Polymers, specifically rubber, have viscoelastic properties (time dependent strain). This, in a nutshell, means that if stress is applied (stretch) over a given period of time, the molecular structure is changed from a pile of sticky intertwined spaghetti, to non sticky rows of straightened spaghetti, making the rubber stronger and more resistant to failure from cracking after the stress is relieved, because the molecules reorientate to their original state with more rigor and defined structure. There are two different approaches to strengthening rubber through its inherent viscoelastic properties: 1) creep, 2) stress relaxation. Both have the same desired affect, which is reorienting the rubber’s molecular structure to make it stronger and more resistant to cracking (failure). Creep is when a constant tension is applied to the rubber and stress relaxation is when constant stress is applied to the rubber, but the rubber is held in constant position and not allowed to continue stretching
Mike,
This is good information, but I don't think it really applies to speaker surround materials. When I read it, I was picturing silly putty. You know the trick that if you pull slowly on a wad of silly putty it will stretch, but if you yank quickly it will snap? That's what you're talking about here, material that is designed to stretch statistically significantly during normal operation.
Speaker surrounds are flexible, yes, but you'll note the that there's enough extra material there -- the bump all the way around -- that allows the cone to travel up and down without pulling the surround material taut, at least if you're operating within the design limits.
