Propeller-based subwoofer

I wanted to make a small (e.g., three-inch diameter) but powerful loudspeaker for low frequencies, and thought one might be able to modulate the movement of a lot of air by varying the pitch of a propeller.

The idea is that you'd make a propeller shape from a moderately flexible material (e.g., HDPE, such as you could cut from a plastic milk jug), glue thin bar magnets to the blades, mount the propeller on a motor shaft, locate it inside a solenoid, and vary the solenoid current to change the propeller pitch. If you make the propeller spin fast enough, and drive the solenoid with an audio waveform, you should be able to hear the audio. (Certainly you'd also get regular old propeller noise. Bummer. Maybe that could be baffled.)

Of course, the air the propeller is driving must come from somewhere, and you don't want sound from back of the propeller to cancel sound from the front, so you'd need to put the whole thing in a duct.

I actually started trying to build one of these things when I was in college, but I never finished it, and I wouldn't tell anyone what I was trying to make, because I didn't want to be laughed at. One very nice thing about getting older is that I don't care about that anymore.

(Looks like I need to learn how to search better. My idea is similar to Ultimate Subwoofer (see link), but in that idea, a fan is modulated by a valve.)