Hi

This is pure speculation but I believe that

• For passive structures it should be doable with a tuned resonator.It might be doable also for non-linear stuctures where a resonace peak can follow excitation by progressive stiffening/eakening and in doing so, allow resonance at more frequencies than one per mode.
• For semiactive systems one can always open/close valves to modify properties into a non-linear fashion.
• For active systems possibilities should be wider.

Sincerely

Claes