Two pounds per square yard of carpet is the usual secondary SB backing for most carpets. Polypropylene is the primary backing for others. Some carpets have fabric or polymer secondary backing, attached with a thin coat of SB latex. Synthetic foam cushion laminated directly to carpet is another form of backing. Carpet installed using tackless strips around the perimeter, over a cushion and stapled to the subfloor is preferable to glues.
Commercial installations use adhesive, in two layers or one if cushioning is built into the carpet.
Carpet cushion of bonded urethane, prime urethane, sponge rubber, synthetic fiber, and rubberized jute are usual cushioning material for carpets. Urethane is made by foaming urethane and methyl butane. Sponge rubber cushion is synthetic rubber material than has been vulcanized or foamed. We have not. Wool on the other hand is a natural healthy product.