Absorbing device for reducing noise vibrations

Abstract

Absorbing device (1) for reducing noise vibrations, suited to be applied to at least one separating structure for absorbing and / or reducing noise vibrations susceptible to crossing through or be propagated by said separating structure; said absorbing device (1) comprising at least one elastically yielding layer (2), extending along a thickness direction (Z) between a base end (3) and a top end (4), opposite to said base end (3); said absorbing device (1) being characterized in that said elastically yielding layer (2) comprises spring means (5) connecting said base end (3) and said top end (4); said absorbing device (1) comprises at least one mass layer (6) mechanically connected to said top end (4), provided with a predetermined mass or density and configured to increase the mass distributed on said top end (4) of said elastically yielding layer (2); said spring means (5) of said elastically yielding layer (2) are tuned along with the mass or density of said mass layer (6) so as to reduce the amplitude of noise vibrations at least at a predetermined frequency.

Description

ABSORBING DEVICE FOR REDUCING NOISE VIBRATIONS DESCRIPTIONTechnical field of the invention

[0001] The present invention concerns an absorbing device for reducing noise vibrations.

[0002] Therefore, the present invention finds advantageous use and purpose in the technical field of manufacturing, producing and / or selling of damping or absorbing devices, intended to be used for reducing undesired noise vibrations, such as in vehicles or between two rooms.

[0003] The device object of the present invention finds advantageous use and purpose in the technical field of soundproofing of environments, whether defined in vehicles (cars, trains, watercrafts, etc.) or in aerospace vehicles (aircrafts, planes, jets, rotorcrafts, helicopters, space systems, etc.) or in buildings of any kind.State of the art

[0004] As it is well known in the state of the art, noise (i.e. undesired acoustic waves) is a problem in many different technical fields.

[0005] For example, often inside dwellings or houses or apartment blocks or otherwise buildings intended to contain people, it is necessary to contain, limit or absorb noise from outside and / or acoustically separate two neighbouring rooms in the same building.

[0006] Moreover, it is well known that in the technical field of transport, be it by means of motor vehicles or aircraft, it is necessary to separate the passenger compartment or cabin / cockpit from external noise, be it from the environment or from the vehicle or aircraft engines or from the external surfaces exposed to airflow.

[0007] For this purpose, many different solutions for acoustically separating two different environments are known, a significant part of which is summarised schematically below.

[0008] For the purpose of the present patent application, the environment in which the noise is produced is called “source side” and the environment in which the noise must be reduced is called “receiver side”. A separating structure (e.g. a wall) is provided between the source side and the receiver side, configured to separate the two sides from one another.

[0009] In general, noise is produced in the source side; vibrations can be produced in the separating structure when the noise waves coming from the source side interact with the separating structure; the vibrations then propagate as noise vibrations inside the receiver side when the separating structure interacts with the fluid in the receiver side.

[0010] Moreover, further vibrations can be originated on the separating structure (e.g.unsteady flow impinging on structure) or remotely and transmitted to separating structure via structure propagation.

[0011] A first strategy for reducing noise provides increasing acoustic absorption on the receiver side (e.g. use of porous materials providing high value of Sound Absorption Coefficient - SAC).

[0012] The Sound Absorption Coefficient (SAC) is used to evaluate the sound absorption efficiency of a material / systems and it is defined as the ratio of absorbed acoustic energy to incident acoustic energy.

[0013] A second strategy for reducing noise provides increasing the Sound Transmission Loss (STL) of the separating structure between the source side and the receiver side (e.g. higher surface mass density, double wall structure, use of sound insulating material, etc.).

[0014] The Sound Transmission Loss (STL) is used to evaluate the sound insulation efficiency of a material / systems and it is a quantification of how much sound energy is prevented from traveling through an acoustic treatment.

[0015] A third strategy for reducing noise provides increasing the damping of the structure-propagated vibrations of the separating structure (e.g. constrained layer damping, viscoelastic materials, vibration absorbers, etc.).

[0016] The technical field of soundproofing is extremely rich in different technical solutions, which usually involve the application of at least one panel of sound-absorbing and / or sound-insulating material on a wall delimiting the room / environment to be soundproofed from sound or vibration.

[0017] Many different materials are known nowadays which are employed as soundproofing barriers and devices.

[0018] For example, with the term “acoustic metamaterials” are identified materials which are engineered to possess properties not found in naturally occurring materials, namely designed with periodic structures that define bandgaps.

[0019] These bandgaps are configured to block sound waves of specific and predetermined frequencies, making acoustic metamaterials highly effective at attenuating unwanted noise frequencies while permitting other frequencies to propagate.

[0020] Anyhow, acoustic metamaterials often require precise, complex an timeconsuming manufacturing processes, making them more expensive to produce compared to conventional materials.

[0021] Moreover, acoustic metamaterials often add extra weight compared to traditional insulation methods, potentially impacting their usage in particular environments, forexample due to the aircraft's fuel efficiency requirements.

[0022] Another example of an absorbing device for reducing noise vibrations lies in usage of carbon fiber, whose composites exhibit an exceptional strength-to-weight ratio.

[0023] Nowadays some absorbing devices are made of composite material comprising carbon fibers; these composites can be combined with damping materials or utilized within multi-layered structures to provide effective sound barriers with minimal weight.

[0024] More in detail, the stiffness and density of carbon fibers contribute to the reduction of vibrations and noise transmission.

[0025] For example, in aircrafts applications, carbon fiber composite absorbing devices provide structural strength while adding minimal weight, which is crucial for maintaining aircraft performance and fuel efficiency.

[0026] Moreover, carbon fiber composite absorbing devices can be molded into complex shapes, allowing for customized soundproofing solutions tailored to specific parts of the aircraft.

[0027] However, even the aforementioned absorbing devices made with carbon fiber composite materials have shown many disadvantages.

[0028] Firstly, the production of carbon fiber composites is more expensive than traditional materials, which can increase the overall cost of the device and hence the cost of the object that will comprise the absorbing device, such as an aircraft.

[0029] Moreover, while being strong, carbon fiber composite materials can be brittle and susceptible to cracking under certain stress conditions, requiring careful handling, design considerations and narrowing the possible uses.

[0030] Moreover, the fabrication of carbon fiber composite panels with integrated soundproofing features involves sophisticated processes and precise quality control, adding to production complexity.

[0031] Another example of materials used in absorbing devices are viscoelastic materials, which are provided with viscous and elastic properties when deformed.

[0032] Absorbing panels made with viscoelastic materials convert vibrational energy into thermal energy, thereby dampening vibrations and reducing noise transmission.

[0033] However, even the absorbing devices made with viscoelastic materials have shown many disadvantages.

[0034] Firstly, viscoelastic materials can be sensitive to temperature changes, potentially affecting their damping properties. In extreme temperatures, they may become either too rigid or too soft, reducing their effectiveness.

[0035] Moreover, over time, viscoelastic materials can degrade, especially whenexposed to harsh environmental conditions, leading to reduced damping performance.

[0036] Another example of known absorbing devices are panels made with foam, which notoriously acts as an absorber and insulator, increasing sound absorption and reducing sound transmission. More in detail, foams used in absorbing devices are usually made of melamine foam, which is designed to absorb sound waves and they are often made from open-cell materials that trap and dissipate sound waves within the foam structure.

[0037] However, even the absorbing devices made with foam have shown many disadvantages.

[0038] Firstly, foams are generally less effective at absorbing low-frequency sounds. In several aircraft types (e.g. turbo-prop, piston-prop, etc.), the principal component of noise pollution is made of low frequencies generated by the aircraft engines.

[0039] Hence, even if commonly used, foams (and in particular melamine foams) are not effective against low frequencies and therefore they are not effective when installed in aircrafts where the principal component of noise pollution is made of low frequencies.

[0040] Moreover, foams are typically good at absorbing sound within a room but are not effective at blocking sound transmission between spaces. Effective soundproofing often requires materials with significant mass to prevent sound waves from passing through, which foams generally lack.

[0041] Moreover, foams can be susceptible to physical damage, such as tearing or compression over time. They may degrade more quickly compared to other more robust soundproofing materials.

[0042] Moreover, many foams can absorb moisture, which can lead to mould growth, degradation, and reduced acoustic performance. This makes them unsuitable for use in damp or humid environments without proper moisture barriers or treatments.

[0043] Moreover, proper installation of devices for soundproofing made with foam requires careful attention to coverage and sealing. Gaps, seams, or improper placement can significantly reduce effectiveness, and achieving optimal installation can be labour- intensive and complex.

[0044] Moreover, while foams can be shaped and coloured to fit specific design needs, they may not always meet aesthetic requirements without additional finishing or covering, which can add to the overall cost and complexity.

[0045] Moreover, foams are rigid and cannot be installed following the shape of walls with special shapes or curves.Purpose of the invention

[0046] The purpose of the present invention is to make available an absorbing devicefor reducing noise vibrations capable of overcoming the drawbacks of the above- mentioned prior art.

[0047] In particular, the main purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of reducing noise, whether produced from outside or propagated through a separating structure, e.g. a wall.

[0048] A further purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of reducing noise at a wide frequency range, in particular at the receiver side.

[0049] Another purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of providing high values of Sound Absorption Coefficient and / or Sound Transmission Loss.

[0050] A further purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of reducing low frequency noise.

[0051] Another purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of reducing noise via reactive or resonant damping effect of both structure- borne and air-borne noise vibrations.

[0052] A further purpose of the present invention is to provide an absorbing device for reducing noise vibrations having a compact and lightweight configuration.

[0053] Another purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of easily tuning the frequencies at which the device is better performing (i.e. higher SAC and STL).

[0054] A further purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of providing dissipation of energy of noise vibrations, for example via structural losses and / or visco-elastic damping and / or visco-thermal damping.

[0055] Another purpose of the present invention is to provide an absorbing device for reducing noise vibrations capable of reducing noise acoustic radiation by means of the system geometry and / or tuning the surface acoustic impedance at the frequency of interest.

[0056] These purposes, together with others that will be better clarified below, are achieved by an absorbing device for reducing noise vibrations according to claim 1.

[0057] Other purposes that will be better described below are achieved by an absorbing device for reducing noise vibrations according to dependent claims.Brief description of the drawings

[0058] The advantages and features of the present invention will become clear from thefollowing detailed description of certain preferred but not limiting embodiments of an absorbing device for reducing noise vibrations with particular reference to the following drawings.- Figure 1 shows an axonometric view of an absorbing device for reducing noise vibrations, according to the present invention.- Figure 2 shows a frontal view of the device according to the invention, along the X axis.- Figure 3 shows a lateral view of the device according to the invention, along the Y axis.- Figure 4 shows a lateral view of an absorbing device according to a further embodiment.- Figure 5 shows an axonometric view of a detail of the elastically yielding layer, with some parts removed to better highlight others.- Figure 6 shows a lateral view of the detail of the elastically yielding layer of figure 5, with some parts removed to better highlight others.- Figure 7 shows a schematic view of the absorbing device according to the invention, according to a second embodiment of the latter.- Figure 8 shows a schematic view of the absorbing device according to the invention, according to a third embodiment of the latter.- Figure 9 shows a schematic view of a transverse section of an aircraft fuselage, internally provided with the absorbing device according to the present invention.- Figure 10 shows a schematic view a particular of the transverse section shown in figure 9, realized at the line identified with reference IX.- Figure 11 shows a graph of the trend of the Sound Absorption Coefficient (SAC) and the Sound Transmission Loss (STL) of an exemplary embodiment of the absorbing device according to the invention.Detailed description of the invention

[0059] The present invention concerns an absorbing device for reducing noise vibrations.

[0060] The absorbing device according to the invention has been identified in the attached drawings with numeric reference 1 .

[0061] The present invention finds advantageous use and purpose in the technical field of manufacturing, producing and / or selling of damping or absorbing devices, intended to be used for reducing undesired noise vibrations, such as in vehicles or between tworooms.

[0062] The device object of the present invention finds advantageous use and purpose in the technical field of soundproofing of environments, whether defined in vehicles (cars, trains, watercrafts, etc.) or in aerospace vehicles (aircrafts, planes, jets, rotorcrafts, helicopters, space systems, etc.) or in buildings of any kind.

[0063] Within the context of this patent application, the term “noise vibrations” or “noise” or “vibrations” must be interpreted as any structure-borne and / or air-borne vibration or group of vibrations which are undesired and may define an unpleasant sound and / or an undesired mechanical stress on a structure or the like.

[0064] The term “absorbing” referred to the device according to the invention must be interpreted as the capability of the device to at least partially take in the vibration / sound energy.

[0065] Therefore, the device according to the invention is suited to be installed in any environment or on any wall or structure that needs to reduce the amount of noise vibrations.

[0066] The absorbing device 1 according to the invention is particularly suitable for being applied in aircrafts, for example for reducing noise vibrations propagating through the fuselage.

[0067] As already mentioned, the absorbing device 1 for reducing noise vibrations according to the invention is suited to be applied to at least one separating structure for absorbing and / or reducing noise vibrations susceptible to crossing through or be propagated by said separating structure.

[0068] The term “separating structure” must be interpreted as defining any wall or structure intended to divide two or more environments, between which a sound and vibrations damping and reduction is required.

[0069] Said absorbing device 1 comprises at least one elastically yielding layer 2, extending along a thickness direction Z between a base end 3 and a top end 4, opposite to said base end 3.

[0070] The absorbing device 1 according to the invention is essentially planar and lies on a lying plane defined by a first axis X, parallel to said mass layer 6, and a second axis Y, orthogonal with respect to said first axis X and parallel to said mass layer 6.

[0071] Preferably, in the following description the width of the absorbing device 1 is defined parallel to the first axis X and the depth of the absorbing device 1 is defined parallel to the second axis Y.

[0072] Preferably, the thickness direction Z is orthogonal to both first axis X and secondaxis Y.

[0073] The elastically yielding layer 2 is preferably suited to be compressed and / o stretched elastically during normal use.

[0074] The elastically yielding layer 2 shall preferably, but not exclusively, have low specific airflow resistance. The term “low” specific airflow resistance must be interpreted as equal or lower to the characteristic impedance of air.

[0075] According to the invention, said elastically yielding layer 2 comprises spring means 5 connecting said base end 3 and said top end 4.

[0076] Moreover, said absorbing device 1 comprises at least one mass layer 6 mechanically connected to said top end 4, provided with a predetermined mass or density and configured to increase the mass distributed on said top end 4 of said elastically yielding layer 2.

[0077] The device according to the invention is configured to be intercepted by noise vibrations, either through the air of the environment in which the device is installed or through the separation structure.

[0078] The device is configured to respond to noise vibrations by elastic deformation thereof. More in detail, the absorbing device 1 is configured to respond as a mass-spring damping system.

[0079] In other words, following and / or during stimulation of the absorbing device 1 by noise vibrations, the absorbing device 1 defines, preferably simultaneously, compression zones and expansion zones.

[0080] Preferably, compression zones and expansion zones are zones of the absorbing device 1 , identified along a lying plane defined by the first axis X and the second axis Y, in which - respectively - the thickness (parallel to the thickness axis Z) is less (for compression zones) or greater (for expansion zones) with respect to the thickness of the device at rest.

[0081] In this way, the particular elastic modulus of the absorbing device 1 , and in particular of the elastically yielding layer 2 and of the mass layer 6, allows the absorbing device 1 to deform in a different manner, unlike the prior art devices which tend to respond by vibrating substantially rigidly.

[0082] In order to achieve the aforementioned purposes, said spring means 5 of said elastically yielding layer 2 are tuned along with the mass or density of said mass layer 6 so as to reduce the amplitude of noise vibrations at least at a predetermined frequency.

[0083] More in detail, the elastic properties of the spring means 5 can be varied along with the mass (or density) of the mass layer 6 on the basis of what frequency (orfrequency band) must be reduced by the absorbing device 1.

[0084] In this manner, the absorbing device 1 according to the invention can be used for reducing a wide range of noise frequencies, and particularly can be used for reducing noise vibrations at low frequencies, for example comprised between 0 and 200 Hz.

[0085] The term “low frequency” must be interpreted, according to the scope of the present patent application, in relative terms. More in detail, a low frequency can be defined as a frequency range, wherein the acoustic wavelength of the noise vibrations is larger than the typical material / system quarter wave resonance, and it can be calculated as: f < c0 / (4 tM), wherein f is the frequency, c0is the speed of sound and tMis the typical material / system thickness.

[0086] In other words, the absorbing device 1 is intended to be used for reducing noise vibrations at low frequencies, wherein the acoustic wavelength of the noise vibrations is larger than the typical material / system quarter wave resonance.

[0087] More in detail, the frequency of the reduced noise vibrations is f < c0 / (4 tM), wherein f is the frequency, c0is the speed of sound and tMis the typical material / system thickness.

[0088] Preferably, the mass layer 6 is realized with a porous material, preferably permeable to airflow (e.g., woven, knitted or non-woven textile, foam, perforated plate, etc.).

[0089] According to a particular aspect of the invention, the absorbing device defines a footprint on a lying plane, orthogonal to the thickness direction Z, which is greater than the footprint (thickness) parallel to the thickness direction Z.

[0090] Preferably, the footprint of the absorbing device 1 measured orthogonally to the thickness direction Z is at least ten times greater than the footprint measured parallel to the thickness direction Z.

[0091] In other words, the absorbing device 1 is preferably substantially leaf-like.

[0092] Hence, the absorbing device 1 can be attached to the separating structure, and preferably the absorbing device 1 is preferably configured to cover substantially all the surface of the separating structure, which needs a noise vibration damping and / or reduction.

[0093] Obviously, the absorbing device 1 according to the invention can be configured to cover only one or more portions of one or more surfaces of the separating structure, without going beyond the scope of protection of this patent application.

[0094] According to the invention, the absorbing system 1 is realized with elastic material and / or bendable material / materials.

[0095] Preferably, the absorbing system 1 is suited to be applied even on shaped and / or irregular surfaces of the separating structure, since the elasticity of the materials the form the absorbing system 1 permit it to bend and follow eventual curves of said surfaces.

[0096] Preferably, said spring means 5 of said elastically yielding layer 2 are configured to provide an elastic modulus lower than the elastic modulus of the material composing said spring means 5.

[0097] In use, the base end 3 of the elastically yielding layer 2 is preferably intended to be faced towards the separating structure, while the mass layer 6 is preferably intended to be faced towards an opposite direction with respect to the base end 3 of the elastically yielding layer 2, in particular towards an inner environment that needs a noise reduction.

[0098] In other words, the elastically yielding layer 2 is realized with a material provided with an elastic modulus, and in particular the Young’s modulus, that is higher with respect to the elastic modulus, and in particular the Young’s modulus, of the elastically yielding layer 2.

[0099] In other words, the elastically yielding layer 2 has a lower Young’s modulus than the material which it is composed of, due to the geometry of the elongated elements 7, and in particular due to the fact that said elongated elements 7 are configured to elastically behave as being compressed, along a compression direction parallel to said thickness direction Z, beyond their critical buckling load.

[0100] Advantageously, the elastically yielding layer 2 and the mass layer 6 are constructed so as to counteract against the propagation of the noise vibrations.

[0101] For example, the absorbing device can behave as an integrated collection of unit cell vibration absorbers. In fact, the elastically yielding layer 2 is built as a collection of unit cells and / or the mass layer 6 is designed to have low bending stiffness so that forces (and hence accelerations, velocities, displacements) are not easily transferred between unit cells.

[0102] This makes the mass layer 6 not oscillating fully synchronously, so reducing effectiveness of acoustic radiation.

[0103] According to a further embodiment of the invention, the mass layer 6 is configured to have a desired mass and / or a discontinuous surface. In such preferred embodiment it can be realized through a weaved / knitted mesh fabric.

[0104] Advantageously, the globally distributed mass of the mass layer 6 keeps the material / system behaves as a distributed vibration and sound absorber.

[0105] Moreover, the reduced material surface of the mass layer 6 reduces the effectiveness of the acoustic radiation, limiting a propagation of the sound waves throughthe environment.

[0106] Preferably, acoustic radiation reduction is achieved by means of tuning the materials surface acoustic impedance at the frequency of interest. This can be achieved for example by tuning / choosing the constitutive parameters of the mass layer 6.

[0107] Therefore, preferably, the elastically yielding layer 2 according to the invention defines a metamaterial, and in particular an acoustic metamaterial.

[0108] Within the meaning of the present description, all features described with reference to the elastically yielding layer 2 are also to be understood as being described with reference to an innovative acoustic metamaterial, which is also an object of the present invention.

[0109] Preferably, said spring means 5 of said elastically yielding layer 2 comprise a plurality of elongated elements 7 extending parallel to one another, between said base end 3 and said top end 4.

[0110] The term “parallel” associated with the elongated elements 7 must be interpreted as extending between said base end 3 and said top end 4. Obviously, each elongated element 7 may be extending along a curved line, defining a lying plane. Preferably, the elongated elements therefore define a plurality of lying planes, which may be parallel to each other or may define a sheaf of planes.

[0111] Preferably, said elongated elements 7 of said spring means 5 are buckled, defining a curved shape, between said base end 3 and said top end 4.

[0112] Preferably, said elongated elements 7 are configured to elastically behave as being compressed, along a compression direction parallel to said thickness direction Z, beyond their critical buckling load.

[0113] In other words, preferably, the elongated elements 7 are compressed beyond their buckling point, and it means that the stiffness of the system (i.e. of the absorbing device) drops significantly and very small changes in load (i.e. due to noise vibrations) on the device can cause large displacements on the elongated elements 7, which respond damping the noise vibrations very effectively.

[0114] Moreover, the post-buckled elongated elements 7 of the elastically yielding layer 2 give responses to the vibrations which are highly nonlinear (since their stiffness decreases or varies with displacement). This non-linear behaviour enables tuneable compliance (by tuning the mass and / or the density of the mass layer 6) and high energy absorption.

[0115] Hence, the absorbing device according to the invention can simultaneously reduce noise vibrations of both structure- borne and air-borne noise vibrations, since thepost-buckled elongated elements 7 behave with extraordinary efficiency.

[0116] Preferably, said elongated elements 7 can be configured to elastically behave as a high static low dynamic stiffness spring.

[0117] Differently, in any case, the elongated elements 7 can be configured to elastically behave as a low static high dynamic stiffness spring.

[0118] Preferably, said mass layer 6 comprises at least first sub-layer 8. According to an embodiment of the invention, the mass layer 6 comprises a second sub-layer 9.

[0119] More in detail, the first sub-layer 8 is mechanically connected to said elastically yielding layer 2 at said top end 4.

[0120] Preferably, the second sub-layer 9 covers said first sub-layer 8 and is provided with a predetermined mass or density.

[0121] The second sub-layer 9 of the mass layer 6 is preferably mechanically attached at least to the first sub-layer 8, for example through fixing means such as glue or similar.

[0122] Preferably, the absorbing device 1 according to the invention comprises a base layer 10 mechanically connected to said elastically yielding layer 2 at said base end 4.

[0123] Preferably, said first sub-layer 8 of said mass layer 6 is configured to mechanically connect said elongated elements 7 of said spring means 5 at said top end 4.

[0124] Preferably, said base layer 10 is configured to mechanically connect said elongated elements 7 of said spring means 5 at said base end 3.

[0125] According to a preferred embodiment of the present invention, the mass layer 6 is realized by the combination of a woven and / or knitted textile (which defines the second sub-layer 9) and a spacer fabric top meshed outer layer (which defines the first sub-layer 8).

[0126] According to this preferred embodiment of the present invention, the mass layer 6 is preferably bonded with the elastically yielding layer 2, in particular a woven and / or knitted textile (defining the second sub-layer 9) and the spacer fabric top meshed outer layer (which defines the first sub-layer 8 and is preferably integral with the elastically yielding layer 2) are attached by means of glue or other well-known equivalents, and the first sub-layer 8 (namely a spacer fabric top meshed outer layer) forms an integral part with the elastically yielding layer 2.

[0127] The mass layer 6 is preferably bonded to the elastically yielding layer 2.

[0128] For example, the mass layer 6 can be bonded to the elastically yielding layer 2 for the whole elastically yielding layer 2 width and / or depth, for a reduced portion of the elastically yielding layer 2 width and / or depth, or it can be wider / deeper than theelastically yielding layer 2 width and / or depth.

[0129] The preferred bonding means for bonding the first sub-layer 8 and the second sub-layer 9 of the mass layer 6 comprise glue or a thermoplastic net. Other bonding means are possible and suitable, as long as the bonding means result in a global connection between the two layers that is permeable to airflow.

[0130] The mass layer 6 and the elastically yielding layer 2, or portions of them, can be produced at the same time, this is the case for instance of spacer fabrics (defining the elastically yielding layer 2) weaved with a suitable top layer (defining the first sublayer 8 of the mass layer 6) that has the desired mass.

[0131] According to the preferred embodiment described above, the mass layer 6 is preferably composed by a material, like a woven, knitted or non-woven textile, that is provided with relatively low bending stiffness.

[0132] The mass layer 6 can also be partially or fully wrapped around the elastically yielding layer 2 to cover also its thickness.

[0133] In a different embodiment, the mass layer 6 can also be a macroscopically homogeneous material (metal, plastic, composite, or similar) that has low permeability to airflow.

[0134] The mass layer 6 shall preferably, but not exclusively, have low specific airflow resistance. The term “low” must be interpreted as equal or lower to the characteristic impedance of air.

[0135] Preferably, the base layer 10 is connected to the separating structure by means of any suitable bonding means, such as adhesive tape, glue, thermoplastic net, etc.

[0136] According to the aforementioned preferred embodiment, the base layer 10 is defined by a bottom meshed outer layer of a spacer fabric, so it forms an integral part with the elastically yielding layer 2.

[0137] The base layer 10 is preferably bonded to the elastically yielding layer 2 for the whole elastically yielding layer 2 width and / or depth, via any suitable bonding means, suited to connect the base layer and the elastically yielding layer 2.

[0138] The base layer 10 can have any footprint with respect to the elastically yielding layer 2; e.g., the base layer can be wider and / or deeper and / or narrower and / or shallower compared to the elastically yielding layer 2.

[0139] As already mentioned, the base layer 10 can be not present, and hence the elastically yielding layer 2 would be directly connected to the separating structure.

[0140] Preferably, said elongated elements 7 define, with said base end 3 a first angle a comprised between 10° and 80°.

[0141] According to the preferred embodiment of the present invention, all said elongated elements 7 define a respective curved shape lying preferably on a plane, which can be defined by said thickness direction Z and the first axis X transverse to said thickness direction Z.

[0142] Differently, said elongated elements 7 define a respective curved shape lying preferably on a plane, which can be defined by said thickness direction Z and said second axis Y, which is transverse to said thickness direction Z.

[0143] Advantageously, said elongated elements 7 can define a respective three- dimensional curved shape, curving both along a plane defined by said thickness direction Z and the first axis X, along a plane defined by said thickness direction Z and said second axis Y and along different intermediate planes.

[0144] Said elongated elements 7 can define three-dimensional shapes such as helicoidal shapes or curly shapes.

[0145] According to a particular embodiment of the present invention, the first sub-layer 8 and the base layer 10 are integrally formed with the elastically yielding layer 2.

[0146] According to a particular (but not limiting) embodiment of the present invention, the elastically yielding layer 2, the first sub-layer 8 of the mass layer 6 (and preferably the base layer 10) are made via warp-knitting (or weft-knitting) of yarns, preferably comprising monofilaments as elongated elements 7.

[0147] More in detail, the elastically yielding layer 2 comprises a spacer fabric, or portions of a spacer fabric.

[0148] Preferably, the elastically yielding layer 2 and / or the first sub-layer 8 of the mass layer 6 and / or the base layer 10 are made in plastic material, for example polyester.

[0149] The elongated elements 7 can be realized in any elastic material, such as polypropylene, polybutylene terephthalate, polyethylene, polyamide, etc.

[0150] According to a different embodiment of the present invention, which is not shown in the attached drawings, each of the plurality of elongated elements 7 can comprise one or more filaments. Hence, the elongated elements 7 can be monofilament or multifilament.

[0151] In a preferred embodiment of the invention, the first sub-layer 8 of the mass layer 6 (and preferably the base layer 10) is made by knitting of yarns, preferably mono or multi-filament yarns.

[0152] According to a preferred embodiment of the invention, the base layer 10 (and preferably also the first sub-layer 8 of the mass layer 6) is defined by a plurality of repeated geometric shapes, such as hexagons or similar geometric shapes.

[0153] In other words, preferably without limiting, the base layer 10 (and preferably also the first sub-layer 8 of the mass layer 6) defines a honeycomb structure.

[0154] Preferably, the buckled shape of the elongated elements 7 is provided by the knitting process and by the thermal stabilization, which preferably takes place after the knitting.

[0155] Preferably, according to a further embodiment of the present invention, two or more absorbing devices 1 may be stacked, in order to define an absorbing system, wherein two or more elastically yielding layers 2 overlap one another, preferably configured to damp or reduce different and / or neighbouring frequencies or frequency bands.

[0156] The absorbing system comprising two or more absorbing devices 1 according to the invention preferably provides reactive and / or resonant elastically yielding layers 2 and mass layers 6 with multiple frequency STL and / or SAC peaks due to the multiple degrees-of-freedom of the system.

[0157] As mentioned above, the Sound Absorption Coefficient (SAC) is used to evaluate the sound absorption efficiency of a material / systems and it is defined as the ratio of absorbed acoustic energy to incident acoustic energy.

[0158] Moreover, the Sound Transmission Loss (STL) is used to evaluate the sound insulation efficiency of a material / systems and it is a quantification of how much sound energy is prevented from traveling through an acoustic treatment.

[0159] Accordingly, the multitude of STL and / or SAC peaks provided by the absorbing device 1 according to the invention and the absorbing system, which comprises two or more absorbing devices 1 , lead to high values of SAC and STL in a broadband frequency range.

[0160] According to a further embodiment of the invention, shown in figure 4, The elongated elements 7 of the spring means 5 of the elastically yielding layer 2 define shapes that are symmetrical with respect to thickness direction Z, i.e. identifying the thickness direction Z as a symmetry axis.

[0161] Operatively, the absorbing device 1 can hence be realized by juxtaposing two or more elastically yielding layers 2 orientating the curved shapes in different directions, such as the example depicted in figure 4, wherein a first and a second elastically yielding layers 2’, 2” are symmetrical with each other.

[0162] Advantageously, juxtaposing two or more elastically yielding layers 2 orientating the curved shapes in different directions permits to increase the shear stiffness of the absorbing device 1 .

[0163] According to a further embodiment of the present invention, which is not shown in the attached drawings, the absorbing device 1 , for example for handling and cleanliness requirements and / or for aesthetic and design requirements, can comprise a covering layer, which is configured to partially or entirely cover at least the mass layer 6, and preferably to cover the mass layer 6 and the visible parts (i.e. lateral edges) of the elastically yielding layer 2 and the visible parts (i.e. lateral edges) of the base layer 10.

[0164] The covering layer shall have airflow resistance low enough to avoid negative effects on the material / system, it is advisable that the specific airflow resistance is lower or equal to the characteristic impedance of air.

[0165] The covering layer can cover the whole absorbing system 1 or only portions of it.

[0166] According to a different embodiment of the invention, the second sub-layer 9 of the mass layer can be mechanically attached to the covering layer.

[0167] In other words, the covering layer can be interposed between the first sub-layer 8 of the mass layer 6 and the second sub-layer 9 of the mass layer 6.

[0168] The absorbing device 1 according to the invention can comprise a further layer, comprising preferably an homogeneous poro-elastic layer, configured to be positioned between the elastically yielding layer 2 and the mass layer 6 and / or covering at least partially the mass layer 6.

[0169] This further layer is configured to provide additional SAC and STL at high noise vibrations frequencies.

[0170] According to the embodiment shown in the attached figure 7, the separating structure S where the absorbing device 1 is intended to be installed, can be provided with perforations P.

[0171] Preferably, according to the embodiment shown in the attached figure 8, the absorbing device 1 can comprise a perforated enclosure 11.

[0172] Preferably, the perforated enclosure 11 is mechanically attached to base end 3 of the elastically yielding layer 2 or to the base layer 10.

[0173] In use, the perforated enclosure 11 is configured to be installed between the elastically yielding layer 2 (or the base layer 10, if present) and the separating structure S.

[0174] Preferably, the perforated enclosure 11 is in aeraulic connection at least with the elastically yielding layer 2.

[0175] Preferably, an airflow F can be insufflated through the absorbing device 1 , in order to vary one or more mechanical and structural features of the absorbing device 1 .

[0176] According to the embodiment shown in figure 7, the airflow F is insufflated through the perforations P on the separating structure S.

[0177] Preferably, the absorbing device 1 has low airflow resistance, allowing the airflow F to pass through its volume.

[0178] According to the embodiment shown in figure 8, the airflow F is insufflated though the perforated enclosure 11.

[0179] The insufflation of the airflow F is for example realized thought compressing means, which are not shown in the attached figures, since they are per se well known to the skilled person in the art and hence will not be further described in the following.

[0180] According to another embodiment of the invention, the airflow F can be discharged and / or sucked into / from the environment where the absorbing device 1 in installed and hence is in contact with the absorbing device 1 itself.

[0181] The effect of the airflow F through the material / system is to make the sound absorption peak due to the reactive / resonant air-borne acoustic damping of the system to be broadband. This effect is achieved thanks to the flow-induced acoustic absorption at the perforations and / or at the mass layer 6 surface.

[0182] The aforementioned particular embodiments allow change the Young’s modulus of the elastically yielding layer 2 by creating a preload to the elastically yielding layer 2, due to the pressure drop across the absorbing device 1 and due to Young’s modulus dependence on preload applied to the elastically yielding layer 2. The pressure of the airflow F introduced into the absorbing device 1 allows loading, if the airflow is directed into the elastically yielding layer, or unloading, if the airflow is directed out of the elastically yielding layer.

[0183] Hence, by varying the pressure drop across the absorbing device 1 by changing the flow rate of the airflow F, it is possible to vary the noise vibrations frequency that the absorbing device 1 can reduce, i.e. the absorbing device 1 is frequency tuneable.

[0184] Operatively, actively changing the airflow F, would result in a pressure drop change, which in turn would modify the Young’s modulus of the elastically yielding layer 2 and so the resonance frequency of the system.

[0185] Moreover, the temperature of the airflow F can be modulated in order to modulate the temperature of the materials composing the absorbing device 1 , and in particular the material composing the elastically yielding layer 2.

[0186] This temperature change affects / tunes the temperature dependent properties of the materials (composing the elastically yielding layer 2) that in turn affect the frequency of the SAC and STL peaks.

[0187] This embodiment would have the advantage of combining air conditioning while tuning the noise reduction performance.

[0188] Operatively, the low frequency SAC / STL peak (i.e. due to reactive / resonant damping effect at low frequencies of both structure-borne vibrations and air-borne sound) tuneable mainly by modifying the elastically yielding layer 2 constitutive parameters and / or the mass layer 6 constitutive parameters.

[0189] The absorbing device 1 can reduce the global level of noise (i.e. also at high frequencies) by means of the sound absorbing / insulating properties of the mass layer 6.

[0190] The SAC and STL provided at high frequencies by the porous and / or reactive absorption properties of the mass layer 6 can be tuned by changing the constitutive parameters of the mass layer 6 or its distance from the separating structure.

[0191] The absorbing device 1 can reduce acoustic radiation by means of the system geometry and by tuning surface acoustic impedance of the absorbing device 1 at a frequency of interest.

[0192] More in detail, the absorbing device 1 can dampen the noise vibrations via structural losses and / or visco-elastic damping and / or visco-thermal damping.

[0193] Moreover, as already mentioned above, the low rigidity (i.e. low resistance to bending) of the materials used to realize the absorbing device 1 allow complex shaped installations.

[0194] More in detail, the absorbing device 1 reduces desired frequency noise vibrations (i.e. provides SAC and STL peak values) in a frequency range that is approximately around the frequency:

[0196] where co is the angular frequency [rad / s], f is the frequency [Hz], K is the spring constant of the elastically yielding layer 2 [N / m] (i.e. material stiffness), m is the mass of the mass layer 6 [kg], Eetr is the Young’s modulus of the elastically yielding layer, Lo is the thickness of the elastically yielding layer 2 [m], p is the density of the mass layer 6 [kg / m3], t is the thickness of the mass layer 6 [m], and msis the surface mass density of the mass layer 6 [kg / m2].

[0197] The low frequency range where the absorbing device 1 provides high SAC and STL can be shifted by changing the Young’s modulus of the elastically yielding layer 2 (Eeff) and / or the elastically yielding layer thickness (Lo), and / or the mass layer 6 density (p), and / or the mass layer 6 thickness (t).

[0198] The aforementioned parameters (Eetr, Lo, p, t) can be changed via tuning of theconstitutive parameters of the mass layer 6 and / or elastically yielding layer 2. In other words, by changing the material and / or the mechanical and / or chemical properties of the materials and / or the geometrical parameters of the elastically yielding layer 2 and mass layer 6, it is possible to tune the desired frequency to reduce the noise vibrations.

[0199] A possible installation example is shown in attached drawings 9 and 10, wherein the absorbing device 1 is operatively associated to an aircraft, and in particular to a fuselage 100 of an aircraft.

[0200] Preferably, the absorbing device 1 is attached to a fuselage 100 wall or to an internal trim wall 101.

[0201] The absorbing device 1 can, for example, be installed on the fuselage panels of a turboprop aircraft.

[0202] In such specific type of aircrafts, one of the possible rotational speeds of the propellers can be considered to be around 1200 rpm with a four-blade propeller. Hence, the Blade Pass Frequency (BPF) can be easily calculated as 1200 / 60*4=80Hz.

[0203] Therefore, the absorbing device 1 is designed to reduce noise vibrations at least in correspondence of the aforementioned BPF and it is installed on the inner side of the fuselage wall (the term “inner” must be interpreted as facing towards the cabin / cockpit interior).

[0204] According to this particular (and obviously non-limiting) example, the absorbing device 1 according to the invention is designed to lead to Sound Absorption Coefficient (SAC) and Sound Transmission Loss (STL) peaks at about 80Hz. Such peaks at the desired frequency are better shown and visible in the attached graph in figure 11 , wherein the SAC curve is identified with the reference sign 201 and the STL curve is identified with the reference sign 202.

[0205] In the same figure 11 , 80Hz zone, where the peaks are present is identified with the reference 203.

[0206] The absorbing device 1 according to the invention is therefore assumed to be installed on fuselage panels, which are normally made of an aluminum alloy sheet, that we assume for example to be of about 1mm thickness.

[0207] In such an example, the base layer 10 is made of the outer meshed face of a warp knitted spacer fabric made of polyester yarns, the elastically yielding layer 2 is made of the spacer filaments / yarns of a warp knitted space fabric made of polyester yarns and the mass layer 6 is made of the combination of: (first sub-layer 8) the outer meshed face of a warp knitted spacer fabric made of polyester yarns and (second sublayer 9) a knitted mesh fabric made of polyester yarns.

[0208] In such example, the Young’s modulus of the base layer 10 and the mass layer 6 (both sub-layers 8, 9) are about 105— 106Pa.

[0209] According to the invention, in such example the Young’s modulus of the elastically yielding layer 2 is about 3000 Pa, i.e. much less than the base layer 10 and the mass layer 6. Since all the layers are made of the same material, it is clear that the elastically yielding layer 2 has a lower Young’s modulus than the material which it is composed of.

[0210] As it is clear from the description, the absorbing device according to the invention is completely passive. The absorbing device according to the invention is not intended to be activated, e.g. electrically.

[0211] The present invention is embodiable in other variations all of which fall within the scope and purpose of the inventive features claimed and described; these technical features may be substituted by different technically equivalent elements. The used materials, the shapes and dimensions of the features of the invention may be substituted by any equivalent per-se known by the person skilled in the art, as long as they are compatible with their intended use.

[0212] The numbers and reference marks inserted in the claims, the description and the drawings have the sole purpose of increasing the clarity of the text and are not to be considered as limiting the technical interpretation of the objects or processes identified by them.

Claims

CLAIMS1. Absorbing device (1) for reducing noise vibrations via reactive or resonant damping effect of both structure-borne and air-borne noise vibrations, suited to be applied to at least one separating structure for absorbing and / or reducing noise vibrations susceptible to crossing through or be propagated by said separating structure; said absorbing device (1) comprising at least one elastically yielding layer (2), extending along a thickness direction (Z) between a base end (3) and a top end (4), opposite to said base end (3); said elastically yielding layer (2) comprises spring means (5) connecting said base end (3) and said top end (4); said absorbing device (1) comprises at least one mass layer (6) mechanically connected to said top end (4), provided with a predetermined mass or density and configured to increase the mass distributed on said top end (4) of said elastically yielding layer (2); said spring means (5) of said elastically yielding layer (2) being tuned along with the mass or density of said mass layer (6) so as to reduce the amplitude of noise vibrations at least at a predetermined frequency; characterized in that said spring means (5) of said elastically yielding layer (2) comprise a plurality of elongated elements (7) extending parallel to one another, between said base end (3) and said top end (4); said elongated elements (7) being configured to elastically behave as being compressed, along a compression direction parallel to said thickness direction (Z), beyond their critical buckling load.

2. Absorbing device (1) for reducing noise vibrations according to claim 1 , characterized in that said spring means (5) of said elastically yielding layer (2) is configured so as to provide an elastic modulus lower than the elastic modulus of the material composing said spring means (5).

3. Absorbing device (1) for reducing noise vibrations according to claim 1 or 2, characterized in that said elongated elements (7) of said spring means (5) are buckled, defining a curved shape, between said base end (3) and said top end (4).

4. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that said mass layer (6) comprises:- a first sub-layer (8), mechanically connected to said elastically yielding layer (2) at said top end (4);- a second sub-layer (9) covering said first sub-layer (8) provided with a predetermined mass or density.

5. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that is comprises a base layer (10) mechanically connected to said elastically yielding layer (2) at said base end (3).

6. Absorbing device (1) for reducing noise vibrations according to claim 5, characterized in that:- said first sub-layer (8) of said mass layer (6) is configured to mechanically connect said elongated elements (7) of said spring means (5) at said top end (4);- said base layer (10) is configured to mechanically connect said elongated elements (7) of said spring means (5) at said base end (3).

7. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that said elongated elements (7) define, with said base end (3) a first angle (a) comprised between 10° and 80°.

8. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that all said elongated elements (7) define a respective curved shape lying on a plane defined by said thickness direction (Z) and a first axis (X) transverse to said thickness direction.

9. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that the absorbing device 1 reduces desired frequency noise vibrations (i.e. provides SAC and STL peak values) in a frequency range that is approximately around the frequency:wherein co is the angular frequency [rad / s], f is the frequency [Hz], K is the spring constant of the elastically yielding layer (2) [N / m], m is the mass of said mass layer (6) [kg], Eetr is the Young’s modulus of said elastically yielding layer (2), Lo is the thickness of said elastically yielding layer (2) [m], p is the density of said mass layer (6) [kg / m3], t is the thickness of said mass layer (6) [m], and msis the surface mass density of the mass layer (6) [kg / m2].

10. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that said elastically yielding layer (2) is realized with a material provided with an elastic modulus, and in particular the Young’s modulus, that is higher with respect to the elastic modulus, and in particular the Young’s modulus, of elastically yielding layer (2).11 . Absorbing device (1) for reducing noise vibrations according to one or moreof the preceding claims, characterized in that said elastically yielding layer (2) is built as a collection of unit cells and / or the mass layer (6) is designed to have low bending stiffness so that forces are not easily transferred between unit cells.

12. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that by changing the material and / or the mechanical and / or the chemical properties of the materials and / or the geometrical parameters of the elastically yielding layer (2) and mass layer (6), it is possible to tune the desired frequency to reduce the noise vibrations.

13. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that said mass layer (6) is composed by a material that is provided with low bending stiffness.

14. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that following and / or during stimulation of the absorbing device (1) by noise vibrations, the absorbing device (1) defines, preferably simultaneously, compression zones and expansion zones; wherein compression zones and expansion zones are identified along a lying plane defined by the first axis (X) and the second axis (Y), in which - respectively - the thickness parallel to the thickness axis Z is less, for compression zones, or greater, for expansion zones, with respect to the thickness of the device at rest.

15. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that it is intended to be used for reducing noise vibrations at low frequencies, wherein the acoustic wavelength of the noise vibrations is larger than the typical material / system quarter wave resonance.

16. Absorbing device (1) for reducing noise vibrations according to one or more of the preceding claims, characterized in that the frequency of the reduced noise vibrations is: f <co / (4 ^M)> wherein f is the frequency, c0is the speed of sound and tMis the typical material / system thickness.

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