In speaker transducers, normally you're trying to repress any bending modes of the piston. These produce diffractive interference effects, or in some materials

In speaker transducers, normally you're trying to repress any bending modes of the piston. These produce diffractive interference effects, or in some materials "ringing" modes like a bell that are very hard to filter away. The industry standard way for measuring and optimizing this stuff is a laser measuring rig from Kippel. You can find Kippel data for most drivers on the market.