Do you ever wonder why we put our piezo material at the nodal point (no displacement) instead at the antinode (highest displacement) in resonant ultrasonic transducers?

Our logic may tell us that the antinodes would be best for piezo material as the crystal would apply force over a longer distance during a cycle.

That would be true if piezoelectric materials applied *force*.

They don't apply force, they apply stress. Does that seem trivial? It's not.

Piezo materials apply stress as the surrounding structure undergoes strain, thus adding energy into the system. The larger the strain of the surrounding structure, the more energy is introduced.