GB2024380A - Acoustic linings for fluid flow ducts - Google Patents

Abstract

Multi-layer acoustic linings for fluid flow ducts include Helmholtz- type resonators, the necks of which are not identifiable as discretely manufactured components distinct from other parts of the lining structure. A core facing layer (7) overlies a compartmented airspace core structure (5) and comprises a number of elongate sheet pieces (15) which are provided with flanges (19). The flanges extend into the core structure, cooperating therewith to form Helmholtz resonator necks (9) within compartments (11) of the core structure. Other configurations are shown and described, including ones capable of resonating in the Helmholtz mode to more than one frequency, and others incorporating both Helmholtz and tube-type resonators. One specific use of such linings is in the intake or exhaust ducts of gas turbine aeroengines. <IMAGE>

Description

SPECIFICATION Multi-layer acoustic linings The present invention relates to multiwlayer or "sandwich" type acoustic linings for fluid flow ducts, such as the intake or exhaust ducts of gas turbine aeroengines. The invention particularly relates to acoustic linings which incorporate resonators of the Helmholtz type.

Acoustic linings incorporating resonators of the Helmholtz type usually comprise an impermeable backing layer, a compartmented-airspace core (which may be multi-layer construction), and a facing sheet through which sound energy from the fluid flow duct can enter the core; some or all of the core compartments function as Helmoltz resonator volumes because sound energy enters them via structures which act as Helmholtz resonator necks.

The present invention arose from efforts to provide acoustic linings which incorporate resonators of the Helmholtz type but which can be manufactured from relatively fewer components than has hitherto been possible with such linings.

In particular it has been thought desirable to provide acoustic linings in which Helmholtz resonator necks, though present, are not identifiable as discretely manufactured components distinct from other parts of the structure.

It is also desirable to provide acoustic linings which will absorb noise in as broad a range of frequencies as possible. Some embodiments of the present invention make a contribution towards this objective in that the linings are so constructed that they are capable of resonating in the Helmholtz mode to more than one frequency.

According to the present invention, a multilayer acoustic lining for fluid flow duct includes Helmholtz-type resonators and has a core facing layer which overlies a compartmented airspace core structure, said facing layer comprising a plurality of elongate sheet pieces having flanges, which flanges extend into said core structure and cooperate with said core structure to define Helmholtz resonator necks within compartments of said core structure.

In more detail the lining comprises: an impermeable backing layer; a compartmented-airspace core structure; a core facing layer overlying the core structure; and Helmholtz resonator necks connecting the interiors of compartments in the core structure to apertures in the core facing layer, whereby said compartments are effective as Helmholtz resonators: wherein the core facing layer comprises a plurality of elongate sheet pieces disposed in spaced-apart side-by-side relationship, the apertures in the core facing layer being defined between adjacent side edges of adjacent sheet pieces, at least one side edge of each sheet piece being in the form of a flange extending into the core structure as part of each one of a plurality of the Helmholtz resonator necks, the remaining parts of said necks being composed of cooperating portions of the core structure.

Both side edges of the sheet pieces may be in the form of a flange extending into the core structure, the sheet pieces therefore being channel-shaped in cross-section. Adjacent flanges of adjacent sheet pieces may thus cooperate to form complementary portions of the Helmholtz resonator necks.

The core compartments may be defined by core wall formations which span the thickness of the core structure between the backing layer and the core facing layer. Compartments which are effective as Helmholtz resonators consist of a single compartment or a group of acoustically interconnected compartments and each Helmholtz resonator neck comprises cooperating portions of the sheetpiece flanges and the wall formations. The core structure comprises at least one layer of compartments.

It is advantageous to provide an acoustic lining whose resonator compartments have two or more resonant frequencies in the Helmholtz mode because, in dealing with noise comprising a plurality of frequencies, the noise absorption efficiency per unit length of such an acoustic lining is greater than that of an acoustic lining in which each compartment has only one resonant frequency in the Helmholtz mode.

Embodiments of the present invention can be constructed to be capable of resonating in the Helmholtz mode to more than one frequency. This can be done by providing that the core compartments and/or the Helmholtz resonator necks are not all of the same volume. For example, the sheet pieces can be arranged and dimensioned such that the acoustic lining is provided with at least two sets of Helmholtz resonator necks, the necks of one set being configured differently from the necks in the other set so as to contain differing volumes of fluid.

The required differences in volume of the resonator necks can be achieved by appropriate selection of the lengths of the resonator necks (by means of appropriate dimensioning of the flanges of the sheet pieces with respect to the depth of their penetration into the core structure), or the spacings between the sheet pieces, or their dispositions relative to the underlying compartments, or any combination thereof.

ln order to provide acoustic linings capable of absorbing both high and low frequency bands, the core facing layer may be provided with additional apertures in the form of perforations in the sheet pieces, whereby compartments immediately underlying the perforations are rendered effective as tube-type resonators or the like.

In accordance with the invention, the disposition of the sheet pieces relative to the underlying comparments, or the spacing between the sheet pieces or the dimensions thereof, may be selected to produc * l variety of different configurations of acoustic lining, for example as outlined in the following paragraphs.

The core structure may comprise, for example, a single layer of substantially identical compartments. The longitudinal dimensions of the sheet pieces of the core facing layer are oriented in the same direction as rows of compartments in the underlying array of compartments, each such row having substantially the same width as the others. For the purposes of the following paragraphs, the core wall formations necessary to produce this regular array of compartments comprise those which form the dividing walls between each row of compartments and its adjacent rows, and those others which form walls extending transversely of the rows.

Where the width of each sheet piece in the core facing layer is appreciable less (by an appropriate amount) that the width of each underlying row of compartments, it is possible to dispose the sheet piece relative to the rows of compartments so that each compartment in each row is provided either with one Helmholtz resonator neck or with two such necks.

In order to provide rows of compartments in which each compartment has one Helmholtz resonator neck, judiciously sized channel-shaped sheet pieces are disposed so that each sheet piece straddles one of the aforesaid dividing walls, such that adjacent flanges of adjacent sheet pieces form the necks in cooperation with core wall formations which extend transversely of the flanges.

Taking a case in which it is desired to achieve differences in volume of the resonator necks at least partially by means of variation of the lengths of the resonator necks, the channel-shaped sheet pieces can be provided with one flange which extends more deeply into the core structure than the other flange of the same sheet piece, the sheet pieces being disposed so that adjacent flanges of adjacent sheet pieces extend into the core structure to the same depth as each other, whereby some rows of compartments are provided with longer resonator necks than the other rows.

On the other hand, taking a case in which it is desired to achieve differences in volume of the resonator necks at least partially by variation of the spacing between the sheet pieces, one way of achieving this is by utilising sheet pieces which are all of identical width, their longitudinal centrelines being laterally offset by appropriate amounts from registration with the faying edges of the aforesaid dividing walls.

In order to provide rows of compartments in which two necks are located on opposing sides of each compartment, judiciously sized channelshaped sheet pieces are disposed centrally of corresponding rows of compartments such that their flanges form the necks in cooperation with the aforesaid dividing walls and with those core wall formations which extend transversely of the flanges.Taking a case in which it is desired to achieve differences in volume of the resonator necks at least partially by means of variation of the lengths of the resonator necks, the channelshaped sheet pieces can be provided with one flange which extends more deeply into the core structure than the otherflange of the same sheet piece, the sheet pieces being disposed centrally of the rows of compartments such that their flanges form the necks in cooperation with the aforesaid dividing walls and with those core wall formations which extend transversely of the flanges, whereby compartments are provided with two resonator necks of differing lengths.On the other hand, taking a case in which is desired to achieve differences in volume of the resonator necks at least partially by appropriate disposition of the sheet pieces relative to the underlying compartments, the channel-shaped sheet rieces can be disposed so that one flange of any particular sheet piece is closer to its respective dividing wall than the other flange of the same sheet piece is to its respective dividing wall, whereby each compartments in each row of compartments is provided with two resonator necks of differing widths.

Where the width of each sheet piece is appreciably greater (by an appropriate amount) than the width of each underlying row of compartments, but appreciably less (by an appropriate amount) than twice the width of each underlying row of compartments, it is possible to dispose the sheet pieces relative to the rows of compartments either so that each compartment in each row is provided with a neck, or so that only those compartments in each alternate row of compartments are each provided with a neck.

In order to provide each compartment in each row with a neck, judiciously sized channel-shaped sheet pieces are disposed so as to straddle alternate ones of the aforesaid dividing walls such that each flange of each sheet piece forms the necks in cooperation with portions of those dividing walls which are not so straddled and with those core wall formations which extend transversely of the flanges.

In order to provide each compartment in each alternate row of compartments with a neck, judiciously sized ch-annel-shaped sheet pieces are disposed so that alternate rows of compartments are straddled by respective sheet pieces such that adjacent flanges of adjacent sheet pieces form the necks in cooperation with those wall formations which extend transversely of the flanges.

It would of course be additionally possible to vary the Helmholtz resonator neck volumes in ways similar to those previously mentioned, and this observation can also be applied to the following introductory paragraphs.

Where some compartments are provided with necks and some are not, compartments which are not so-provided may be acoustically connected to one or more adjacent compartments which are so provided, thereby to increase the available resonant volume, and/or they may be utilized as quarter-wave resonators or the like as aforesaid.

It is possible to provide each compartment in each row of compartments with resonator neck using sheet pieces each having only one flange., the width of the sheet pieces being appreciably less (by an appropriate amount) than the common width of each underlying row of compartments. In this case the sheet pieces are disposed so that the unflanged edge of each sheet piece is in substantial registration with the faying edge of one of the aforesaid dividing walls, each flange forming the necks in a row of compartments in cooperation with another of the aforesaid dividing walls, and with those wall formations which extend transversely of the flanges.

By appropriate variations in the design of the core structure, it is possible to produce further variations of acoustic lining design according to the invention. For example, the core structure may comprise first and second layer of compartments.

Preferably, the first layer comprises compartments effective as Helmholtz resonators and the second layer comprises compartments effective as quarter-wave resonators or the like, the second layer being superimposed on the first layer and separated therefrom by means of an interlayer composed of sheet material; in this design the core facing layer is perforated and overlies the second layer of compartments, the sheet pieces are channel-shaped in cross-section, having their side edges in the form of flanges, the flanges extend through the thickness of the second layer of compartments to meet with the sheet material of the interlayer, whereby rectangular duct-like enclosures are formed between the channelshaped sheet pieces and the interlayer, the second layer of compartments being contained within these enclosures, the necks of the Helmholtz resonators in the first layer of compartments are formed by the flanges of adjacent sheet pieces in conjunction with core wall formations extending transversely of the flanges, and the interlayer has apertures therein whereby the Helmholtz resonator volume in the first layer of compartments communicate with their respective resonator necks.

Preferably, accustic linings such as those mentioned above should be installed in an axial fluid flow duct su h that the sheet pieces extend their major dimensions circumferentially around the duct. However, if the acoustic lining is provided with an additional sound permeable facing sheet supnrimposed on the core facing layer, as described in our co-pending patent applications nos. 41134/77 and 17147/78, circumferential orientation of the sheet pieces is made less necessary.

Specific embodiments of the invention will now be described, bl way of example only, with reference to the accompanying drawings, in which: Figure 1 A isa part-sectional perspective view showing partof an acoustic lining incorporating resonators of he Helmholtz type and provided with a core fadng layer comprising a plurality of sheet pieces which are channel-shaped in cross section; Figure 1 B isa part-sectional perspective view showing part cf an acoustic lining similar to that shown in Figure 1A, modified so as to be responsive to two frequency bands in the Helmholtz mode; Figure 2A is a cross-sectional scrap view of thea acoustic lining shown in Figure 1A; Figure 2B is a cross-sectional scrap view of the acoustic lining shown in Figure 1 B;; Figure 2C is an alternative configuration to that shown in Figure 2B; Figures 3a and 3b show one way in which the core structure of the acoustic lining in Figures 1 and 2 may be produced; Figures 4b and 4b show an alternative way of producing the core structure; Figure 5 is a part-sectional perspective view of another acoustic lining having channel-shaped sheet pieces in its core facing layer but in which the core structure is of the honeycomb type; Figure 6 is a cross-sectional scrap view of the acoustic lining shown in Figure 5; Figures 7 to 11 B show various other designs of acoustic linings all having core facing layers comprising channel-shaped sheet pieces; Figure 12 shows an acoustic lining in which the sheet pieces of the core facing layer are L-shaped in cross-section;; Figure 13 shows an acoustic lining having a two-layer core structure, one layer being composed of compartments acting as Helmholtz resonators, and the other being composed of compartments acting as quarter-wave resonators or the like; Figure 14 shows how an additional sound permeable facing sheet can be supported on the core facing layer of an acoustic lining which is otherwise identical to that shown in Figure 10; and Figure 1 5 shows an alternative way of supporting the addition facing sheet on the core facing layer.

Referring now to Figures 1 and 2, a multi-layer acoustic lining 1 suitable for incorporation in the intake or exhaust ducts of gas turbine aeorengines comprises an impermeable backing layer 3, a compartmented airspace single-layer core structure 5, and a core facing layer 7 overlying the core structure 5. The acoustic lining 1 incorporates resonators of the Helmholtz type, because Helmholtz resonator necks 9 connect the interiors of compartments 11 within the core structure 5 to apertures 13 in the core facing layer 7, the compartments 11 therefore being effective as Helmholtz resonators.

Compartments 11 in core structure 5 are arranged in a regular array and are defined by core wall formations comprising two sets of wall members 21, 22 respectively which span the thickness of the core structure 5 between backing layer 3 and core facing layer 7. In this particular embodiment, the wail members in each set are disposed parallel to each other and the sets intersect each other at right angles, thus forming compartments in the shape of rectangular prisms.

The core facing layer 7 comprises a number of elongate sheet pieces 1 5 arranged in side-by-side spaced-apart relationship. Their longitudinal dimensions are oriented in the same direction as the rows of compartments in the underlying array of compartments, each row having substantially the same width as the others,. The apertures 13 in core facing layer 7 are defined between adjacent side edges 1 7 of adjacent sheet pieces 1 5 and portions 23 of wall member 22. The side edges 1 7 of the sheet pieces 15 are each in the form of a flange which extends into core structure 5 as part of each one of a number of the Helmholtz resonator necks 9.Since both side edges 17 or the sheet pieces 1 5 are in the form of a flange, the sheet pieces 1 5 are channel-shaped in crosssection.

It will be seen that each Helmholtz resonator neck 9 consists of cooperating portions of adjacent sheet piece flanges, (which form the long sides 1 9 of each neck 9), together with portions of wall members 22 extending transversely of the flanges (which form the short sides 23 of each neck 9). Because of the specific shapes of the elements comprising the acoustic lining 1, the apertures 1 3 and the Helmholtz resonator necks are rectangular in plan view.

Since this configuration provides rows of compartments 11 having one Helmholtz resonator neck 9 for each compartment, the width of each sheet piece 1 5 must obviously be appreciably less (by an appropriate amount) than the width of one of the underlying rows of compartments, the sheet pieces 1 5 being disposed so that each one straddles one of the wall members 21 which form the dividing walls between each row of compartments and its adjacent rows.

Figures 1 B and 2B show an acoustic lining 2 which is similar to lining 1 shown in Figure 1A except that it is capable of resonating to two frequencies in the Helmholtz mode. Since the structure of the lining 2 is so similar to that of lining 1, the common structure will not be described in detail. However, it is pointed out that resonance at two frequencies is achieved by making necks 1 0, 12 of different lengths: because of this different configuration they contain different volumes of air. The differing lengths of necks are achieved by dimensioning flanges 36, 38 so as to extend into core structure 6 by differing amounts. Thus, the flanges 38 extend more deeply into the core structure 6 than the other flanges 36.

Those flanges 36 and 28 which are adjacent to each other extend into the core structure to the same depth as each other, so that each alternate row of compartments 1 6 is provided with longer resonator necks 12 than the other rows of compartments 14.

In Figures 1 B and 2B, the differing volumes of the resonator necks 10 and 12 are achieved by making them of different lengths. However, it is also possible to achieve differences in resonator neck volume by variation of the spacing between the sheet pieces, and this is illustrated in Figure 2C.

In Figure 2C it is shown that the widths, w of the resonator necks 40 are smaller than the widths Wof necks 42. The dimensions wand W are of course equal to the spacings between adjacent sheet pieces 44 and 44'. instead of making sheet pieces 44, 44' of different widths, they are of identical width (being less than the width of each underlying row of compartments, as in Figures 1 A to 2B), the differing dimensions w and Wbeing achieved by laterally offsetting the longitudinal centrelines of the sheet pieces by appropriate amounts dfrom registration with the frying edges 46 of the dividing walls.

It will also be realised that instead of varying the spacing of identical sheet pieces to achieve resonator necks of differing volumes as illustrated in Figure 2C, the sheet pieces themselves could be manufactured in different widths, or alternatively the widths of the rows of compartments could be varied. However, this would have the consequence of increasing the cost and complexity of the manufacturing process.

The core structure 5 (Figures 1A, 2A) may be fabricated from any one of a number of different types of wall members.

For example, wall memblers 21, 22 in Figures 1 A and 2A may comprise appropriately slotted strips of sheet material as shown in Figures 3a and 3b. Figure 3a shows a typical wall member 21 provided with a set of slots 25, all of length A.

Figure 3b shows a typical wall member 22 provided with a first set of slots 27, also of length A, and a second set of slots 29, all of length B. The sets of wall members 21 and 22 are both of depth D and are assembled together at right angles to each other so that portions 3'1 of wall members 21 fit inside slots 27 of wall nnembers 22, lvhilst portions 33 of wall members 22 fit inside slots 25 of wall members 21.

To complete manufacture of the acoustic lining, the wall members 21,22 are secured to the backing layer 3 and the flanges of the channelshaped sheet pieces 1 5 are inserted into slots 29 and secured therein.

Figures 4a and 4b show one alternative way of constructing the core structur e 5 from a plurality of identical wall members 35 comprising corrugated strips of sheet mat erial. Figure 4a shows a perspective view of part of a wall member 25. In order to form compartments having a rectangular prism shape, the corrugations have a square-wave configuration. The wail members 35 are assembled to form the core 'structure 5 by butting corners 37 of the corrucations together as shown in Figures 46 and bounding them as necessary to produce a structurt3,cf the required rigidity. All portions 39 of wall rneambers 35 are provided with slots 41 (shown in trotted lines). for receipt of the flanges of the sheet pieces 15.

Preferably, the slots 41 are machi-ned as required using slitting discs or other suitalal e means after the core structure has been made from the wall members 35.

In the above description, the c:ore structure 5 is fabricated from intersecting or cc)rrugated wall members which define compartments 11 shaped like rectangular prisms. However, it would equally be possible to utilise an array of honeycomb cells for core structure 5 instead, using the usual well known technique for fabrication of such structures.

Figures 5 and 6 show an acoustic lining 43 in which the core structure 45 comprises just such an array of honeycomb cells 47. As in Figures 1 and 2, the sheet pieces 49, which collectively comprise the core facing layer 51, are channelshaped, having their side edges 53 in the form of flanges which fit into slots 55 in the cell walls and cooperate with those walls to form Helmholtz resonator necks 57. In order to produce an acoustic lining capable of absorbing a high frequency band as well as a low frequency band, the sheet pieces have perforations 50 so that those compartments 47 which underlie the perforations 50 can act as tube-type resonators.

It will be noted from Figures 5 and 6 that the widths of the sheet pieces and their dispositions are such that alternate rows of cells are straddled by respective sheet pieces, whereby only each alternate row of cells is provided with a Helmholtz resonator neck in each cell in the row.

Figure 7 is a view similar to Figure 6 showing a section through an acoustic lining 59 in which the core structure 61 is like that described in Figures 1 to 4 but in which, as in Figures 5 and 6, only those cells or compartments in each alternate row of compartments are provided with resonator necks 63. As in Figures 5 and 6 the width of each sheet piece 65 is appreciably greater (by an appropriate amount) than the width W of each underlying row of compartments, but appreciably less (by an appropriate amount) than twice the width W, the channel-shaped sheet pieces 65 being disposed so that alternate rows of the compartments are straddled by respective sheet pieces such that adjacent flanges 67 of adjacent sheet pieces 65 form the necks 63 in cooperation with those wall members 69 which extend transversely of the flanges 67.

In Figures 1, 2, 4, 6 and 7 the sheet -pieces 17, 49 and 65 respectively are disposed relative to the underlying compartments so as to provide them with resonator necks which are located centrally with respect to the widths of the compartments.

However, as shown in Figures 8 and 9, it is equally possible, using, for example, components of the same size as those shown in Figures 1 and 2 and Figures 7 respectively, to dispose the channelshaped sheet pieces 71 and 73 so as to provide the compartments with resonator necks located at any position with respect to the widths W of the compartments, for example along one side of the compartments as shown.

It is also possible using channel-shaped sheet pieces 75 (Figure 10) of appropriate width to dispose them so that instead of straddling each alternate row of underlying compartments, they straddle only alternate ones of the dividing walls 79. Thus, each flange 79 of each sheet piece 75 forms a resonator neck 81 or 83 in cooperation with portions of those dividing walls 85 which are not straddled by the sheet pieces 75, and also with portions of those walls 87 which extend transversely of the flanges 79. Unlike the configurations shown in Figures 5 to 7 this arrangement provides each compartment in each row of compartments with a resonator neck 81 or 83, disposed along one side of the compartment as shown, the necks 81 and 83 being on opposing sides of the dividing walls 85.This construction thus provides each compartment with a neck, as for Figure 1, but requires fewer sheet pieces in the core facing sheet, thus giving a manufacturing advantage.

In Figure 1 lA is shown yet another acoustic lining configuration utilising channel-shaped sheet pieces 89 sized and disposed re"iafive to t'ne underlying compartments so as to provide each compartment in each row of compartments with two resonator necks 91,93 located on opposing sides of each compartment. Each sheet piece 89 has a width appreciably less (by an appropriate amount) than the width of the underlying row of compartments and is disposed centrally of the row so that its flanges 95 form the resonator necks 91, 93 in cooperation with dividing walls 97 between rows of compartments and walls 99 which extend transversely of the flanges 99.

In Figure 11 B, an acoustic lining is shown having the same basic structure as that of Figure 1 1A, but differences in resonator neck volume are achieved by variation of their lengths. Two necks 48, 52 of different lengths are located on opposing sides of each compartment 54, this structure being achieved by making identical channel-shaped sheet-pieces 56 "odd-legged" as in Figures 1 B and 2B; i.e. one flange 58 of each sheet-piece 56 extends more deeply into the core structure than the other flange 60.

By anology with Figure 2C, it is obvious from Figure 11 B that the required differences in volume of the resonator necks could also be achieved, either partially or completely, by positioning the sheet pieces so that one flange is closer to its respective dividing wall than the other flange is to its dividing wall, thus producing a structure in which each compartment in each row of compartments is provided with two resonator necks of different widths.

The provision of two necks for each core compartment has the effect of providing the compartments with a higher overall resonant frequency as compared with core compartments provided with only one neck, the linings under comparison otherwise being of identical construction and dimensions. This higher overall resonant frequency results when, at that frequency, two necks effectively act together as a single volume in exciting the whole compartment into resonance.The embodiment of Figure 11 B provides an acoustic lining capable of absorbing noise in three frequency bands centred respectively on a first çonant frequency produced by the combl.,rion of necks 52 with compartments 54, a second resonant frequency produced by the combination of necks 48 with compartments 54, and a third resonant frequency produced by the combination of necks 52 and 48 with compartments 54.

Figure 1 2 illustrates that it is possible to provide each compartment in each row of compartments with a resonator neck using sheet pieces 101 each having one flange 103, the width of each sheet piece 101 being appreciably less (by an appropriate amount) than the width of the underlying row of compartments. In this case the unflanged edge 105 of each sheet piece 101 is positioned in substantiai registration with the faying edge 107 of an underlying dividing wall 109 which separates two adjacent rows of compartments. Each flange 103 cooperates with the dividing walls 109 and the walls 113 which extend transversely of the flanges 1 03 to form the resonator necks 111 along one side of the compartments.

Figures 1 to 1 2 are concerned with acoustic linings having core structures comprising only one layer of compartments or cells. Figure 13 shows an acoustic lining 11 5 in which the core structure is composed of two layers 11 7 and 11 9. The first layer 11 7 comprises compartments 121 which are effective as Helmholtz resonators and the second layer 11 9 comprises smaller compartments 123. The compartments 123 are designed to be effective as quarter-wave resonators or the like, the channel-shaped sheet pieces 127 which comprise the core facing layer being suitably perforated.Layer 11 9 is superimposed on layer 11 7 but the compartments 1 23 are acoustically separated from compartments 121 by an interlayer 125 made of a sheet material. The flanges 1 29 of the sheet pieces 127 extend through the thickness of the layer 119 of compartments 123 to meet with the interlayer 125, rectangular duct-like enclosures thus being formed between the sheet pieces 1 27 and interlayer 125, the compartments 123 being contained within these enclosures.It will be seen that the necks 131 of the Helmholtiresonator compartments 121 are formed by the flanges 129 of adjacent sheet pieces 127 in conjunction with walls 133 which extend transversely of the flanges 1 29. Resonator necks 1 31 communicate with resonator compartments 1 21 via suitable apertures 135 in the interlayer 1 25.

In the arrangement shown in Figure 13 it is convenient that the second layer 11 9 of the core structure is a honeycomb structure of the type shown in Figures 5 and 6, compartments 123 being in fact individual honeycomb cells.

In the above-described constructions the lengths and widths of the resonator necks are important design parameters because the greater the volume of a neck, the larger the mass of air contained in it and therefore the lower the resonant frequency of the underlying resonator volume. Thus, the widths and lengths of the necks will be chosen according to the frequencies it is desired to eliminate, as will the volumes of the compartments.

Note also that the greater the widths of the apertures in the core facing layer the greater the turbulence generated in any flow of fluid over it.

Thus, when designing such linings for incorporation in, for example, the flow ducts of gas turbine aeroengines, care must be taken that the widths of the resonator necks, which of course determine the widths of the sound receiving apertures, are not so great as tp seriously increase the internal drag of the engine and hence reduce its efficiency by an unacceptable amount.

It is assumed here that fluid flow over the core facing layer is in a direction substantially at right angles to the longitudinal extent of the sheet pieces.

One way of producing an aerodynamically better surface for acoustic linings according to the present invention is to superimpose an additional sound permeable but aerodynamically smoother facing layer on the core facing layers described with reference to Figures 1 to 1 3. An an example, Figure 14 shows an acoustic lining identical to that shown in Figure 10 with the exception that a sound permeable facing sheet 137 overlies the sheet pieces 75 which make up the core facing layer, and is spaced from them by means of wires, rods or thin tubes 1 39. Alternatively the required spacing could be achieved by producing raised ridges 141 in the sheet pieces 75 as shown in Figure 15, these ridges performing the same function as the wires etc. 139, but at a saving in structural weight, complexity and manufacturing costs.The facing sheet 1 37 is aerodynamically smoother than the core facing layer, which of course has a number of turbulence-producing apertures in it in the form of the entrances to the resonator necks 81,83.

As explained in our co-pending application number 41134/77, the sound permeable facing sheet may be made of a sheet material which is inherently permeable to air, such as fibrous metallic sintered material or felt, or an airimpermeable sheet material rendered permeable by having many small holes formed through its thickness, such as perforated sheet metal, Although the effect of spacing facing sheet 137 away from sheet pieces 73 is to reduce the total acoustic resistance of the facing sheet plus the core facing layer as compared with the case in which the facing sheet is in contact with the core facing layer, the latter configuration could, of course, nevertheless be adopted with some penalty in increased acoustic resistance of the acoustic lining.

Referring more generally to the forgoing descriptions of embodiments, it should be noted that in the case of those configurations of acoustic lining in which some but not all compartments in a particular layer of core structure are provided with a resonator neck, those compartments which are not so provided may be acoustically connected to one or more of those adjacent compartments which are so provided in order to increase the avaiiable resonant volume and thereby increase the number of frequencies which can be absorbed.

Alternatively (as shown in Figures 5 and 6), or in addition, those compartments which are not provided with resonator necks may be utilized, for example, as normal quarter-wave resonators if suitable perforations are provided in the core facing layer, thus making possible the construction of compact acoustic liners having the capability of absorbing at least two frequency bands, i.e. a high frequency band and a low frequency band.

Although versions of the invention involving acoustic linings having Helmholtz resonator necks of various volumes have only been specifically described in connection with Figures 1,2 and 11, the same principle can of course be applied to any of the other embodiments of the invention described above, in order to produce acoustic linings having a wider frequency response in the Helmholtz mode.

It will be evident to those skilled in the art that all the above-described embodiments can be constructed from various types and thicknesses of sheet metal according to well-known design criteria. However, the acoustic linings shown could also be built up using various polymers and/or fibre reinforced composite materials as also known.

The components of the acoustic linings described herein may be joined to each other by suitable means appropriate to the materials of which they are constructed, such as by brazing or suitable adhesive substances.

Claims (11)

1. A multi-layer acoustic lining for a fluid flow duct, said lining including Helmholtz-type resonators which are constructed such that the Heimholtz resonator necks are not identifiable as descretely manufacture components distinct from other parts of the lining structure, said lining having a core facing layer which overlies a compartmented airspace core structure, said facing layer comprising a plurality of elongate sheet pieces having flanges, which flanges extend into said core structure and cooperate with said core structure to define Helmholtz resonator necks within compartments of said core structure.

2. A multi-layer acoustic lining for a fluid flow duct comprising: an impermeable backing layer; a compartmented-airspace core structure; a core facing layer overlying the core structure; and Helmholtz resonator necks connecting the interiors of compartments in the core structure to apertures in the core facing layer, whereby said compartments are effective as Helmholtz resonators:: wherein the core facing layer comprises a plurality of elongate sheet pieces disposed in spaced apart side-by-side relationship, the apertures in the core facing layer being defined between adjacent side edges of adjacent sheet pieces, at least one side edge of each sheet piece being in the form of a flange extending into the core structure as part of each one of a plurality of the Helmholtz resonator necks, the remaining parts of said necks being composed of cooperating portions of the core structure.

3. An acoustic lining according to claim 1 or claim 2 in which both side edges of the sheet pieces are in the form of a flange extending into the core structure, the sheet pieces being channelshaped in cross-section.

4. An acoustic lining according to claim 3 in which adjacent flanges of adjacent sheet pieces cooperate to form complementary portions of the Helmholtz resonator necks.

5. An acoustic lining according to any one of claims 1 to 4 in which the core compartments effective as Helmholtz resonators are not all of the same volume, thereby rendering the lining capable of resonating in the Helmholtz mode to a plurality of frequencies.

6. An acoustic lining according to any one of claims 1 to 5 in which the resonator necks are not all of the same volume thereby rendering the lining capable of resonating in the Helmholtz mode to a plurality of frequencies.

7. An acoustic lining according to any one of claims 1 to 6 in which the core facing layer is provided with perforations in order to render effective as tube-type resonators or the like core compartments immediately underlying the perforations.

8. An acoustic lining according to any one of claims 1 to 7 in which some core compartments are adapted to act as tube-type resonators and some are adapted to act as Helmholtz-type resonators.

9. An acoustic lining according to any one of claims 1 to 8 in which at least some of the core compartments are acoustically connected with at least one other core compartment.

10. An acoustic lining according to any one of claims 1 to 9 in which an additional sound permeable facing layer is superimposed on the core facing layer.

11. Multi-layer acoustic linings substantially as described in this specification with reference to and as illustrated by the accompanying drawings.