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In a method of producing a double-walled structure, such as a silencer for use in a ventilation duct system, use is made of an arrangement having a forming head for forming helically-wound lock-seam tubing. In the forming head, a perforated metal strip is helically wound together with an air-permeab

In a method of producing a double-walled structure, such as a silencer for use in a ventilation duct system, use is made of an arrangement having a forming head for forming helically-wound lock-seam tubing. In the forming head, a perforated metal strip is helically wound together with an air-permeable cloth strip such that the tubing fed out of the forming head has an outer cover of said cloth. The tube assembly consisting of the perforated tube and the cloth cover is provided with an outer tubular layer of insulating material and then inserted in a casing which makes the silencer complete.

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[ What we claim and desire to secure by Letters Patent is:] [1.] A method for producing a double-walled structure, comprising the steps of:applying a strip of fluid-permeable material to a perforated strip to form a composite strip;helically forming said composite strip in a forming head to form a t

[ What we claim and desire to secure by Letters Patent is:] [1.] A method for producing a double-walled structure, comprising the steps of:applying a strip of fluid-permeable material to a perforated strip to form a composite strip;helically forming said composite strip in a forming head to form a tube having an inside surface and an outside surface, said fluid permeable material tightly covering the outside surface of the perforated strip as formed into said tube;cutting said tube into a predetermined length;applying a filler material on the outside surface of said tube such that said tube and filler material form an assembly; andenclosing said assembly in a casing.