An Acoustically Driven Vortex Cannon
The vortex cannon is a physics demonstration in which an impulse of air is pushed through an orifice in the end of a cylinder, resulting in a well-defined ring that travels subsonically through the air. We developed a vortex cannon driven by a subwoofer and a sawtooth waveform. Using a fog machine, we were able to generate 8-10 smoke rings per second. The design of this demonstration was described in The Physics Teacher (see #101 in Publications).
Here is high-speed video of a vortex being generated: VortexRing.avi