Plasma Loudspeaker

Lightning provides nature's own entertainment system that has fascinated (and perhaps terrified) mankind for centuries. The ionized channel that results from electrical discharges between the opposing charges at the Earth's surface and cloud leads electric currents greater than 10000A that rise within a few microseconds but rarely last more than a fraction second per flash. With it comes significant gas heating (Gas Temp = 20-30 000 K) leading to molecular excitation, light emission from relaxation of excited molecular states and shock fronts resulting from the rapid thermal expansion that propagates for several kilometers as a sound wave.
A similar effect can be achieved in the laboratory albeit at a greatly reduced scale. Here the ionized channel is created and controlled through standard laboratory equipment with an audio modulating signal creating pure sound waves. This talk describes the electro-acoustic mechanism that makes a gas sing. The energy transfer mechanism between the current and the acoustic emission (similar to lightning) is described and a demonstration gives the student a feel for the clarity of its acoustic emission.