ALVEOLAR STRUCTURE OF AN ACOUSTIC DAMPING PANEL COMPRISING AT LEAST ONE ATTACHED ELEMENT CONFIGURED FOR VIBRATING AT A DESIRED FREQUENCY, METHOD FOR PRODUCING SAID ALVEOLAR STRUCTURE

An alveolar structure of an acoustic damping panel which is configured for damping at least one sound wave of a sound frequency, wherein the alveolar structure comprises at least one attached element, separate from partitions, and configured for vibrating at a frequency substantially equal to the sound frequency of the sound wave to be damped. Also a method for producing such an alveolar structure, an acoustic damping panel comprising at least one such alveolar structure, and an aircraft comprising at least one such acoustic damping panel.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of French Patent Application Number 2300864 filed on Jan. 31, 2023, the entire disclosure of which is incorporated herein by way of reference.

FIELD OF THE INVENTION

The present invention concerns an alveolar structure of an acoustic damping panel comprising at least one attached element configured for vibrating at a desired frequency, and a method for producing said alveolar structure. The invention also concerns an acoustic damping panel comprising at least one such alveolar structure, and an aircraft comprising at least one such acoustic damping panel.

BACKGROUND OF THE INVENTION

According to an embodiment of the prior art, a propulsion unit comprises a nacelle and a turbofan engine positioned inside the nacelle. Certain surfaces of the nacelle and the turbofan engine comprise acoustic damping panels for damping nuisance noise. According to an embodiment visible inFIG.1, an acoustic damping panel10comprises at least one permeable layer12, at least one alveolar structure14and a solid layer16. In the remainder of the description, a layer is described as permeable if it is porous or comprises openings or holes passing through it.

Such an acoustic panel10uses the principle of a Helmholtz resonator. Thus the alveolar structure14has cells14.1, the volume of which is adjusted as a function of the frequency range of the sound waves to be damped.

For certain frequencies, the acoustic damping panel10has a single alveolar structure14, as illustrated inFIG.1. For other frequencies, in particular low frequencies, the acoustic damping panel10may comprise two superposed alveolar structures separated by a permeable layer.

These acoustic damping panels offer relatively good performance for a given frequency range situated in the high frequencies. In order to damp different frequency ranges, some of which are situated in the low frequencies, it is necessary to provide acoustic damping panels with different thicknesses, in particular acoustic damping panels with a large thickness suitable for low frequencies, which has undesirable consequences in terms of mass, bulk and production.

SUMMARY OF THE INVENTION

The present invention is intended to overcome some or all of the drawbacks in the prior art.

To this end, the invention concerns an alveolar structure of an acoustic damping panel which is configured for damping at least one sound wave of given sound frequency, the alveolar structure extending between first and second surfaces, the alveolar structure comprising a plurality of cells separated by partitions which each have a first edge at the first surface, a second edge at the second surface, and third and fourth edges connecting the first and second edges.

According to the invention, the alveolar structure comprises at least one attached element, separate from the partitions and configured for vibrating at a frequency substantially equal to the sound frequency of the sound wave to be damped.

Using an alveolar structure with a relative small and constant thickness, this solution allows damping of sound waves over several frequency ranges, in particular at least one in the low frequencies.

According to another characteristic, the attached element is a tab having a first end connected to a partition of the alveolar structure, and a free second end, said tab being configured for vibrating at a frequency substantially equal to the sound frequency of the sound wave to be damped.

According to another characteristic, the alveolar structure comprises three tabs fixed to the same partition and aligned in a direction substantially parallel to the third and fourth edges of the partition, approximately centered between these third and fourth edges and evenly distributed between the first and second edges.

According to another characteristic, the partitions each have a length measured in a direction parallel to the third and fourth edges, and a width measured in a direction parallel to the first and second edges. In addition, each tab has a first part pressed against and fixed to the partition and extending from the first end, and a second part detached from the partition and extending from the second end, the first and second parts being separated by a fold line, wherein each tab has a thickness of the order of 50 to 500 μm, a constant width of the order of 10 to 90% of the width of the partitions, the second part having a length of the order of 5 to 80% of the length of the partitions.

According to another characteristic, the alveolar structure comprises single partitions each comprising a single layer of material, and double partitions each comprising two layers of material glued together. In addition, all single partitions of at least one considered zone of the alveolar structure each comprise at least one tab.

According to another characteristic, the attached element is a material strip having two ends connected to the partitions of the alveolar structure and passing through at least one cell, said material strip being configured for vibrating at a frequency substantially equal to the sound frequency of the sound wave to be damped.

According to another characteristic, the material strip has upper and lower edges which are substantially parallel with one another and with the first and second edges of the partitions.

According to another characteristic, for each cell of the alveolar structure through which it passes, the material strip has an over-length allowing it to vibrate.

According to another characteristic, each material strip has a height substantially equal to one third of that of the partitions, and a thickness of the order of 20 to 500 μm, each material strip having, in each cell through which it passes, a curved central part and first and second side parts which are flat and coplanar and arranged on either side of the central part.

According to another characteristic, each material strip is flat and has fringes.

According to another characteristic, the alveolar structure comprises several material strips passing through all cells of at least one considered zone of the alveolar structure.

According to another characteristic, the alveolar structure comprises at least first and second zones, the first zone being configured for damping at least one sound wave having a first sound frequency, the second zone being configured for damping at least one sound wave having a second sound frequency different from the first sound frequency; the alveolar structure comprising in the first zone at least one attached element separate from the partitions and configured for vibrating at a frequency substantially equal to the first sound frequency.

According to another characteristic, at least one partition of the second zone has at least one cavity configured such that said partition of the second zone vibrates at a frequency substantially equal to the second sound frequency.

According to another characteristic, the alveolar structure comprises, in the second zone, at least one attached element separate from the partitions and configured for vibrating at a frequency substantially equal to the second sound frequency.

The invention also concerns a method for producing an alveolar structure according to one of the preceding characteristics, the production method comprising a step of cutting out sheets, steps of folding and gluing the sheets, a step of stacking the sheets so as to obtain a stack of sheets, and a step of stretching the stack so as to obtain the alveolar structure. According to the invention, the production method comprises a step of positioning the produced separate elements on each sheet before the stacking step or alternately with the stacking step, the attached elements being positioned after depositing of a new sheet.

The invention also concerns an acoustic damping panel comprising at least one alveolar structure according to one of the preceding characteristics, and an aircraft comprising at least one such acoustic damping panel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an embodiment shown inFIG.2, an aircraft20comprises a fuselage22, two wings24arranged on either side of the fuselage22, and propulsion units26fixed below the wings24. Each propulsion unit26comprises a nacelle28and a turbomachine30positioned inside the nacelle28.

According to an embodiment visible inFIG.3, the propulsion unit26comprises an air intake28.1, a secondary exhaust duct32channeling a secondary flow of air, which is delimited by an inner wall34(also referred to as IFS for “inner fixed structure”) and by an outer wall36(also referred to as OFS for “outer fixed structure”). According to one configuration, the air intake28.1, the inner wall34or the outer wall36comprises at least one acoustic damping panel38which has an outer surface SE in contact with the secondary air flow and an inner surface SI opposite the outer surface SE.

Although it has been described as applying to a secondary exhaust duct32, the invention is not limited to this application. Thus, the acoustic damping panel38can be positioned on any skin which has an outer surface SE in contact with an exterior environment Ext in which sound waves propagate during operation, such as for example a lip and an air inlet duct of an aircraft nacelle, a fan casing of an aircraft nacelle or any other surface of the propulsion unit26or aircraft20. Irrespective of the configuration, the aircraft20comprises at least one acoustic damping panel38.

According to an embodiment visible inFIG.4, an acoustic damping panel38comprises, from the outer surface SE to the inner surface SI, a permeable structure40, one face of which forms the outer surface SE, at least one alveolar structure42and a solid layer44, one face of which forms the inner surface SI.

The solid layer44comprises at least one thin plate made of metal or composite material which is impermeable to sound waves.

The permeable structure40, also called the acoustically resistive layer, may be made of metal or composite material and comprise one or more layer(s). The permeable structure40is permeable to at least one sound wave propagating into the external environment Ext.

The permeable structure40and the solid layer44are not described further here since they may be identical to those of the prior art.

According to a configuration visible inFIG.4, the acoustic damping panel38comprises a single alveolar structure42between the permeable structure40and the solid layer44. According to another configuration, the acoustic damping panel38comprises, between the permeable structure40and the solid layer44, several alveolar structures42superposed on one another and separated by a permeable structure, also called a septum, or several juxtaposed alveolar structures42.

Naturally, if a single alveolar structure42is provided, a thinner acoustic damping panel may be obtained.

The alveolar structure42extends between a first surface S1in contact with or oriented towards the permeable structure40, and a second surface S2in contact with or oriented towards the solid layer44. The alveolar structure42comprises a plurality of partitions46which each have a first edge46.1at the first surface S1, a second edge46.2at the second surface S2, and third and fourth edges46.3,46.4(visible inFIG.5) connecting the first and second edges46.1,46.2, which extend between the first and second surfaces S1, S2and are substantially mutually parallel. The partitions46delimit between them cells48which are each open at a first end at the first surface S1, and at a second end at the second surface S2.

According to one configuration, the cells48have identical hexagonal cross-sections (in a transverse plane parallel to the first or second surface S1, S2). Thus the alveolar structure42forms a honeycomb. Of course, the invention is not limited to this configuration. The cells48may have different, non-hexagonal cross sections.

Irrespective of the configuration, the alveolar structure42has a plurality of cells48,48′ separated by partitions46which each have a first edge46.1at the first surface S1, a second edge46.2at the second surface S2, and third and fourth edges46.3,46.4connecting the first and second edges46.1,46.2; two adjacent partitions46are connected at their third and fourth edges46.3,46.4. The cells48,48′ are dimensioned in volume so as to attenuate, using the Helmholtz resonator principle, a range of sound waves having relatively high frequencies situated in a range from 1,500 to 5,000 Hz.

According to an embodiment, the alveolar structure42has a volume mass of the order of 20 to 150 kg/m3, partitions46which have a width W46(dimension separating the third and fourth edges46.3,46.4) of the order of 4 to 20 mm and a length L46(dimension separating the first and second edges46.1,46.2) of the order of 10 to 60 mm, and also hexagonal cells48.

Each partition46has a first face F46oriented towards a first cell48, and a second face F46′ oriented towards a second cell48′, a thickness E corresponding to a dimension separating the first and second faces F46, F46′, a length L46corresponding to a distance separating the first and second edges46.1,46.2, and a width W46corresponding to a dimension separating the third and fourth edges46.3,46.4.

According to an embodiment, certain partitions46, called single partitions, each comprise a single layer of material. Certain partitions46, called double partitions, each comprise two layers of material glued together.

According to a characteristic of the invention, an acoustic damping panel, which is configured for damping at least one sound wave having a given sound frequency, comprises an alveolar structure42which comprises at least one attached element, separate from partitions46and configured for vibrating at a frequency substantially equal to the sound frequency of the sound wave to be damped. The phrase “substantially equal” means that the vibrational frequency lies in a frequency range of +/−10% of the sound frequency.

Thus as well as damping sound waves at high frequencies using the principle of a Helmholtz resonator, the alveolar structure42is configured for damping sound waves over another frequency range, in particular low frequencies, thanks to the vibrating attached elements.

According to an embodiment visible inFIGS.5,6and8, the attached element is a tab50having a first end50.1connected to a partition46of the alveolar structure42, and a free second end50.2configured for vibrating at a frequency substantially equal to a sound frequency of a sound wave to be damped.

Each tab50has a first part52.1pressed against and fixed to the partition46and extending from the first end50.1, and a second part52.2detached from the partition46and extending from the second end50.2, the first and second parts52.1,52.2being separated by a fold line52.3.

According to a configuration, all single partitions46of at least one considered zone of the alveolar structure42each comprise at least one tab50.

According to an arrangement visible inFIGS.5and6, the alveolar structure42comprises three tabs50fixed to the same partition46and aligned in a direction substantially parallel to the third or fourth edges46.3,46.4of the partition46, approximately centered between these third and fourth edges46.3,46.4and evenly distributed between the first and second edges46.1,46.2.

According to an embodiment, each tab50is made of metal (such as an aluminum alloy for example) or composite material (based on aramid fibers for example), and has a thickness of the order of 50 to 500 μm. It has a constant width (dimension of the fold line52.3) of the order of 10 to 90% of the width W46, and the second part52.2has a length (dimension between the fold line52.3and the second end50.2) of the order of 5 to 80% of the length L46. Such a tab50vibrates at a frequency of the order of 500 to 1,500 Hz. Thus the acoustic damping panel38comprising at least one such tab50is configured for damping sound waves over two frequency ranges, namely high frequencies above 1,500 Hz and low frequencies of the order of 500 to 1,500 Hz.

Naturally, the invention is not limited to this number of tabs50per partition46, or to the geometry, this arrangement or this material for the tabs50. Thus the tabs may be attached to a single face F46, F46′ of a partition46, or to both faces.

Thus depending on the sound frequency of the sound wave to be damped, the person skilled in the art will determine the material, arrangement and geometry of the tabs50such that they vibrate at a vibrational frequency substantially equal to the sound frequency of the sound wave to be damped.

According to an embodiment visible inFIG.7, a method for producing at least one alveolar structure42comprises a first step of cutting out rectangular sheets54; a second step of folding the sheets54along the fold lines56corresponding to the third and fourth edges of the partitions46so as to obtain strips58,58′ which correspond to the partitions46of the alveolar structure42to be obtained; a third step of gluing some strips58′ onto one or the other of the faces of each sheet54, wherein even-numbered strips58(corresponding to single partitions46) are not glued to either of the two faces of the sheets54, while odd-numbered strips58′ (corresponding to double partitions46) are glued alternately to the first face and then to the second face of the sheet54; a step of stacking the sheets54so as to obtain a stack60of sheets54; in some cases a fourth step of trimming the stack60to a dimension substantially equal to the length L46of the partitions46; and finally a final step of stretching the stack60so as to obtain the alveolar structure42.

These various steps may be automated.

As illustrated inFIG.8, the method for producing the alveolar structure42comprises a step of positioning the tabs50, which step may be automated. According to an operating mode, this step may be carried out on each sheet54before the stacking step or alternately with the stacking step, the tabs50being positioned after depositing of a new sheet54.

According to another embodiment visible inFIGS.9to11, the attached element is a material strip62having two ends62.1,62.2connected to the partitions46of the alveolar structure42and passing through at least one cell48, said material strip62being configured for vibrating at a frequency substantially equal to the sound frequency of the sound wave to be damped.

This material strip62has upper and lower edges64.1,64.2which are substantially parallel with one another and with the first and second edges46.1,46.2of the partitions46.

For each cell48of the alveolar structure42through which it passes, the material strip62comprises a curved central part66.1and first and second side parts66.2,66.3which are flat and coplanar and arranged respectively between the first end62.1and the central part66.1, and between the second end62.2and the central part66.1. As a variant, the central part66.1forms a V-shape when viewed from above. Whatever variant is used, for each cell48, the material strip62comprises an over-length between its first and second ends68.1,68.2, enabling it to vibrate. According to an operating mode, for each material strip62, each central part66.1is obtained by folding.

According to a configuration, the alveolar structure42comprises a single material strip62positioned in a single cell48. According to another configuration, the alveolar structure42comprises several material strips62passing through all cells48of at least one considered zone of the alveolar structure42. As illustrated onFIGS.9and11, a same material strip62may pass through several cells48.

According to a configuration, each material strip62is approximately centered relative to the first and second edges46.1,46.2of the partitions46, and has a height (dimension measured perpendicularly to the upper and lower edges64.1,64.2) which is substantially equal to one third of the length L46of the partitions46. Each material strip62is made of metal (such as an aluminum alloy for example) or composite material (based on aramid fibers for example), and has a thickness of the order of 20 to 500 μm. For each cell48, each material strip62comprises a semi-cylindrical central part66.1with an axis substantially parallel to the third and fourth edges46.3,46.4of the partitions46and a radius of the order of 0.5 to 3 mm.

Such a material strip62is configured to vibrate at a frequency of the order of 500 to 1,500 Hz. Thus the acoustic damping panel38comprising at least one such material strip62is configured for damping sound waves having a frequency of the order of 500 to 1,500 Hz.

Of course, the invention is not restricted to this configuration for the material strips62. Thus as illustrated inFIGS.12and13, each material strip62′ is flat and has fringes68separated by straight cutouts70which are parallel with one another and with the third and fourth edges46.3,46.4of the partitions46, and which extend from a first edge of the upper and lower edges64.1,64.2and are remote from a second edge, different from the first edge, of the upper and lower edges64.1,64.2.

Thus depending on the sound frequency of the sound wave to be damped, the person skilled in the art will determine at least one characteristic (location, geometry, material etc.) of at least one material strip62,62′ such that it vibrates at a vibrational frequency substantially equal to the sound frequency of the sound wave to be damped.

As illustrated inFIGS.11and14, the method for producing the alveolar structure42comprises a step of positioning the material strips62,62′, which step may be automated. According to an operating mode, the material strips62,62′ are interposed between the sheets54during the stacking step.

Irrespective of operating mode, the method for producing the alveolar structure42comprises a step of positioning the attached elements50,62,62′, which step may be automated. According to an operating mode, this step may be carried out on each sheet54before the stacking step or alternately with the stacking step, the attached elements50,62,62′ being positioned after depositing of a new sheet54.

According to an embodiment visible inFIG.15, an acoustic damping panel, which is configured for damping at least one sound wave with a given sound frequency, comprises an alveolar structure42which comprises at least one partition46having at least one cavity72configured such that the partition46vibrates at a vibrational frequency substantially equal to the given sound frequency.

According to a non-limitative operating mode, each cavity72is obtained by material removal.

According to a configuration, each cavity72is remote from the first, second, third and fourth edges46.1to46.4. However, at least one cavity72may extend up to at least one first edge of the first, second, third and fourth edges46.1to46.4. For example, at least one cavity72may extend up to at least the first or second edge46.1,46.2.

According to a configuration, each cavity72is a blind cavity and opens only onto one of the first and second faces F46, F46′ of the partition46. According to another configuration visible inFIG.15, each cavity72is a through cavity and opens onto the first and second faces F46, F46′ of the partition46.

According to a configuration, at least one partition46comprises a single cavity72. According to another configuration, at least one partition46comprises several cavities72.

According to an arrangement, all single partitions46of at least one considered zone of the alveolar structure42each comprise at least one cavity72,72′.

According to an embodiment, the alveolar structure42has a volume mass of the order of 20 to 150 kg/m3, partitions46which have a width W46of the order of 4 to 20 mm, and a length L46of the order of 10 to 60 mm, and hexagonal cells48dimensioned to damp sound waves of a frequency situated in a range from 1,500 to 5,000 Hz.

At least some partitions, in particular the single partitions46, each comprise two through cavities72,72′ (connecting the first and second faces F46, F46′) which each have a length L72, measured in a first direction parallel to the third and fourth edges46.3,46.4, of the order of 10 to 90% of the length L72, and a width W72, measured in a second direction parallel to the first and second edges46.1,46.2, of the order of 0.1 to 1 mm. The two cavities72,72′ of a first partition46are offset relative to one another in the second direction, spaced apart by a distance of the order of 10 to 90% of the width W72, and are substantially centered relative to the first and second edges46.1,46.2and relative to the third and fourth edges46.3,46.4. Such cavities72,72′ allow the partition46to vibrate at a vibrational frequency of the order of 500 to 1,000 Hz.

Thus an alveolar structure42with cells48having a volume adapted for damping sound waves of a first frequency using the principle of a Helmholtz resonator comprises partitions46, some of which each comprise a cavity72configured to enable said partitions to vibrate at a second frequency and thus damp sound waves of the second frequency.

Knowing the frequency of the sound wave to be damped and the characteristics of the partitions46, including in particular the material, thickness, length and width, the person skilled in the art is able to determine the characteristics of the cavity (or cavities)72,72′ to be produced on at least one partition46such that said partition46vibrates at a given frequency substantially equal to the frequency of the sound wave to be damped.

According to a configuration, all single partitions46vibrate and all have the same cavities72,72′ and are all configured to vibrate at the same frequency.

According to another configuration visible inFIG.16, the alveolar structure42has at least first and second zones Z1, Z2, at least one partition46of the first zone having at least one cavity72configured to allow the partition46of the first zone Z1to vibrate at a first frequency, at least one partition46of the second zone having at least one cavity72′ configured to allow the partition46of the second zone Z2to vibrate at a second frequency different from the first frequency.

According to an operating mode visible inFIG.16, the method for producing the alveolar structure42visible inFIG.15comprises at least one step of producing cavities72,72′, which may be carried out either before or after the step of stacking the sheets54, in all cases before the step of stretching. When the cavities72are through cavities, the step of producing the cavities72is carried out after the step of stacking, so that cavities72are created simultaneously on several superposed sheets54. The step of producing the cavities72may also be automated. As a non-limitative example, the step of producing the cavity72is carried out by an ultrasonic cutting process, an ultrasonic cutting head74being fixed to the end of an articulated arm76.

According to an arrangement, the alveolar structure42comprises at least first and second zones, the first zone being configured for damping at least one sound wave having a first sound frequency, the second zone being configured for damping at least one sound wave having a second sound frequency different from the first sound frequency. In the first zone, the alveolar structure42comprises at least one attached element separate from the partitions46, such as a tab50, a material strip62,62′ for example, configured for vibrating at a frequency substantially equal to the first sound frequency.

In the second zone, the alveolar structure42comprises at least one attached element separate from the partitions46, such as a tab50, a material strip62,62′ for example, configured for vibrating at a frequency substantially equal to the second sound frequency, or at least one partition provided with at least one cavity72configured such that the partition vibrates at a frequency substantially equal to the second sound frequency.

Thus it is possible to design an alveolar structure which has a relatively small and constant thickness and several zones, each of which is designed to damp sound waves at a given frequency.