I believe the purpose of a tuning fork is to produce a single pure frequency of vibration.  How do two coupled vibrating prongs isolate a single frequency?  Is it possible to produce the same effect using only 1 prong?  Can a single prong not generate a pure frequency?  Does the addition of more prongs produce a "more pure" frequency?

The two prong system only supports a single standing wave mode, why is that?

I am by no means an expert in tuning fork design, but here are some physical considerations:


Different designs may have different "purities," but don't take this too far. It is certainly possible to tune to something not a pure tone; after all, orchestras usually tune to instruments, not tuning forks.
Whatever mode(s) you want to excite, you don't want to damp with your hand. Imagine a single bar. If you struck it in free space, a good deal of the power would go into the lowest frequency mode, which would involve motion at both ends. However, clamping a resonator at an antinode is the best way to damp it - all the energy would go into your hand. A fork, on the other hand, has a natural bending mode that will not couple very well to a clamp in the middle.

