Microphones

Microphones are transducers; they convert energy from one form to another.
Mics convert sound waves from an acoustic form into an electrical form
The electrical equivalent of the amplitude of the wave is the voltage of the signal
Acoustical compression results in a positive voltage, and acoustical rarefaction results in a negative voltage; if the mic was picking up a sine wave, the electrical signal would also be a sine wave
The current is the electrical equivalent of air particle movement. The electrons oscillate around a fixed point, just as air particles do. There is an alternating current (AC).
In DC (direct current) systems, the resistance of a device impedes the flow of electrons through it. Resistance has a direct relationship with voltage and current, as described by Ohm's law…

I = The current flowing through a device
V = The voltage across the device
R = The device's resistance

V = IR
I = V ÷ R
R = V ÷ I

AC systems replace resistance with impedance. Impedance is more complex, as it contains both resistance and reactance components.
The reactance varies with the frequency of the signal.