Measure the Capacitance of a condenser

[Luckyman_]

TL;DR Summary

I built a condenser and I want to measure the capacitance of it when reduce the distance between the two plate

when the distance between the two piece of aluminum variate, the capacitance of this self-made capacitor also variate, and I would like the measure the variation of the capacitance caused by a sound wave. If I thought about a De Sauty Bridge but it doesn't fit my requirements. The is this relation between C and ε0 *A/d where ε0 and A are fixed in my case.

 

[Baluncore]

Welcome to PF.

If you charge the capacitor to a DC voltage, then disconnect the source of charge. Measure the AC voltage as the separation of the plates change, due to sound waves.
Use a FET follower as a voltage buffer.
https://en.wikipedia.org/wiki/Microphone#Condenser