From Ted Weber's "Let's Talk Speakers":
OK, let's start with basics. What are the main differences in sound between comparable speakers, one with a ceramic magnet, one with AlNiCo? Also, how does the size of the voice coil affect the sound.
Jeff, good questions. The whole 'AlNiCo mojo' is about smooth compression at high average levels, such as what you would have running the amp flat out. AlNiCo (Aluminum-Nickel-Cobalt) is an alloy magnet and all alloy magnets are easier to demagnetize than comparable Ceramic (Strontium Ferrite) magnets. What this means is that as the voice coil starts moving in response to the input signal, it generates a magnetic field of its own that tries to demagnetize the magnet. As its effect lowers the available magnetic field of the AlNiCo magnet, the speaker becomes less efficient, the voice coil moves less, etc. The physics of it is that the small magnets near the surface of the magnet poles (called 'domains') begin to change state, or flip directions. The result is smooth compression, the same kind of operating curve compression that occurs in a tube amplifier. The ceramic magnet, on the other hand, doesn't compress or demagnetize as easily, so the voice coil moves to its mechanical limit and won't go any farther. This is why some players say ceramics sound a little edgey at high average levels as opposed to AlNiCo. However, by properly designing the entire magnetic circuit, Ceramics can be made to behave quite well for desireable guitar amp tone and dynamics. You might compare the two magnetic circuits to solid state amps versus tube amps, where the solid state amp gives it all its got then clips hard, while a tube amp compresses nice and smooth. The extension of this idea, then, is that with the AlNiCo, like the tube amp, you can seem to have a louder average volume since it gets compressed smoothly. By the way, the compressing or demagnetization that occurs with the AlNiCo is not permanent. It springs right back to its design operating point.
A voice coil is like an electric motor. The bigger the voice coil, the more wire used, the more torque or pulling power you have to move the cone. With the proper match of components, you can get more sensitivity, wider frequency response, and more power handling ability.