Mechanical solution
That was my thought as well- until I multiplied 19 by 60 and realized that we were talking about 1200 rpm, which is pretty fast. The motor would make a lot of noise, as would the piston. My plan was to use a piece of truck inner tube for a sheet of rubber, stretch it over a cylinder, put a hook in the middle of it and use a connecting rod attached to a wheel a la locomotives to generate the sound, but there were too many parts that would make noise. Another engineer suggested using a ball instead of a diaphragm, as you wouldn't need to worry about its orientation inside the pipe, but I was still worried about noise. Besides, then you can't change the amplitude (volume).

What I have in mind is to make a square wave and use it to power an electromagnet behind a sheet of rubber with a large chunk of iron attached to the center. I'll want it to grab at the iron, then let it go, then grab and let go- the rubber sheet will be a LOT less responsive than a traditional paper speaker, so it will have more of a sine wave to it (I think). I may need to use a fish weighing scale to determine the k value (spring constant) for the rubber and see if I can get it to somewhere near a natural (resonant) frequency near 19 Hz, but then I may not bother. If I get mushy sine waves, so be it.

One of the engineers suggested using a timer chip (a 555 chip, to be precise) to do this. I think that will be what I try. He also said I can get an amplifier chip for less than a buck that will pump it up a lot- and if I need more than that I'll feed it through a MOSFET to pump out the power. (See Ampere's Law- I need current going through it to produce a good strong field, as well as a LOT of windings.)

I acquired today four chunks of 12" diameter PVC pipe, about 18" long, with junctions to fit over them to retain the rubber sheet. I need to find my breadboard to make the trial circuit, and once I get that figured out I'll make actual circuit boards for it and install rheostats and whatnot for control- and then see about getting some suitable iron for the electromagnet. I'll need something about 1/2" diameter bent into a C shape, with the ends of the C pretty close together. I may be able to get one of the machine shops in the area to do that for me.

I'll let you know what happens. This may take a couple of weeks.
( The Resident Loon Not a demographic. Do not measure., Thu 1 May 2008, 2:30, Reply)