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Sound waves, particularly in the atmosphere, whose frequencies of pressure variation and of vibration are below the audible range, that is, lower than about 20 Hz. Earthquake and seismic waves are elastic waves which occur at infrasonic frequencies in the Earth's crust and in the oceans and seas. The physical laws of propagation in the atmosphere are essentially the same as for audible sound. The local speed of infrasound in air at ambient temperatures near 20°C (68°F) is about 340 m/s (1115 ft/s), the same as for audible sound.
At frequencies less than about 1.0 Hz, infrasound propagates through the atmosphere for distances of thousands of kilometers without substantial loss of energy. Sounds at these frequencies are almost always present at measurable intensities. Those of natural origin have many causes, including tornadoes, volcanic explosions, earthquakes, the aurora borealis, waves on the seas, large meteorites, and lightning discharges. When the wind blows, turbulent pressure fluctuations in the atmosphere occur at amplitudes up to tens of pascals, at infrasonic frequencies. People are unaware of these pressures via the sensation of hearing.
Sufficiently strong infrasound is “audible,” contrary to simple acoustic tradition. The threshold sound pressure level (the least intensity for audibility) is about 92 dB at 16 Hz, and increases 12 dB per octave to about 140 dB at 1.0 Hz. However, there is no sensation of tone. Listeners variously describe audible infrasound as pumping, popping effect, or chugging. For vibration at very low frequencies, motion sickness of people in boats must have been one of the earliest noticeable effects. The human body is particularly sensitive to vibrations and infrasound near 7 Hz, at which frequency there is an overall mechanical resonance of organs in the abdominal and chest cavities. See Atmospheric acoustics, Sound
elastic waves similar to sound waves but at frequencies below those audible to humans. The upper limit of the infrasonic region is usually assumed to be 16—25 hertz (Hz). The lower limit of the infrasound range is indefinite. Vibrations of tenths and even hundredths of a hertz—that is, with periods dozens of seconds long—are of practical interest. Infrasound exists in the noises of the atmosphere, the forests, and the seas; its source is atmospheric turbulence and wind (for example, the so-called voice of the sea is infrasonic vibrations created by the wind vortices on the crests of ocean waves). Infrasonic vibrations are also produced by lightning discharges (thunder), explosions, and gunfire.
Tremors and vibrations at infrasonic frequencies are observed in the earth’s crust from a great variety of sources including explosions, avalanches, and transportation.
The absorption of infrasound in various mediums is typically small, so that infrasonic waves in air, water, and the earth’s crust can propagate over great distances. This phenomenon is of practical use in locating strong explosions or the position of guns that are firing. The long-range propagation of infrasound in an ocean makes possible the prediction of natural calamities, such as tsunamis. The sounds of explosions, which contain a large number of infrasonic frequencies, are used to study the upper atmospheric layers and the properties of the aqueous medium.
Infrasound is received and measured by means of special microphones, hydrophones, geophones, or vibrometers.
REFERENCESShuleikin, V. V. Fizika moria, 4th ed. Moscow, 1968.
Cole, R. Podvodnye vzryvy. Moscow, 1950. (Translated from English.)
I. G. RUSAKOV