A sound absorptive element comprising at least one acoustic reso ance membrane formed by a layer of polymeric nanofibers

Technical field

The invention relates to a sound absorbing means which contains at least one acoustic resonance membrane formed by a layer of polymeric nanofibers.

Background art

Sound absorbing materials are generally used in many different fields and their main task is to provide hygiene of the environment from the point of view of undesired or harmful sound. The design of a sound absorbing material suitable for the particular application is based on a range of frequencies of unwanted sound, which is to be absorbed or damped.

To absorb sounds of high frequencies, especially porous materials are used, such as melamine, polyurethane and metal foams or non-woven fabrics made from mineral or polymeric fibers. Nevertheless, for absorbing sounds of lower frequencies are these materials unsuitable due to great material thickness needed in such cases. To absorb sounds of low frequencies, principally structures based on resonance principle are employed, in which by the resonance of one of the elements is acoustic energy converted into thermal energy. However, these structures absorb only sounds of certain frequency, while for other frequencies is their absorption very poor. Therefore, different combinations of perforated panel, a sound absorbing material and possibly air gap are most commonly used.

The general objective is to combine the above-mentioned characteristics into one acoustic system which would be able to absorb sounds of low as well as of high frequencies. In this sence is for example from CZ PV 2005-226 or analogical WO 2006108363 known a layered sound absorbing non-woven fabric, which comprises a layer of nanofibers with diameter up to 600 nanometers and a surface weight of 0,1 to 5 g.m ^"2 and at least another layer of fibrous material, these layers being formed by cross laying. The layer of nanofibers fulfills the function of acoustic resonance membrane resonating at low frequency, whereas the layer of another material provides not only sufficient damping, by which means the maximum quantity of sound energy gathered in the resonator is converted into heat, but at the same time it is also capable of absorbing sounds of higher frequencies. However, in practical applications, this textile absorbs with good results especially sounds of frequencies in relatively narrow range from approximately 700 to 1300 Hz.

The goalof the invention is therefore to eliminate or at least reduce the disadvantages of the present state of the art and to propose sound absorbing means that would be capable of absorbing with good results sounds in as broadest frequency range as possible.

Principle of the ^" invention

The goal of the invention is achieved by a sound absorbing means which contains resonance membrane formed by a layer of polymeric nanofibers, which is attached to a frame. The resonance membrane is then, upon impact of sound waves of low frequency, brought into forced vibrations, whereby the kinetic energy of the membrane is converted into thermal energy by friction of individual nanofibers, by the friction of the membrane with ambient air and possibly with other layers of material arranged in its proximity, and part of the energy is also transmitted to the frame, by which means the vibrations of the resonance membrane are damped.

According to the need and/or requirements on sound absorbing means and its properties, the frame can be arranged along at least part of the resonance membrane circumference and/or at least on part of the area of at least one surface of the resonance membrane. In one of the variants this frame is rigid, whereby it can form part of the support to which the sound absorbing means is applied.

In a second variant the frame is flexible, whereby it is formed, for example, by a grid or a mesh of formations of material in solid state which penetrate at least partially into the thickness of the resonance membrane and are interconnected with the nanofibers by enwrapping part of these nanofibres or, on the contrary, are at least partially enwrapped by the material of the nanofibers, or due to the adhesive properties of the material of the nanofibers. Also, the grid, as well as the mesh of formations, can be regular in order to obtain uniform properties over the whole area of the sound absorbing material.

In all the variants of the frame it may be also advantageous if it is spatially shaped, for example according to the surface of the support, on which the sound absorbing means is to be applied.

Formations of material in solid state are points and/or linear formations and/or planar formations - as required, whereby as the frame it is possible to use, for example, even only one linear formation, which is arranged along at least a part of the resonance membrane circumference.

To obtain desired sound absorbing characteristics, the resonance membrane is connected to the frame with positive, zero or negative tension.

Description of drawings

In the enclosed drawings Fig. 1a schematically shows a cross section of sound absorbing means according to the invention with one resonance membrane, Fig. 1b shows a cross section of sound absorbing means according to the invention with four resonance membranes, Fig. 1c shows another embodiment of sound absorbing means according to the invention with one resonance membrane, Fig. 1d shows the principle of the sound absorbing means according to the embodiment shown at Fig. 1c, Fig. 2a shows another embodiment of sound absorbing means according to the invention with one resonance membrane, Fig. 2b shows another variant of the sound absorbing means according to Fig. 2a, Fig. 2c shows another variant of the sound absorbing means according to Fig. 2a, Fig. 3a shows another embodiment of sound absorbing means according to the invention with one resonance membrane, Fig. 3b shows another variant of the sound absorbing means according to Fig. 3a, Fig. 3c showsanother variant of the sound absorbing means according to Fig. 3a, Fig. 4 shows another variant of the frame of the sound absorbing means according to Fig. 3c, Fig. 5 shows a graph of sound absorption coefficients in dependence on the frequency of sound for three variants of the sound absorbing means according to Fig. 3c, Fig. 6a shows a graph of sound absorption coefficients in dependence on the frequency of sound for individual layer of polyurethane foam and two embodiments of sound absorbing means according to the invention, one of which contains a layer of this polyurethane foam, Fig. 6b shows a cross section of this sound absorbing means, Fig. 7a shows a graph of sound absorption coefficients in dependence on the frequency of sound for individual layer of non-woven textile and sound absorbing means according to the invention which contains this layer of non- woven textile, Fig. 7b shows a cross section of this sound absorbing means, Fig. 8a shows a graph of sound absorption coefficients in dependence on the frequency of sound for individual layer of polyurethane foam other than at Fig. 6a and two varinats of sound absorbing means with two resonance membranes according to the invention, one alternative of which contains a layer of this polyurethane foam, Fig. 8b shows a cross section of this sound absorbing means with two acoustic membranes which does not contain a layer of this polyurethane foam, and Fig. 8c shows a cross section of embodiment of the sound absorbing means which comprises a layer of polyurethane foam.

Examples of embodiment

The principle of the invention consists in using resonance membraneformed by layer of polymeric nanofibers which is due to its small interfibrous spaces and planar arrangement brought into forced vibrations upon impact of sound waves of low frequency. As a result of mutual friction of individual nanofibers, friction of the membrane with ambient air and possibly with other layers of material arranged in its proximity, the kinetic energy of this membrane is converted into thermal energy, and its vibrations are gradually damped. Moreover, in the embodiment of the sound absorbing means according to the invention, part of the kinetic energy of the membrane is transmitted to the frame, to which the membrane is fixedly attached, and other part is converted into thermal energy due to increased friction in its inner structure, which is caused by the fact that the neighbouring parts of the membrane, separated at least partially by the frame or its elements, may vibrate with mutually different periods and/or deviations.

According to requirements, a separate layer of polymeric nanofibers is used, or a layer of polymeric nanofibers arranged on suitable carrying layer, on which it was deposited during its formation through electrostatic spinning, or to which it was transported after being formed, such as for example a textile, a grid, a mesh, metal or plastic foil (including bubble foil), a layer of foam material, a layer of aerogel, a layer containing aerogel as one of its components, etc., or a layer containing any combination of these materials. The carrying layer can be arranged in a direction towards the support, to which the sound absorbing means according to the invention is to be applied, or on contrary, in a direction away from it, when it protects the layer of polymeric nanofibers from mechanical damage, and at the same time it can constitute or co-constitute the face side of this means. Thus in both cases this layer - due to mutual friction with the layer of the polymeric nanofibers - also contributes to damping of this layer and conversion of its kinetic energy into thermal energy. In addition, some underlay layers can serve also to damp at least partially sounds with higher frequencies. If needed, the layer of polymeric nanofibers is connected to the carrying layer by means of suitable binder and/or by lamination.

In the embodiment shown at Fig. 1a the membrane is formed by layer of polymeric nanofibers and is arranged in inner space of the rigid frame 2, to which it is fixedly attached along its circumference or at least its part/parts, preferably by means of suitable binder, for example melt binder. As the melt binder it is possible to use also the material of the layer of nanofibers and/or of the rigid frame 2, or, as the case may be, the material of not shown carrying layer on which the membrane is arranged. The inner space 3 of the rigid frame 2 between the membrane Λ and the support 4, to which the sound absorbing means is applied, is in the illustrated variant void, however in other variants it can be filled at least partially with a material, preferably with material for absorbingsounds with higher frequencies, such as for example a textile, a foam material, an aerogel, a bubble foil, a composite, etc., or a combination of any of these materials. The rigid frame 2 is equipped with not shown means of connecting to the support 4, e.g. with the help of connecting elements, or glue or adhesive, etc.

In a not shown variant of embodiment the membrane 1 is arranged on the upper or lower side of the rigid frame 2.

To increase the extent of damping sounds with low frequencies, in the inner space of the rigid frame 2 and/or on its upper and/or lower side there are arranged more membranes i formed by layers of polymeric nanofibers, whereby these membranes 1 can be arranged mutually parallel, the spacings between them being the same or different, or at least one of them is arranged skewed in relation to the others and/or in at least partial contact with at least one of them. All the membranes 1 may be identical as to their surface weight, material and diameter of the nanofibers, or at least one of them can differ from the others in at least one of these parameters. In the embodiment shown in Fig. 1b there are four identical membranes 1, H, 12 and 13 in total, which are arranged mutually parallel with identical spacings between them in the inner space of the same rigid frame 2, as in the variant shown in Fig. 1 a, to which they are fixedly attached along their circumference or at least on its part/parts, for example by means of suitable binder. Any of the spaces between these individual membranes 1, 11., 12 and 13 and between the lower membrane 13 and the support 4, to which the sound absorbing means according to the invention is applied, can be at least partially filled with a material, preferably with one of the above-mentioned materials for damping sounds with higher frequencies, or a combination of these materials. In the embodiment shown in Fig. 1c the membrane Λ is by suitable binder attached to the rigid frame 2 formed by regular grid 21 , whose empty spaces 22, or meshes, this membrane overlaps. In these empty spaces 22 the membrane is freely movable, whereby upon the impact of sound waves of low frequency it may move in each of them differently, e.g. with different period and/or deviation, which results in the increase in friction between the nanofibers in its structure and contributes to damping (see Fig. 1d). In not shown variants of embodiment the frame 2 is composed of regular or irregular grid 2J. having in principle any size and/or shape of inner spaces 22, or meshes, and their orientaion, or it is composed of two such grids 21 , each of which is arranged on one of the surfaces of the membrane . In all the variants of embodiment the grid 21. is according to requirements arranged on at least part of the surface of the membrane -

Apart from the flat planar grids 21 , the frame 2 of the sound absorbing means according to the invention can be spatially shaped, for example according to the surface of the support, to which this sound absorbing means is applied - see e.g. Fig. 4. In the variants in which the sound absorbing means according to the invention comprises more layers, for the purpose of increasing its cohesion, at least some of its layers may be mutually interconnected by a binder, for example melt binder, or by lamination. As a melt binder it is possible to use directly material of one of the layers, for example layer of polymeric nanofibers, which is with advantage for example polypropylene, polyethylene, polyamide, polyester, etc., their copolymer/s, or, as the case may be, a mixture of these materials, etc.

The layer of polymeric nanofibers which is fixedly attached to the frame 2 can be, if needed, further attached, for example even by means of this frame 2, to another frame 2, which is part of the surface of the support 4 and/or is arranged on the surface of the support 4, to which the sound absorbing means according to the invention is applied.

Owing to the fact that the rigid frame 2, to which the membrane 1/membranes 1, H , 12 and 13 is/are fixedly attached, is not suitable for all the applications of the sound absorbing means according to the invention, it is also possible to use flexible variant of the frame 20. Such a frame 20 is then composed, for example, of a grid 21 made of flexible or elastic material, or composed _; of at least one formation of suitable material in solid state, but preferably of mesh of formations 201 of suitable material in solid state, which are not mutually fixedly interconnected and which penetrate through the whole thickness of the layer of polymeric nanofibers, or at least through part of it, at the same time enwrapping part of its nanofibers. In other embodiments the flexible frame 20, or its formations 201 is attached to the layer of polymeric nanofibers by means of suitable binder, or by means of material of nanofibers, which is either melted and in this mannerconnected to the formations 201 of the frame 20, whereby these formations are at least partially enwrapped with the material of the nanofibers, or which has adhesive properties. In both cases the frame 20 is arranged at least on part of the surface of at least one surface of the membrane Λ . At least some of the formations 201 also serve to attach the membrane to the support 4 or to the rigid frame 2, wherein they also reinforce it and partially protect it from mechanical damage, such as abrasion. These formations 201 consist, for example, of points, which can be located only along the circumference of themembrane (Fig. 2a) or at least on one of its parts and/or on its whole surface, namely either evenly (Fig. 2b) or unevenly (Fig. 2c). Besides, these formations 201 are in another embodiment composed of linear formations, such as fibers and/or strips, which may be located only along the circumference of the membrane or at least on one of its parts and/or the whole of its surface, namely either randomly (Fig. 3a) and/or in regular mesh (Fig. 3b) and/or in regular grid (Fig. 3c). The density and shape of the mesh of these formations 201 determine the resonance frequency of the acoustic means according to the invention. Furthermore, it is advantageous if the density of the mesh is increased in the area of expected increased mechanical strain of the membrane 1 , e.g. in proximity to its orifices (e.g. orifices for manipulation, installation, etc.) and/or in the area of its attachment to the rigid frame 2, etc. In another alternative the flexible frame 20 is composed of at least one linear and/or planar formation 201. which may be arranged along at least part of the circumference of the membrane Λ or on part of its surface. Also, these alternatives can be in principle combined in any suitable manner on one or both surfaces of the layer of polymeric nanofibers. In addition, it is also possible to mutually combine various variants of the rigid frame 2 and/or of the flexible frame 20, whereby it is possible, for example, to arrange the layer of polymeric nanofibers between two grids 21 with the same shape and/or size and/or orientation of empty spaces 23, or meshes, or, on the contrary, between two grids 21 , which differ at least in one of these parameters. In another variant of embodiment the layer of polymeric nanofibers, which is fixedly attached to the flexible frame 20, is further attached, for example also by means of this flexible frame 20, to the rigid frame 2.

A suitable material of formations 201 of the flexible frame is, for example, a polymer or mixture of polymers, or, as the case may be, an inorganic substance on basis of glass, glass-derived silicates, crystallic silicates, metal oxides, semi-metal oxides, or an organic-inorganic substance containing an metal alkoxide, alkaline metal alkoxide, alkoxide of alkaline earth metal, transient metal alkoxide, semi-metal alkoxide, metal salt, alkaline metal salt, alkaline earth metal salt, transient metal salt, semi-metal salt, organometallic compound, organometallic chelate, silicate, metal, metal oxide, etc., or a mixture thereof.

In all the above-described embodiments any of the membranes is attached to the rigid frame 2 and/or the flexible frame 20 either with initial tension, in tight or loose manner, or with positive, zero or negative tension, by which means it is possible to adapt its acoustic characteristics at least partially to the parameters of sound/sounds in the environment in which the sound absorbing means according to the invention is to be applied. In addition to this, its sound absorbing characteristics can be also modified by surface weight and/or material of the layer of polymeric nanofibers and/or diameter of its nanofibers and/or shape and/or size and/or density of empty spaces 22, or meshes of theframe 2, 20 in which the layer of polymeric nanofibers can move, or by mutual arrangement of more membranes 1_.

In order to achieve uniform acoustic characteristics within the whole area of the sound absorbing means according to the invention, it is advantageous if the layer/layers of polymeric nanofibers is as homogeneous as possible, both in direction of its width and in direction of its length. Nowadays, the highest homogeneity in both directions is achieved by its production by the so called nozzle-less electrostatic spinning, in which a polymer solution or melt is subject to spinning in an electric field from the surface of the spinning electrode of an oblong shape - for example a cylinder (see e.g. EP 1673493) or a cord (see e.g. EP 2059630 or EP 2173930). This type of electrostatic spinning is applied commercially, for example, in the technology Nanospider™ of the company Elmarco.

Fig. 5 shows a graph of coefficients of sound absorption a in dependence on frequency of sound for three variants of sound absorbing meansaccording to the invention in the embodiment according to Fig. 1c. The black curve then indicates sound absorption coefficient a of the sound absorbing means comprising membrane 1_ formed by layer of nanofibers of polyvinylalcohol (PVA) having surface weight of 5,7 g.m ^"2 , which is attached to the rigid frame 2 formed by a grid having dimension of meshes 4,1 x 4,3 mm (i.e. 17.63 mm ² ), which is located at distance of 30 mm from the support 4. The light grey curve indicates sound absorption coefficient a of the sound absorbing means comprising membrane formed by layer of nanofibers of polyvinylalcohol (PVA) having surface weight of 5,7 g.m ^"2 , which is attached to the rigid frame 2, composed of a grid with dimension of meshes 9,0 x 14,2 mm (i.e. 127,80 mm ² ), which is located at distance of 30 mm from the support 4. The dark grey curve then indicates sound absorption coefficient a of the sound absorbing means comprising membrane 1. formed by layer of nanofibers of polyvinylalcohol (PVA) having surface weight of 1 ,7 g.m ^"2 , which is attached to the rigid frame 2 composed of a grid with dimension of meshes 4,1 x 4,3 mm (i.e. 17,63 mm ² ), which is located at distance of 30 mm from the support 4.

Fig. 6a shows a graph of coefficients of sound absorption a in dependence on the frequency of sound for individual layer of polyurethane foam and two variants of the sound absorbing means in the embodiment according to Fig. 1c. The light grey curve indicates sound absorption coefficient a of the individual layer of polyurethane foam having thickness of 25 mm and surface weight of 640 g.m ^"2 located 25 mm from the support 4. The dark grey curve indicates sound absorption coefficient a of the sound absorbing means according to the invention, which comprises resonance membrane 1_ formed by layer of nanofibers of polyamide 6 (PA6) ,with surface weight of 1 g.m ^"2 , which is attached to the rigid frame 2 composed of a grid with dimension of meshes 6,5 x 6,7 mm, i.e. 43,55 mm ² ), whereby on the side of this sound absorbing means turned away from the support 4 is further arranged a covering layer formed by calendered non-woven textile made from polyester (PES) and viscose fibers (VI) having surface weight of 25 g.m ^"2 . This means is positioned at distance of 30 mm from the support 4. Finally, the black curve indicates sound absorption coefficient a of the sound absorbing means according to Fig. 6b, which comprises resonance membrane Λ formed by layer of nanofibers of polyamide 6 (PA6) having surface weight of 1 g.m ^"2 , which is attached to the rigid frame 2 composed of a grid with dimension of meshes 6,5 x 6,7 mm (i.e. 43,55 mm ² ), and is at the same time overlayed by covering layer formed by calendered non- woven textile made from polyester (PES) and viscose (VI) fibers with surface weight of 25 g.m ^"2 . The rigid frame 2, formed by a grid, is at the same time attached to another rigid frame 2 which is arranged on the surface of the support 4 and whose inner space 3 is filled with a layer of polyurethane foam with thickness of 25 mm and surface weight of 640 g.m ^"2 - see Fig. 6b.

Fig. 7a shows a graph of sound absorption coefficient a in dependence on the frequency of sound for individual layer of non-woven textile and for a variant of the sound absorbing means in the embodiment according to Fig. 1 c. The grey curve then indicates sound absorption coefficient a for individual layer of non-woven polyester fabric having thickness of 10 mm and surface weight of 100 g.m ^"2 . The black curve then indicates sound absorption coefficient a of the sound absorbing means according to the invention which contains resonance membrane Λ formed by layer of nanofibers of polyacrylonitrile (PAN) having surface weight of 1 g.m ^"2 , which is attached to the rigid frame 2 formed by a grid with dimensions of meshes 6,5 x 6,7 mm (i.e. 43,55 mm ² ), whereby on the side of this sound absorbing means turned away from the support 4 is further arranged a covering layer formed by calendered non-woven textile made from polyester and viscose fibers having surface weight of 25 g.m ^"2 . The rigid frame 2, composed of a grid, is at the same time attached to another rigid frame 2, which is arranged on the support 4 and whose inner space 3 is filled with a layer of polyester non-woven fabric having thickness of 10 mm with surface weight 100 g.m ^"2 - see Fig. 7b.

Fig. 8a shows a graph of sound absorption coefficient a in dependence on the frequency of sound for individual layer of polyurethane foam and two variants of the sound absorbing means according to Fig. 1c, which comprises two resonance membranes 1 and 11.. The light grey curve indicates sound absorption coefficient a of the individual layer ofpolyurethane foam having thickness of 50 mm and surface weight of 1280 g.m ^"2 . The dark grey curve indicates sound absorption coefficient a of thesound absorbing means according to the invention which comprises resonance membrane 1 formed by layer of nanofibers of polyamide 6 (PA6) having surface weight of 1 g.m ^"2 , which is attached to the rigid frame 2 composed of a grid having dimensions of meshes 6,5 x 6,7 mm (tj. 43,55 mm ² ), and a second resonance membrane H, arranged under it and formed by layer of nanofibers of polyamide 6 (PA6) having surface weight of 12,5 g.m ^"2 , which is attached to the rigid frame 2, composed of a grid having dimensions of meshes 6,5 x 6,7 mm (i.e. 43,55 mm ² ). On the side of this sound absorbing means, turned away from thesupport 4 is further arranged a covering layer 5 formed by non-woven textile made from polyester and viscose fibers having surface weight of 25 g.m ^"2 . Both rigid frames 2 composed of a grid are at the same time attached to another rigid frame 2, which is arranged on the support 4 and whose inner space 3, having a height of 50 mm, is void. The black curve then indicates sound absorption coefficient a of the same sound absorbing means according to the invention, in which the inner space 3 of the frame 2, arranged on the surface of the support 4, is filled with a layer of polyurethane foam having thickness of 50 mm and surface weight of 1280 g.m ^"2 .

The sound absorbing means according to the invention can be used, for example, for the production of acoustic bodies, jalousie, wallpapers, tiling, ceilings, screens, curtains and separating walls for interiors, or, as the case may be, segment or profile elements for transportation industry (e.g. door panels, wheel archs, paneling of a boot or engine compartment or a cabin), materials for noise reduction of noisy devices, for the production of earphones, etc. In addition, any of tits layers may be provided with suitable surface treatment, for example for increasing flame resistance, water resistance, electrical conductivity, etc., by means of plasma treatment, spray application, spreading, etc.