What is it exactly that determines the frequency of a piezoelectric material?

Question

Why is it that the frequency gets smaller the larger the crystal instead of the opposite? You would think that the more ions, the more energy, the louder something gets so the higher the frequency.

In:

Answer 1

piezoelectric crystals physically resonate much like a tuning fork, a large tuning fork can fit a longer wavelength, and hence will have a lower frequency than a small one.