Sound absorbing device

Claim 1

**WARNING** start of CLMS field may overlap end of DESC **. 4 is retained in position by an inner wall 5 formed from a single strip of metal shaped into a helical configuration with adjacent edges interlocked, by the same arrangement as is used for the outer wall 3. The inner wall 5 is formed with a plurality of perforations 6. Between the mass 4 of sound absorbent material and the inner wall 5 is located a thin sheet 7 - of material which is sound transmitting but is impervious to liquid or gas. A suitable material is a polyethylene terephthalate known as Melinex (Registered Trade Mark). When a stream of gas passes along the passage 2 any noise in the gas will be transmitted through the perforations 6 in the inner wall 5 and through the sheet 7 into the sound absorbent material 4 where the noise will be absorbed. However, since the thin sheet 7 is impervious to liquids and gases and to particles, there will be no transfer of liquids or gases or particles from the gas stream to the sound absorbent material 4. Therefore, the sound absorbing properties of the material 4 will not be affected by impurities. Particles or liquids in the gas stream may become trapped in the perforations 6 or attached to the surface of the inner wall 5 or the sheet 7 and these particles and liquids can be removed by passing a cleaning fluid, such as water or steam, along the duct. The construction of the outer wall 3 is more clearly illustrated in Figs. 3 and 4 which show the way in which adjacent edges of the single strip of metal 10 forming the outer wall 3 are shaped so that they interlock together. One edge 11 of the strip 10 is bent to form a ridge 12 with a U-shaped cross section, one arm 13 of the U extending substantially parallel to the outer surface 14 of the sheet and spaced therefrom. The other edge 15 of the strip is bent to form a ridge 16 having a substantially C-shaped cross section. This ridge 16 projects above the outer surface 14 of the strip 11. One arm 17 of the C extends substantially parallel to the outer surface 14 and is spaced from the central portion 18 of the C. The two ridges 12 and 16 are adapted to interlock, as illustrated, the outer arm 13 of ridge 12 engaging within the hollow formed within the ridge 16. By compressing together the two ridges the adjacent edges of the strip 10 are securely held together. Since the ridges interlock closely there is no gap between the adjacent edges of the strip. Hence, the strip 10 is formed into a continuous outer wall 3 which is very strong and rigid and no noise is transmitted through the wall. The construction of the inner wall 5 is substantially identical and, therefore, the inner wall is continuous, except for the perforations 6 in the sheet forming the inner wall. Fig. 4 is a plan view of the region A and illustrates the outer surface of the ridge 16 when the adjacent edges of the strip 10 are securely interlocked together. The inner and outer walls can each be formed by a continuous process which bends the edges of the strip 10 to form the ridges 12 and 16 and links the ridges together to form a continuous wall which has no apertures in it through which sound can escape, and which is very strong and rigid. The sheet 7 is also a continuous strip of thin material which is wound over the outer surface of the inner wall 5 in a spiral with adjacent edges overlapping in order to provide a continuous membrane preventing the passage of liquids, gases or particles from a gas stream passing along the passage 2 into the sound absorbent material 4. Fig. 5 illustrates an alternative form of construction of the sound absorbing device in which a pod 21 is located within the passage 2. The pod comprises an axially extending cylindrical duct 22 supported on legs 23 bolted to the inner surface of inner wall 5. The cylinder 22 has an outer wall 24 and inner wall 25 each formed by the same construction as the walls 3 and 5 together with a mass 26 of sound absorbent material similar to the material of the mass 4. The walls 24 and 25 are perforated and the mass 26 of sound absorbent material is surrounded by a sheet of moisture impervious material like the material of the sheet 7. The pod 21 provides additional sound absorption. In practice, the sound absorbing device described may be supplied with the pod 21 as an optional feature, the pod being stored within the passage 2 for transport, together with its fixing bolts. WHAT WE CLAIM IS:- 1. A sound absorbing device comprising a duct having a rigid outer wall formed from a continuous strip of metal shaped into a helical configuraiton with adjacent edges interlocked so as to form a continuous rigid wall and an annular mass of sound absorbent material located within said outer wall and forming a passage extending axially along said duct.

Claim 3

A device as claimed in either of the preceding claims in which said duct has a perforated inner wall extending along the passage, said sound absorbent material being located between said inner and outer walls.

Claim 5

A device as claimed in any one of claims 1, 2 or 4 in which, for each continuous strip of metal, the two edges are each formed with a ridge, the two ridges having cross sections such that they interlock to form a continuous wall.

Claim 7

A device as claimed in any one of the preceding claims comprising an additional sound absorbing unit extending axially within said passage.