For my experiment, I am trying to mechanically excite a quartz crystal tuning fork and measure the voltage across the tuning fork. When the TF is driven at its resonance frequency, there should be more charge accumulation and hence greater voltage.
I have just encountered a mystery though. A quartz crystal tuning fork has a dip and a peak: series and parallel resonances. When I was electrically driving the tuning fork, I was able to perfectly understand them and depending on the measuring scheme I was employing, either the series or parallel resonance peaked and the other one dipped.
Yet, for the mechanical excitation scheme, when measuring the voltage across the tuning fork, I wasn't sure which of the series and parallel resonances would peak. When I started measuring the voltage value, I ended up seeing the series resonance peaking (left plot of 2nd image).
Strangely, when I just swapped TF lead 1 and TF lead 2, the amplitude profile was basically turned upside down: the series resonance dipped and the parallel resonance peaked (right plot of 2nd image).
Also, when I swapped shear piezo lead 1 and shear piezo lead 2, I saw that the amplitude plot's peaks reversed.
I wasn't able to find anything related to my observations in the existing literature.