Hi, just very quickly. If I use an underpowered speaker on an overpowered amp, and turn the volume up very loud, what exactly would happen? Distortion maybe? Also, would this damage the speaker (presumably)? Would this damage the amp in any way? Thanks.
The simple answer is that you would probably do more damage to your hearing in the first instance. Yes the speakers will get damaged if driving them to the amps maximum. You are in charge of the volume control and if you know there's a big imbalance and you damage your kit then there's only one to blame.
There is a lot of kit out there that has speakers below the maximum available from the amp that work perfectly together.
I have a 2.1 system, want to make it 5.1. I bought a centre speaker a while back without doing too much research. If it’s significantly underpowered, I just want to confirm that it will do no damage to my amp? And if it is crap, then I could use it temporarily without any worries.
The more speakers you add to a receiver then the more the power is diluted and as most receivers are just quoted with two channels driven it's unlikely you'll do any damage. Figures quoted from most manufacturers when all speakers are engaged have to be taken with more than a pinch of salt.
Long term high power will eventually overheat the drivers and the voicecoils will fail.
Square waves from a clipping (underpowered) amplifier will create large transient currents as the speaker attempts to stop and start very quickly, causing the speaker to fail. The increase in distortion will often increase the amount of in band signal and this will saturate particularly the HF driver as per failure mode 1.
High volumes can cause the voice coil to leave the safety of the magnetic flux field created by the magnet. They then become just a coil of wire instead of an electromagnetic component and quickly burn out.
Matching an amplifier to a speaker in a domestic environment is not really an issue, as you need to be about a factor of 3-5 out before issues will arise. IE: running a 30w speaker on a 100w amplifier or vice versa will be absolutely fine unless you deliberately try to make it fail by running it at high distortion levels for long periods of time.
Home audio is not designed to be idiot proof. If it sounds or smells bad, turn it down! If you enjoy your ears bleeding buy a suitable high power system and if you can't recognise high distortion buy an active system that will have more protection built in and will be harder to break.