Feedback (or howl-round) is caused when, at a microphone, the volume is greater from the speakers than from the source itself. That means that output from speakers is returning back into the system and getting amplified again and again. Very quickly this turns into unacceptable levels of noise.

You will reduce the occurrences of feedback if your system is set up correctly in the first place by the correct positioning of speakers and careful setting of a graphic equaliser across the sound.

There are other things you can do to minimise the likelihood of feedback, these include:

• Train users in proper microphone technique.
  When people are closer to the microphone the system operator can turn down the volume, reducing the risk of feedback.
  
• Take care when placing monitor speakers and setting the level.
  If people cannot hear their microphone in the monitor mix it is sometimes better to turn down whatever is blocking the sound rather than just turning up their microphone.
  
• Turn off any unused microphones.
  Less open mics will mean less chances for feedback to start and also more chance of finding the problem mic quicker. It is worth bearing this in mind when selecting a sound desk - some have "mute" buttons that will let you turn on and off a microphone without moving the fader.
  
• Understand the difference between directional and omni-directional microphones.
  This is a particular issue for radio mics on a presenter / preacher who likes to move around. A directional mic only picks up sound in a tightly controlled area which can reduce the chance of feedback.