What causes the loud squealing or distortion from a microphone?
The main cause of microphone feedback - the loud squeal you sometimes hear - is that a loop of sound has been created by the microphone picking up sound from the PA system speaker, which is then sent back to the speaker!
Try not to use the microphone while standing directly in front of the speaker, and don't point the microphone at a speaker. Experiment with good and bad places to stand in the room before your event. If you can't move, try angling the speakers very slightly to the left or right.
If you do get microphone noise, quickly turn down the microphone volume to stop it. And if you have a mixing desk, try turning down the MID or HI EQ on that channel.
Another cause of microphone noise or distortion is having the volume up too high.
If you're not using the microphone correctly the microphone won't pick up enough signal and the microphone volume on the receiver and/or mixer will need to be turned up loudly to compensate and you're more likely to get a distorted sound.
If you're using a handheld microphone, don't hold or touch the top of the microphone. Your voice should flow across the top, so either hold it just below your chin or out horizontally in front of you and speak into the top. If you hold it down by your tummy or wave it around you won't get a good result.