This could be tough depending on how much sand you are quaking. May I suggest using pneumatics to pressurize something like innertubes? The problem is that the waveform would be full of compression/ pressurization artifacts, the advantage is that its pretty cheap.
Ummm, Servos don't have a negative signal, rather the position is maintained relative to the pulse width via the feedback circuit to the position sensor (which is usually a mechanical potentiometer). So you send out pulses at some regular cycle length, eg 50 Hz, and the width of the pulse...
I've been looking at "how to" vids that show how an air horn works: basically a thin diaphragm of metal, or even balloon rubber, is made to vibrate by passing compressed air over its surface.
Can anyone tell me how you control the frequency of the vibration to control the tone produced. Q...