Microphones on Shabbat

Hi,

There are four main reasons to prohibit speaking into a classic microphone on Shabbat:

Causing a microphone to broadcast a sound via its reaction to one's voice may involve completing the microphone's functionality or building it, which would be biblically prohibited;

Causing the electrical current to rise and fall may involve biblical prohibitions, even though the circuit remains "on" throughout the operation of the microphone;

We may not engage in activities which appear to be prohibited ("marit ayin"), and since microphones are normally turned on only when needed, listeners may not realize that this sound system was turned on before Shabbat;

If the sound system were to malfunction, one might forget it is Shabbat and adjust it.

(Rav Moshe Feinstein, Igrot Moshe Orach Chaim 4:84)

Have a great day,
Mordechai