I think walters needs this question asked to him again:
What are you trying to achieve?
I mean why would you want one formula for input, RC time, and phase? To me it is senseless to try to combine everything into one formula.
I only look at the RC time. Input is just input. and if the input is a speaker, check it's resistance, and then treat it as a resistor.
The low frequency resonance of a speaker causes it to be a lousy microphone.
Try making an allpass filter, a variable phase shift or a comb filter with just a speaker.
I know what you can do with the speaker: spin it! :lol:
In fact, connect a speaker to a circuit of some sorts, and to a motor, and see if you can power the motor from the techno music in the room! :lol: :lol:
it requires a large depletion layer region (large area).
Also varacters are used in HF/UHF ckts for tuning , that reqs only pf variation
i hope u have visted the links i suggested.
A 0.22uF capacitor's effective value in a filter circuit can be changed with PWM. With 100% duty-cycle it is 0.22uF. With 10% duty-cycle it is 0.022uF. With 1% duty-cycle it is ..., you figure it out. Of course, the frequency of the PWM is very high so can't be heard.
You also didn't hear me describe how PWM can vary the capacitance over a wide range of any capacitor. I've used switched-capacitor lowpass filter ICs for years. They switch internal capacitors similar to PWM and work perfectly.