What is the Resonance of a Linear Mechanical Resonator, Peak Amplitude of Displacement or Peak Amplitude of Velocity?

Electromechanical resonators are an essential component in many sensors. These systems are essentially mechanical resonators that are driven by harmonic electrostatic forces, and their response is often measured using capacitive sensing. There is some ambiguity in the literature with respect to the resonance response of mechanical resonators. Should resonance be associated with the peak amplitude of velocity or should it be associated with the peak amplitude of displacement? Another ambiguity relates to the definition of phase: Should phase relate velocity or displacement to the driving force? These two issues are addressed here, and it is shown why resonance should be associated with peak amplitude of velocity and not with peak amplitude of displacement.