what causes the howling sound in the public address systems? and how we can reduce or avoid it?

Answer 1

The first two answerers are correct about the cause. Increasing the distance between microphone and loudspeaker, if possible, is good.

The simplest thing you can try, is to put the micrphone as close to the person's mouth as possible (but not directly in front) and then turn down the PA amp's volume knob.

Replace the micrphone (probably an omni-directional) with a cardioid or super cardioid microphone. These mics reject sounds originating sideways to the mic.

Or you could try a "noise cancelling" microphone. These need to be placed very close to the person's mouth. They minimize any sounds originating at distances more than a foot.

Answer 2

It's called feedback. It happens when the sound coming out of the speakers is picked up by the microphone(s) and is amplified again. Be sure that your microphones are not pointed at any speakers. Move the speakers forward or the microphones back. Also, sometimes you can adjust the EQ or tone to reduce the frequencies that are feeding back.

Answer 3

It's positive feedback. The signal gets broadcast from speakers. The microphone hears the signal and re-broadcasts it and you get an ugly squeal.

To avoid this, put the mike in a room where you can't hear the PA system--or at least try to shield the mike from getting too much feedback from the speakers.

Answer 4

It's called karaoke. I happens when bad singing coming from the speakers in a bar is heard by drunks, and the microphone is subsequently left unattended. It can be avoided by hiding the microphone or, if that's not an option, going to another bar :-)