Microphones (sound reinforcement)   

Both dynamic and condenser types of microphones are commonly used in sound reinforcement applications. Special microphones are often useful such as lavalier, hand-held, and wireless types.
Condenser microphones need to be provided with a polarizing voltage which is typically provided by the mixing console and called phantom power. This is usually 48 volts and is sent down the mic cable. If a condenser microphone in the system is not working properly check that the phantom power is enabled on the mixing console for the input channel the microphone is operating into. 
To minimize feedback problems in live sound reinforcement situations microphone directivity and placement must be taken into account. Generally, unidirectional microphones with cardioid pickup patterns are most useful. Careful positioning of these microphones so that the sound returning from speakers and onstage monitors is to the rear of the microphone where there is maximum rejection will minimize feedback and maximize gain. 

It may be neccessary in a live sound situation to split the signal coming from a microphone. This is typically the case where a separate monitor mixer is being used or a recording of the event is desired. In this case the microphones signal is split and sent to the main mixer and another mixer for monitoring and/or recording. 

The simplest method for splitting a microphone signal is by using a Y adaptor which physically splits the microphone signal into two or more outputs. This approach has several problems which can occur. Firstly, if one of the splits happens to short out in the cable or at the mixer for some reason, the other line will most likely also be shorted out as well. So if one system goes down they all go down. 

The other more common problem with this simple solution is that by connecting one microphone to two systems they become susceptible to ground loops which provide unwanted hum. This can be avoided by lifting the shield connector at the input of all but one of the destinations (house console, monitor mixer, remote recording mixer, etc.). 

A more effective solution to the problem of splitting microphone signals is the use of splitter transformers. These avoid ground loop problems by physically separating the source signal from the splits. 
See Also:
microphone selection 
microphone accessories 
mic placement 
mic frequency characteristics 
polar response 
wireless microphones