There has recently been some discussion about clipping in the Component vs. Non component 6x9 thread and I wanted to move discussion here so we didn't further ruin that thread.
There were bits and pieces of good info in that thread but no one really put them all together. There was also some information that was flat out wrong. Let's get some facts straight before we move on.
-A speaker will fail for 2 reasons
1) Over reaching it's thermal limits
2) Over reaching it's mechanical limits
-Clipping an amp will cause it to produce maximum power in excess of it's rating
-No speaker, in history, has ever died because it was underpowered......EVER
To start off, a signal that a speaker sees is an oscillating wave. That is, one that goes up and down. The farther each wave gets from it's zero point, the base line, the higher the amplitude. Each peak of the wave is the maximum power of that signal. Let's assume that this "peak" in our case is 100watts. Our signal from the headunit is matched to the input sensitivity (gain) of the amp so even at fill tilt, the signal is clean and not clipped so we have a pure 100watts. Now, this amp we are using in this example is rated at 100watts and if that rating is correct it will only produce 100watts, no more. So there is a cap at 100watts.
Now let's get destructive and wildly turn up the gain like an idiot who thinks it's just another volume knob. Now the amp will output 100watts before the headunit reaches it's maximum volume setting. So if we keep turning up the volume on the headunit, past the point where the amp is outputting 100watts, it will "clip" the signal. The wave will no longer be smooth and sloping. The peaks will be clipped off. The first picture below is what the signal SHOULD look like. The second picture is a clipped signal.
Notice the smooth peaks of the waves are now flat.The peak still represents 100watts but the duration at which peak power is sustained has been increased. Peak power is identical but the power over time has been DRASTICALLY increased. For example, I'm a cross country runner. I can run 15mph no problem. But I can't sustain that pace for an entire mile (4min mile) because I'll burn myself out. My body can recover from a burst to that speed but it can't sustain it. The same goes for a speaker. It can sustain brief moments at peak power because it can dissipate the heat effectively over time. But when peak power is sustained for a long time the speaker can't dissipate all the heat and will thermally fail. (think smoke)
Additionally, the flat spots on the clipped wave represent pauses in the cone's movement. So during the duration of the clip the speaker is at rest and not producing sound. It should be quite evident why this is not pleasant to the ears.