Diffraction of Spark-Produced Acoustic Impulses

A transient electrical discharge in air serves as a convenient source of acoustic impulses for studies of wave diffraction. “N waves,” with initial shocks about 0.01 atm in magnitude and with durations of less than 30 μsec at 10 cm distance, are produced by sparks having discharge energies of 0.05 J. They are detected with wide-band condenser microphones, which utilize a 14-mil metallized Mylar diaphragm supported partially by the thin layer of air trapped on a machined brass surface. The receiving response is nominally flat to above 12 MHz. Phenomena which are demonstrated here include the diffraction of N waves incident upon circular disks and narrow rectangular plates.