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Over Powering: gets the most blame, but you have to do it for a long time to cause speaker failure.

Under Powering: is the most common. A distorted signal delivers far more power than a clean signal. Smaller amps distort easier than a large ones.

Transients: Sudden loud sounds like dropping a microphone or plugging and/or unplugging cables when the amp is on can damage speakers.

Feedback: is the loud squeal that is heard when a microphone is pointed too close to a speaker or the volume is too loud. Feeding back for more than several seconds may burn out speakers.