GB2267359A

GB2267359A - Improvements in attenuating bends

Info

Publication number: GB2267359A
Application number: GB9211129A
Authority: GB
Inventors: Alan Trevor Fry
Original assignee: Sound Attenuators Ltd
Current assignee: Sound Attenuators Ltd
Priority date: 1992-05-26
Filing date: 1992-05-26
Publication date: 1993-12-01
Anticipated expiration: 2012-05-26
Also published as: GB9211129D0; GB2267359B

Abstract

A bend attenuator has a gas passage 113 of larger cross-sectional area in that part of the duct which is on the outside of the curve of the bend compared to the cross-sectional area 112 of the gas passage on the inside of the curve of the bend. The attenuating material can be on the surfaces of the duct (as shown) but is not necessarily so positioned. The duct may be divided into more than the two passages. The upstream ends of the attenuating material are streamlined. <IMAGE>

Description

IMPROVEMENTS IN ATTENUATING BENDS This invention relates to a ducted gas-filled acoustic bend attenuator and in particular to an improved arrangement of acoustic attenuating material in the duct of the attenuator.

It is known to attenuate duct-borne noise by means of duct sections which contain at least one mass of acoustic attenuating material. Usually the material lines the inside of the duct wall causing a localised restriction in the cross-sectional area available for gas flow through the duct. Further masses of attenuating material (usually known as "pods" or "splitters") are disposed inside the lined wall to further restrict the area available for gas flow. The more attenuating material there is in the duct, the better is likely to be the acoustic attenuation produced, whereas the less attenuating material there is in the duct the less resistance to gas flow there is likely to be. Resistance to gas flow is usually measured in terms of pressure loss, the aim normally being to achieve a low level of pressure loss commensurate with good acoustic performance. The design of a commercially successful acoustic attenuator thus involves a careful balancing of acoustic performance against pressure loss figures, subject always to manufacturing cost considerations.

One way of reducing the pressure loss without reducing the mass of acoustic material in the duct is to aerodynamically shape the leading and trailing ends of each inserted mass and it is known to provide smoothly curved leading ends and tapered trailing ends for this purpose.

In the specification of International Patent Application PCT/GB90/01799 we describe the advantage that can be achieved in acoustic attenuating ducts by providing a stepped discontinuity in the width of the gas passage in the exit region of both straight and bend attenuators.

We have now found that a further improvement in performance of a bend attenuator results if in addition to the stepped discontinuity in gas passage width, a gas passage of larger cross-sectional area is provided in that part of the duct which is on the outside of the curve of the bend compared to the cross-sectional area of the gas passage on the inside of the curve of the bend In a duct of rectangular cross-section, we prefer a wider gas passage to be provided on the outside of the curve of the bend compared to the width of the gas passage on the inside of the curve of the bend.

In a simple case where the cross-section of the bend attenuator is rectangular and is divided by a central splitter into a shorter gas passage (on the inside of the bend) and a longer gas passage (on the outside of the bend) - the invention can be seen as modifying the attenuation located in the duct so that the longer passage has a larger cross-sectional area than the shorter passage. The modification can be just in the central splitter and can then be thought of as a simple moving of the central splitter inwardly somewhat without increasing the volume of attenuating material in the duct. Alternatively, or in addition, the modification can be achieved by increasing the volume of attenuating material flanking the inner shorter gas passage by increasing the thickness of the material lining the duct on the inside of the bend or the thickness in the adjacent splitter. In either, event the net effect is to more closely match the attenuating effect of the inner and outer gas passages so that a more closely comparable level of attenuation occurs in each passage.

The acoustic attenuating material can be any of the prior art forms and mention can be made of foamed materials (e.g. rubber, plastics or minerals), fibrous materials (e.g. mineral fibre mats or pads), particulate materials (bonded into aggregate blocks or contained in air permeable housings) or cork. Depending on the velocity of the gas flowing in the gas passage, it may be necessary to face the attenuating material with a surfacing sheet to protect against erosion. Suitable prior art materials would be films or foils of metal or plastics, mineral fibre tissue, woven or non-woven cloths, nets or meshes, and perforated sheets of metal or plastics.

A preferred bend attenuator according to this invention has splitters on the upstream side of the bend which are each thicker than the corresponding splitter on the downstream side of the bend and a gas passage which is wider on the outside of the bend than on the inside. At the bend, where slabs of attenuating material of different thickness meet, the transition between adjacent slab ends may be smoothed by the inclusion of radiused bridge pieces.

The invention will now be further described, by way of example, with reference to the accompanying drawing, in which: Figures 1 and 2 are schematic sections of prior art bend attenuators, Figures 3 and 4 are attenuators similar to Figures 1 and 2, respectively, but modified according to this invention, and Figure 5 is a schematic section through a further attenuator according to this invention.

Figure 1 shows a bend attenuator 5 with slabs 20 and 21 of sound attenuating material lining the inner 10 and outer 11 walls of the bend attenuator 5 provided with coupling flanges 6 and 7. A central splitter slab 23 is disposed between the slabs 20 and 21 to leave an inner gas passage 12 and an outer gas passage 13. Each slab 20, 21 and 23 has a rounded nose 20a, 21a and 23a at the inlet end close to flange 6 but its trailing end is provided by a discontinuity (e.g. as shown by using thinner slabs 20b, 21b and 23b close to the flange 7). Instead of using wallflanking slabs 20, 21, these may be moved in somewhat from the walls 10, 11 to provide further gas passages between the slabs and the walls (i.e. to give rise to a four passage bend attenuator rather than the two passage bend attenuator 5 shown in Figure 1). Where the slabs 20, 21 are moved in from the walls 10, 11 the central slab 23 can be dispensed with to give a three-passage bend attenuator 8 as shown in Figure 2 (which is generally as shown in Figure 6 of the aforementioned International application). In Figure 2, the third (or outermost) passage is numbered 14.

In all the aforementioned forms of bend attenuator, if the cross-sectional areas of all the gas passages are the same, it will be found that an acoustic short circuit arises by virtue of the shortest passage (i.e. passage 12 as shown in Figures 1 and 2) presenting less attenuating material to the gas stream passing along it, so that the overall performance of the bend attenuator is downgraded.

We have now found that this degrading of performance can be avoided by deliberately reducing the cross-sectional area available to gas flow in the inner or innermost gas passage thereby forcing more gas to flow through the longer and outer (or outermost) gas passage(s). This modification is illustrated schematically in Figures 3 and 4 where broken lines are used to represent the position of attenuating material in the Figure 1 and Figure 2 embodiments and one hundred has been added to the reference numerals used in the Prior Art Figures. Thus duct 105 shows the duct of Figure 1 modified according to the invention and Figure 4 shows the duct 108 of Figure 2 so modified.

The reduction in cross-sectional area of the passage 112 relative to passage 12 can be achieved by making the slabs flanking the inner passage thicker as shown in Figures 3 and 4 and/or by moving slabs of the same thickness inwardly towards the wall 110 (as shown with slab 121 in Figure 4).

In Figure 3, slabs 123 and 123b are widened (compared with slabs 23 and 23b) to reduce the cross-sectional area of passage 112 compared to passages 12, 13 and 113.

In Figure 4, slab 121 is moved somewhat further from wall 111 but not thickened as compared to slab 21 so that passage 113 is smaller in cross-sectional area than passages 13, 14 and 114. Passage 114 has a larger crosssectional area than passage 14. Slab 120 is thickened compared to slab 20 to reduce the cross-sectional area of passage 112 compared to that of passage 12. Slabs 120b and 121b are also shown displaced in Figure 4.

In Figure 5, three airways 212, 213 and 214 are shown with the innermost airway 212 flanked by wall-mounted pads 220, 220b and an inner splitter 223, 223b, the intermediate airway 213 flanked by the inner splitter 223, 223b and an outer splitter 224, 224b and the outermost airway 214 being flanked by the outer splitter 224, 224b and wall-mounted pads 225, 225b.

The adjacent ends of the upstream and downstream parts 223 and 224 of each splitter are thicker than the respective downstream parts 223b and 224b to provide output airways that are wider than the respective input airways, plus thicker inlet end splitters, plus thinner outlet splitters after the bend. Further, radiused bridge pieces 250 link the adjacent ends of the slabs which form each splitter and the inner wall pads, these bridge pieces being located on the inside curve of each airway.

As with the Figures 3 and 4 embodiments. the crosssection of the input end of each airway is greater than the cross-section of the respective output end and the cross sections of the input ends of the three airways and the cross-sections of the output ends of the three airways increase the further out radially, with respect to the bend, that airway is located.

In all the attenuators shown in the drawings tail-end tapering of the slabs is dispensed with in favour of the stepped arrangement featured in the specification of PCT/GB90/01799.

The invention can also be applied to ducts of circular cross-section either by eccentrically mounting the pod in the duct to encourage gas flow around the outside of the bend or by using a pod of non-circular cross-section to achieve the same end

Claims

1. A bend attenuator having a stepped discontinuity in gas passage width in the direction of gas flow through a curved duct of the attenuator and a gas passage of larger cross-sectional area in that part of the duct which is on the outside of the curve of the bend compared to the crosssectional area of the gas passage on the inside of the curve of the bend.

2. An attenuator according to claim 1, with a duct of rectangular cross-section, in which a wider gas passage is provided within the duct on the outside of the curve of the bend compared to the width of the gas passage on the inside of the curve of the bend.

3. An attenuator according to claim 1 where the crosssection of the curved duct is rectangular and is divided by a central splitter of attenuating material into a shorter gas passage (on the inside of the bend) and a longer gas passage (on the outside of the bend) the attenuating material being located in the duct so that the longer passage has a larger cross-sectional area than the shorter passage.

4. An attenuator according to claim 3, in which a comparable thickness of attenuating material flanks the inside duct on both the inside and the outside of the curve.

5. An attenuator according to claim 3, in which a greater thickness of attenuating material flanks the inside of the duct on the inside of the curve compared to the thickness of such material flanking the inside of the duct on the outside of the curve.

6. An attenuator according to claim 4 or claim 5, in which the thicknesses of attenuating material flanking the duct and forming the central splitter are chosen to match the attenuating effect of the inner and outer gas passages so that a more closely comparable level of attenuation occurs in each gas passage.

7. An attenuator according to any preceding claim, in which acoustic attenuating material selected from foamed materials, fibrous materials, particulate materials, or cork is used in the duct.

8. An attenuator according to claim 7, in which the attenuating material is faced with a surfacing sheet to protect against erosion.

9. An attenuator according to claim 8, in which films or foils of metal or plastics, mineral fibre tissue, woven or non-woven cloths, nets or meshes, or perforated sheets of metal or plastics are used as a surfacing sheet.

10. A bend attenuator according to any preceding claim having at least one splitter on an upstream side of the bend which is thicker than the or a corresponding splitter on a downstream side of the bend and a gas passage which is wider on the outside of the bend than on the inside.

11. An attenuator according to claim 10, in which where slabs of attenuating material of different thickness meet, the transition between adjacent slab ends is smoothed by the inclusion of radiused bridge pieces.

12. A bend attenuator substantially as herein described with reference to Figure 3, 4 or 5 of the accompanying drawings.