INFRASONICS

vibrational or stress waves in elastic media, having a frequency below those of sound waves that can be detected by the human eari.e., below 20 hertz. The range of frequencies extends down to geologic vibrations that complete one cycle in 100 seconds or longer. Naturally occurring infrasonic vibrations take the form of tidal motion or earth tremors. The monitoring of very small earth tremors using a seismograph has value as a means of providing early warning of volcanic activity or of serious earthquake shocks. Artificial tremors may be generated with underground explosions in order to identify and map underlying rock structures. With an array of seismic detectors, a computational form of holography may be achieved. The development of extremely sensitive detectors has provided a means of monitoring underground nuclear-explosion tests, thus contributing to the maintenance of the nuclear test ban treaty. Often associated with severe earthquakes are infrasonic disturbances of the atmosphere that may extend to 50 km (30 miles) above the Earth's surface. These waves can travel considerable distances around the globe. Human perception of low-frequency sound waves propagating in air does not have a well-defined cutoff point. Above about 18 hertz sound waves appear to have tonality; below this frequency the individual compression waves may be distinguished. Driving an automobile with an open window may generate an infrasonic resonance. The sonic boom of supersonic aircraft contains significant levels of infrasound. In certain circumstances occupational exposure to infrasound may be severe: transformer rooms, compressor plants, and engine rooms may all produce levels that are extremely high and cause discomfort. The mechanisms by which infrasonics may be perceived by humans and their physiological effects are incompletely understood.