I need to generate a pressure waveform or signal of about 2 to 4 psi (a square or sinusoidal signal would be fine) and the frequency needs to be variable, from 1 hz to maybe 25 hz. I've thought about running a small hobby motor connected to a piston pump or bellows (the pressure can be conducted through air or a fluid like water). I basically want this pressure signal to be delivered through a flexible tube to a pressure transducer.

One off-the-shelf item that sort-of does this is an aquarium air bubbler. These things have a bellows that's articulated or vibrated with the help of a small magnet that moves back and forth across an air gap of a transformer that is connected directly to the main 120 V ac power. What I'd like to do is power this thing with a variable-frequency power supply. I'm thinking the current is relatively low - maybe 100 ma.

I know this looks a lot like I need a signal generator, but I think I need more drive current (and a lot higher voltage) than your typical signal generator.

Any ideas?

 

How about a speaker with a waterproof diaphragm?

 

Well, those type of AC-powered devices are designed to operate over a fairly narrow range of frequencies; usually 47Hz to 63Hz. If you tried using a lower frequency, the coil would rapidly enter saturation, and it would "look" like a dead short to the AC input. You would then have a blown fuse, or smoke. Maybe both.

I like your "hobby motor driving a bellows/piston/diaphragm" idea much better. If you have a bass speaker in an enclosure sitting around, that might work.

 

Some of the "higher end" aquarium pumps are actual piston pumps with a crankshaft and motor. Replacing the original motor with a small DC motor and using PWM to drive it should give you what you need. Or better yet, try one of these....http://www.aquariumguys.com/battery-pumps.html