Sound absorbing device

(72) Inventors ROBERT IRWIN

HUGH RICHARD LISSENBOROUGH

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(54) SOUND ABSORBING DEVICE (71) We, Eurovib (Acoustic Products) Limited, of 1 Shortlands Road, Kingston upon Thames, Surrey, a British Company, do hereby declare the invention, 5 for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: —

The present invention relates to sound absorbing devices for reducing the noise in gas streams, for example streams of air in ventilation systems.

It is known to pass gas streams contain15 ing noise along ducts, the wall of which include sound absorbing material. The outer wall of such a duct is usually made from sheet metal rolled to form a circular cross sectional duct and welded along the longitudinal joint between the edges of the sheet. This construction requires a sheet of metal of the required length of the duct to be cut and then shaped as required. If any gaps are left in the welded joint these can provide passages for noise to escape from the device. The sound absorbent material is usually located inside the outer wall.

In known sound absorbing devices, the gas stream makes direct contact with the sound absorbent material. This can result in chemical reaction between the gas and the sound absorbent material taking place and the deposit on the sound absorbent material of particles carried within the gas stream. Each of these factors tends to reduce the sound absorbing properties of the sound absorbent material.

The object of the present invention is to provide an improved sound absorbing device.

According to the present invention a sound absorbing device comprises a duct having a rigid outer wall formed from a 45 continuous strip of metal shaped into a helical configuration 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 along 50 said duct.

According to a preferred embodiment of the invention a sheet of sound transmitting and liquid and gas impervious material extends over the surface of said annular mass 55 of sound absorbent material forming said passage.

In order that the invention may be more readily understood reference will now be made to the accompanying drawings in 60 which: —

FIG. 1 is a perspective view of a sound absorbing device embodying the invention;

FIG. 2 is a side view of the device illustrated in Fig. 1 sectional along an axial 65 plane;

FIG. 3 is an enlarged view of the region A in Fig. 2;

FIG. 4 is a plan view of the region A; and 70

FIG. 5 is an end view of the device illustrated in Figs. 1 and 2 modified by the addition of an internal axially extending pod.

The sound absorbing device illustrated in 75 Figs. 1 and 2 consists of a cylindrical duct 1 formed with an axial passage 2 along which a gas stream may be passed. The duct 1 has an outer wall 3 formed from a continuous strip of metal shaped into a 80 helical configuration. The adjacent edges of the strip are shaped so that they are interlocked as described below with reference to Figs. 3 and 4, and the strip forms a continuous outer wall for the duct. 85

Inside the outer wall 3 is a mass 4 of sound absorbent material, for example glass fibre or other mineral wool. The mass 4 is annular in cross section so that it does not extend into the’ axial passage 2. The mass 90 is retained in position by an inner wall 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 10 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 ab25 sorbent 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 55 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 60 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 65 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 70 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 75 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. 80

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 85 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 90 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 95 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 100 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. 105

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. 110