Acoustic Absorption

Abstract

An acoustic panel (for absorbing sound) includes a first sheet with spaced microperforations, a second sheet with microperforations more widely spaced than the microperforations of the first sheet, and a first cellular core sandwiched between the first sheet and the second sheet. The panel can be spaced from a surface, such as a wall. A second cellular core can be provided between the second sheet and a third sheet. The third sheet is preferably solid without microperforations but can have microperforations. Noise Reduction Coefficient (NRC) can be 0.8.

Claims

1. An acoustic panel for absorbing sound, the acoustic panel comprising: at least a first sheet having spaced microperforations; a second sheet having microperforations more widely spaced than the microperforations of the first sheet; and a first cellular core sandwiched between the first sheet and the second sheet.

2. The acoustic panel of claim 1, wherein the microperforations provide a larger open area of the first sheet than a respective open area provided by the microperforations of the second sheet, the open area of each said sheet determined by diameter and/or spacing of the respective microperforations.

3. The acoustic panel of claim 2, wherein the larger open area of the first sheet is substantially provided by closer spacing of the microperforations of the first sheet compared to respectively wider spacing of the microperforations of the second sheet.

4. The acoustic panel of claim 3, wherein the first sheet includes at least some of the microperforations of a different diameter compared to the diameter of at least some of the microperforations of the second sheet.

5. (canceled)

6. The acoustic panel of claim 1, wherein there are substantially half as many microperforations per unit area through the second sheet than there are microperforations per unit area through the first sheet.

7. The acoustic panel of claim 6, wherein there is at least one microperforation in the first sheet for each said cell in the first cellular core.

8. The acoustic panel of claim 6, wherein there are approximately half as many microperforations per unit area in the second sheet as the number of cells per unit area in the first cellular core, whatever the respective microperforation diameters are in the first and second sheets.

9. The acoustic panel of claim 1, further including a third sheet spaced from the first sheet and the second sheet such that the second sheet is intermediate between the first sheet and the third sheet, and a second cellular core is between the second sheet and the third sheet.

10. The acoustic panel of claim 1, wherein the cells of the first cellular core are the same or smaller in diameter than cells of the second cellular core.

11. The acoustic panel of claim 1, wherein the cells of the first cellular core are of smaller depth to absorb relatively higher frequency acoustic waves in combination with the microperforated first sheet, than a total thickness of the panel in combination with the microperforated second sheet.

12. The acoustic panel of claim 1, wherein approximately 50% of the cells in the first cellular core are closed at their bases, or wherein approximately 50% of the cells in the first cellular core are open at their bases corresponding to the microperforations of the second sheet.

13. (canceled)

14. (canceled)

15. The acoustic panel of claim 1, wherein the respective microperforations are between 0.1 mm and 2.0 mm diameter, preferably between 0.1 mm and 1.0 mm diameter, more preferably between 0.3 mm and 0.8 mm diameter.

16. The acoustic panel of claim 1, wherein the cells of the respective first or second cellular core are bonded to a respective internal face of the respective sheet.

17. (canceled)

18. (canceled)

19. A method of absorbing multiple sound frequencies by employing a first microperforated sheet in association with a primary cell depth of a first cellular core to absorb a peak (high) frequency of sound, and a second microperforated sheet in association with a secondary cell depth provided by the first cellular core in combination with a second cellular core, or the first cellular core and an airgap, to additionally absorb a second (low) peak frequency.

20. The method of claim 19, including providing a larger open area of the microperforations of the first sheet than the microperforations of the second sheet.

21. The method of claim 19, including connecting a proportion of the cells of the first cellular core to cells of the second cellular core or by connecting a proportion of the cells of the first cellular core to a space between the second sheet and a rear surface to provide an increased resonance depth, the connecting provided by the microperforations in the second sheet.

22. The method of claim 21, including connecting approximately or substantially 50% of the cells of the first cellular core to respective cells of the second cellular core or to the space between the second sheet and the rear surface.

23. The method of claim 19, wherein at least twice as many said microperforations are provided per unit area in the first sheet than in the second sheet.

24. The method of claim 19, including providing at least some of the microperforations in the first sheet of a different diameter compared to the diameter of at least some of the microperforations of the second sheet.

25. (canceled)

26. The method of claim 19, including providing at least some of the microperforations in the first sheet of the same diameter compared to the diameter of at least some of the microperforations of the second sheet.

27. The method of claim 19, including providing at least one microperforation in the first sheet for each respective said cell in the first cellular core, and providing approximately half as many microperforations per unit area in the second sheet as the number of cells per unit area in the first cellular core, whatever the respective microperforation diameters are in the first and second sheets.

28. (canceled)

29. The method of claim 19, including providing a third sheet spaced from the first sheet and the second sheet such that the second sheet is intermediate between the first sheet and the third sheet, and a second cellular core is between the second sheet and the third sheet.

30. The method of claim 19, including providing the cells of the first cellular core of smaller depth in combination with the first sheet to absorb relatively higher frequency acoustic waves than a total thickness of the panel in combination with the microperforated second sheet.

31. The method of claim 19, including providing approximately 50% of the cells in the first cellular core closed at their bases, or providing approximately 50% of the cells in the first cellular core open at their bases corresponding to the microperforations of the second sheet.

32. (canceled)

33. (canceled)

34. A microperforated panel absorber comprising: a first sheet, a second sheet and a first core structure therebetween; the first sheet having microperforations; the second sheet having microperforations; the first core structure having primary cells.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0102] One or more embodiments of the present invention will hereinafter be described with reference to the accompanying drawings, in which:

[0103] FIG. 1 shows a known single layer honeycomb core acoustic panel with microperforations in a facing sheet and a solid rear sheet. NB. Perforation holes are shown perfectly in register with underlying cells for convenience only. There is no necessity to specifically align the microperforations of the sheets with the cells in any particular manner.

[0104] FIG. 2 shows a side sectional view of a single layer cellular core acoustic panel spaced from a solid rear surface such as a wall or ceiling, according to an embodiment of the present invention.

[0105] NB: microperforations in both the first and second microperforated sheets are shown perfectly in register with honeycomb cells in the cellular core layer for convenience only. There is no necessity to align first or second microperforated sheets with the cells in any particular way.

[0106] FIG. 3 shows an exploded view of a single layer cellular core acoustic panel with perforated front and rear sheets, according to an embodiment of the present invention.

[0107] FIG. 4a shows a side sectional view of a double cellular core acoustic panel with non-perforated rear (third) sheet, according to a further embodiment of the present invention.

[0108] FIG. 4b shows a side sectional view of a double cellular core acoustic panel with microperforated rear (third) sheet according to a further embodiment of the present invention.

[0109] FIG. 5 shows a graph of acoustic absorption from test results for an example of a known acoustic panel having one microperforated (front) face, a non-perforated back face and a single honeycomb cellular core sandwiched therebetween.

[0110] FIG. 6 shows a graph of test results of acoustic absorption for a dual core layer acoustic panel according to an embodiment of the present invention.

[0111] FIG. 7 shows a graph of test results of acoustic absorption for a single core layer acoustic panel with airgap behind, according to a further embodiment of the present invention.

[0112] FIG. 8 shows a graph of test results of acoustic absorption for a dual core layer acoustic panel according to a further embodiment of the present invention, optimised to give high Noise Reduction Coefficient (0.8).

[0113] FIG. 9 shows a graph of acoustic absorption from test results of an example of a known acoustic panel having a zone of reduced depth within part of the cellular core of the panel.

[0114] FIG. 10 shows a graph of absorption for a panel according to an embodiment of the present invention having first and second microperforated sheets bonded to each side of a first cellular core, with airgap behind, showing a clear double peak of absorption as tested to ASTM C423-08 standard in comparison with a variety of non-perforated honeycomb panels.

[0115] ASTM C423-08 is a standard test method for sound absorption and sound absorption coefficients by the reverberation room method.

[0116] FIG. 11 shows only panels Ayre #4, Ayres #5 and Ayres #6 from FIG. 11.

DESCRIPTION OF PREFERRED EMBODIMENT

[0117] FIG. 1 shows a known single core layer acoustic panel 10 having a facing sheet 12 with microperforations 14 (e.g. FIG. 5).

[0118] The panel 10 is backed by a solid rear sheet 16. A honeycomb core 20 is sandwiched between the facing and rear sheets. The core has cells 24 defined by cell walls 22. Each microperforation 14 enables sound to pass to one of the cells underlying the respective microperforation. Such a panel has absorption at and around a single peak frequency due to the fixed cell size and single central microperforation per cell.

[0119] FIG. 2 shows a side sectional view of an embodiment of the present invention. An acoustic panel 110 for absorbing sound includes a first sheet 112, a second sheet 116, a single layer cellular core 120 sandwiched therebetween and a solid surface 126 to the rear across an airgap (e.g. FIG. 8). Primary Cell Depth (PCD) and Secondary Cell Depth (SCD) are indicated. Preferably the The first sheet has microperforations 114 through to cells 124 of the cellular core 120. Each cell is defined by at least one cell wall 122.

[0120] The acoustic panel has a second sheet 116 with microperforations 118 at a greater spacing between the microperforations than the spacing of the microperforations of the first sheet. It will be appreciated that the microperforations in the second sheet are at a greater spacing than the respective spacing of the microperforations of the first sheet.

[0121] Spacing of the microperforations 114 need not be limited to only one perforation per cell. Spacing of the microperforations 118 preferably should be such that only half of the cells 124 have a respective perforation at their bases.

[0122] For one or more forms of the present invention, the microperforations in the first sheet and the microperforations in the second sheet (and/or in any third sheet etc.), may be of the same diameter. Alternatively, the microperforations in one said sheet may be of a different diameter to the microperforations in any other said sheet.

[0123] FIG. 3 shows an exploded view of an acoustic panel 210 according to an embodiment of the present invention. A first (facing) sheet 212 has microperforations 214. A second (rear) sheet 216 has microperforations 218 at a larger spacing between perforations than that of the first sheet.

[0124] A cellular core 220, having a thickness 228, is provided intermediate the first sheet 212 and the second sheet 216. In a finished product, the first sheet and the second sheet would each be bonded to the core.

[0125] FIG. 4a shows a side sectional view of a double core layer acoustic panel 310 for absorbing sound according to an embodiment of the present invention (e.g. FIG. 7).

[0126] The panel includes a first sheet 312 having microperforations 314, second sheet 316 also with microperforations 318, the first and second sheets sandwiching therebetween a cellular core 320 of cells 322. Each cell has at least one cell wall 324 and a cell depth (Primary Cell Depth ‘PCD’ or first cell depth).

[0127] The microperforations of the second sheet (which can be termed an intermediate sheet or layer or septum/septum sheet) are spaced at greater distances apart than the microperforations of the first sheet.

[0128] A second cellular core 328 having cells 330 with a least one cell wall 332 can be sandwiched between the second (intermediate) layer and a third sheet or rear layer 334. Each cell 330 has a cell depth (Secondary Cell Depth ‘SCD’ or second cell depth).

[0129] Preferably the third sheet is solid without perforations. However, it will be appreciated that the acoustic panel can have more layers of cellular core and intermediate microperforated sheets to selectively absorb more sound frequencies.

[0130] For example, FIG. 4b shows an embodiment of the acoustic panel 410 present invention having two layers of cellular core with a rear (third) sheet 434 with microperforations 436 therethrough to connect to an open space/air gap 438 to a solid surface 440 behind the acoustic panel.

[0131] The acoustic panel 410 includes a first sheet 412 having microperforations 414, second sheet 416 also with microperforations 418, the first and second sheets sandwiching therebetween a cellular core 420 of cells 422. Each cell has at least one cell wall 424.

[0132] A second cellular core 428 has cells 430 with a least one cell wall 432 can be sandwiched between the second (intermediate) sheet and a third sheet or rear layer 434.

[0133] The third sheet or rear layer/sheet 434 includes microperforations 436.

[0134] The cells 422 provide a primary or first cell depth (Primary Cell Depth ‘PCD’). The cells 428 provide a secondary or second cell depth (Secondary Cell Depth ‘SCD’). The open space/air gap between the rear (third) sheet 434 and the surface 440 behind the acoustic panel provides a tertiary or third cell depth (Tertiary Cell Depth ‘TCD’).

[0135] Preferably the open area provided by the microperforations at the rear of the acoustic panel is smaller than the open area of the first sheet and of the second sheet.

[0136] FIG. 5 shows by way of comparative example test results of acoustic frequency absorption for a known single layer honeycomb core acoustic panel having a microperforated front sheet and a solid back/rear sheet.

[0137] The panel is in this embodiment is 40 mm thick, with a 0.9 mm thick microperforated aluminium face panel and a solid (non-microperforated) 0.3 mm thick rear sheet with a single layer aluminium honeycomb core. The noise reduction coefficient (NRC) tested as 0.5.

[0138] Test results for the panel relating to FIG. 5 were as shown in Table 1 below:

TABLE-US-00001 TABLE 1 ⅓ Octave RT for RT for room Sound Centre Frequency Empty Room with Sample Absorption Hz Sec. Sec. Coefficient 100 4.4 4.3 0.02 125 6.4 4.7 0.09 160 6.5 5.0 0.14 200 7.9 5.5 0.17 250 8.6 4.9 0.27 315 8.7 3.2 0.62 400 8.4 2.2 1.02 500 7.8 2.2 1.05 630 6.8 2.4 0.86 800 5.7 2.6 0.64 1 k 4.5 2.7 0.47 1.25 k 4.1 2.8 0.35 1.6 k 3.7 2.9 0.22 2 k 3.4 3.0 0.12 2.5 k 3.4 3.1 0.08 3.15 k 3.1 3.0 0.05 4 k 2.7 2.6 0.04 5 k 2.2 2.1 0.09

[0139] FIG. 6 shows a graph of test results of acoustic absorption for an acoustic panel according to an embodiment of the present invention.

[0140] The embodiment the subject of the test results represented by FIG. 6 is a panel with a front facing sheet of a particular hole spacing/open area of microperforation, an inner second sheet (e.g. septum layer) of a wider hole spacing/smaller open area, to that of the first sheet, and a non-perforated third sheet (rear/back face).

[0141] The acoustic panel tested has a 40 mm overall thickness (OT) with microperforated facing (first) sheet and microperforated intermediate (second) sheet with a solid rear (third) sheet, all of aluminium. The two cellular cores are honeycomb style cores, preferably of aluminium.

[0142] In this embodiment, the test results provide an absorption graph having two absorption peaks (Primary P and Secondary S); one a higher frequency peak corresponding to the facing hole size & spacing (open area) and upper cellular core cell depth (first or Primary cell depth ‘PCD’), and the lower frequency peak corresponding to the intermediate (septum) second sheet hole size & spacing (open area) and total panel depth (second or Secondary Cell Depth SCD).

[0143] The second microperforated sheet of FIG. 6 is the same as the first microperforated sheet of FIG. 5, giving a similar low frequency peak.

[0144] The first microperforated sheet of the FIG. 6 panel has a much higher open area than the second microperforated sheet which, combined with the shallower depth of the primary cells, results in the additional higher peak frequency compared to FIG. 5—thereby dramatically increasing Noise Reduction Coefficient (NRC) from 0.5 to 0.7.

[0145] The test results yielded the following tabulated data (Table 2) represented in the graph of FIG. 6 for a 40 mm thick, dual cellular core (honeycomb), acoustic panel of aluminium sheets and aluminium cellular core, with a nil space behind the acoustic panel:

TABLE-US-00002 TABLE 2 ⅓ Octave RT for RT for room Sound Centre Fequency Empty Room with Sample Absorption Hz Sec. Sec. Coefficient 100 4.9 4.1 0.13 125 5.6 5.0 0.07 160 6.8 5.6 0.09 200 8.6 6.1 0.15 250 9.5 5.6 0.23 315 9.2 4.0 0.44 400 8.6 2.6 0.82 500 8.1 2.2 1.05 630 7.2 2.0 1.09 800 5.8 2.0 0.99 1 k 4.9 1.9 1.01 1.25 k 4.1 1.7 1.06 1.6 k 3.8 1.8 0.90 2 k 3.6 2.1 0.60 2.5 k 3.3 2.5 0.32 3.15 k 3.0 2.5 0.19 4 k 2.6 2.3 0.13 5 k 2.0 1.9 0.10 6.3 k 1.6 1.6 0.05 8 k 1.2 1.2 0.02 10 k 0.9 0.9 0.11

[0146] The results show a significant increase in Noise Reduction Coefficient (NRC). The overall NRC is 0.70, being an improvement in noise reduction over the 0.5 NRC panel of FIG. 5.

[0147] Utilising one or more embodiments of the present invention, the absorption peaks can be tailored using a mathematical model, so that acoustic panels can be tailored to absorb particular frequencies. Further layers of perforated intermediate sheets and cellular cores of other sized cells can be added, absorbing more peak frequencies.

[0148] For example, the first microperforated sheet of the FIG. 6 panel has a much higher open area than the second microperforated sheet which, combined with the shallower depth of the primary cells, results in the additional higher peak frequency—thereby dramatically increasing Noise Reduction Coefficent (NRC) from 0.5 to 0.7.

[0149] The low frequency peak of the test results shown in the graph of FIG. 7 is produced by the same second microperforated sheet and second cell depth or secondary cell depth (SCD) (labelled S) as the peak low frequency of FIG. 6 (which is also the same first microperforated sheet and cell depth of FIG. 5).

[0150] Absorption of both peak frequencies (relating to the first or primary dell depth ‘PCD’ labelled P, and the second or secondary cell depth ‘SCD’ labelled S) of FIG. 8 are slightly reduced compared to FIG. 6, which both have the same first and second microperforated sheets, and the peak low frequency is shifted slightly to higher frequency as a consequence of using an airgap behind the panel instead of an additional layer of cellular core. These results show a reduction in noise reduction coefficient (NRC) to 0.60 for this 20 mm thick panel (with 20 mm airgap behind) compared to the 40 mm thick double-layer panel of FIG. 6. However NRC is still higher than the single-layer 40 mm panel of FIG. 5 (NRC=0.50).

[0151] FIG. 8 shows by way of comparative example test results of acoustic frequency absorption for a dual layer honeycomb core acoustic panel having a microperforated front sheet, a microperforated septum/intermediate sheet and a solid back/rear sheet, similar to the subject panel of FIG. 6 with first and second microperforated sheets modified for high NRC.

[0152] The panel is 40 mm thick overall, with 0.8 mm thick microperforated aluminium facing and septum sheets and a solid (non-microperforated) 0.3 mm rear sheet with a honeycomb core (preferably of aluminium) between the facing and septum sheet and a second honeycomb core (preferably of aluminium) between the septum sheet and the rear sheet. The noise reduction coefficient (NRC) tested as 0.8.

[0153] Test results for the panel relating to FIG. 8 are as shown in Table 3 below:

TABLE-US-00003 TABLE 3 ⅓ Octave RT for RT for room Sound Centre Fequency Empty Room with Sample Absorption Hz Sec. Sec. Coefficient 100 4.8 4.0 0.15 125 5.9 4.6 0.17 160 7.9 5.4 0.21 200 9.0 5.4 0.26 250 9.2 4.9 0.33 315 8.9 3.6 0.55 400 8.1 2.3 1.03 500 7.4 2.0 1.18 630 6.3 2.2 0.99 800 5.3 2.1 0.96 1 k 4.5 1.8 1.05 1.25 k 4.0 1.7 1.11 1.6 k 4.0 1.9 0.90 2 k 3.7 2.3 0.56 2.5 k 3.3 2.5 0.35 3.15 k 2.9 2.5 0.24 4 k 2.5 2.2 0.20 5 k 1.9 1.8 0.19

[0154] In particular, FIG. 8 shows a graph of test results of acoustic absorption for an acoustic panel according to a preferred embodiment of the present invention. The embodiment the subject of the test results represented by FIG. 8 is a panel with a front facing sheet of a particular hole spacing/open area of microperforation, an inner second sheet (e.g. septum layer) of a wider hole spacing/smaller open area, to that of the first sheet, and a non-perforated third sheet (rear/back face).

[0155] In this embodiment, the test results provide an absorption graph having two absorption peaks; one a higher frequency (S) peak corresponding to the facing hole size & spacing (open area) and upper cellular core cell depth, and the lower frequency (P) peak corresponding to the intermediate (septum) second sheet hole size & spacing (open area) and total panel depth. The spacing of the microperforations in the first (facing) sheet is 6.0 mm and the second (septum) sheet is 13.25 m, with 9.5 mm diameter honeycomb cells in both cell layers and a non-perforated rear face.

[0156] FIG. 9 shows a graph of acoustic absorption from test results of an example of an existing acoustic panel having a single microperforated facing (first) sheet and a zone of reduced depth within part of the cellular core of the panel giving two peaks of absorption—one peak at a lower frequency corresponding to full honeycomb cell depth, and a second peak at a higher frequency corresponding to reduced cell depth.

[0157] In the panel under test in respect of FIG. 9, the reduced depth area was produced by inserting a higher-density section of honeycomb having a non-perforated sheet on the lower side, this is crushed into the upper surface of the lower-density honeycomb cellular core of the “mother panel” during panel manufacture.

[0158] The panel was of 40 mm overall thickness with a 0.7 mm thick microperforated aluminium facing panel. NRC was tested as 0.6.

[0159] The graph of absorption in FIG. 9 shows a shoulder (Sh) on the high frequency side of the primary peak P, compared to a standard panel—resulting in increased Noise Reduction Coefficient (NRC) compared to a standard panel of FIG. 5.

[0160] According to at least one embodiment of the present invention, three sheets (3 planes) are separated from each other by two respective layers of honeycomb (aluminium or other material) cellular core, one cellular core between the front (facing) first sheet and the second (intermediate) sheet, and the second cellular core between the second (intermediate) sheet and the third (rear/back) sheet.

[0161] It will be appreciated that the present invention can have more than three sheets and more than two cores, and the rear sheet of the acoustic panel can be solid or microperforated, depending on the required application.

[0162] The spacing of the microperforations is preferably such that at least one microperforation of the facing (first) sheet leads to each and every individual cell within the first cellular core.

[0163] The spacing of the microperforations within the intermediate (second) sheet is preferably such that each perforation leads from only approximately 50% of the cells of the first cellular core into the cells of the second cellular core e.g. secondary cells. Other proportions of the cells are possible, such as between 80% and 20%, depending on the required application and sound frequencies to be absorbed.

[0164] The cell diameter of the first cellular core (upper layer, say) is preferably the same or smaller than the cell diameter of the second cellular core.

[0165] Preferably, approximately 50% of all of the cells of the first cellular core (upper cells) are closed at their bases, and the other 50% of the upper cells have holes (microperforations) at their bases leading to the respective cells of the second cellular core. Each successive core from the first to the second to the third, and so on, may need cells of increasing diameter compared to the previous core.

[0166] Preferably the spacing of the microperforations in the first (facing) sheet is between 2 mm and 20 mm, more preferably between 3 mm and 15 mm, yet more preferably between 5 mm and 10 mm. By way of example, an embodiment of the present invention was subjected to testing and provided the test result graph shown in FIG. 6.

[0167] An alternative embodiment of the present invention provides an acoustic panel having a microperforated first (facing) sheet, a microperforated second (rear) sheet, and a cellular core sandwiched therebetween.

[0168] Such a ‘single core layer’ panel having microperforated facing and rear sheets is particularly, though not solely, suited for applications where there will be a surface behind the acoustic panel, such as a wall, ceiling or rear sheet of another panel.

[0169] Such a ‘single core layer’ acoustic panel absorbs two peak frequencies as does the two layer version of the present invention; a higher frequency corresponding to the microperforated facing hole size & spacing and the depth of the honeycomb, and a lower frequency corresponding to the microperforated back face hole size and spacing and the combined depth of the ‘honeycomb’ panel and space to the surface behind the acoustic panel.

[0170] The test results yielded the following tabulated data (Table 4) represented in the graph of FIG. 9 for a 20 mm thick, single cellular core (honeycomb), acoustic panel of microperforated facing and rear aluminium sheets and an aluminium cellular core, with a 20 mm space behind the acoustic panel:

TABLE-US-00004 TABLE 4 ⅓ Octave RT for RT for room Sound Centre Fequency Empty Room with Sample Absorption Hz Sec. Sec. Coefficient 100 4.5 4.7 0.00 125 5.4 5.2 0.04 160 6.8 6.2 0.06 200 8.5 6.9 0.10 250 8.8 6.7 0.12 315 9.2 6.0 0.20 400 8.9 4.5 0.36 500 8.2 3.4 0.57 630 7.1 2.5 0.86 800 5.8 2.2 0.95 1 k 4.8 2.0 0.93 1.25 k 4.4 1.9 0.96 1.6 k 4.0 1.9 0.90 2 k 3.8 2.1 0.67 2.5 k 3.6 2.5 0.41 3.15 k 3.1 2.6 0.22 4 k 2.7 2.5 0.16 5 k 2.2 2.1 0.14

[0171] Table 5 below shows a comparison of basic specifications for various honeycomb core panels subjected to absorption testing.

TABLE-US-00005 TABLE 5 Panel Panel Thickness Weight number # (mm) Kg/m.sup.2 Description 1 10 4.86 Non-perforated decorative laminate both sides 2 20 5.37 Non-perforated decorative laminate both sides 3 10 3.41 Non-perforated aluminium both sides 4 20 3.96 Non-perforated aluminium both sides 5 20 7.79 Micro perforated aluminium facing, solid back 6 20 5.78 Micro perforated aluminium both sides

[0172] As shown in Table 5, six different types of panel were tested, panels #1 to #6. The panels had a range of thicknesses, constructions and finishes. The results are shown in the graphs in FIG. 10.

[0173] All six panels were installed covering a bare bulkhead. Two of the panels (#1 and #2) were retested with the addition of two inches of 3 pcf fibreglass placed between the bulkhead and the panel.

[0174] FIG. 10 shows results of testing on panels #1 to #6 conducted to ASTM C423-08.

[0175] Those test results show single peak improvement in absorption for panel #5, having microperforations on one side, and double peak improvement in absorption coefficient for mid-range frequencies for the acoustic panel with microperforations on both sides plus an airgap behind the panel, panel #6.

[0176] The chart of FIG. 10 shows peaks exhibiting significant absorption between 500 Hz and 1600 Hz for microperforated panels #5 and #6 being much higher than the other non-perforated panels in the test.

[0177] The microperforated sheets and core depths used in the double peak panel matched those used in FIG. 7, except the airgap to the rear of the panel was 50 mm instead of 20 mm.

[0178] It can be seen that the high frequency peak remained close to 1600 Hz, whereas the low frequency peak shifted from 800 Hz to 500 Hz due to the higher secondary cell depth (total depth of panel and airgap).

[0179] Table 6 below shows the Absorption Coefficient values at various frequencies (left hand column) for panels #1 to #6 as reflected in the graph shown in FIG. 10. The Ayres #6 panel has two spaced layers of perforated sheets and shows two absorption peaks in the chart, per at least one embodiment of the present invention.

[0180] In the legend in FIG. 10, “2 in 3 pcf FG—Ayres #1 @ 2 in” is the “Ayres #1 @ 2 in” non-perforated panel with 2 inches of 3 lb/ft.sup.3 fibreglass in the 2 inch airgap behind the panel, and “2 in 3 pcf FG—Ayres #2 @ 2 in” is the “Ayres #2 @ 2 in” non-perforated panel with 2 inches of 3 lb/ft.sup.3 fibreglass in the 2 inch in the airgap behind the panel.

[0181] FIG. 11 shows only the traces for Ayres #4, Ayres #5 and Ayres #6 panels from Table 6 and FIG. 10, the Ayres #6 panel having the aforementioned two spaced layers of perforated sheets and showing two absorption peaks in the chart.

TABLE-US-00006 TABLE 6 Ayres 2 in 3 pcf Ayres 2 in 3 pcf Ayres #3 Ayres Ayres Ayres #6 #1 @2 in FG-Ayres #2 @2 in FG-Ayres @2 in #4 @2 in #5 @2 in @2 in Test Absorp #1 @2 in Absorp #2 @2 in Absorp Absorp Absorp Absorp Freq Coeff Absorp Coeff Coeff Absorp Coeff Coeff Coeff Coeff Coeff (Hz) Sab/m{circumflex over ( )}2 Sab/m{circumflex over ( )}2 Sab/m{circumflex over ( )}2 Sab/m{circumflex over ( )}2 Sab/m{circumflex over ( )}2 Sab/m{circumflex over ( )}2 Sab/m{circumflex over ( )}2 Sab/m{circumflex over ( )}2 25 31.5 −0.073 −0.066 −0.029 0.110 −0.008 −0.074 −0.089 −0.049 40 0.028 0.037 0.081 0.138 0.083 0.056 0.018 −0.004 50 0.137 0.111 0.120 0.149 0.126 0.117 0.160 0.001 63 0.093 0.161 0.127 0.077 0.042 0.282 0.153 0.187 80 0.008 0.025 0.008 0.050 0.056 0.021 0.091 0.038 100 0.007 0.163 0.159 0.131 0.003 0.198 0.162 0.064 125 0.182 0.480 0.248 0.546 0.096 0.289 0.324 0.061 160 0.517 0.486 0.286 0.506 0.352 0.368 0.311 0.142 200 0.242 0.179 0.312 0.260 0.466 0.277 0.314 0.289 250 0.164 0.098 0.240 0.174 0.219 0.259 0.159 0.398 315 0.068 0.077 0.157 0.153 0.162 0.223 0.160 0.606 400 0.046 0.075 0.161 0.173 0.155 0.192 0.242 0.794 500 0.046 0.057 0.151 0.168 0.128 0.178 0.363 0.855 630 0.050 0.051 0.149 0.169 0.134 0.181 0.591 0.768 800 0.051 0.043 0.108 0.115 0.137 0.120 0.891 0.672 1000 0.034 0.061 0.053 0.056 0.111 0.064 0.891 0.630 1250 0.062 0.098 0.035 0.032 0.113 0.045 0.535 0.827 1600 0.046 0.085 0.019 0.026 0.068 0.018 0.304 0.813 2000 0.046 0.064 0.013 0.030 0.052 0.023 0.199 0.551 2500 0.038 0.060 0.021 0.018 0.046 0.008 0.133 0.337 3150 0.048 0.036 0.027 0.047 0.046 0.014 0.088 0.215 4000 0.044 0.006 0.026 0.041 0.034 −0.011 0.040 0.142 5000 0.085 −0.020 0.009 0.058 0.028 −0.033 0.052 0.114 6300 0.092 −0.041 −0.022 0.073 0.012 −0.091 −0.004 0.112 8000 0.260 −0.027 0.038 0.162 0.023 −0.152 −0.027 0.166 10000 0.303 −0.075 −0.013 0.128 0.018 −0.261 −0.064 0.310