Acoustic barrier cap in acoustic honeycomb

Abstract

An acoustic honeycomb structure includes cells in which an acoustic barrier is positioned within the honeycomb cells using a friction-locking insertion process to provide a multi-degree-of-freedom (MDOF) acoustic liner with multiple acoustic resonator depths. A solid polymer film forms an acoustic barrier cap. The acoustic barrier cap is frictionally locked to the cell walls at one or more cell depths to form an acoustically reflective hard wall, which forms the effective bottom end of the acoustic resonator.

Description

Background Technology 1. Technical Field

[0002] This invention generally relates to acoustic systems for attenuating noise. The invention relates to cabins and other structures using cellular fabrication that can be used to reduce noise generated by aircraft engines or other noise sources. More specifically, the invention relates to acoustic structures in which acoustic reflective solid barriers are inserted into one or more cellular cells to provide internal terminations of the acoustic cells, the internal terminations defining the acoustic depth of the cells.

[0003] 2. Description of related technologies

[0004] It is widely accepted that the best way to deal with excessive noise generated by a particular source is to treat the noise at the source. This is typically achieved by adding acoustic damping structures (acoustic treatments) to the structure of the noise source. One particularly problematic noise source is the jet engine used on most passenger aircraft. Acoustic treatments are typically incorporated into the engine nacelle (including the intake, bypass, and exhaust structures). These acoustic treatments include acoustic linings containing relatively thin acoustic materials or meshes with millions of holes that create acoustic impedance to the acoustic energy generated by the engine.

[0005] Honeycomb structures are a popular material for aircraft and aerospace vehicles due to their relative strength and light weight. For acoustic applications, such as engine nacelles, acoustic materials are added to honeycomb structures such that the honeycomb cells are acoustically enclosed at the ends furthest from the engine, and covered with an acoustically permeable covering at the ends closest to the engine. Enclosing the honeycomb cells creates acoustic resonators that provide attenuation, damping, and / or suppression of noise. The specific frequency of noise attenuated by a given honeycomb cell or resonator is directly related to the depth of the cell. Typically, as the noise frequency decreases, the cell depth must increase to provide sufficient damping or suppression.

[0006] A typical acoustic liner has a honeycomb core sandwiched between a solid sheet or skin and a perforated or otherwise sound-permeable sheet or skin. The perforated sheet is located closest to the noise source, and the solid sheet forms the bottom of the acoustic resonators. In this type of acoustic liner, all honeycomb cells have the same depth. Such an acoustic liner, where all acoustic resonators have the same depth, is called a single-degree-of-freedom (SDOF) acoustic liner. An SDOF liner provides sound damping only near a specific sound frequency.

[0007] A fundamental challenge for acoustic engineers designing acoustic liners for jet engines is fabricating acoustic structures that provide sufficient suppression or damping of sound frequencies across the entire range of noise generated by jet engines. Multiple SDOF acoustic liners with different resonator depths can be combined to attenuate noise over a wider frequency range. However, acoustic liners have been developed where the effective resonator depth within a single liner varies. Such multi-resonator-depth acoustic liners are called multi-degree-of-freedom (MDOF) acoustic liners. MDOF acoustic liners have been found to effectively dampen jet engine noise over a much wider frequency range compared to the possibilities offered by SDOF acoustic liners.

[0008] One method of manufacturing an MDOF acoustic liner involves positioning individual solid inserts within a cell. The solid inserts are positioned at varying distances between the cell edges to provide an acoustic barrier forming the bottom of the acoustic resonator. For example, see U.S. Patent No. 8,651,233, where solid inserts are positioned at different locations within the cell to provide an MDOF acoustic liner with multiple resonator cavity depths, ideally suited for damping a relatively wide range of sound frequencies.

[0009] The solid insert used to form the bottom of the acoustic resonator must have sufficient rigidity to function as an acoustic barrier or rigid wall that reflects substantially all sound waves in the attenuated or damped frequency range. The solid insert must also be able to withstand the high temperatures exposed to those found in jet engine acoustic linings. The solid insert should be as lightweight as possible while still providing the desired acoustic reflectivity.

[0010] Acoustic spacers are located inside the cellular cell to provide additional noise attenuation characteristics for the resonator. Each acoustic spacer is typically constructed of a thin polymer fabric or a perforated polymer membrane. Acoustic spacers do not act as acoustic barriers or rigid walls. Instead, they provide attenuation or damping of sound waves passing through them. One method of positioning acoustic spacers within a cellular cell involves inserting individual lightweight spacer fabric blocks into the cellular cell to form spacer caps having anchoring flanges glued to the cellular wall. The use of spacer caps is described in U.S. Patent Nos. 7,434,659; 7,510,052; 7,854,298; 8,066,098; 8,607,924; 8,651,233; 8,857,566; 9,016,430; and 9,469,985.

[0011] Another method for positioning acoustic septa in a cellular cell involves inserting individual solid polymer membrane blocks into the cellular cell to form septa caps, the septa caps also having anchoring flanges glued to the cellular walls. The solid polymer membrane is perforated to form the acoustic septa before or after insertion into the cellular cell. See, for example, U.S. Patent No. 8,413,761.

[0012] The process of positioning the spacer cap within a cell requires that the spacer cap be frictionally locked within the cell to secure it in place before it is permanently bonded to the cell walls. Frictional locking of the spacer cap is a critical aspect of this type of spacer insertion procedure. Insufficient frictional locking can cause the spacer cap to shift or otherwise move during processing. Any shift in the spacer cap makes it difficult to apply adhesive evenly during bonding. Shifting also causes uncontrolled changes in acoustic properties. In the worst case, insufficient frictional locking can cause the spacer cap to detach completely from the cell. Summary of the Invention

[0013] According to the present invention, it has been discovered that the friction-locking insertion process, already used for positioning acoustic spacers within a cellular cell, can also be used for positioning acoustic barriers within a cellular cell to provide an MDOF acoustic liner with multiple acoustic resonator depths. The invention is based on the discovery that a solid polymer film of a specific thickness and shape can form an acoustic barrier cap. This acoustic barrier cap can be frictionally locked and bonded to the cell wall to form an acoustically reflective hard wall, which forms the effective bottom end of the acoustic resonator.

[0014] This invention relates to an acoustic structure designed to be located close to a noise source, such as a jet engine or other power source. The structure includes a honeycomb having a first edge and a second edge, the first edge being closest to the noise source. The honeycomb comprises multiple cells, each cell having a left side and a right side. Each cell is formed by a lower wall extending between the first and second edges of the honeycomb and an upper wall also extending between the first and second edges of the honeycomb. The lower wall includes a lower left portion, a lower right portion, and a lower central portion. The upper wall includes an upper left portion, an upper right portion, and an upper central portion. A left joint is formed along the left side of the cell at the junction of the lower and upper walls. A right joint is formed along the right side of the cell at the junction of the lower and upper walls. The depth of the cell is equal to the distance between the first and second edges of the honeycomb.

[0015] As a feature of the invention, an acoustic barrier cap is inserted into at least one cell to provide an acoustically reflective hard wall forming the acoustic bottom of the cell. The acoustic barrier cap is a solid polymer film that has been folded to form a planar acoustic barrier portion and a wing portion surrounding the acoustic barrier portion. The planar acoustic barrier portion extends transversely to the upper and lower walls of the cell. The planar acoustic barrier portion has a top side located closest to a first edge of the cell and a bottom side located closest to a second edge of the cell. The planar acoustic barrier portion is surrounded by boundaries consisting of an upper right boundary portion, an upper central boundary portion, an upper left boundary portion, a lower right boundary portion, a lower central boundary portion, and a lower left boundary portion.

[0016] As another feature of the invention, the wing portion of the acoustic barrier cap includes an upper right wing, an upper central wing, and an upper left wing, all of which protrude from the upper boundary of the planar acoustic barrier portion. The wing portion further includes a lower right wing, a lower central wing, and a lower left wing, all of which protrude from the lower boundary of the planar acoustic barrier portion.

[0017] An acoustic barrier cap is inserted into the unit, causing the upper right wing to be frictionally locked to the upper right end of the upper wall, the upper central wing to be frictionally locked to the upper central end of the upper wall, and the upper left wing to be frictionally locked to the upper left end of the upper wall. The lower right wing is frictionally locked to the lower right end of the lower wall, the lower central wing is frictionally locked to the lower central end of the lower wall, and the lower left wing is frictionally locked to the lower left end of the lower wall.

[0018] This invention relates to a precursor structure formed when an acoustic barrier cap is frictionally locked within a cellular cell. The invention also relates to an acoustic structure formed when an acoustic barrier cap is permanently bonded to a cellular cell, and methods for manufacturing the precursor and final acoustic structures.

[0019] The foregoing discussion of the present invention, as well as many other features and accompanying advantages, will be better understood by referring to the following detailed description in conjunction with the accompanying drawings. Attached Figure Description

[0020] Figure 1 is a perspective view of an exemplary acoustic structure according to the present invention.

[0021] Figure 2 is an enlarged view of a portion of the exemplary acoustic structure shown in Figure 1.

[0022] Figure 3 is a simplified view showing the acoustic barrier cap inserted into the cell to form a precursor structure, where the septum is frictionally locked within the cell.

[0023] Figure 4 is a simplified view illustrating an exemplary method for applying adhesive to the flap portion of an acoustic barrier cap.

[0024] Figure 5 shows a first exemplary acoustic barrier cap for insertion into a hexagonal cellular cell.

[0025] Figure 6 shows a second exemplary acoustic barrier cap for insertion into a hexagonal cellular cell.

[0026] Figure 7 is an exploded view of an exemplary acoustic liner.

[0027] Figure 8 depicts an exemplary acoustic liner located near a noise source.

[0028] Figure 9 is a simplified view showing the orientation in a cell according to an embodiment of the invention, wherein acoustic barrier caps are positioned at different heights within the same cell.

[0029] Figure 10 is a simplified cross-sectional view of a turbofan jet engine, showing an exemplary location for the acoustic liner positioning. Detailed Implementation

[0030] An exemplary acoustic structure according to the invention is generally shown as 10 in Figures 1, 2, and 7. The acoustic structure 10 includes a honeycomb 12 having a first edge 14 and a second edge 16, the first edge being located closest to a noise source. The honeycomb 10 includes cells 18. Each cell 18 has a left side 20 and a right side 22. A lower wall 24 and an upper wall 26 each extend between the first edge 14 and the second edge 16 to define each cell 18. The lower wall 24 and the upper wall 26 preferably extend parallel to each other between the first edge 14 and the second edge 16. The lower wall 24 includes a lower left end portion 28, a lower right end portion 30, and a lower central portion 32. The upper wall 26 includes an upper left end portion 34, a lower right end portion 36, and an upper central portion 38. A left joint 40 is formed on the left side of each cell at the junction of the lower left end portion 28 and the upper left end portion 34. A right joint 42 is formed on the right side of each cell at the junction of the lower right end portion 30 and the upper right end portion 36.

[0031] The depth of each cell 18 is equal to and defined by the distance between the first edge 14 and the second edge 16 (also referred to as the core thickness). Each cell 18 has a cell size equal to the area surrounded by the lower wall 24 and the upper wall 26, as measured at the first edge 14 of the cell and as measured perpendicular to the cell wall.

[0032] The acoustic structure 10 includes acoustic barrier caps 44. Each acoustic barrier cap 44 is a solid polymer diaphragm block that has been folded to form a planar acoustic barrier portion 46 and a wing portion 48 surrounding the planar acoustic barrier portion 46. The acoustic barrier caps 44 are positioned within the unit 18 between a first edge 14 and a second edge 16 to provide an acoustic cavity with a depth less than the depth of the unit 18. The acoustic cavity depth is the distance between the planar acoustic barrier portion 46 and the first edge 14. The planar acoustic barrier portion 46 is oriented laterally to the unit wall. Preferably, the planar acoustic barrier portion 46 is oriented substantially perpendicular to the unit wall. Substantially perpendicular means at an angle of 90° ± 10°.

[0033] An exemplary polymer membrane insert is shown as 50 in FIG. 3. The insert is then folded and inserted into cell 18 to form an acoustic barrier cap 44. Insert 50 includes a planar acoustic barrier portion 52 and a flap portion 53 surrounding the planar acoustic barrier portion 52. The planar acoustic barrier portion has a boundary 54 (shown in dashed lines). Boundary 54 has an upper right boundary portion 60, an upper central boundary portion 62, an upper left boundary portion 64, a lower right boundary portion 66, a lower central boundary portion 68, and a lower left boundary portion 70. When insert 50 is placed in the cellular cell, the planar acoustic barrier portion 52 has a top side 56 located closest to the first edge of the cellular cell. When insert 50 is placed in the cellular cell, the bottom side 58 of the insert is located closest to the second edge of the cellular cell (see FIG. 2). During insertion into cellular cell 18, flap portion 53 folds toward the top side 56 of planar acoustic barrier portion 54.

[0034] The wing portion 53 of the insert 50 includes: an upper right wing 72 protruding from the upper right boundary portion 60; an upper central wing 74 protruding from the upper central boundary portion 62; an upper left wing 76 protruding from the upper left boundary portion 64; a lower right wing 78 protruding from the lower right boundary portion 66; a lower central wing 80 protruding from the lower central boundary portion 68; and a lower left wing 82 protruding from the lower left boundary portion 70.

[0035] When the insert 50 is placed into the cell unit 18 to form the acoustic barrier cap 44, the flaps are frictionally locked to the cell wall as follows: the upper right flap 72 is frictionally locked to the upper right end portion 36 of the upper wall 26; the upper central flap 74 is frictionally locked to the upper central portion 38 of the upper wall 26; the upper left flap 76 is frictionally locked to the upper left end portion 34 of the upper wall 26; the lower right flap 78 is frictionally locked to the lower right end portion 30 of the lower wall 24; the lower central flap 82 is frictionally locked to the lower central portion 32 of the lower wall 24; and the lower left flap 82 is frictionally locked to the lower left end portion 28 of the lower wall 24.

[0036] Cellular 12 can be made of any conventional materials used to manufacture cellular panels, including metals, ceramics, and composite materials. Exemplary composite materials include glass fibers, resin-impregnated aramid paper (such as... Various combinations of graphite fibers and suitable matrix resins are available. Matrix resins capable of withstanding relatively high temperatures (350°F to 500°F) are preferred for acoustic panels in jet engines. Honeycombs made of metallic or ceramic materials can operate at higher temperatures than honeycombs made of composite materials. However, composite honeycombs are more preferred for jet engine acoustic panels because they are relatively lightweight. Composite honeycombs are commercially available and can be used for extended periods at temperatures from 350°F to 500°F, with short-term capabilities up to 700°F. Such high-temperature honeycombs utilize a combination of glass fabric fiber supports and high-temperature resins (such as polyamide-imide resins or polyimide resins) for the prepreg resin matrix, node adhesives, and coating resins. A preferred exemplary type of glass fiber reinforced hexagonal polyimide honeycomb is available from Hexcel Corporation (Casa Grand, Arizona) under the trade name... HRH-327 obtained.

[0037] Cellular cell 18 has a cell perimeter, shown as dashed line 84 in Figure 1. The cell perimeter 84 is defined by an upper wall 26 and a lower wall 24. The upper right portion 36 and the upper left portion 34 each form a portion of the cell perimeter 84 larger than the upper central portion 38. The lower right portion 30 and the lower left portion 28 form a portion of the cell perimeter 84 larger than the lower central portion 32. This type of irregular hexagon is preferred.

[0038] Insert 50 is specifically designed for insertion into irregular hexagonal honeycomb cells 18. The upper right wing 72 and upper left wing 76 are each larger than the upper central wing 74. The lower right wing 78 and lower left wing 82 are each larger than the lower central wing 80. This wing configuration matches the corresponding walls that frictionally lock with the wing during insertion of insert 50 into cell 18.

[0039] The acoustic barrier cap according to the invention can be inserted into cells with shapes different from the irregular hexagons formed by cell 18, provided that the shape of the insert is changed to accommodate different cell geometries. The cell shape can be a regular hexagon or other cell shapes suitable for manufacturing acoustic panels. For example, the acoustic honeycomb can be a flexible honeycomb where the cell walls form a combination of convex and concave curvatures, which allows the honeycomb to be more easily formed into non-planar acoustic panels. Preferred flexible honeycombs are available from Hexcel Corporation (Dublin, California). Flexible honeycomb. Flexible honeycomb can be made from a variety of suitable materials, including 5052 or 5056 aluminum, aramid / phenolic composites, and glass fiber / phenolic composites.

[0040] This invention applies to cell sizes ranging from 0.1 square inches to 1.0 square inches. Cell sizes smaller than 0.1 square inches are too small to allow for the insertion of acoustic barrier caps. Cell sizes larger than 1.0 square inches require membranes that are too thick to be folded and inserted into the cell. The cell size is the area surrounded by the upper wall 26 and the lower wall 24, as measured at the first edge 14. Preferred cell sizes range from 0.3 square inches to 0.6 square inches. Particularly preferred are cellular cells with a hexagonal cell having a distance (DC) between opposite walls of 0.38 inches ± 0.05 inches.

[0041] To provide a suitable acoustic barrier cap, insert 50 must have sufficient size, shape, and flexibility to be folded and inserted into the cell. The folded insert must also exhibit sufficient resilience to provide adequate frictional locking of the acoustic barrier cap within the cell to allow for subsequent processing, including the application of adhesives to permanently bond the acoustic barrier cap within the cell. Insert 50 must also be made of a polymer capable of withstanding the high temperatures typically exposed to those found in jet engine acoustic linings.

[0042] The planar acoustic barrier portion 52 of the insert 50 must have sufficient stiffness so that the resulting acoustic barrier cap 44 functions as the bottom of the acoustic cavity and reflects most of the sound entering the unit 18. When forming the planar acoustic barrier portion 46 of the acoustic barrier cap, the planar acoustic barrier portion 52 must have sufficient stiffness to provide an acoustic reflection coefficient of at least 0.75 for sound wave frequencies in the range of 500 Hz to 4000 Hz. More preferably, the reflection coefficient of the acoustic barrier cap will be at least 0.8 for sound wave frequencies in the range of 500 Hz to 4000 Hz. The reflection coefficient is determined by the equation R = (Z-1) / (Z+1), where R is the reflection coefficient and Z is the frequency-dependent normalized impedance of the planar acoustic barrier portion. A reflection coefficient of 1 is equal to 100% reflection of a sound wave at a given frequency.

[0043] It has been found that polyetheretherketone (PEEK) membranes previously used to manufacture perforated acoustic diaphragm caps (see U.S. Patent No. 8,413,761) can also be used to manufacture suitable acoustic barrier caps, provided that the above-mentioned criteria regarding size, shape, resilience (friction locking), insertion flexibility, and acoustic stiffness are met.

[0044] PEEK is a crystalline thermoplastic polymer that can be processed to form films in either amorphous or crystalline phases. Compared to crystalline PEEK films, amorphous PEEK films are more transparent and easier to thermoform. Amorphous PEEK films are formed by heating the amorphous PEEK film to a temperature above the glass transition temperature (T0) of amorphous PEEK. g The temperature is maintained for a sufficient time to achieve approximately 30% to 35% crystallinity. Amorphous PEEK membranes exhibit better chemical resistance and abrasion resistance than amorphous membranes. Amorphous PEEK membranes are also more flexible and have greater resilience than amorphous membranes. Resilience is the force or bias applied to a folded membrane to restore it to its original pre-folded (flat) shape. Amorphous PEEK membranes are preferred for manufacturing acoustic barrier caps. PEEK membranes are available from SEFAR America Inc. (Deep, NY) under the trade names SEFAR PETEX, SEFARNITEX, and SEFAR PEEKTEX. PEEK sheets or membranes are also available from Victrex USA (Greenville, South Carolina), a manufacturer of PEEK membranes, under the trade name... PEEK TM Polymer procurement.

[0045] Polymer films other than PEEK films can be used, provided they exhibit similar properties in terms of resilience (friction locking), insertion flexibility, acoustic stiffness, and thermal stability. For example, polyimide films are used as an alternative to PEEK films in the manufacture of acoustic barrier caps. A variety of suitable polyimide films are available from DuPont Chemical Company (Midland, Michigan) under trade names... Polyimide films are obtained. Films made of polyetherketone or polyphenylene sulfide are also suitable.

[0046] The thickness of the polymer film used to manufacture the insert should be from 0.003 to 0.035 inches, and the thickness of the polymer film increases as the cell size increases from 0.1 square inches to 1.0 square inches. For cells 18 with cell sizes from 0.4 to 0.5 inches, a preferred polymer film thickness is from 0.010 to 0.025 inches. This preferred film thickness has been found to provide a particularly useful combination of insert foldability, frictional locking of the acoustic barrier cap, and a high acoustic reflection coefficient. Insert 50 preferably has a polymer film thickness from 0.003 to 0.009 inches. Such inserts are preferably used to manufacture acoustic barrier caps inserted into hexagonal honeycombs with cell sizes from 0.1 to 0.6 square inches.

[0047] For hexagonal cell sizes ranging from 0.4 to 1.0 inches, a thickness of 0.010 to 0.035 inches for the insert is preferred. Such a thicker insert is shown as 50T in Figure 4. Insert 50T has the same basic shape as insert 50 (Figure 3), except that the fins are slotted to form sub-fins, which make the thicker insert more flexible. The sub-fin portions ensure that the thicker insert 50T has the flexibility and resilience required for insertion and frictional locking within the cell.

[0048] The reference numbers used to identify the various elements of insert 50T correspond to the numbers used to identify the elements of insert 50. A “T” has been added to the corresponding numbers in Figure 4 to reflect that they are the same as the elements described for insert 50, except that insert 50T is thicker. Therefore, the previous description of the various numbered elements of insert 50 also applies to the corresponding numbered (T) elements illustrated in Figure 4. The wing portion 53T of insert 50T includes additional slots that divide each of winglets 72T, 74T, 76T, 78T, 80T, and 82T into a first sub-wing portion and a second sub-wing portion. In Figure 4, the sub-wing portions are identified by using winglet numbers followed by “a” or “b” to identify the individual sub-wing portions.

[0049] The size and shape of the planar acoustic barrier portion 52 (52T) will be the same as or slightly smaller than the size and shape of the unit. Preferably, the distance D (DT) between the opposing upper boundary portion 62 (62T) and the lower central boundary portion 68 (68T) will be 85% to 99% of the corresponding distance between the opposing upper central wall portion 38 and the lower central wall portion 32. The slots in the insert S (ST) that separate the fins from each other should terminate at or near the boundary of the planar acoustic portion 54 (54T). The slots should terminate at a distance from the boundary 54 (54T) equal to 0 to 50% of the fin width W (WT). Preferably, the slots should terminate at a distance from the boundary 54 (54T) equal to 2% to 20% of the fin width.

[0050] The width W (WT) of the wing portion 53 (53T) can vary depending on a number of factors, including the flexibility (thickness) of the polymer membrane of the unit size, the number of winglets in the wing portion, and the adhesive used to permanently bond the acoustic barrier cap to the unit wall. A wing portion width of approximately 0.1 inches to 0.5 inches is suitable. Preferably, the wing portion width W (WT) will be 5% to 35% of the distance D (DT) between the opposing upper and lower central boundary portions.

[0051] The slots S(ST) separating the flaps from each other can be U-shaped, as shown in 90 and 90T, or V-shaped, as shown in 92 and 92T. Preferably, the upper left flap 76 (76T) and the lower left flap 82 (82T) are separated from each other by V-shaped slots, and the upper right flap 72 (72T) and the lower right flap 78 (78T) are also separated from each other by V-shaped slots. It has been found that the V-shaped slots at these locations effectively facilitate proper folding and frictional locking of the membrane inserts in the cellular cell.

[0052] The insert, featuring a combination of U-shaped and V-shaped slots as shown in Figure 3, is made from a 0.006-inch thick amorphous PEEK film. The insert is used for... Acoustic barrier caps are formed in HRH-327 cells, where the DC of the cell is 0.38 inches. For acoustic frequencies ranging from 500Hz to 2000Hz and 3500Hz to 4000Hz, the acoustic barrier caps exhibit a reflection coefficient of approximately 0.8. The inserts, featuring a combination of U-shaped and V-shaped slots as shown in Figure 4, are made of a 0.010-inch thick amorphous PEEK film. The inserts are used for... Acoustic barrier caps are formed in the HRH-327 cell, where the DC of the cell is 0.38 inches. The acoustic barrier caps exhibit a reflection coefficient that increases from 0.8 at 500 Hz to 0.9 over the entire range from 500 Hz to 4000 Hz. Thicker inserts (0.010 inches) are particularly preferred because they provide a relatively high reflection coefficient over a wider frequency range compared to thinner inserts (0.006 inches). This increase in reflection coefficient characteristics is unexpected given the relatively small increase in PEEK film thickness (0.004 inches).

[0053] The fin portion of the insert can be perforated to increase its surface area, thereby enhancing its adhesive bond with the unit wall. The perforation provides an increased surface area and openings into which adhesive can enter to improve the bond between the fin portion and the unit wall. The perforations or holes can be drilled mechanically or using chemicals. Preferably, the perforations are made by laser drilling holes through a relatively thin polymer film. Preferably, the polymer film is laser-drilled to provide the desired number of perforations before the insert forms the acoustic barrier cap. One advantage of this procedure is that the flat insert surface makes it easier to focus the laser beam onto the polymer film during the drilling operation.

[0054] Figure 5 illustrates an exemplary method for inserting an acoustic barrier cap into a cellular cell to form a precursor structure, wherein the acoustic barrier cap is frictionally locked within the cellular cell. The reference numbers used to identify the cellular structure in Figure 5 are the same as those in Figure 1, except that they include "P" to indicate that the structure is a precursor structure in which the acoustic barrier cap has not yet been permanently bonded to the cell wall.

[0055] As shown in Figure 5, the polymer film 81 is cut to form an insert of appropriate size, such as the insert 50 shown in Figure 3. A plunger 83 of appropriate size is used to force the insert 50 into the cell. A cap-folding die (not shown) can be used to facilitate the insertion process. The cap-folding die has a die opening whose size and shape are intended to pre-fold and form an acoustic barrier cap before insertion into the cell. The use of a cap-folding die is preferred but not mandatory. It is possible to use a cell as a die and form an acoustic barrier cap by simply forcing the insert 50 into the cell using the plunger 83. The edges of many cell panels tend to be relatively serrated because, during manufacturing, the panels are typically cut from larger cell blocks. When a flat insert film is forced directly into the cell, these serrated cell edges tend to trap, tear, and contaminate the acoustic barrier cap. Therefore, if the cell is used as a die for folding and forming the acoustic barrier cap, the cell edges should be as smooth as possible.

[0056] It is important to select the size, shape, and flexibility of the polymer membrane, as well as the size / shape of the plunger and die (or, if no die is used, just the plunger), to provide sufficient frictional contact between the fin portion and the unit wall to hold the acoustic barrier cap in place during subsequent processing of the precursor structure. The amount of frictional locking or retention should be sufficient to prevent the acoustic barrier cap from falling out of the cell, even if the precursor structure is accidentally dropped during processing.

[0057] Frictional locking of the acoustic barrier cap to the unit wall is achieved by varying the fin portion size, number of fins, polymer film thickness, polymer film stiffness / resilience, slot size, and slot shape until a sufficient level of frictional locking is reached. For example, frictional locking tends to decrease with increasing fin number and / or slot size. Frictional locking tends to increase with increasing polymer film thickness, polymer film stiffness / resilience, and fin size. As illustrated above for inserts 50 and 50T, specific combinations of these parameters have been found to provide sufficient frictional locking of the acoustic barrier cap within the precursor cellular structure.

[0058] The degree of frictional locking between the acoustic barrier cap and the cell wall can be measured by placing a test weight onto the acoustic barrier cap and determining whether any movement of the cap occurs. For example, the acoustic barrier cap is considered to have sufficient frictional locking force to the cell wall if it passes the following test: Place a test weight (27 grams) on top of the dry acoustic barrier cap from the insert side. Sufficient frictional locking force is achieved when the dry cap supports the 27 grams without slipping off the cell. In the exemplary test, the 27-gram test weight is a steel bar with a diameter of 0.368 inches and a length of 2.00 inches.

[0059] The acoustic barrier cap 44P is held in place within the precursor structure 10P in Figure 5 solely by friction locking. As previously mentioned, the friction locking must be sufficient to firmly hold the spacer caps in place until they can be permanently bonded using a suitable adhesive. The adhesive used can be any conventional adhesive used in the manufacture of cellular panels. Preferred adhesives include those that are stable at high temperatures (350° to 500°). Exemplary adhesives include epoxy resins, acrylic resins, phenolic resins, cyanoacrylates, bismaleimides, polyamide-imides, and polyimides.

[0060] Various known adhesive application procedures can be used to apply adhesive to the wing portion / unit wall interface. An important consideration is that the adhesive should be applied in a controlled manner. The adhesive should be applied to the wing portion at the interface with the unit wall to a minimum. An exemplary adhesive application procedure is shown in Figure 6. In this exemplary procedure, the cell 12P is simply dipped into a liquid adhesive pool 91 such that only the wing portion 48P is immersed in the adhesive. This dipping procedure can be used to accurately apply the adhesive to the wing portion / unit wall interface, provided that the acoustic barrier cap is precisely rubbed locked at the same level before dipping. For acoustic barrier caps at different levels, multiple dipping steps are required. Alternatively, a brush or other site-specific application techniques can be used to apply the adhesive. Some of these techniques can be used to coat the core wall with adhesive before inserting the acoustic barrier cap. Alternatively, the adhesive can be screen-printed onto the wing portion before insertion into the core.

[0061] The impregnation procedure depicted in Figure 6 for applying the adhesive is preferred because the adhesive tends to be capillarily drawn upwards into the interface between the fin portion and the cell wall. This upward capillary action of the adhesive fills any air gaps between the fin portion and the cell wall, ensuring that the acoustic barrier cap provides maximum sound wave reflection. Once the adhesive is in place, it is cured according to known procedures or otherwise solidified to permanently bond the acoustic barrier cap to the cell wall.

[0062] The acoustic structure according to the invention can be used in a variety of situations requiring noise attenuation. The acoustic structure is particularly well-suited for use in conjunction with power generation systems where noise attenuation is often a problem. Because honeycomb is a relatively lightweight material, the acoustic structure of the invention is particularly suitable for use in aircraft systems. Exemplary applications include the nacelle of a jet engine, the cowling of a large turbine or reciprocating engine, and related acoustic structures. An exemplary turbofan jet engine is shown as 100 in FIG. 10. The jet engine 100 includes a nacelle 102. Acoustic panels or linings according to the invention can be placed, for example, at locations 104, 106, and 108 to provide damping or attenuation of noise generated by the jet engine.

[0063] The basic acoustic structure of the present invention is typically thermoformed into the final shape of an engine compartment, and then an outer material skin or sheet is bonded to the outer edge of the formed acoustic structure using one or more adhesive layers. This finished sandwich panel is cured in a retaining tool that maintains the complex shape of the compartment during bonding. For example, as shown in FIG7, the acoustic structure 10 is bonded to a solid, acoustically impermeable sheet or skin 80 at a second edge 16, and an acoustically perforated skin or sheet 82 is bonded to a first edge 14 to form an acoustic panel or acoustic liner. The bonding of the solid skin 80 and the perforated skin 82 is typically completed in a bonding tool under elevated temperature and pressure. A bonding tool is typically required to maintain the desired shape of the acoustic structure during panel formation.

[0064] In Figure 8, a portion of an exemplary acoustic panel 112 is shown in place around a portion of a cabin containing a jet engine or other noise source. The jet engine or other noise source is schematically shown as 110. The acoustic panel 112 includes an acoustic structure 114, a sound-permeable skin 116, and a solid, sound-impermeable skin 118. An acoustic barrier cap 120 is present in some of the cell cells 122 to form an acoustic resonator, the depth of which is equal to the distance from the sound-permeable skin 116 to the planar acoustic portion of the acoustic barrier cap 120. Other cell cells 124 do not include acoustic barrier caps, such that the effective resonator depth is equal to the distance from the sound-permeable skin 116 to the solid skin 118. The acoustic panel 112 is an example of an MDOF acoustic liner type that can be manufactured according to the invention by reducing the depth of some of the cell cells using acoustic barrier caps.

[0065] Another exemplary acoustic panel is shown as 130 in Figure 9. Acoustic panel 130 includes an acoustic structure 132, a sound-permeable skin 134, and a solid, sound-impermeable skin 136. Acoustic barrier caps 138 and 140 are located in units 142 and 146, respectively, to provide resonator cavities of different depths. Unit 148 does not include an acoustic barrier cap. This type of MDOF design, which provides multiple resonator depths, allows for fine-tuning of the noise attenuation characteristics of the acoustic structure. The multi-resonator depth configuration shown in Figure 9 is merely intended as an example of the many possible multi-level acoustic barrier cap arrangements possible according to the invention. As those skilled in the art will understand, the number and variety of different possible acoustic barrier cap placement levels are extremely large and can be customized to meet specific noise attenuation requirements.

[0066] Exemplary embodiments of the present invention have been described thus. Those skilled in the art should note that these are merely exemplary embodiments, and various other alternatives, adaptations, and modifications can be made within the scope of the invention. Therefore, the present invention is not limited to the preferred embodiments and examples described above, but is limited only by the following claims.

Claims

1. An acoustic structure precursor wherein an acoustic barrier cap is frictionally locked within a cell of a honeycomb to form an acoustic cavity that attenuates noise generated from a source once the acoustic barrier cap is adhesively bonded within the cell to form an acoustic hard wall, the acoustic structure precursor comprising: A) a honeycomb comprising a first edge and a second edge, the first edge being located closest to the source, the honeycomb comprising a cell having a left side and a right side, the cell defined by a lower wall extending between the first edge and the second edge and an upper wall also extending between the first edge and the second edge, the lower wall comprising a lower left end portion, a lower right end portion, and a lower central portion located between the lower left end portion and the lower right end portion, the upper wall comprising an upper left end portion, an upper right end portion, and an upper central portion located between the upper left end portion and the upper right end portion, wherein a left junction is formed along the left side of the cell at the intersection of the lower left end portion and the upper left end portion, wherein a right junction is formed along the right side of the cell at the intersection of the lower right end portion and the upper right end portion, wherein the cell has a depth defined by the distance between the first edge and the second edge, and wherein the cell has a cell area defined by the area surrounded by the upper wall and the lower wall, the cell area measured at the first edge being from 0.1 square inches to 1.0 square inches; B) an acoustic barrier cap comprising a solid polymer film that has been folded to form a planar acoustic barrier portion and a flap portion surrounding the planar acoustic barrier portion, the solid polymer film having a thickness from 0.010 inches to 0.035 inches, the acoustic barrier cap positioned within the cell between the first edge and the second edge to provide an acoustic cavity having a depth less than the depth of the cell, and wherein the acoustic barrier cap has sufficient stiffness to provide an acoustic reflection coefficient of at least 0.75 for acoustic wave frequencies in the range from 500 Hz to 4000 Hz, wherein: a) the planar acoustic barrier portion extends transverse to the upper wall and the lower wall, the planar acoustic barrier portion having a top side located closest to the first edge, a bottom side located closest to the second edge, the planar acoustic barrier portion having boundaries comprising: an upper right boundary portion, an upper central boundary portion, an upper left boundary portion, a lower right boundary portion, a lower central boundary portion, and a lower left boundary portion; and b) the flap portions comprise an upper right flap protruding from the upper right border portion, an upper center flap protruding from the upper center border portion, an upper left flap protruding from the upper left border portion, a lower right flap protruding from the lower right border portion, a lower center flap protruding from the lower center border portion, and a lower left flap protruding from the lower left border portion, wherein the flap portions provide a frictional lock of the acoustic barrier cap within the cell, wherein the upper right flap is frictionally locked to the upper right end portion of the upper wall, the upper center flap is frictionally locked to the upper center portion of the upper wall, and the upper left flap is frictionally locked to the upper left end portion of the upper wall, and wherein the lower right flap is frictionally locked to the lower right end portion of the lower wall, the lower center flap is frictionally locked to the lower center portion of the lower wall, and the lower left flap is frictionally locked to the lower left end portion of the lower wall.

2. The acoustic structure precursor of claim 1, wherein the upper and lower walls form a hexagonal cell.

3. The acoustic structure precursor of claim 2, wherein each of the upper right flap, the upper center flap, the upper left flap, the lower right flap, the lower center flap, and the lower left flap is divided into a first sub-flap portion and a second sub-flap portion.

4. The acoustic structure precursor of claim 2, wherein the cell has a cell perimeter defined by the upper and lower walls, wherein the upper right and left end portions each form a greater portion of the cell perimeter than the upper center portion, and wherein the lower right and left end portions form a greater portion of the cell perimeter than the lower center portion, and wherein the upper right and left flaps are each greater than the upper center flap, and wherein the lower right and left flaps are each greater than the lower center flap.

5. The acoustic structure precursor of claim 3, wherein the cell has a cell perimeter defined by the upper and lower walls, wherein the upper right and left end portions each form a greater portion of the cell perimeter than the upper center portion, and wherein the lower right and left end portions form a greater portion of the cell perimeter than the lower center portion, and wherein the upper right and left flaps are each greater than the upper center flap, and wherein the lower right and left flaps are each greater than the lower center flap.

6. The acoustic structure precursor of claim 1, wherein the upper left and lower left flaps are separated from each other by a V-shaped notch, and wherein the upper right and lower right flaps are separated from each other by a V-shaped notch.

7. The acoustic structure precursor of claim 1, wherein the solid polymer film is selected from the group consisting of a polyether ether ketone film, a polyimide film, a polyether ketone film, and a polyphenylene sulfide film.

8. An acoustic structure comprising the acoustic structure precursor of claim 1 and an adhesive that binds the flap portions with the upper and lower walls to form the acoustic hard wall.

9. The acoustic structure according to claim 8, wherein the acoustic structure comprises a sound-transparent sheet attached to a first edge of the honeycomb and a solid, sound-impermeable sheet attached to a second edge of the honeycomb.

10. A method for manufacturing an acoustic structure precursor, wherein an acoustic barrier cap is frictionally locked within a cell of a honeycomb to form an acoustic cavity, and once the acoustic barrier cap is adhesively bonded within the cell to form an acoustic hard wall, the acoustic cavity attenuates noise generated from a source, the method comprising the steps of: A) A cellular network is provided, the cellular network including a first edge and a second edge, the first edge being located closest to the source, the cellular network including cells having a left side and a right side, the cells being defined by a lower wall extending between the first edge and the second edge and an upper wall also extending between the first edge and the second edge, the lower wall including a lower left end portion, a lower right end portion and a lower central portion located between the lower left end portion and the lower right end portion, the upper wall including an upper left end portion, an upper right end portion and an upper central portion located between the upper left end portion and the upper right end portion, wherein a left joint is formed along the left side of the cell at the junction of the lower left end portion and the upper left end portion, wherein a right joint is formed along the right side of the cell at the junction of the lower right end portion and the upper right end portion, wherein the cell has a depth defined by the distance between the first edge and the second edge, and wherein the cell has a cell size defined by the area surrounded by the upper wall and the lower wall, the cell size being from 0.1 square inches to 1 square inch when measured at the first edge; B) Provide a solid polymer membrane that can be folded to form an acoustic barrier cap, the acoustic barrier cap including a planar acoustic barrier portion and a wing portion surrounding the planar acoustic barrier portion, the thickness of the solid polymer membrane being from 0.010 inches to 0.035 inches, wherein: a) The planar acoustic barrier portion has boundaries, the boundaries including an upper right boundary portion, an upper central boundary portion, an upper left boundary portion, a lower right boundary portion, a lower central boundary portion, and a lower left boundary portion; and b) The wing portion includes an upper right wing protruding from the upper right boundary portion, an upper central wing protruding from the upper central boundary portion, an upper left wing protruding from the upper left boundary portion, a lower right wing protruding from the lower right boundary portion, a lower central wing protruding from the lower central boundary portion, and a lower left wing protruding from the lower left boundary portion; and C) The solid polymer film is positioned within the unit to form the acoustic barrier cap, wherein the planar acoustic barrier portion extends transversely to the upper and lower walls, the planar acoustic barrier portion having a top side closest to the first edge and a bottom side closest to the second edge, the acoustic barrier cap being located between the first and second edges to provide an acoustic cavity, the depth of the acoustic cavity being less than the depth of the unit, and wherein the acoustic barrier cap has sufficient stiffness to provide acoustic protection for frequencies from 500 Hz to 4000 Hz. An acoustic reflection coefficient of at least 0.75 for sound wave frequencies in the Hz range, wherein the wing portions provide frictional locking of the acoustic barrier cap within the unit, wherein the upper right wing is frictionally locked to the upper right end portion of the upper wall, the upper central wing is frictionally locked to the upper center of the upper wall, and the upper left wing is frictionally locked to the upper left end portion of the upper wall, wherein the lower right wing is frictionally locked to the lower right end portion of the lower wall, the lower central wing is frictionally locked to the lower center portion of the lower wall, and the lower left wing is frictionally locked to the lower left end portion of the lower wall.

11. The method for manufacturing an acoustic structural precursor according to claim 10, wherein the upper wall and the lower wall form hexagonal units.

12. The method for manufacturing an acoustic structure precursor according to claim 11, wherein each of the upper right wing, the upper central wing, the upper left wing, the lower right wing, the lower central wing, and the lower left wing is respectively divided into a first sub-wing portion and a second sub-wing portion.

13. The method for manufacturing an acoustic structure precursor according to claim 11, wherein the unit has a unit perimeter defined by the upper wall and the lower wall, wherein the upper right portion and the upper left portion each form a portion of the unit perimeter that is larger than the upper central portion, wherein the lower right portion and the lower left portion form a portion of the unit perimeter that is larger than the lower central portion, wherein the upper right wing and the upper left wing are each larger than the upper central wing, and wherein the lower right wing and the lower left wing are each larger than the lower central wing.

14. The method for manufacturing an acoustic structure precursor according to claim 12, wherein the unit has a unit perimeter defined by the upper wall and the lower wall, wherein the upper right portion and the upper left portion each form a portion of the unit perimeter that is larger than the upper central portion, wherein the lower right portion and the lower left portion form a portion of the unit perimeter that is larger than the lower central portion, and wherein the upper right wing and the upper left wing are each larger than the upper central wing, and wherein the lower right wing and the lower left wing are each larger than the lower central wing.

15. The method for manufacturing an acoustic structural precursor according to claim 10, wherein the solid polymer membrane is selected from polyetheretherketone membranes, polyimide membranes, polyetherketone membranes, and polyphenylene sulfide membranes.

16. The method for manufacturing an acoustic structure precursor according to claim 10, the method comprising the additional step of adhesively bonding the wing portion to the upper and lower walls of the unit.

17. A method for manufacturing an acoustic structure, the method comprising the steps of: An acoustic structural precursor according to claim 1 is provided, and the wing portion is adhesively bonded to the upper and lower walls of the unit.

18. The method for manufacturing an acoustic structure according to claim 17, wherein the upper wall and the lower wall form hexagonal units.

19. The method for manufacturing an acoustic structure according to claim 18, wherein each of the upper right wing, the upper central wing, the upper left wing, the lower right wing, the lower central wing, and the lower left wing is respectively divided into a first sub-wing portion and a second sub-wing portion.

20. The method for manufacturing an acoustic structure according to claim 17, wherein the solid polymer membrane is selected from polyetheretherketone membranes, polyimide membranes, polyetherketone membranes, and polyphenylene sulfide membranes.

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