What causes that screeching sound in PA systems?

A basic PA (public address) system includes a microphone, an amplifier, and one or more speakers. With these three elements, there's always a chance of feedback. Feedback occurs when sound from the speakers is picked up by the microphone, amplified, and sent through the speakers once again, like this:

For example, imagine placing the microphone right in front of the speaker. Now, tap the microphone. The sound travels through the amplifier, comes out of the speaker, and re-enters the microphone, repeating the cycle. This loop occurs so fast that it generates its own frequency, which we hear as a howling sound. The space between the microphone and speakers affects the frequency of this howl, since it controls how quickly the sound can loop through the system.

You can experiment with this on your computer if it has both speakers and a microphone. In Windows, you’ll need to activate both the microphone and speakers via the volume control (found by double-clicking the speaker icon in the system tray).

Make sure that neither the microphone nor speakers are muted, and that both are at full volume (if the microphone control is not visible, enable it in Properties). Once everything is set up, tapping the microphone should be audible through the speakers. Now, move the microphone close to the speakers and increase the speaker volume until you get feedback. Try adjusting the distance between the microphone and speakers to see how it changes the sound. Just be sure to avoid doing this at 2 a.m. when others are trying to sleep, and don’t forget to let the dog outside...

If you're setting up a sound system and want to prevent feedback, there are a few key guidelines to follow:

Explore these links to learn more: