Acoustic Observatory -- Low-Cost Check out my Acoustic Observatory It uses well-known and obvious principles of operation to "see" thunder at great distances. It cost about $50 to build, including the red laser diode -- but not including the front surface mirror which has been lying around my "lab" since the late nineties. (It's a small unmounted mirror, 1" x 1"; I wouldn't have paid more than ~$20 for it.) My goal with this simple device was to "see" thunder from storms approaching from west of Washington, D.C. "See" the thunder, that is, before I could hear it, maybe hours before I could hear it. Also I wanted to "see" certain long-wave sounds called "microbaroms" that, according to the Wikipedia, are created by storms at sea. I hope, if hurricanes come up the east coast this fall, to see plausible signals of microbaroms, which supposedly travel thousands of miles. I'm posting this because the laser-and-mirror method of this Acoustic Observatory seems like a low-cost way to detect certain small signals. Seismic signals, for instance, very small ones as come from traffic a quarter-mile distant might be detectable. You can see a schematic diagram of a possible Seismic Observatory.