For a spring and mass system, I've learnt 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??