Sinuous springs are made from two-dimensional S-shaped wires and are also known as "No-Sag springs," "S coils," "Zigzag (zig zag) wire" or "zigzag (zig zag) springs". They are commonly used in ottomans, chairs (seat and back) and sofas (seat and back) and take the place of coil springs. Sinuous springs install quickly and easily and don't require a time-consuming hand-tie job. They are fastened with clips and no-sag nails, and they run every few inches apart from front-to-back or top-to-bottom on seats, backs, ottoman, etc.

http://www.perfectproductsonline.com/sinuoussprings.html

Sinuous Spring Suspension refers to “S coils” which are piecese of heavier gauge steel formed into a continuous “S” shape. The “S coils” span front to back on the seat cushion, every few inches across the entire piece, top to bottom under the back seat cushions and are attached to the frame with clips and nails. A durable suspension that provides slightly firmer support for your sofa. More often than not, most of the comfort in your sofa comes from the cushion components, and this is a great, reliable suspension.

http://www.sofagroups.com/blog/tag/sinuous-spring-suspension

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Note added at 32 mins (2014-01-20 10:32:42 GMT)
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Wellenfederung heißt, dass Stahl wellenförmig gebogen wird und die Wellen parallel wie eine Hängematte in den Rahmen (eines Sofas) eingehangen sind.

http://www.fremdwort.de/suchen/bedeutung/Wellenunterfederung