Sheep, your right about the thermal issues, these magnets will lose there magnetism at less that 100 degrees celcius, newer magnets have coatings that help prevent this, but in the end these will never be infinite duty cycle speakers at full power, they will need to cool off a bit after an extended period of full volume.
Davemcc, the way I refer this to spark timing is that all engines are timed so the flame front reaches the piston exactly at tdc, so the piston is just starting to move back down and the flame will assist in this.
If your spark timing is far too early, or an even better example, if you have detonation, the flame front reaches the piston as its still moving up, which puts huge stress on the parts because of the opposing force of momentum with the combustion chamber pressure.
This sudden change in direction is very rough unlike the smooth push of a properly firing charge, if the piston is at tdc its already overcome its momentum and just starting to make its way back down as the flame pushes it (relatively) gently down the cylinder, thats efficient. slamming it back down before that energy has dissipated is not.
Same thing with a speaker, you'll notice when the speaker is off it stays in a certain position, this is because of the spring like action of the supension. As the diaphram reaches the end of its 'stroke', the suspension is pulling it back. unlike a crankshaft, this is more flexible in terms of when its best to start the return stroke (because the force builds gradually), so the speaker can operate effectively under different frequencies where it might be returning earlier/ later, within certain design limits of course.
So now that the suspension has stopped the cone, the return signal begins pulling the diaphragm back down.
Now, if you pump too much power into those speakers, the problem is the magnet cant push/ pull with enough force, so the diaphram doesnt make it to the top of its stroke before the return signal comes. The diaphram is then pulled back before the suspension has had the chance to absorb its momentum.
This is like detonation in your engine. The result is a 'slamming' back of the cone, causing a sudden drop in pressure infront and pressure spike behind. (or vice versa depending on whether its at the top or bottom of its stroke) this creates a sharp cracking sound. Thats one kind of distortion, theres different kinds dependning on the frequence and input power but this kind of thing is the reason for most high volume distortion (signal noise is an entirely seperate issue).
That voice coil creates a very powerful magneti field, but sadly the ferromagnet doesn't. When the diaphragm is being pushed out, you could think of it like pushing against a large object.
If you were top push really hard againt a big sponge, you would be pushed back a bit, but mostly your arms would be thrust into the sponge. The weak field of a ferromagnet can also be 'penetrated' this way simply because the interia is too great to push the diaphragm away fast enough. it still pushes but its sort of half-assed.
A neodymium magnet is more like a wall, and pushing it results in a greater push back, meaning more output. the idea is theres no extra input, its just a more efficient force.
Its like pushing too like-pole ferromagnets together, it requires a force (intertia of the diaphragm) but it can be done pretty easily. neodmyium magnets are much stronger, unless they are very small I strongly doubt you will be able to push them together. This way they overcome intertia much more effectively.
A Neodymium magnet, with higher strength, pushes/ pulls the diaphram faster, so even with higher energy, the coil is moving fast enough that it does reach the top of its stroke before the return signal comes, meaning it can operate efficiently without distortion at higher energies (and is simply more efficient at lower energys because it pushes/ pulls the cone faster with the same signal, causing an increase in dB).
Thats why all the best speakers are using them, but they're cutting corners, theyre using something like N40-N45 magnets, which definately are strong but N55 is the highest strength magnets you can get and you really can tell.
-Matt
