Amps take a dc current and change it to an ac current, which then runs to the sub, through the voice coils. The current running through the voice coil makes it an electro magnet. The frequency of a sound wave wanted to be produced is determined by the frequency of the current running through the voice coils, which determines how fast it switches between positive and negative polarity. The when the voice coil switches one way, it is attracted to the magnet, when it's the opposite polarity, it's repelled, and want's to move out.
Furthermore, it isn't the size of the magnet that matters. BL (motor strength) is more important. There are different magnet materials used now and size isn't a big factor. It is possible to have too much motor power, and resulting in overdamping the driver.
Yes, materials can be a big factor actually. You'd be suprised at the strength of a teeny pice of magnet X, compared to a big chunk of magnet Y.
Jon, I get so sick of seeing people think certain subs are gonna be uber loud because they have a 150, 200 oz magnet. Some companies, *cough, Audiobahn, love to advertise oversized magnets on their subs, and draw people with little knowledge into them.
I couldntagree more, and jonathan said it well in another post about how the IDmax is extremely efficient with a reltively low amount of power, Honestly that is one of the best subs I have ever heard and the magnet is half the size of some of those audiobahn subs!
Look at speakers like the Morel Hybrids and DLS loudspeakers. They're integrating ferrite and neodymium magnets so that they can use a smaller magnet, making installs MUCH easier in today's smaller cars due to less mounting depth. It's providing a BL of 7+ on these speakers, which is pretty high for a 6.5" driver. The Adire Audio Shiva uses a small magnet.