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Audio signals are a/c voltages. Speakers will fry if they receive d/c voltage. Some of the older solid state amps could send moemtary d/c voltage spikes to speakers when you first fire the amp up. Most newer quaility amps have delays or protection built into them to prevent this from happening. If you use an amp that doesn't have enough power for your application the tendency is to turn it up beyond it's clean power rating which then starts sending a form of a square wave instead of a sine wave. Depending how bad you are clipping the amp, the square wave is seen to the speaker as some dc voltage. The voice coil overheats and that is bad news for the speaker or driver. Even is you have an amp that is powerful enough for your application it has the protential to send transient peaks that are capable of causing speaker/driver damage. Some amp and powered speakers have clipping L.E.D.'s that show when the amp is clipping. This helps give you a warning as to what your speaker may be seeing. Some have protection built into the amps just for this reason. I like to use a limiter to help prevent to much clipping. I have found that karaoke disks can often have very different output levels from song to song. A limiter will help level things out. I used to rebuild or recone speakers years ago. The only break in I would do was hooking up an low amplification signal generator set for low frequency in order to have the cone move in and out to make sure the coil in the speaker was properly aligned and not rubbing. May of the home speakers (woofers) have a foam surround at the outer edge of the cone to allow more cone movement for low end frequencies. This foam rots from age and humitity. It will also stretch if the speaker is over driven with much power at the low frequency spectrum and this will somtimes cause the voice coil to rub a little in the magnetic gap that it travels in. If it starts rubbing the coating on the windings of the voice coil the coating will start coming offf and it is possible for the coil to burn in spots until it opens up. The speaker is dead that point. Hope this helps.
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