From ProAV Magazine, this is a handy refresher on feedback.  Favorite points: Real World Fixes 4,5,7, Tips for Mic Users and Multiple Mics.  Feedback is one of those things best to fix at its source.  Feedback exterminators are nice safety nets, but you should always limit feedback potential from the start.

For sound system users and listeners, nothing is more annoying than feedback. Here are some tips for preventing and minimizing feedback in your sound system.

THE GOAL
Prevent the sound emanating from the speakers from re-entering the microphones.

WHAT IS FEEDBACK?
Feedback is the squeal or howling sound created when sound from a loudspeaker is picked up by a microphone and then fed back into the sound system. Feedback "modes" are very narrow in bandwidth and can be numerous in rooms with poor acoustics. They usually occur between 50 Hz and 2,500 Hz.

FEEDBACK FACTORS
•Loudspeakers too close to microphones
•Type of microphone
•Too many open microphones
•Indiscriminatelyboosting tone controls
•Too many hard, reflective room surfaces (glass, marble, wood, etc.)

ELECTRONIC SOLUTIONS
1. The panic fix. Reduce master gain. You'll kill feedback, but you'll also affect the sound level.
2. Good. Add a graphic equalizer. Most graphic equalizers will cut a frequency band 1/3 or 1/6 octave wide, between 6 and 18 dB. However, you'll also alter the sound quality.
3. Better. Add a parametric equalizer. Use a narrow bandwidth filter and sweep to find the offending frequency.
4. Best. Use an automatic feedback controller. Feedback suppressors, eliminators, etc. automatically sweep the response and look for resonance build-ups that could cause a feedback loop.
When detected, a very narrow band "notch" filter is deployed at that frequency. The resulting effect on sound quality ranges from minimal to inaudible.

REAL-WORLD FIXES
For System Operators
1. Mount the microphones in fixed positions.
2. Place the speakers in front of the microphones so there's no direct path back to the microphone.
3. Aim the speakers so the sound doesn't reflect directly off a wall and back toward the mic.
4. Use cardioid or hyper-cardioid microphones, and point the mics away from the speakers.
5. Position stage monitors off-axis from mics.
6. Treat room surfaces to make them as sound absorbent as possible to minimize reflections. If possible, use acoustical absorbing tiles in the ceiling, put down carpeting, and hang curtains.
7. Educate the microphone user. When non-professional microphone users hear feedback, they usually put a hand over the mic capsule, which makes feedback worse by making the pickup pattern more omnidirectional. Instruct users to be aware of where the loudspeakers are, and to keep the mic away from them.

For Mic Users
1. Get closer to the microphone. Speak loudly and clearly so that the system doesn't have to amplify the sound so much.
2. Point the mic away from the loudspeakers.
3. If you're using a wireless mic, move away from the loudspeaker.
4. NEVER put your hand over the microphone.

MULTIPLE MICS?
Mute or turn down the gain of any microphone that's not in use. Noise gates can be helpful for doing this.

NAG AND PAG
Using the inverse square law, you can calculate how much system gain you need and the point at which feedback is most likely to occur.

NEEDED ACOUSTIC GAIN (NAG) is the gain in decibels required with sound reinforcement to achieve an equivalent acoustic level at the farthest listener that's equal to what the nearest listener would hear without sound reinforcement.

POTENTIAL ACOUSTIC GAIN (PAG) is the maximum achievable gain in decibels of a sound system before the risk of feedback occurs.

AUTOMATIC FEEDBACK SUPPRESSION
How It Works:
1. Feedback is identified by a DSP algorithm that distinguishes it from music - usually by identifying signals with little or no harmonic content.
2. A shallow, narrow notch filter is deployed.
3. If necessary, the notch will continue to deepen and narrow until the feedback is eliminated.
4. With many systems, multiple channels can be automatically deployed.
5. Some systems use fixed (or "static") filters, and some use dynamic (or "floating") filters that release the notch once feedback has been attenuated.

But Be Careful:
Many transducers will produce harmonic content even with a pure sinusoidal frequency. The DSP may fail to identify the feedback. Also, some non-feedback sounds naturally have weak harmonics and can be incorrectly identified as feedback, triggering a notch.

Tips When Using:
1. Place automatic feedback suppression last (after all system EQ and delay changes).
2. Avoid deep, narrow notches. Use shallow, wide notches instead. (Remember, you only need to bring the open loop gain below unity gain to keep things stable).
3. Only keep static filters that are necessary.
4. Only leave a couple of floating filters active and restrict their depth.