Resonance frequency increasing with mass?

[bulgogii]

For a spring and mass system, I've learned the resonance frequency of a system increases with decrease in mass.

Not sure if this case is also related but when I attach a coin sized metal onto a piezoelectric material of a smaller size, the resonance frequency range actually goes up (from observation by measuring the electrical impedance(ohms) of the piezoelectric material with varying in frequency range). The resonance frequency range keeps going up when I continue attaching metal pieces with an adhesive.

Ain't the resonance frequency range suppose to go down with increase in mass? or is it the stiffness affecting the result??

 

[paisiello2]

First of all, when you say resonance frequency do you actually mean natural frequency? that is, the fundamental frequency that the system vibrates in, absent any driving forces?

Secondly, it's very likely that by gluing the larger size metal onto the other smaller material that you are changing the stiffness characteristics as well.

 

[bulgogii]

Thx for the reply, the metal is vibrated using a small piece of pzt material(acts as actuator&sensor simultaneously) glued to it. the PZT material is vibrated by inputing AC voltage(1 volt) from 10kHz to 90kHz using a function generator. Maybe its the natural frequency of the PZT material?

But as you've mentioned, the change in the resonance frequency range is probably due to the change in the stiffness by attaching additional metal pieces

 

[paisiello2]

You didn't answer my first question.

 

[bulgogii]

Yes~ I meant natural frequency when I say resonance frequency