I want to do a project where sound controls and electromagnet, which in turn controls some magnetic fluid. Nothing fancy here. Just Audio ---> Magnet.
Here is what I think I know so far....

A voice coil for a speaker is just a basic electromagnet. Via some testing, it doesn't seem to be a very powerful or efficient one. It doesn't need to be since it just bounces off the string permanent magnet in the speaker housing.
I have also deduced that a moderate audio amp will produce up to 6V A/C on the outputs. This of course varies with the input.
I have also found for sale some pretty powerful (they claim a 200lb holding force) electromagnets that run off of a single D cell. Which is IIRC 1.5V max.
Therefore I should be able to power an electromagnet off of an amp and get what I want....
BUT....
I am concerned that my HZ are way to high to effectively move the ferrofluid. All tweeter and no bass if you get my drift. It seems like I would need to bring the Hz way down to push around this stuff.
Any help here?