FR2504520A1 - Composite formable panels for acoustic isolation of HF noise - involving macro:cellular layer with shaped perforation of one cover - Google Patents

Abstract

Sheet insulation suitable for subsequent forming incorporates a rigid macrocellular layer enclosed between relatively thin cover sheets of which at least one face is partly perforated and the cells so exposed are opt. filled with a rigid open-cell foam. Pref. one of the faces of the cellular layer is of ductile or thermoplastic material which can be postformed conically into the cell voids and, in some such cases, ruptured to allow sound to directly enter the core cavities. Pref. the rigid macrocellular layer, of ductile metal or thermoplastic material, is covered by a layer of similar thickness of porous microcellular foam covered in turn by a 'septum' layer, the layers typically being 12, 10 and 0.8 mm thick respectively. Pref. the perforated layer is covered by an ancillary, unperforated layer of e.g. 22 microns thick polyethylene film. The composite sheet can be reshaped using a compression press. Used for cladding for acoustic insulation of high intensity moise sources such as gear box drives or combustion motors for heavy vehicles, to suppress sound reflection or reverberation without involving excessive wt..

Description

 The present invention relates to a formable sound insulation and absorption material, intended in particular for the attenuation of the noises emitted by motor vehicles, more particularly the noises emitted by the intense sources which constitute the gearboxes and the motors of the weights. heavy, and further relates to a method of forming such a material.

 In the field of acoustics, it is known that vibrations or impact noises from machines are transmitted by conductivity in materials and also propagate in the air, in the form of sound waves. More precisely, we know that low frequencies preferentially go through the solid channel and that high frequencies essentially take the air way. In the case of machines such as the internal combustion engines of road vehicles, attempts are generally made to isolate the noises at the source, that is to say to prevent their transmission in the materials, for example by mounting the engines on anti-vibration devices. However, it remains essential to determine the attenuation of airborne noise at a certain distance from their source, that is to say around the engines.

 To this end, taking into account that an airborne noise undergoes, when passing through a wall, a weakening all the more important as the weight of this wall is higher per unit area, we have already tried to isolate the motors by surrounding them with a shell made of a dense material, for example metal sheet. A good weakening of the acoustic pressure is thus obtained through the shell, but, on the other hand, the reflection coefficient on the latter is particularly high. As a result, the driver and any passengers in the vehicle are fairly well protected against the noise emitted by the engine, but that said reflection of the noise waves and their local reverberation on the ground cause, depending on the state of the latter, genes and major nuisances for the environment.

 We have therefore tried to achieve a certain reduction in noise around the motors by adding to the aforementioned shell a relatively soft and necessarily porous coating intended to reduce the noise level of the waves reflected before their reverberation or their rebound on the ground. There was thus obtained a partial attenuation of the noise waves and an improvement as regards the nuisance caused by the engines of vehicles on the environment. It should however be noted that said porous coating becomes dirty over time, which results in a reduction in its absorbing power at the same time as a progressive increase in harmful reflections. This ultimately results in almost total ineffectiveness of the fouled coating as an element for attenuating airborne noise, although it is still of some interest as an element for damping the vibrations of the shell.

 To avoid such fouling of the porous coating, a waterproof, flexible and as light as possible membrane has been attached to the latter. I1 is clear that this avoids any fouling of the porous coating, but to the detriment of its qualities of acoustic absorption because this added membrane constitutes a new reflecting element which significantly aggravates the nuisances suffered by the environment.

 The invention aims to remedy the drawbacks indicated above by providing a formable sound insulation and absorption material, in particular for motor vehicles, making it possible not only to improve sound insulation by compensating for the mass effect of the material. by a rigidity effect, but also to improve the acoustic absorption by producing in the material an absorbent face using cells of the “honeycomb” type to directly trap the incident sound waves as in resonators, the invention aiming in addition to allow the forming of said material and to allow in particular the production of self-supporting walls comprising the latter.

 The Applicants have in fact surprisingly discovered that the honeycomb structures used in aeronautical construction for their rigidity and lightness qualities notably improve the sound insulation at equal density, especially at low frequencies.

 According to the invention, the formable sound insulation and absorption material against incident noise waves, comprises a rigid layer of cells juxtaposed and taken transversely between two sheets forming lids sealing the two ends said cells, the latter possibly being filled with a hard plastic foam with open cells.

According to other characteristics:
- Some or all of the covers situated on the incidence side of the noise waves are each shaped into a horn towards the inside of the associated cell, the axis of the horn corresponding substantially to the axis of said cell;
- some or all of the horns have one or more small orifices opening into the associated cell;
- The material comprises a thin protective membrane, waterproof and elastic, arranged externally on the sheet with lids forming cones.

 Also according to the invention, a composite material comprising a layer of the aforementioned material further comprises a thick layer of a porous material, light and soft, disposed on the cover sheet of the rigid layer which is situated opposite the side. incidence of noise waves, and a thin layer of a tight, heavy and flexible material, which is arranged on the porous layer opposite the rigid layer, said layers being linked to each other.

 Furthermore, in accordance with the invention, in the process for forming the aforementioned material, two half-molds are produced having imprints of substantially complementary shapes, a coating film is placed on the imprint of the lower half-mold, the upper face of which is coated with a resin, then a flat layer of juxtaposed cells previously filled with a hard open cell foam is placed on the half-mold thus prepared, a coating film is placed on the layer of cells, the underside of which is is coated with a resin, and the half-molds are pressed against each other to determine the bonding and the polymerization at the same time as the forming of the layer of cells.

According to other characteristics of the process:
- Cones are produced by coating one of the half-molds with roughness on which the corresponding film bears during pressing;
- Ports are made by coating one of the half-molds with points on which the corresponding film bears during pressing.

Other characteristics and advantages of the present invention will emerge more clearly from the description which follows, given with reference to the appended drawings in which
Figure 1 shows a partial cross-sectional view of a section of the formable material according to the invention;
2 shows a schematic perspective view with partial cutaway of a section of the composite material according to the invention, which comprises a layer of the simple material shown in Figure 1;
3 shows a partial cross-sectional view of the material shown in Figure 1, some lids being shaped into a horn possibly having a small orifice;
FIG. 4 represents a view similar to that of FIG. 3, the sheet with lids in cones comprising a protective membrane; ;
FIG. 5 represents a graph of the acoustic insulation as a function of the frequency for three known materials;
FIG. 6 represents a graph of the acoustic insulation as a function of the frequency for a known composite material and for an embodiment of the composite material according to the invention;
FIG. 7 represents a graph of the acoustic absorption as a function of the frequency for the three variants of the material according to the invention which are respectively represented in FIGS. 1, 3 and 4;
Figure 8 shows a partial cross-sectional view of a mold illustrating an embodiment of the process for forming the material shown in Figure 1;
FIG. 9 represents a view similar to that of FIG. 8, the lower half-mold being coated with asperities forming cones; and
FIG. 10 represents a view similar to that of FIG. 8, the lower half-mold being coated with spikes forming orifices.

 In these drawings, the same references designate the same elements.

 Referring to FIG. 1, a formable sound insulation and absorption material according to the invention is designated as a whole by 1 and essentially comprises a rigid layer 2 of cells 3 juxtaposed and taken transversely between two sheets 4 and 5 forming lids 6 closing the two ends of said cells 3.

As will be explained in more detail with reference to the method of forming the rigid material 1, the cells are optionally filled with a hard plastic foam with open cells.

 Referring to Figure 2, a composite material according to the invention comprises a layer of rigid material 1 and further comprises a thick layer 7 of a porous, light and soft material, and a thin layer 8 of a waterproof material , heavy and flexible. According to the invention, the layers 1, 7 and 8 are linked to each other and are superposed in a succession determined with respect to the general direction of the incident noise waves. More specifically, the porous layer 7 is arranged on the sheet 5 with lids which is located opposite the incidence side of the noise waves, and the waterproof layer 8, which forms a barrier which can be called "septum", is placed on the porous layer 7 opposite the rigid layer 1.

Before describing certain forms in more detail
of simple and composite materials
according to the invention, it is necessary to consider various known materials, in particular those which may constitute one of the layers 1,7 or 8 of the composite material.

 During the research which led to the invention, the Applicants studied and measured the sound insulation determined by a certain number of known simple or composite materials. FIGS. 5 and 6 graphically represent the sound insulation obtained as a function of the frequency for many of these materials. The sound insulation is expressed in decibels (dB) and plotted on the ordinate on a linear scale, while the frequency is expressed in hertz (Hz) and plotted on the abscissa on a logarithmic scale.

 For a bare sheet metal having a thickness of 8/10 mm and a weight of approximately 6 kg / m2, - we obtain substantially the curve in dashed lines A, visible in FIG. 5, which shows that the sound insulation determined by such a sheet decreases from around 8 dB for very low frequencies, reaches a quasi-zero value for 50 Hz, and then regularly increases to around 40 dB for 8000 Hz. We therefore see that a simple reflective sheet provides relatively poor sound insulation at medium frequencies and poor at low frequencies.

 When, in known manner, a thin damping coating is added, for example a bituminous screed, to the sheet of the aforementioned type, a total weight of 9 to 10 kg / m2 is obtained. The corresponding curve B, which is shown in phantom in Figure 5, shows that such a coating provides a slight improvement in the sound insulation for all frequencies, more particularly for low frequencies, but that this insulation is still very weak in low frequencies (from 10 dB to 15 dB for a frequency below 100 Hz).

 In the case of a known composite material in which a relatively thick porous layer is added to the sheet and covered with a tight, flexible and heavy membrane, the curve in broken lines D visible in FIG. 6 is obtained substantially. This curve shows that the sound insulation is clearly improved for the medium and high audible frequencies, but that it remains bad for the low frequencies since it presents a value not exceeding 20 dB for a frequency lower than 200 Hz.

 From the above, it turns out that the known simple or composite materials do not make it possible to obtain suitable insulation with respect to airborne noise at low frequencies.

 In addition, we know that honeycomb materials, which combine rigidity with lightness, are increasingly used, especially in aSronautique construction. However, these cellular materials have never been used until now as acoustic insulation materials.

 The Applicants have however sought to verify the acoustic qualities of a cellular material, with a view to producing one of the layers of a composite material for acoustic insulation and absorption. For a honeycombed plastic material of the laminated type, having a weight of approximately 3 kg / m 2, as shown in FIGS. 1 and 2, the insulation curve is obtained substantially in solid line. Readable in FIG. 5. D ' surprisingly, this curve C shows that the sound insulation offered by such a cellular material has a value greater than 45 dB for very low frequencies, decreases slowly to a minimum of the order of 18 dB for approximately 400 Hz and then increases rapidly to reach around 46 dB for 8000 Hz. If we compare the results of this simple honeycomb material with those of bare sheet metal (curve A) and damped sheet metal (curve B), we see that the insulation acoustic offered by said honeycomb material is much higher for the low frequencies, hardly weaker between 300 Hz and 600 Hz and still slightly higher beyond 800 Hz. In addition, the curves A and B tend asymptotically towards a limit of the order 40 dB for 80 00 Hz, while the curve C passes through a value of 46 dB for this same frequency and then presents a slope which is still clearly increasing.

 Faced with these unexpected results, the Applicants carried out the mounting of said honeycomb material in place of the sheet previously used in a composite assembly such as that described above as allowing the curve D to be obtained.

Consequently, the composite material according to the invention essentially comprises a rigid cellular layer 1, a porous layer 7 and a layer of septum 8, as has already been indicated with reference to FIG. 2.

 Referring to curve E shown in solid lines in FIG. 6, it can be seen that the acoustic insulation offered by this new composite material with a honeycomb layer has a value of approximately 52 dB for very low frequencies, decreases to a minimum of around 26 db for around 250 Hz and then increases very quickly, already reaching a value of 53 dB for 1000 Hz.

By comparing these results with those of the known composite material corresponding to curve D, it can be seen that the composite material according to the invention provides significantly improved acoustic insulation for the whole frequency range and more particularly very improved insulation for the low and medium frequencies.

 However, the aforementioned results essentially relate to acoustic insulation, that is to say the attenuation of airborne noise beyond the screen material, and do not take into account the power of reflection of said material. For example, in the particular case of road vehicles, the composite material described above can form a shell surrounding the gearbox or the engine and adequately protect the driver and any passengers against the noise waves emitted by the box or the engine, but the reflection of these waves on said material causes their reverberation on the ground and, therefore, significant discomfort and nuisance for the environment.

To provide a solution to this problem,
Applicants have shaped some or all of the lids 6 located on the incidence side of the noise waves, so as to obtain, for example by driving in an asperity or appropriate tip and deformation of each of the selected lids, a horn oriented towards the inside of the associated cell 3. The horns thus obtained can have various shapes, pointed, cylindrical, or even flared, so as to constitute small resonators or closed sound pipes intended to trap the waves of incident noise. Preferably, the axis of each horn corresponds substantially to the axis of the associated cell. For the same layer of rigid material 1, the length and the cross section of the horns can differ as a function of the dominant frequencies of the noises to be attenuated.

 Based again on the theory of resonators and sound pipes, the Applicants have considered drilling, at the bottom of some or all of the horns, one or more small orifices opening into the associated cell.

Each orifice may be axial or parallel to the axis of the horn or have an appropriate inclination relative to the latter. Under these conditions, the horn forms a substantially open sound pipe, while its associated cell forms a closed sound pipe, these two sound pipes helping to trap the waves of incident noise in the cell layer.

 It is obvious that the dimensions of the cells as well as the number and dimensions of the horns and their possible orifices are chosen as a function of the various frequencies constituting the noises to be attenuated.

 Referring to Figures 1 and 2, we have already described the rigid layer 1 as comprising cells 3 juxtaposed and taken transversely between two sheets 4 and 5 forming the lids 6 of said cells. With reference to FIGS. 3 and 4, certain lids 6, forming part of the sheet 4 situated on the side of the incidence of the noise waves, are formed into horns 9, some of which have an axial orifice 10.

 Referring now to FIG. 7, this graphically represents the acoustic absorption as a function of the frequency, measured at the level of the sheet 4, situated on the incidence side of the noise waves, and determined by the embodiments of the trapping layer 1 according to the invention. The alpha sound absorption is expressed as a percentage and plotted on the ordinate on a linear scale, while the frequency is expressed in hertz and plotted on the abscissa on a logarithmic scale.

 For the simplest embodiment which is visible in FIG. 1 and in which the lids 6 of the sheet 4 are not deformed, the curve in dashed lines F is obtained. This curve shows that the acoustic absorption is not that by 10 ó on average up to 1000 Hz, that it fairly quickly reaches a plateau of around 40% for 2000 Hz at 4000 Hz and that it then increases suddenly to reach 662 for 5000 Hz.

 When the lids 6 are shaped according to a certain number of horns 9 each comprising an axial orifice 10, the curve in phantom lines G is obtained. This curve shows that the acoustic absorption is already around 16% for 125 Hz, increases rapidly up to a maximum level of 86S for 800 Hz to 1000 Hz and then slowly decreases to 68oxo for 5,000 Hz.

 It can therefore be seen that the sound absorption offered by the trapping layer with lids forming perforated horns according to the invention has an extremely high value, which is equal to or greater than 68% for the frequency range between 400 Hz and 5000 Hz.

 It should be noted, however, that the horns 9 are particularly subject to fouling and that the same is true for the cells 3 when the horns are pierced. To avoid this drawback, the trapping layer of the rigid material 1 according to the invention preferably comprises a thin elastic membrane 11 constituted by a film, for example of polyethylene, having a thickness of 22 microns and arranged externally on the sheet 4 (see figure 4).

 Under these conditions, we obtain the curve in solid line H which shows that the sound absorption is very low for 125 Hz, slowly reaches 10 ó for 200 Hz and 12 ó for around 300 Hz, then increases rapidly to a maximum of 94 ó for 2000 Hz and then decreases until reaching 71 ó for 5000 Hz.

 A comparative examination of the curves G and H shows that the presence of the membrane 11 causes a marked reduction in the acoustic absorption for the medium frequencies and a slight increase in the latter for the frequencies above 1600 Hz. In addition, after curve H, the sound absorption value remains at a level of around 70 6 for frequencies above 5000 Hz.

 It is obvious that the choice of the characteristics of thickness, weight and elasticity of the membrane is a function of the frequencies to be absorbed.

 In addition to the qualities of insulation and sound absorption studied above, the composite material according to the invention offers a certain number of advantages. For example, the rigidity and the lightness of the layer of the cellular material 1 make it possible to avoid the ribs and other reinforcing members which are usually associated with the sheet.

 In general, the trapping layer 1 can be made wholly or partly by a light metal or by a plastic material, in which case the cells can be caught between prepolymerized flat sheets. The porous layer 7 may consist of a woven or non-woven textile material, for example felt, or of a plastic foam. The reflection layer 8 can consist of a flexible and heavy waterproof material such as rubber.

 By way of example, according to a possible embodiment of the composite material according to the invention, the rigid layers 1, porous 7 and waterproof 8 may have respective thicknesses of the order of 12 mm, 10 mm and 8/10 of mm, the elastic membrane 11 having a thickness of less than 25 microns. It is obvious that the relative thicknesses of the various layers as well as their constituent materials are advantageously chosen essentially as a function of the frequencies of the noise waves to be attenuated.

 Furthermore, it has previously been indicated that the cellular material according to the invention is intended in particular for the attenuation of the noises emitted by motor vehicles.

Consequently, it is particularly desirable to be able to produce the simple or composite insulating materials according to the invention, not only in the form of flat walls such as those described above, but also in the form of curved walls, for example of the self-supporting type.

 To this end, the Applicants have developed a process for forming the simple material according to the invention, in which two upper and lower half-molds 12 and 13 are produced (FIG. 8) having respective imprints 14 and 15 of substantially shapes complementary corresponding to the desired shape for the material. We then have on the imprint 15 of the lower half-mold 13 a film or covering sheet 4 whose upper face 16 is coated with a resin, then placed on the half mold 13 thus prepared a flat layer 2 of juxtaposed cells 3 previously filled with a hard foam 17 with open cells, there is then on the layer 2 of cells 3 a film or covering sheet 5 whose lower face 18 is coated of a resin, and the half-molds 12 and 13 are pressed against each other to determine the bonding and the polymerization at the same time as the forming of the layer 2 of cells 3.

 While the forming takes place, the presence of the hard foam 17 makes it possible to avoid any prohibitive deformation of the cells 3, while preventing the resin associated with the covering sheets 4 and 5 from being distributed unevenly between said cells. In addition, the open cells of the foam 17 contribute to the acoustic insulation and absorption of the material thus formed.

 To make the horns 9 previously described with reference to FIGS. 3 and 4, one of the half-molds, preferably the lower half-mold 13, is coated with roughness 19 (FIG. 9) against which the facing film or sheet 4 bears. or 5 during pressing. The protrusions 19 can obviously be produced in a plate 20, removably placed on the lower half-mold 13, the upper face 21 of this plate having a shape substantially complementary to that of the cavity 14 of the upper half-mold 12 and its lower face 22 having a shape which matches that of the imprint 15 of the lower half-mold 13.

 In the same way, to make the orifices 10, the lower half-mold 17 can be coated with tips 23 (FIG. 10) against which the facing film or sheet 4 or 5 bears during pressing. The tips 23 can be attached to a removable plate 24, the upper and lower faces 25 and 26 of which are identical to the corresponding faces of the plate 20, that is to say respectively have shapes substantially complementary to those of the imprints 14 and 15 of the half-molds 12 and 13.

 During the implementation of the above method, the plates 20 and 24 can obviously be used separately during successive pressing passes, but it is also possible to produce the horns 9 and the orifices 10 in a single pass. In the latter case, each tip 23 can be made in one piece with an associated roughness 19, for example in the extension of the latter. Alternatively, each tip 23 can be slidably mounted at the top of the associated roughness 19, in which case it is preferable to interpose a sealed and gulpable bag of the balloon type between the plate and the half-mold, or even d 'use such a balloon instead of said half-mold during pressing.

 In general, in the formable material according to the invention, the layer 2 of juxtaposed cells 3 may consist of any rigid material having sufficient mechanical strength for the intended use. With regard to the sheet forming simple lids, this can be relatively flexible, while the sheet with lids made of cones must retain the shape which is communicated to it during pressing.

 In all cases, the implementation of the method according to the invention makes it possible to form the simple cellular material, to which can be attached in a manner known per se, on the one hand, the elastic membrane 11 and, on the other hand, the porous layer 7 and the waterproof layer 8, thus making it possible to produce the composite material according to the invention.

 whether simple or composite, the material according to the invention can be used for all types of insulation and acoustic absorption but, because it is formable, it is particularly well suited to the production of shells intended for surround sources of intense noise such as heat engines, especially motor vehicles.

 It is understood that the present invention has only been described and shown for explanatory purposes but in no way limitative and that any useful modification may be made to it, in particular in the field of technical equivalences, without going beyond its ambit.

Claims (9)

 1. Formable acoustic insulation and absorption material vis-à-vis incident noise waves, characterized in that it comprises a rigid layer of cells juxtaposed and taken transversely between two sheets forming lids sealing the two ends of said cells, the latter possibly being filled with a hard plastic foam with open cells.  2. Material according to claim 1, characterized in that some or all of the covers situated on the incidence side of the noise waves are each shaped into a horn towards the inside of the associated cell, the axis of the horn corresponding substantially to the axis of said cell.  3. Material according to claim 2, characterized in that some or all of the horns have one or more small orifices opening into the associated cell.  4. Material according to one of claims 2 or 3, characterized in that it comprises a thin waterproof and elastic protective membrane g, disposed externally on the sheet of lids forming cones.  5. Composite material comprising a layer of the material according to any one of claims 1 to 4, characterized in that it further comprises a thick layer of a porous material, light t soft, disposed on the sheet of lids the rigid layer which is situated opposite the incidence side of the noise waves, and a thin layer of a tight, heavy and flexible material, which is disposed on the porous layer opposite the rigid layer, said layers being related to each other.  6. A method of forming the material according to claim I, characterized in that two half-molds having imprints of substantially complementary shapes are produced, there is a coating film on the imprint of the lower half-mold, the upper face of which is coated with a resin, then a flat layer of juxtaposed cells previously filled with a hard foam with open cells is placed on the half-mold thus prepared, a covering film is placed on the layer of cells lower is coated with a resin, and the half-molds are pressed against each other to determine the bonding and the polymerization at the same time as the forming of the cell layer.  7. Method according to claim 6, characterized in that the cores are produced by coating one of the half-molds with roughness on which the corresponding film bears during pressing.  8. Process according to claim 6, characterized in that orifices are produced by coating one of the half-molds with spikes on which the corresponding film bears during pressing.  9. Application of the material according to any one of claims 1 to 5 to the production of shells of the nipples to surround the thermal engines, in particular of motor vehicles.