kg/m³ and 100 kg/m³, and preferably a density of between 30 kg/m³ and 70 kg/m³. The density is preferably defined as the ratio of the mass of the first sound absorbing layer to the total volume of the first sound absorbing layer, wherein the total volume comprises the volume of the structural material of the first sound absorbing layer and the volume of the pores. The present embodiment has the advantage that an air flow resistivity can be obtained, which in combination with the previously described thicknesses of the first sound absorbing layer, yield the required air flow resistance.

According to an embodiment of the present invention, the first sound absorbing layer has a Young’s modulus smaller than 300 kPa and preferably smaller than 200 kPa Preferably the first sound absorbing layer is isotropic in terms of Young’s modulus. Preferably, the Young’s modulus is defined as the Young’s modulus along the thickness direction. The present embodiment has the advantage that good sound absorbing properties are combined with good sound insulating properties thereby improving the overall soundproofing properties of the system, in particular the transmission loss.

According to an embodiment of the present invention, the tortuosity of the first sound absorbing layer is between 1 and 5, more preferably between 2.5 and 3.5. Such tortuosity values improve the soundproofing properties of the system.

According to an embodiment of the present invention, the porosity of the first sound absorbing layer is above 0.9, more preferably above 0.93. Such porosity values improve the soundproofing properties of the system.

Preferred embodiments regarding the first cover layer:

The first cover layer is preferably configured to be directed away from the source of noise.

According to an embodiment of the present invention, the first cover layer has an areal weight of less than 400 g/m², preferably of less than 300 g/m², the areal weight of the first cover layer being preferably higher than 30 g/m², more preferably higher than 50 g/m² and most preferably higher than 70 g/m². Providing a first cover layer with a higher areal weight increases the mechanical strength thereof and simultaneously tends to increase the acoustic performance of the soundproofing system.

According to an embodiment of the present invention, the average thickness of the first cover layer is larger than 30 μm, more preferably larger than 50μm and most preferably larger than 70μm. According to an embodiment of the present invention, the average thickness of the first cover layer is smaller than 400 μm, preferably smaller than 300 μm. Providing a first cover layer with a greater thickness increases the mechanical strength thereof and simultaneously tends to increase the acoustic performance of the soundproofing system.

According to an embodiment of the present invention, the first cover layer comprises a foil, for example a polymeric foil such as a polyethylene (PE) or a thermoplastic polyurethane (TPU) foil.

According to an embodiment of the present invention, the first cover layer covers the first sound absorbing layer over a predetermined surface area, the first cover layer being not adhered, i.e. not glued or otherwise fastened, to said first sound absorbing layer or only over at most 10% of said predetermined surface area. The present embodiment clarifies that it is preferable that the first cover layer is substantially loose with respect to the first sound absorbing layer, for example trapping one or more pockets of air between the first cover layer and the first sound absorbing layer. It has been found that a better acoustic performance is obtained in this way. The embodiment wherein the first cover layer is not adhered at all to the first sound absorbing layer, has the advantage that the first soundproofing system can be more easily manufactured.

Preferred embodiments regarding the intermediate barrier layer:

According to a first embodiment, the intermediate barrier layer has a bending modulus (measured according to ASTM D790 procedure B typel ) along any direction perpendicular to the thickness direction, higher than 8 N/mm and preferably higher than 20 N/mm, According to the present first embodiment, the intermediate barrier layer is a stiffening layer arranged to maintain structural stability of the soundproofing system, for example in situations wherein the soundproofing system is not well supported by a supporting surface, for example when the soundproofing system is positioned vertically, i.e. with the thickness direction of the soundproofing system perpendicular to the gravitational acceleration direction, for example when positioning the soundproofing system in a side-fender. The soundproofing system can thus be clamped easily between two walls to close the gap between these two walls without requiring any further support element. Preferably, the average thickness of the intermediate barrier layer is larger than 1 mm, and preferably smaller than 5 mm.

The intermediate barrier layer may be hollow. In this way the bending modulus can be increased without increasing the weight of the barrier layer. According to a second embodiment, alternative to the first embodiment, the intermediate barrier layer is a flexible layer, for example a foil such as a polymeric foil for example a polyethylene (PE) or a thermoplastic polyurethane (TPU) foil. Preferably, the intermediate barrier layer is substantially the same layer as the first cover layer, for example in terms of composition/structure, i.e. both are for example the same polymeric foil optionally having a different average thickness. In a first implementation, the intermediate barrier layer has an average thickness which is larger, for example at least double, than the average thickness of the first cover layer or of the second cover layer or of the first and the second cover layer. In a second implementation, the average thickness of the intermediate barrier layer is between 80% to 120%, preferably between 90% to 110% of the average thickness of the first cover layer. In general, the average thickness of the intermediate barrier layer is preferably smaller than 400 μm, preferably smaller than 300 μm, but preferably larger than 30μm, preferably larger than 50μm, preferably larger than 70μm, most preferably larger than 200 μm. In general, the intermediate barrier layer preferably has an areal weight of less than 400 g/m², preferably of less than 300 g/m², the areal weight of the intermediate barrier layer being preferably higher than 30 g/m², preferably higher than 50 g/m², preferably higher than 70 g/m², preferably higher than 150 g/m², most preferably higher than 200 g/m².

According to an embodiment of the present invention, the intermediate barrier layer is adhered to said second sound absorbing layer or to said first sound absorbing layer or to said first and to said second sound absorbing layer. The present embodiment has the advantage of simplifying the handling of the modified ABA-complex, for example upon inserting the modified ABA-complex into the bag formed by the cover layers. The present embodiment is particularly advantageous when the intermediate barrier layer is quite thin and flexible and thus difficult to handle separately.

Preferred embodiments regarding the second cover layer:

The second cover layer is preferably configured to be directed towards the source of noise.

According to an embodiment of the present invention, the second cover layer has an areal weight of less than 400 g/m², preferably of less than 300 g/m², the areal weight of the second cover layer being preferably higher than 30 g/m², more preferably higher than 50 g/m² and most preferably higher than 70 g/m². Providing a second cover layer with a higher areal weight increases the mechanical properties thereof and simultaneously tends to increase the acoustic performance of the soundproofing system.

According to an embodiment of the present invention, the average thickness of the second cover layer is larger than 30μm, more preferably larger than 50μm and most preferably larger than 70μm. According to an embodiment of the present invention, the average thickness of the second cover layer is smaller than 400 μm, preferably smaller than 300 μm.

According to an embodiment of the present invention, the second cover layer covers the second sound absorbing layer over a predetermined surface area, the second cover layer being not adhered, i.e. not glued or otherwise fastened, to said second sound absorbing layer or only over at most 10% of said predetermined surface area. The present embodiment clarifies that it is preferable that the second cover layer is substantially loose with respect to the second sound absorbing layer, for example trapping one or more pockets of air between the second cover layer and the second sound absorbing layer. It has been found that a better acoustic performance is obtained this way. The embodiment wherein the second cover layer is not adhered at all to the second sound absorbing layer, has the advantage that the first soundproofing system is more easily manufactured.

Preferred embodiments referred to as “double impermeable cover layers”:

In these preferred embodiments, the first and the second cover layers are both substantially impermeable to air.

According to an embodiment of the present invention, not only the first but also the second cover layer is substantially air- impermeable, i.e. has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1.0 I/m². s, preferably less than 0.5 l/m².s and comprises preferably a polymeric foil such as a polyethylene (PE) or a thermoplastic polyurethane (TPU) foil.

According to an embodiment of the present invention, not only the first but also the second sound absorbing layer has a second airflow resistance which is higher than 3000 Ns/m³ but lower than 50 000 Ns/m³. According to a further embodiment of the present invention, said second air flow resistance is higher than 4000 Ns/m³, preferably higher than 5000 Ns/m³ and more preferably higher than 6000 Ns/m³. According to a further embodiment of the present invention, said second air flow resistance is smaller than 40 000 Ns/m³, preferably smaller than 30 000 Ns/m³ and more preferably smaller than 25 000 Ns/m³. The present embodiment has the advantage that an improved overall acoustic performance is obtained.

According to an embodiment of the present invention, the airflow resistance of the second sound absorbing layer is between 80% to 120%, preferably between 90% to 110% of the airflow resistance of the first sound absorbing layer. The present embodiment has the advantage that an improved overall acoustic performance is obtained.

According to an embodiment of the present invention, the second cover layer is substantially the same layer as the first cover layer, for example in terms of composition/structure, i.e, both are for example the same polymeric foil optionally having a different average thickness. In a first implementation the average thickness of the first cover layer is between 80% to 120%, preferably between 90% to 110% of the average thickness of the second cover layer. In a second implementation, the average thickness of the first cover layer is larger, for example by a factor between 1.33 to 5, than the second cover layer. This implementation has the advantage to provide a more tear-resistant first cover layer whilst minimizing the overall areal weight of the soundproofing system.

According to an embodiment of the present invention, the second sound absorbing layer has an average thickness which is larger than 4 mm, preferably larger than 6 mm and more preferably larger than 8 mm. According to a further embodiment, the second sound absorbing layer has an average thickness which is smaller than 30 mm, preferably smaller than 25 mm and more preferably smaller than 20 mm. The present embodiment has the advantage that the required air flow resistance of the second sound absorbing layer can be easily obtained using material properties as described below. Moreover good soundproofing properties can be obtained with relatively limited total thicknesses of the soundproofing system.

According to an embodiment of the present invention, the second sound absorbing layer comprises a foam layer, in particular a polymeric foam layer and/or a fibrous layer such as a woven or a non- woven, for example a felt.

The present embodiments have the advantage of efficiently absorbing sound energy.

According to an embodiment of the present invention, the second sound absorbing layer has a density of between 20 kg/m³ and 100 kg/m³, and preferably a density of between 30 kg/m³ and 70 kg/m³. The density is preferably defined as the ratio of the mass of the second sound absorbing layer to the total volume of the second sound absorbing layer, wherein the total volume comprises the volume of the structural material of the second sound absorbing layer, and the volume of the pores if any. The present embodiment has the advantage that an air flow resistivity can be obtained, which in combination with the previously described thicknesses of the second sound absorbing layer, yield the required air flow resistance.

According to an embodiment of the present invention, the second sound absorbing layer has a Young’s modulus smaller than 300 kPa and preferably smaller than 200 kPa. Preferably, the Young’s modulus is defined as the Young’s modulus along the thickness direction. The present embodiment has the advantage that good sound absorbing properties are combined with good sound insulating properties thereby improving the overall soundproofing properties of the system, in particular the transmission loss.

According to an embodiment of the present invention, the tortuosity of the second sound absorbing layer is between 1 and 5, more preferably between 2.5 and 3.5. Such tortuosity values improve the soundproofing properties of the system.

According to an embodiment of the present invention, the porosity of the second sound absorbing layer is above 0.9, more preferably above 0.93. Such porosity values improve the soundproofing properties of the system.

According to an embodiment of the present invention, the average thickness of the first sound absorbing layer is between 80% to 120%, preferably between 90% to 110% of the average thickness of the second sound absorbing layer. Providing the intermediate barrier layer between substantially equally thick sound absorbing layers, provides a balance between the amount of sound absorption and the amount of sound insulation, thereby obtaining overall better acoustic performance.

Preferred embodiments referred to as “single impermeable cover layer”: In these preferred embodiments, the first cover layer is substantially impermeable to air whilst the second cover layer is permeable to air.

According to an embodiment of the present invention, alternative to the above mentioned embodiment regarding “double impermeable cover layers”, the second cover layer is substantially air permeable, i.e. has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of at least 100 l/m².s, preferably of at least 150 l/m².s, and said second sound absorbing layer has a second airflow resistance which is smaller than 1000 N.s/m³, preferably smaller than 500 N.s/m³ and more preferably smaller than 200 N.s/m³. The present embodiment is particularly advantageous to increase the ratio of sound absorption to the sound insulation, in comparison to the above mentioned embodiments regarding “double impermeable cover layers”. The present “single impermeable cover layer” embodiments are particularly advantageous when the soundproofing system is positioned with the second cover layer being positioned closer to the engine noise source than the first cover layer, because the engine sound will be efficiently absorbed in this way.

According to an embodiment of the present invention, the second sound absorbing layer has an average thickness which is larger than 4 mm, preferably larger than 6 mm and more preferably larger than 8 mm. According to a further embodiment, the second sound absorbing layer has an average thickness which is smaller than 30 mm, preferably smaller than 25 mm and more preferably smaller than 20 mm. With such thicknesses, the second sound absorbing layer can provide good sound absorbing properties which substantially contribute to the overall soundproofing properties of the system. Absorbing a substantially portion of the noise reduces the overall noise level in the “noise” (engine) compartment so that less noise can pass through the soundproofing system.

According to an embodiment of the present invention, the second sound absorbing layer comprises a foam layer, in particular a polymeric foam layer. Preferably according to the present embodiment, the second sound absorbing layer is a porous layer, for example comprising a network of open cells. The present embodiments have the advantage of efficiently absorbing sound energy.

According to an embodiment of the present invention, the porosity of the second sound absorbing layer is above 0.9, more preferably above 0.93.

According to an embodiment of the present invention, the average thickness of the first sound absorbing layer is between 80% to 120%, preferably between 90% to 110% of the average thickness of the second sound absorbing layer. Providing the intermediate barrier layer between substantially equally thick sound absorbing layers, provides a balance between the amount of sound absorption and the amount of sound insulation, thereby obtaining overall good acoustic performance.

Brief description of the figures Figure 1 is a perspective view of a car showing the positioning of an embodiment of the soundproofing system in the side-fender as well as on the wheel arch.

Figure 2 is a detailed view of the positioning of an embodiment of the soundproofing system on the wheel arch.

Figure 3a is a cross-sectional exploded view of an embodiment of the soundproofing system positioned in a side fender.

Figure 3b is a cross-sectional view of the embodiment according to Figure 3a wherein the soundproofing system is clamped in the side fender.

Figure 4a is a cross-sectional exploded view of an embodiment of the soundproofing system positioned on a wheel arch.

Figure 4b is a cross-sectional view of the embodiment according to Figure 4a wherein the soundproofing system is fixed onto the wheel arch.

Figures 5-10 are graphs depicting the Transmission Loss as a function of frequency for an open hole, for a soundproofing system of the state of the art provided in the open hole, for a steel plate provided in the open hole, and for different embodiments of the soundproofing system according to the present invention provided in the open hole.

Detailed description

Hereinafter the invention will be described in certain embodiments and in reference to the accompanying figures. The present invention is however not limited by the following description.

Figure 1 is a perspective view of a car 1 showing the positioning of an embodiment of the soundproofing system 2 in the side- fender 3 as well as on the wheel arch 4. Airborne sound emanates from different sources in the car, such as from the engine 5 and from the wheels 6 The soundproofing system 2 is provided onto the wheel arch 4 and into the side fender 3 in order to limit the propagation of sound in the passenger compartment 7 in particular by providing sound insulation and sound absorption.

Figure 2 is a detailed perspective view of the positioning of an embodiment of the soundproofing system 2 on the wheel arch 4.

Figure 3a is a cross-sectional top exploded view of an embodiment of the soundproofing system 2 positioned in a cavity 8 of the side fender 3. The cross-section has been taken along line l-l in figure 1 . The cavity 8 of the side fender 3 is delimited by opposing lateral walls 13, wherein one of the opposing lateral walls 13 is for example an outside panel of the car 1. The soundproofing system 2 interconnects the opposing lateral walls 13 of the side fender cavity 8 thereby dividing the cavity 8 into a first part 9 and a second part 10, wherein the first part 9 is closer to the engine noise source 17 and further away from the passenger compartment 7 than the second part 10, The soundproofing system 2 comprises a stack of layers comprising:

- a first cover layer 11 which has an areal weight of less than 500 g/m²; the first cover layer 11 having an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1.0 I/m². s, i.e. the first cover layer 11 is substantially air- impermeable,

- a second cover layer 12 which has an areal weight of less than 500 g/m²;

- a first sound absorbing layer 14 which has a first airflow resistance, measured in accordance with ISO 9053-1 :2018, Part 1 , wherein the first air flow resistance is higher than 3000 Ns/m³ but lower than 50 000 Ns/m³, and wherein the first sound absorbing layer 14 is arranged between the first cover layer 11 and the second cover layer 12;

- a second sound absorbing layer 15 arranged between the first sound absorbing layer 14 and the second cover layer 12; and - an intermediate barrier layer 16 which has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1.0 l/m².s, i.e. the intermediate barrier layer 16 is substantially air-impermeable, wherein the intermediate barrier layer 16 is arranged between said first sound absorbing layer 14 and said second sound absorbing layer 15.

The intermediate layer is preferably a stiffening layer, avoiding the undesired sagging of the soundproofing system 2 in the side fender cavity 8. As illustrated in Figure 3b the soundproofing system 2 is indeed clamped between the two lateral walls 13 and no further support is needed in order to avoid said sagging.

The soundproofing system 2 is positioned in the side-fender such that the first cover layer 11 lays adjacent to the second part 10 of the side-fender cavity 8 and such that the second cover layer 12 lays adjacent to the first part 9 of the side fender cavity 8, The second cover layer 12 can be substantially air-impermeable, in which case the second sound absorbing layer 15 preferably has a second airflow resistance, measured in accordance with ISO 9053-1 :2018, Part 1 , wherein the second air flow resistance is higher than 3000 Ns/m³ but lower than 50 000 Ns/m³. In an alternative embodiment, the second cover layer 12 is substantially air permeable, i.e. has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of at least 100 l/m².s, preferably of at least 150 l/m².s, in which case said second sound absorbing layer 15 preferably has a second airflow resistance which is smaller than 1000 N. s/m³, preferably smaller than 500 N. s/m³ and more preferably smaller than 200 N.s/m³. The first cover layer 11 and the second cover layer 12 are interconnected such as to form a bag, enclosing the remaining layers of the stack of layers (further referred to as the modified ABA- complex). In the soundproofing system 2 shown in figure 3a, the modified ABA-complex is releasable from the bag, i.e. the bag is not adhered or otherwise fastened to the modified ABA-complex, The modified ABA- complex is also loose with respect to the bag i.e. a substantial amount of air pockets is provided between the ABA-complex and the bag.

Figure 3b shows the soundproofing system 2 clamped in a side fender 3, In this embodiment the layers of the modified ABA-complex may be loose or not loose with respect to the bag. Air pockets between the modified ABA-complex and the bag may be minimized, for example by gluing the ABA-complex to the bag over a substantial areal, or for example by providing a vacuum between the bag and the modified ABA-complex.

Figure 4a is a cross-sectional side exploded view of an embodiment of the soundproofing system 2 positioned on a wheel arch 4. The cross-section has been taken along line ll-ll in figure 1. The soundproofing system 2 shown in figure 4a is for example composed in the same way as the soundproofing system 2 as shown in figure 3a. The wheel arch 4 provides a surface 19 on which the soundproofing system 2 can be supported. Providing the intermediate barrier layer 16 as a stiffening layer is thus not advantageous when applying the soundproofing system 2 to the wheel arch 4. The sound source in the wheel arch 4 is mainly the sound 18 coming from the wheels 6, although also the engine 5 is a sound source from which sound impacts the second cover layer 12 of the soundproofing system 2 in figure 4a.

Figure 4b shows the soundproofing system fixed onto the wheel arch. The soundproofing system 2 is fastened to the supporting surface 19 of the wheel arch 4 by means of screws 20,

Experimental part

The invention will be now described in more details with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

1. USED MATERIALS The following materials have been used in the Examples 1 - 12 and in the Comparative Examples C.E.1 to C.E.3.

1 1 Cover layers

- Cover 1: TPU film Prochimir® TC 5563 (commercially available from Prochimir); UV-stabilized TPU ether based monolayer film, thickness: 250 μm, density: 1179 kg/m³, areal weight: 302 g/m², air permeability: <1.0 l/m².s (at pressure 200 Pa).

- Cover 2: PP film Prochimir® TC3005 (commercially available from Prochimir); thickness: 40 μm, density: 910 kg/m³, areal weight: 36 g/m², air permeability: <1.0 I/m².s (at pressure 200 Pa).

- Cover 3: PP film Prochimir® TC3005 (commercially available from Prochimir); thickness: 75 μm, density: 910 kg/m³, areal weight: 68 g/m², air permeability: <1.0 l/m².s (at pressure 200 Pa).

- Cover 4: EPDM mass Tarkett®, reference 7445025 (commercially available from Tarkett GDL SA); thickness: 2.7 mm, density: 1667 kg/m³, areal weight: 4.500 g/m².

- Cover 5: Non-woven Fibertex® V56 (commercially available from Fibertex); thickness: 0.42 mm, density: 0.24 g/cm³, areal weight: 102 g/m², air permeability: 178 l/m².s (at pressure 200 Pa).

- Cover 6: Non-woven Fibertex® V39 (commercially available from Fibertex); thickness: 0,50 mm, density: 0.20 g/m³, areal weight: 98 g/m², air permeability: 782 I/m².s (at pressure 200 Pa).

1.2. Sound absorbing layers

- Absorber 1 : dBR®Seal M50 flexible polyurethane foam with a semi- closed cell structure (commercially available from Recticel); density: 50 kg/m³, air permeability: 17 I/m².s (ISO 9237, thickness 10 mm, 200 Pa, 20 cm²), airflow resistivity: 618736 Ns/m⁴, Young modulus: 150 kPa, porosity: 0.95,: tortuosity 3.0

- Absorber 2: Situseal® T46065 flexible polyurethane foam (commercially available from Recticel), density 45 kg/m³, air permeability < 333 l/m².s (ISO 9237, thickness 10 mm, 200 Pa, 20 cm²), airflow resistivity: 294 135 Ns/m⁴, Young modulus: 150 kPa, porosity: 0.95, tortuosity: 3.0

- Absorber 3: dBR® P220 flexible polyurethane foam with excellent acoustic absorbing properties (commercially available from Recticel); density: 25 kg/m3, air permeability: 50-160 l/m².s (ISO 9237, thickness 10 mm, 200 Pa, 20 cm2), air flow resistivity: 112000 N.s/m⁴, porosity: 0.97.

- Absorber 4: dBR® P210 flexible polyurethane foam (commercially available from Recticel); density: 24 kg/m3, air permeability: 750 l/m².s - 1250 l/m².s (ISO 9237, thickness 10 mm, 200 Pa, 20 cm²), airflow resistivity: 8650 N.s/m⁴, porosity: 0,98,

- Absorber 5: Meltblown PP-Fiber 8510® (commercially available from Monadnock), density: 21.7 kg/m³, air flow resistivity: 71 600 Ns/m⁴.

1.3. Intermediate barrier layer

- Barrier 1 : TPU film Prochimir® TC 5563 (commercially available from Prochimir) UV-stabilized TPU ether based monolayer film; thickness: 250 μm, density: 1179 kg/m³, areal weight: 302 g/m², air permeability: <1.0 l/m².s (at pressure 200 Pa).

- Barrier 2: AkyLux® fluted Structure twin wall polypropylene sheets (commercially available from DS Smith); thickness: 2 mm, density: 125 kg/m³, areal weight: 250 g/m², bending modulus: 30 N/mm²

- Barrier 3: TPU-Ester film Prochimir® TC 5018 CX03 (commercially available from Prochimir); thickness: 30 μm, density: 1210 kg/m³, areal weight: 36 g/m², air permeability <1.0 I/m².s (at pressure 200 Pa).

2 SAMPLE PREPARATION

Different stacks of layers were produced using the above mentioned individual layers. Twelve different stacks of layers were produced according to the present invention. Three comparative examples were also produced. The lay-up of the fifteen stacks of layers are shown in the table below.

Table 1 : Composition of the examples and comparative examples.

Each one of the fifteen stacks of layers were cut in the following dimensions:

• Thickness of each layer: depending of the composition (see table 1 ) · Length and Width: 80 mm x 80 mm All the layers in a stack of layers were stacked without glueing unless stated otherwise. The first and the second cover layers were not sealed to each other at the edges, thereby not forming an enclosing bag. In example 7, the PU film 30 μm was laminated on the second sound absorbing layer by use of a heat laminator. In example 2, the first cover layer was glued on the first sound absorbing layer and the second cover layer was glued on the second sound absorbing layer, using the glue type ‘3M-90 glue spray’, in an amount of +/- 25 g/m². In example 3, all the layers were glued to each other, using glue type '3M-90 glue spray’, in an amount of +/- 25 g/m². In example 4, the first noise absorbing layer and the second noise absorbing layer were glued to the intermediate barrier layer, using glue type ‘3M-90 glue spray’, in an amount of +/- 25 g/m². In comparative example 2 (C.E.2), Cover 4 is glued on Cover 1 of cover layer 1 , using the glue type ‘3M-90 glue spray’, in an amount of +/- 25 g/m², and a second Cover 4 is glued on Cover 1 of cover layer 2, using the glue type ‘3M-90 glue spray’, in an amount of +/- 25 g/m². 3 DETERMINATION OF PARAMETERS

In the present subsection, the testing methods used to determine material parameters of the different layers of the stack of layers is described. Wherever these parameters are described or claimed in the present patent application, these parameters are preferably determined using the test methods described in the present sub-section.

In the present invention, the transmission loss values of airborne sound at 1/3 octave bands between 500Hz and 8kHz is measured for each example stack of layers according to the testing procedure set out below. The testing procedure is based on the ISO 15186-1 Rev.10/2003 principle. In particular, the transmission loss was measured in an alpha cabin. The alpha cabin is the send room where a test sound (pink noise) is produced using 4 loudspeakers. For each measurement, one of the example stack of layers is mounted in the door of the alpha cabin by means of the associated test panel. The test panel comprises two metal sheets of 1 mm each, in which a 60 mm diameter hole provides undisturbed access in-and-out of the alpha cabin. The example stack of layers, is placed between the two metal sheets and covers the entire hole, thereby disturbing access in-and out of the alpha cabin. In front of the test panel is an open receiving area, where an intensity probe is positioned at 50mm from the test panel. The intensity probe receives the sound transmitted through the test panel. The transmission loss as a function of frequency, is subsequently calculated by subtracting the frequency spectrum of the sound emitted in the alpha cabin, i.e. the intensity of the emitted sound in dB as a function of frequency, from the frequency spectrum of the sound measured by the intensity probe, i.e. the intensity of the received sound in dB as a function of frequency. The transmission loss of the undisturbed test panel, i.e. without a stack of layers provided in the test panel, is referred to as the transmission loss of the hole. The insertion loss of a stack of layers can be obtained by subtracting the transmission loss of the hole (indicated in Table 3) from the transmission loss of the example stack of layers.

In the present invention, the term ‘global transmission loss’ for an example stack of layers is determined as follows: Lglobal = 10 * wherein Li is the transmission loss value measured for the

example stack of layers at the i^th 1/3 octave band of the n 1/3 octave bands from 500Hz to 8kHz, and wherein the ‘transmission loss’ at the i^th 1/3 octave band of the n 1/3 octave bands from 500Hz to 8kHz, is defined as wherein p_i is the pressure value measured for the

example stack of layers at the i^th 1/3 octave band and wherein p₀ is 2*10^-5 Pa.

In the present invention, the density is measured according to ISO 845 REV. 07/2020.

In the present invention, the porosity is measured according to the publication: "Méthode de la masse manquante” as publicly available in the Journal of Applied Physics 101 (12), 2007.

In the present invention, the tortuosity is measured according to the publication: “Frequency dependent tortuosity measurement by means of ultrasonic tests”, by Paolo Bonfiglio and Francesco Pompoli, ICSV14-Cairns-Australia, 2007.

In the present invention, the air permeability is measured according to EN ISO 9237 Rev. 12 10/1995.

In the present invention, the air flow resistance is measured according to ISO 9053.

In the present invention, the bending modulus is measured according to ASTM D 790 B.

In the present invention, the Young modulus is measured according to ISO 18437-5.

4 TEST RESULTS The test results for the different examples and comparative examples, and for a steel plate of 1 mm and for the empty hole, are indicated in Tables 2 and 3.

Table 2: Transmission loss (TL in dB) of Examples 1 to 5, of comparative

Examples 1 and 2 and of a 1 mm steel plate.

Table 3: Transmission loss (TL in dB) of Examples 6 to 12, of comparative Example 3 and of an open hole.

The test results indicated in Tables 2 and 3 are also shown in the graphs of Figures 5 to 10, The test results obtained for the empty hole are not shown in the graphs. They can be used to convert the transmission losses into insertion losses, namely by subtracting the transmission losses obtained for the empty hole from the other transmission losses.

The global transmission losses (global TL) indicate the overall acoustic performance of the soundproofing system. For a 1 mm steel plate the global TL is quite high and is more particularly equal to 52,7 dB. A drawback of such a steel plate is that it cannot be easily be built in in a vehicle. Moreover, it has a high weight so that its acoustic performance per areal weight is very small, namely only 7 dB.m²/kg. The complex composition of Example 1, on the contrary, has a much lower areal weight, namely an areal weight of only 1,95 kg/m², but still provides a global transmission loss which is similar to the global transmission loss of the steel plate. The acoustic performance per areal weight is thus much better than that of a steel plate and is more particularly about four times as high.

The transmission losses in dB are indicated in Figure 5 for the different frequencies (1/3th octaves) for the steel plate and for Examples 1 and 5 and for comparative Example C.E.1. It can be seen that the TL values for the complex of Example 1 are, at lower frequencies, a little bit higher than the TL values for the steel plate and are, at higher frequencies a little bit lower.

The complex of Example 1 was made with a 250 μm TPU foil as barrier layer and as the first and the second cover layer. In the complex of Example 5 the two cover layers were replaced by lighter cover layers, namely respectively by a 75 μm and a 40 μm PP foil. The areal weight of the complex was thus reduced to 1 ,565 kg/m². In the higher frequency region (higher than 4000 Hz), the TL values remained substantially the same. In the lower frequency region (lower than 4000 Hz), the TL values were reduced somewhat, although not excessively. The complex of Example 5 is thus still very suitable for being used as soundproofing system in a vehicle, especially in view of its good acoustic performance per areal weight, which was equal to about 32 dB.m²/kg.

In comparative Example C.E.1 the two cover layers have been removed from the complex of Example 1. It can be seen in Figure 1 that this resulted in a substantial reduction of the TL values. Even with the thinner cover layers of Example 5, much better TL values were obtained, especially in the higher frequency region.

In Example 1 , all of the layers were loose, i.e. they were not adhered to one another. The effects of adhering the different layers or some of them together can be seen in Figure 6.

In Example 2 only the two cover layers were glued to the respective sound absorbing layers. This resulted in a lowering of the TL values over the whole frequency range. It is thus preferred not to glue the cover layers to the sound absorbing layers. In Example 4 only the barrier layer was glued to the two adjoining sound absorbing layers. This resulted in a very slightly increased global TL. In the lower frequency range the TL values were a little bit lower whilst in the higher frequency range they were a little bit higher. An advantage of this embodiment is that the barrier layer and the two sound absorbing layers can be first laminated or adhered (glued) to one another so that a complex is obtained which is easier to handle when applying the cover layers on the outer surfaces of this complex. The complex of barrier layer and sound absorbing layers can also be cut first easily into the required shape before applying the cover layers. These cover layers are preferably adhered to one another along their edges to form a closed bag. This can be done by a simple sealing/welding operation, in particular by a thermal or ultrasonic welding operation.

In Example 3 all the layers were adhered to one another. This still resulted in a good global TL value. The advantage of such an embodiment is that all the layers can first be adhered/laminated onto one another and can then be cut easily in to required shapes.

Figure 7 shows the effect of an additional mass, namely an EPDM layer, on both surfaces of the complex of Example 1. It can be seen that in the higher frequency region (frequencies higher than 4000 Hz) the additional EPDM layers in comparative Example C.E.2 had no effect on the TL values. In the lower frequency region (frequencies lower than 4000 Hz), they had however some effect although not a large effect. In the soundproofing system of the present invention it is thus not required to apply one or more mass layers (having in particular an areal weight of higher than 500 g/m²) to achieve the required TL values. A drawback of such mass layer is that it considerably increases the areal weight of the soundproofing system. In comparative Example C.E.2 the areal weight increased to 11 ,75 kg/m², which is even higher than the areal weight of the steel plate, and this for achieving only a similar global TL. By using no mass layer in the complex of Example 1 , substantially the same global TL could be achieved but the acoustic performance per areal weight was much higher, namely four to five times higher.

Figure 8 shows the effect of using a different barrier layer. In Example 5 a 250 μm TPU barrier layer was used in combination with two thin PP cover layers. In Example 7 the TPU barrier layer of Example 5 was replaced by thinner foil layer, namely by a 30 μm PU foil layer which is laminated, for support, onto one of the sound absorbing layers. Moreover, the sound absorbing layers were replaced by sound absorbing layers which had a smaller air flow resistance, namely an AFR of only 3677 N.s/m³ instead of 7734 N.s/m³. The TL values of Example 7 were reduced somewhat compared to the TL values of Example 5, which could be due to the reduced AFR value of the sound absorbing layers so that the thinner barrier layer would have no or only a small effect on the TL values.

In Example 6 the flexible 250 μm TPU barrier layer of Example 5 was replaced by a stiffer, hollow barrier layer, having the same areal weight of 250 g/m². The global TL value was slightly reduced. In the higher frequency region (frequencies higher than 4000 Hz), the TL values were all reduced. In the lower frequency region (frequencies lower than 4000 Hz), the TL values were only reduced in a narrower region, which might be due to resonance effects in the hollow barrier layer, or due to the stiffness of the barrier layer.

The stiffer, hollow barrier layer of Example 6 provides the required stiffness to the soundproofing system so that it can be clamped more easily, without additional supports, between two lateral walls. In the next examples, use was thus always made of this stiffer barrier layer.

Example 8 differs only from Example 6 by the fact that the two sound absorbing layers of 12,5 mm were replaced by a sound absorbing layer of 5 mm and one of 20 mm. in Figure 8 it can be seen that this resulted in reduced TL values. Also the global TL value was reduced from 46,8 dB to 41 ,1 dB.

Figure 9 shows the effect of replacing one impermeable cover layer by an air permeable cover layer and by replacing the air resistant sound absorbing layer underneath this air permeable cover layer by a more open sound absorbing layer, i.e. by a sound absorbing layer having a much lower air flow resistance. In Examples 11 and 12, the second sound absorbing layer and the second cover layer were indeed replaced by more open layers in view of enhancing the sound absorbing properties of the system. This also reduced the global TL value but, in practical conditions, when larger surfaces are covered with the soundproofing system, the complexes of these examples may reduce the general noise level in the noise emission area and would thus have a larger effect on the noise level in the passenger compartment.

In Example 12 the second sound absorbing layer was not so open and still had a relatively high AFR of about 1395 N.s/m³ The second cover layer was quite open. The TL values were smaller than the TL values of Example 6 over the whole frequency range. In Example 11 better TL values were achieved. Compared to Example 12 the second sound absorbing layer was more open whilst the second cover layer was more closed.

Figure 10 shows the effects of increasing or reducing the thickness of the two sound absorbing layers. Compared to Example 6, the thickness of the two sound absorbing layers was increased from 12,5 mm in Example 6 to 20 mm in Example 10. By this higher thickness, and higher areal weight, only a very small increase of the global TL value was obtained, namely an increase from 46,8 to 47,2 dB, As appears from Figure 10, the TL values were in particular increased in the lower frequency range whilst they were reduced in the higher frequency range. In Example 9 the thickness of the two sound absorbing layers was reduced to 5 mm. This resulted in a substantial decrease of the global TL value and in a general decrease of the TL values over the entire frequency range. In comparative Example C.E.3 only one sound absorbing layer was used having a thickness of about 9,3 mm. The same cover layers were used as in Example 9. The soundproofing system of comparative Example C.E.3 had a similar total thickness than that of Example 9. By the presence of the barrier layer in the complex of Example 9, and by the higher AFR values of the two sound absorbing layers, the TL values obtained by the sound proofing system of Example 9 were considerably higher than those obtained by the comparative sound proofing system. The global TL value increased in particular from 21,1 to 34,2 dB.

Claims

1 , A soundproofing system (2) for a vehicle (1), which system comprises a stack of layers comprising:

- a first cover layer (11 ) which has an areal weight of less than 500 g/m²;

- a second cover layer (12) which has an areal weight of less than 500 g/m²;

- a first sound absorbing layer (14) which is arranged between the first cover layer (11 ) and the second cover layer (12); and

- a second sound absorbing layer (15) arranged between the first sound absorbing layer (14) and the second cover layer (12), characterised in that the first cover layer (11 ) has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1.0 l/m².s; in that said first sound absorbing layer (14) has a first airflow resistance, measured in accordance with ISO 9053-1 :2018, Part 1 , which is higher than 3000 Ns/m³ but lower than 50 000 Ns/m³; in that said stack of layers comprises an intermediate barrier layer (16) which is has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1 .0 I/m².s; and in that the intermediate barrier layer (16) is arranged between said first sound absorbing layer (14) and said second sound absorbing layer (15).

2. The soundproofing system (2) according to claim 1 , characterised in that said first airflow resistance is higher than 4000 Ns/m³, preferably higher than 5000 Ns/m³ and more preferably higher than 6000 Ns/m³.

3. The soundproofing system (2) according to claim 1 or 2, characterised in that said first air flow resistance is smaller than 40 000 Ns/m³, preferably smaller than 30 000 Ns/m³ and more preferably smaller than 25 000 Ns/m³,

4. The soundproofing system (2) according to any one of the claims 1 to 3, characterised in that the first sound absorbing layer (14) has an average thickness which is larger than 4 mm, preferably larger than 6 mm and more preferably larger than 8 mm

5. The soundproofing system (2) according to any one of the claims 1 to 4, characterised in that the first sound absorbing layer (14) has an average thickness which is smaller than 30 mm, preferably smaller than 25 mm and more preferably smaller than 20 mm

6. The soundproofing system (2) according to any one of the claims 1 to 5, characterised in that said intermediate barrier (16) layer is adhered to said first sound absorbing layer (14).

7. The soundproofing system (2) according to any one of the preceding claims, characterised in that said first sound absorbing layer (14) comprises a foam layer, in particular a polymeric foam layer, and/or a fibrous layer, in particular a woven or a non-woven layer.

8. The soundproofing system according to any one of the preceding claims, characterised in that said first sound absorbing layer (14) has a density of between 20 kg/m³ and 100 kg/m³, and preferably a density of between 30 kg/m³ and 70 kg/m³.

9. The soundproofing system (2) according to any one of the preceding claims, characterised in that said first sound absorbing layer (14) has a Young modulus smaller than 300 kPa and preferably smaller than 200 kPa.

10. The soundproofing system (2) according to any one of the preceding claims, characterised in that said first cover layer (11 ) has an areal weight of less than 400 g/m², preferably of less than 300 g/m², the areal weight of the first cover layer (11 ) being preferably higher than 30 g/m², more preferably higher than 50 g/m² and most preferably higher than 70 g/m².

11 . The soundproofing system (2) according to any one of the claims 1 to 10, characterised in that said first cover layer (11 ) comprises a polymeric foil.

12. The soundproofing system (2) according to any one of preceding claims, characterised in that said first cover layer (11 ) covers area the first sound absorbing layer (14) over a predetermined surface area, the first cover layer (11 ) being not adhered to said first sound absorbing layer (14) or only over at most 10% of said predetermined surface area.

13. The soundproofing system (2) according to any one of the preceding claims, characterised in that said intermediate barrier layer (16) has a bending modulus (measured according to ASTM D790 procedure B typel ) along any direction perpendicular to the thickness direction higher than 8 N/mm and preferably higher than 20 N/mm.

14. The soundproofing system (2) according to any one of the preceding claims, characterised in that the second sound absorbing layer (15) has an average thickness which is larger than 4 mm, preferably larger than 6 mm and more preferably larger than 8 mm.

15. The soundproofing system (2) according to any one of the preceding claims, characterised in that the second sound absorbing layer (15) has an average thickness which is smaller than 30 mm, preferably smaller than 25 mm and more preferably smaller than 20 mm.

16. The soundproofing system (2) according to any one of the preceding claims, characterised in that the intermediate barrier layer (16) is adhered to said second sound absorbing layer (15).

17. The soundproofing system (2) according to any one of the preceding claims, characterised in that said second sound absorbing layer (15) comprises a foam layer, in particular a polymeric foam layer, and/or a fibrous layer, in particular a woven or a non-woven layer.

18. The soundproofing system (2) according to any one of the preceding claims, characterised in that said second sound absorbing layer (15) has a density of between 20 kg/m³ and 100 kg/m³, and preferably a density of between 30 kg/m³ and 70 kg/m³.

19. The soundproofing system (2) according to any one of the preceding claims, characterised in that said second sound absorbing layer (15) has a Young modulus smaller than 300 kPa and preferably smaller than 200 kPa.

20. The soundproofing system (2) according to any one of the preceding claims, characterised in that said second cover layer (15) has an areal weight of less than 400 g/m², preferably of less than 300 g/m², the areal weight of the second cover layer being preferably higher than 30 g/m², more preferably higher than 50 g/m² and most preferably higher than 70 g/m².

21 . The soundproofing system (2) according to any one of the preceding claims, characterised in that said second cover layer (12) covers the second sound adsorbing layer (15) over a predetermined surface area, the second cover layer (12) being not adhered to said second sound adsorbing layer (15) or only over at most 10% of said predetermined surface area,

22. The soundproofing system (2) according to any one of the preceding claims, characterised in that said second cover layer (12) has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of less than 1 .0 I/m².s and comprises preferably a polymeric foil.

23. The soundproofing system (2) according to the preceding claim, characterised in that said second sound absorbing layer (15) has a second airflow resistance which is higher than 3000 Ns/m³ but lower than 50 000 Ns/m³,

24 The soundproofing system (2) according to the preceding claim, characterised in that said second air flow resistance is higher than 4000 Ns/m³, preferably higher than 5000 Ns/m³ and more preferably higher than 6000 Ns/m³.

25. The soundproofing system (2) according to claim 23 or 24, characterised in that said second air flow resistance is smaller than 40000 Ns/m³, preferably smaller than 30000 Ns/m³ and more preferably smaller than 25000 Ns/m³

26. The soundproofing system (2) according to any one of the claims 1 to 21 characterised in that said second cover layer (12) has an air permeability, measured in accordance with ISO 9237 (10/1995), at a pressure of 200 Pa, of at least 100 l/m².s, preferably of at least 150 l/m².s, and said second sound absorbing layer (15) has a second airflow resistance which is smaller than 1000 N. s/m³, preferably smaller than 500 N.s/m³ and more preferably smaller than 200 N.s/m³

27. The soundproofing system (2) according to any one of the preceding claims, characterised in that the first cover layer (11) is interconnected to the second cover layer (12) such as to form a bag enclosing the first sound absorbing layer (14), the intermediate barrier layer (16) and the second sound absorbing layer (15).

28. The soundproofing system (2) according to any one of the preceding claims, characterised in that said intermediate barrier layer (16) has an areal weight of less than 500 g/m², preferably of less than 400 g/m² and more preferably of less than 300 g/m².