I found this linked on another site, and thought it may be interesting to have a look at

The most insanely powerful motor in the world.....

Back in the 70's, disk drives looked like washing machines. The disk pack loaded in from the top and had ten platters. So the head assembly had twenty heads and weighed a couple pounds. Moving that massive head asembly back and forth to seek a track at 200 inches per second required a seriously powerful motor. You'd typically see linear motors with magnets that weighed 100 pounds and more, and the force was enough to break your arm if it were to extend when you weren't in a safe position. Serious horsepower, and good from DC up to a few hundred Hertz.

You can find these kinds of disk drives at electronics salvagers and buy the motors pretty cheap, if you're interested. Make a helluva sub for single digit frequencies; Great for impressing your buddies!