Patent Description:
Acoustic panels may be used in various applications to attenuate noise. An acoustic panel, for example, may be configured with a nacelle of an aircraft propulsion system to attenuate noise generated by a gas turbine engine. Such an acoustic panel typically includes a honeycomb core connected between a perforated face skin and a solid, non-perforated back skin. The honeycomb core includes a plurality of resonating chambers. These resonating chambers are tuned by selecting a desired chamber length and, thus, core thickness that corresponds to a specific target frequency of noise to be attenuated. Increasing the core thickness, for example, will typically tune the resonating chambers for attenuating lower frequency noise. Conversely, decreasing the core thickness will typically tune the resonating chambers for attenuating higher frequency noise.

Recent trends in aircraft engine design such as higher bypass ratios, larger fan diameters, slower rotating fans and/or fewer number of fan blades have resulted in those aircraft engines generating relatively low frequency noise. Relatively strict space constraints for those engines, however, typically limit or prohibit increasing the thickness of an acoustic panel to tune its resonating chambers for such relatively low frequency noise. Reducing the thickness of the acoustic panel also has the concern of compromising structural integrity.

There is a need in the art for acoustic and structural panels with increased structural integrity and reduced thicknesses.

<CIT> discloses a prior art structured panel as set forth in the preamble of claim <NUM>.

<CIT> discloses a prior art acoustic panel with vertical stiffeners.

From a first aspect, there is provided a structured panel as recited in claim <NUM>.

There is also provided a method for manufacturing a structured panel for attenuating sound as recited in claim <NUM>.

The present disclosure includes structured panels such as, but not limited to, acoustic panels for attenuating sound; e.g., noise. Each structured panel may include one or more multi-layered structures with one or more structural reinforcements, such as rib structures, for increasing rigidity, strength, stability (i.e., resistance to buckling) and/or other metrics of structural integrity of that panel. For ease of description, the following disclosure will first describe a general panel configuration without structural reinforcements and then describe how one or more multi-layered structures and associated structural reinforcement(s) may be added to one or more elements (e.g., components) of the panel to increase rigidity, strength, stability and/or other metrics of structural integrity of that panel.

<FIG> is a partial, perspective schematic illustration of an acoustic panel <NUM> for attenuating noise. This acoustic panel <NUM> may be configured to attenuate noise generated by an aircraft propulsion system such as, for example, a turbofan propulsion system or a turbojet propulsion system. With such a configuration, the acoustic panel <NUM> may be configured with a nacelle of the propulsion system. The acoustic panel <NUM>, for example, may be configured as or with an inner or outer barrel, a translating sleeve of a thrust reverser, a blocker door, etc. Alternatively, the acoustic panel <NUM> may be configured with another component / structure of the aircraft such as its fuselage or a wing. Furthermore, the acoustic panel <NUM> may be configured to also or alternatively attenuate aircraft related noise other than that generated by the propulsion system. The acoustic panel <NUM> of the present disclosure, however, may alternatively be configured for non-aircraft applications.

The acoustic panel <NUM> extends longitudinally along an x-axis. The acoustic panel <NUM> extends laterally along a y-axis. The acoustic panel <NUM> extends vertically along a z-axis. The term "vertical" is used herein to describe a depthwise panel direction and is not limited to a gravitational up/down direction. Furthermore, for ease of illustration, the x-y plane is shown as a generally flat plane. However, in other embodiments, the x-y plane and, thus, the acoustic panel <NUM> may be curved and/or follow an undulating geometry. For example, the x-y plane and, thus, the acoustic panel <NUM> may be arcuate, cylindrical or conical with or without radial undulations. Thus, the vertical direction may change at different locations along the x-y plane; e.g., the vertical direction may be a radial direction for a cylindrical or conical acoustic panel.

The acoustic panel <NUM> includes a perforated first (e.g., face) skin <NUM>, a solid non-perforated second (e.g., back) skin <NUM> and a cellular core <NUM>. Briefly, the cellular core <NUM> is disposed and extends vertically between the first skin <NUM> and the second skin <NUM>. The cellular core <NUM> is also connected to the first skin <NUM> and the second skin <NUM>. The cellular core <NUM>, for example, may be fused, adhered, welded, brazed and/or otherwise bonded to the first skin <NUM> and/or the second skin <NUM>. The cellular core <NUM> may also or alternatively be mechanically fastened to the first skin <NUM> and/or the second skin <NUM>. Alternatively, the cellular core <NUM> may be formed integral with the first skin <NUM> and/or the second skin <NUM> as a monolithic body using, for example, additive manufacturing. However, the present disclosure is not limited to any particular manufacturing methods.

The first skin <NUM> may be configured as a relatively thin sheet or layer of material that extends longitudinally and laterally along the x-y plane. This first skin material may include, but is not limited to, a polymer, a fiber reinforced composite (e.g., fiberglass composite, carbon fiber composite, aramid fiber composite, composite reinforced by any combination of glass, carbon, aramid or other fibers, etc.), metal, alloys, metal matrix composite, ceramic, or ceramic matrix composite, or a combination thereof. Referring now to <FIG>, the first skin <NUM> has a vertical thickness <NUM> that extends vertically between opposing side surfaces <NUM> and <NUM>. The first skin <NUM> includes a plurality of perforations <NUM>; e.g., apertures such as through-holes (see also <FIG>). Each of these perforations <NUM> extends generally vertically through the first skin <NUM> between its side surfaces <NUM> and <NUM>. In other embodiments, the perforations <NUM> may be non-uniform through the thickness <NUM> and/or may be non-perpendicular to the side surfaces <NUM> and/or <NUM>.

The second skin <NUM> may be configured as a relatively thin sheet or layer of (e.g., continuous and uninterrupted) material that extends longitudinally and laterally along the x-y plane (see <FIG>). This second skin material may include, but is not limited to, a polymer, a fiber reinforced composite (e.g., fiberglass composite, carbon fiber composite, aramid fiber composite, composite reinforced by any combination of glass, carbon, aramid or other fibers, etc.), metal, alloys, metal matrix composite, ceramic, or ceramic matrix composite, or a combination thereof. The second skin material may be the same as or different than the first skin material. The second skin <NUM> has a vertical thickness <NUM> that extends vertically between opposing side surfaces <NUM> and <NUM>. This vertical thickness <NUM> may be substantially equal to or different (e.g., greater or less) than the vertical thickness <NUM> of the first skin <NUM>. The thickness <NUM> of the first skin <NUM> and/or the thickness <NUM> the second skin <NUM> may be uniform or non-uniform along the x-y plane.

Referring to <FIG>, the cellular core <NUM> extends longitudinally and laterally along the x-y plane. Referring again to <FIG>, the cellular core <NUM> has a vertical thickness <NUM> that extends vertically between opposing core sides, which sides are respectively abutted against the first skin <NUM> and the second skin <NUM> and their side surfaces <NUM> and <NUM>. The vertical thickness <NUM> may be substantially greater than the vertical thicknesses <NUM> and <NUM> of the first skin <NUM> and/or the second skin <NUM>, respectively. The vertical thickness <NUM>, for example, may be at least ten to forty times (<NUM>-40x), or more, greater than the vertical thicknesses <NUM> and <NUM>; however, the acoustic panel <NUM> of the present disclosure is not limited to such an exemplary embodiment.

Referring to <FIG>, the cellular core <NUM> includes a plurality of solid non-perforated walls <NUM> (e.g., cavity sidewalls) and one or more arrays of corrugations <NUM>. The walls <NUM> and corrugations <NUM> are arranged together to configure the cellular core <NUM> as an open cavity (e.g., open cell) structure. This open cavity structure forms a plurality of cavities <NUM> (each including divided sub-cavities 128A and 128B) vertically between the first skin <NUM> and the second skin <NUM>. Each of these cavities <NUM> may be fluidly coupled with one or more respective perforations <NUM> in the first skin <NUM> (see <FIG>).

Referring to <FIG>, each of the walls <NUM> has a length that extends longitudinally along the x-axis. Each of the walls <NUM> has a thickness that extends laterally along the y-axis. Referring now to <FIG>, each of the walls <NUM> has a height <NUM> that extends vertically between the first skin <NUM> and the second skin <NUM>.

Each of the walls <NUM> is at least partially (or completely) connected to or otherwise engaged with the first skin <NUM> and/or the second skin <NUM>. Each of the exemplary walls <NUM> of <FIG> is orientated substantially perpendicular to the first skin <NUM> and the second skin <NUM>; e.g., at a ninety-degree angle to the skins <NUM> and <NUM>. However, in other embodiments, one or more of the walls <NUM> may be angularly offset from the first skin <NUM> and/or the second skin <NUM> by a non-ninety-degree angle; e.g., an acute angle or an obtuse angle.

The walls <NUM> are arranged generally parallel with one another; see also <FIG>. The walls <NUM> are laterally spaced from one another along the y-axis so as to respectively form the cavities <NUM> between the walls <NUM>. Each of the walls <NUM> shown in <FIG> therefore respectively forms lateral sides of adj acent cavities <NUM> on either side of the respective wall <NUM>. Each of the walls <NUM> thereby also fluidly separates those cavities <NUM> on either side of the wall <NUM>.

Referring to <FIG>, the corrugations <NUM> in each array are disposed and extend laterally between a laterally adjacent pair of the walls <NUM>; see also <FIG>. Each of the corrugations <NUM> includes a solid non-perforated baffle <NUM> and a porous (e.g., perforated) septum <NUM>. In another exemplary embodiment, one or more or each of the corrugations <NUM> includes only porous (e.g. perforated) septa <NUM>, or only solid non-perforated baffles <NUM> in an alternating periodic or non-periodic pattern along the y-axis or the x-axis or both.

Referring to <FIG> and <FIG>, the baffle <NUM> has a width extending laterally between opposing lateral sides. These lateral sides are at least partially (or completely) connected to or otherwise engaged with a respective laterally adjacent pair of the walls <NUM>. Referring to <FIG>, the baffle <NUM> has a length extending diagonally (e.g., vertically and longitudinally) between opposing top and bottom ends <NUM> and <NUM>. Note, the terms "top" and "bottom" are used above to describe ends of the baffle <NUM> as situated in the drawings and are not intended to limit the baffle <NUM> or the acoustic panel <NUM> to such an exemplary gravitational orientation.

The septum <NUM> has a width extending laterally between opposing lateral sides. These lateral sides are connected to or otherwise engaged with a respective laterally adjacent pair of the walls <NUM>. The septum <NUM> has a length extending diagonally (e.g., vertically and longitudinally) between opposing top and bottom ends <NUM> and <NUM>. Note, the terms "top" and "bottom" are used above to describe ends of the septum <NUM> as situated in the drawings and are not intended to limit the septum <NUM> or the acoustic panel <NUM> to such an exemplary gravitational orientation.

The septum <NUM> includes one or more perforations <NUM>. In the exemplary embodiment of <FIG>, the perforations <NUM> are configured as through holes. However, in other embodiments, the perforations <NUM> may be formed by interconnected pores in the septum <NUM> where the septum material, for example, has an open cell porous structure.

The top end <NUM> of the baffle <NUM> is connected at least partially (or completely) to or otherwise engaged with the first skin <NUM>. This top end <NUM> is also longitudinally connected to the top end <NUM> of the septum <NUM> at an interface <NUM> between the baffle <NUM> and the septum <NUM>. The bottom end <NUM> of the baffle <NUM> is connected to or otherwise engaged with the second skin <NUM>. This bottom end <NUM> is also longitudinally connected to the bottom end <NUM> of a septum <NUM> of an adjacent one of the corrugations <NUM> at an interface <NUM>. With the foregoing configuration, the baffle <NUM> extends vertically between the first skin <NUM> and the second skin <NUM> and longitudinally between the septums <NUM>. The baffle <NUM> is therefore angularly offset from the first skin <NUM> and the second skin <NUM> by an included angle <NUM>; e.g., between <NUM>-<NUM> degrees. This angle <NUM> is an acute angle such as, but not limited to, about forty-five degrees (<NUM>°).

The top end <NUM> of the septum <NUM> is at least partially (or completely) connected to or otherwise engaged with the first skin <NUM>. This top end <NUM> is also longitudinally connected to the top end <NUM> of the baffle <NUM> as described above. The bottom end <NUM> of the septum <NUM> is at least partially (or completely) connected to or otherwise engaged with the second skin <NUM>. This bottom end <NUM> is also longitudinally connected to the bottom end <NUM> of a baffle <NUM> of an adjacent one of the corrugations <NUM> at an interface; e.g., the interface <NUM>. With the foregoing configuration, the septum <NUM> extends vertically between the first skin <NUM> and the second skin <NUM> and longitudinally between the baffles <NUM>. The septum <NUM> is therefore angularly offset from the first skin <NUM> and the second skin <NUM> by an included angle <NUM>; e.g., between <NUM>-<NUM> degrees. This angle <NUM> may be an acute angle such as, but not limited to, about forty-five degrees (<NUM>°). The angle <NUM> may be substantially equal to the angle <NUM> as shown in <FIG>. Alternatively, the angle <NUM> may be different from the angle <NUM>; e.g., a larger or smaller acute angle, or a right angle. For example, the angle <NUM> may be about ninety degrees and the angle <NUM> may be about forty-five degrees as shown in <FIG>. In another example the angle <NUM> may be about ninety degrees and the angle <NUM> may be about forty-five degrees.

Referring to <FIG>, each of the cavities <NUM> extends longitudinally between and is formed by a longitudinally adjacent pair of the baffles <NUM>. Each septum <NUM> is disposed within and divides a respective one of the cavities <NUM> into fluidly coupled sub-cavities 128A and 128B. More particularly, the perforations <NUM> in the septum <NUM> fluidly couple the sub-cavities 128A and 128B together.

Each of the cavities <NUM> forms a resonance chamber <NUM>. A length <NUM> of the resonance chamber <NUM> extends diagonally (e.g., longitudinally and vertically) between the first skin <NUM> and the second skin <NUM> and through a respective one of the septums <NUM>. The length <NUM> of the resonance chamber <NUM> therefore is longer than the vertical thickness <NUM> of the cellular core <NUM>. This enables noise attenuation of relatively low frequency noise without increasing the vertical thickness <NUM> of the cellular core <NUM> and, thus, a vertical thickness of the acoustic panel <NUM>. For example, each resonance chamber <NUM> may receive noise waves through the perforations <NUM> in the first skin <NUM>. The resonance chamber <NUM> may reverse the phase of one or more frequencies of those sound waves using known acoustic reflection principles and subsequently direct the reverse phase sound waves out of the acoustic panel <NUM> through the perforations <NUM> to destructively interfere with other incoming noise waves.

The cellular core <NUM> may be constructed from any suitable material or materials. The cellular core <NUM>, for example, may be constructed from a polymer, a fiber reinforced composite (e.g., fiberglass composite, carbon fiber composite, aramid fiber composite, composite reinforced by any combination of glass, carbon, aramid or other fibers), metal, alloys, metal matrix composite, ceramic, or ceramic matrix composite, or a combination thereof. One or more of components of the cellular core <NUM> may be constructed from the same or a like material. Alternatively, one or more of the components of the cellular core <NUM> may be constructed from a different material than one or more of the other components of the cellular core <NUM>. Furthermore, the cellular core <NUM> may be constructed from the same material(s) as the first skin <NUM> and/or the second skin <NUM>, or a different material or materials.

Referring to <FIG>, one or more elements 156A-B (generally referred to as "<NUM>") of the acoustic panel <NUM> and, more particularly, the cellular core <NUM> may be configured with a multi-layered structure 158A-B (generally referred to as "<NUM>") with one or more structural reinforcements 160A-G (generally referred to as "<NUM>"). These structural reinforcements <NUM> are provided to increase rigidity, stability, strength and/or structural integrity of the corresponding element(s) <NUM> as well as the acoustic panel <NUM> as a whole. Examples of the one or more elements <NUM> include, but are not limited to: (A) one, some or each of the baffles <NUM>; (B) one, some or each of the septums <NUM>; (C) one, some or each of the arrays of corrugations <NUM>; (D) one, some or each of the walls <NUM>; and (E) a combination of any two or more of (A) to (D).

<FIG> illustrate a portion of the acoustic panel element 156A configured with the multi-layered structure 158A. This multi-layered structure 158A includes a first layer 162A (e.g., ply of material) and a second layer 164A (e.g., ply of material).

The first layer 162A may be a contoured layer. The first layer 162A of <FIG>, for example, is configured with one or more of the structural reinforcements 160A. Each structural reinforcement 160A of <FIG> is configured as a rib 166A. Each rib 166A may be a stamped, molded and/or otherwise formed in the first layer 162A to project out from an exterior surface <NUM> / side of the acoustic panel element 156A / multi-layered structure 158A. Each rib 166A extends along a trajectory 170A, where the trajectories 170A of the ribs 166A may be parallel with one another as shown in <FIG>; however, in other embodiments, the trajectories 170A of some of the ribs 166A may be non-parallel. Note, the term "trajectory" may describe a centerline that follows along a length of a feature, where that length is greater than other dimensions (e.g., a width and/or a thickness) of the feature. Each trajectory 170A of <FIG> is straight-line trajectory. However, in other embodiments, the trajectory 170A of one or more of the ribs 166A may alternatively be a curved or otherwise convoluted line trajectory.

The second layer 164A may be a flat or non-contoured layer. The second layer 164A of <FIG>, for example, is configured without any structural reinforcements (e.g., ribs). More particularly, this second layer 164A is configured with a flat, planar and/or uninterrupted exterior surface <NUM>, which exterior surface <NUM> is opposite the exterior surface <NUM>. Of course, where the acoustic panel element 156A is configured as one of the arrays of corrugations <NUM>, the exterior surface <NUM> may include a plurality of bends therein. However, each portion of the exterior surface <NUM> associated with a baffle <NUM> and/or a septum <NUM> may be flat, planar and/or uninterrupted as shown, for example, in <FIG>, <FIG>, <FIG>, and <FIG>-.

Referring again to <FIG>, the second layer 164A is bonded and/or otherwise attached to the first layer 162A. This attachment may be a direct attachment as shown in <FIG>. Alternatively, the attachment may be an indirect attachment with, for example, one or more intermediate layers between the first layer 162A and the second layer 164A.

<FIG> illustrate a portion of the acoustic panel element 156B configured with the multi-layered structure 158B. This multi-layered structure 158B includes a first layer 162B (e.g., ply of material) and a second layer 164B (e.g., ply of material).

The first layer 162B may be a contoured layer. The first layer 162B of <FIG>, for example, is configured with one or more of the structural reinforcements 160B. Each structural reinforcement 160B of <FIG> is configured as a rib 166B. Each rib 166B may be a stamped, molded and/or otherwise formed in the first layer 162B to project out from the exterior surface <NUM> / side of the acoustic panel element 156B / multi-layered structure 158B. Each rib 166B extends along a trajectory 170B, where the trajectories 170B of the ribs 166B may be parallel with one another as shown in <FIG>; however, in other embodiments, the trajectories 170B of some of the ribs 166B may be non-parallel.

The second layer 164B may also be a contoured layer. The second layer 164B of <FIG>, for example, is configured with one or more of the structural reinforcements 160C. Each structural reinforcement 160C of <FIG> is configured as a rib 166C. Each rib 166C may be a stamped, molded and/or otherwise formed in the second layer 164B to project out from the exterior surface <NUM> / side of the acoustic panel element 156B / multi-layered structure 158B. Each rib 166C extends along a trajectory 170C, where the trajectories 170C of the ribs 166C may be parallel with one another as shown in <FIG>; however, in other embodiments, the trajectories 170C of some of the ribs 166C may be non-parallel. The trajectories 170C of the ribs 166C in the second layer 164B may also be parallel with the trajectories 170B of the ribs 166B in the first layer 162B; however, in other embodiments, the trajectories 170B and 170C of some of the ribs 166B and 166C may be non-parallel as shown, for example, in <FIG>. As shown in <FIG>, each rib 166C in the second layer 164B may be (e.g., longitudinally, laterally and/or vertically) aligned with a closest one of the ribs 166B in the first layer 162B. The rib 166C thereby completely overlaps the rib 166B. However, in other embodiments, the rib 166C may partially overlap the rib 166B as shown, for example, in <FIG>. In still other embodiments, the rib 166C may not overlap any of the ribs 166B as shown, for example, in <FIG>.

Referring again to <FIG>, the first layer 162B is bonded and/or otherwise attached to the second layer 164B. This attachment may be a direct attachment as shown in <FIG>. Alternatively, the attachment may be an indirect attachment with, for example, one or more intermediate layers between the first layer 162B and the second layer 164B.

Each structural reinforcement <NUM> describe above includes a single discrete rib 166A-C (generally referred to as "<NUM>"). However, in other embodiments, one or more of the structural reinforcements <NUM> in the first layer 162C-F and/or in the second layer 164C-F may each include a plurality of ribs as shown, for example, in <FIG>. The structural reinforcement 160D of <FIG>, for example, includes a first rib 166D and a second rib 166E. The first rib 166D extends along a first trajectory 170D and the second rib 166E extends along a second trajectory 170E. The first trajectory 170D and the second trajectory 170E are straight-line trajectories; however, in other embodiments, one or both of these trajectories 170D and 170E may alternatively be curved or otherwise convoluted line trajectories. The first trajectory 170D of the first rib 166D is non-parallel with the second trajectory 170E of the second rib 166E. The first trajectory 170D and the first rib 166D of <FIG>, for example, are perpendicular and coincident with the second trajectory 170E and the second rib 166E. Of course, in other embodiments, an included angle between the first and the second trajectories 170F and <NUM> and ribs 166F and <NUM> of the structural reinforcement 160E may be acute (or obtuse) as shown in <FIG> for example. Referring again to <FIG>, the first rib 166D intersects and thereby runs into the second rib 166E. The first rib 166D of <FIG>, for example, bisects the second rib 166E and the second rib 166D bisects the first rib 166E.

Each of the ribs 166A-G (generally referred to as "<NUM>") shown in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG> and <FIG> is configured as a hollow rib. The rib 166A of <FIG> defines an empty channel <NUM> in an interior side of the first layer 162A such that this channel <NUM> separates the rib material of the first layer 162A from the adjacent material of the second layer 164A, or the channel <NUM> of <FIG> may be alternatively filled with another material to provide a solid rib, which may be perforated or non-perforated. However, in other examples, falling outside the wording of the claims, one or more of the ribs <NUM> described herein may be configured as a solid rib <NUM> as shown, for example, in <FIG>. In yet further embodiments, one or more of the ribs <NUM> described herein may be configured as a perforated rib 166A or non-perforated rib 166B with a solid opposite surface <NUM> or a perforated opposite surface <NUM> as shown, for example, in <FIG>.

In some embodiments, each layer <NUM>, <NUM> of the multi-layered structure <NUM> may be configured from composite material. Each layer <NUM>, <NUM>, for example, may be formed from one or more plies of fiber reinforcement within a polymer matrix. The one or more plies of fiber reinforcement within the first layer <NUM> may be discrete (e.g., not touching and/or formed part of) the one or more plies of fiber reinforcement within the second layer <NUM>. For example, each layer <NUM>, <NUM> may be laid up in a separate step. However, the present disclosure is not limited to the foregoing exemplary materials and/or layup.

In some embodiments, the first layer <NUM> and the second layer <NUM> may each partially define one or more of the cavities <NUM>. For example, referring to <FIG>, the first layer <NUM> may partially define one or more of the sub-cavities 128A. In these embodiments, the first layer ribs <NUM> project partially into the cavities <NUM> and, more particularly, the sub-cavities 128A. The second layer <NUM>, by contrast, may partially define one or more of the opposing sub-cavities 128B. However, in other embodiments, the first and the second layers <NUM> and <NUM> may be reversed such that the first layer <NUM> may partially define one or more of the sub-cavities 128B and the second layer <NUM> may partially define one or more of the opposing sub-cavities 128A. Accordingly, the first layer ribs <NUM> will project partially into one or more of the sub-cavities 128B.

Claim 1:
A structured panel (<NUM>), comprising:
a first skin (<NUM>) with a plurality of perforations (<NUM>);
a second skin (<NUM>); and
a core (<NUM>) forming a plurality of cavities (<NUM>) vertically between the first skin (<NUM>) and the second skin (<NUM>), each of the cavities (<NUM>) respectively fluidly coupled with one or more of the perforations (<NUM>), and the cavities (<NUM>) comprising a first cavity (<NUM>),
characterised in that
an element (<NUM>; 156A; 156B) of the core (<NUM>) is configured with a multi-layered structure (<NUM>; 158A; 158B), the multi-layered structure (<NUM>; 158A; 158B) includes a first layer (<NUM> A-F) and a second layer (<NUM> A-F) attached to the first layer (<NUM> A-F), and the first layer (<NUM> A-F) is configured with a first rib (166A; 166B; 166D; 166F) projecting into the first cavity (<NUM>), wherein the first rib (166A; 166B; 166D; 166F) defines a channel (<NUM>) in an interior side of the first layer (162A-F) such that the channel (<NUM>) separates the material of the first layer (162A-F) from adjacent material of the second layer (164A-F), and wherein the channel (<NUM>) is an empty channel (<NUM>) or is filled with another material.