There are some amazing videos on You Tube which beautifully illustrate the properties of ferrofluids.
I firmly believe that where there is science, there is beauty.

So, here's the beauty:







.........and now for the science:






The clips are not camera trickery or time lapse photography, it's merely the application and manipulation of magnetic forces on a liquid that is loaded with metal particles.
In a ferrofluid, the carrier (or liquid) is usually a synthetic oil - the fact that its oil is fairly apparent in the videos above. The particles are normally Magnetite (Fe₃ O₄).

So, why do they put ferrofluids into loudspeakers then?
1. It conducts heat away from the voice-coil. These are made of extremely thin, fragile wires which can burn out if over stimulated. It helps to make a speaker more efficient.

2. It's relative density ('stiffness') keeps the voice coil concentrically aligned with the magnet

3. That density assists with damping the moving parts of the motor (as well as those that aren't supposed to move).

I don't know when ferrofluids were first used in loudspeakers
- anyone?