Hi djag67,
I missed your question about comparing springs to foam and the simple answer is that they are not equivalent at all. The āart and scienceā involved in the theory of how springs work is amazingly complex much less how they will respond with various layers of foam above them. You can get an idea of some of the variations in how different springs work and some comparisons with other materials in post #16 here and post #10 here and post #2 here.
Phoenix