infrasound

infrasound

soundlike waves having a frequency below the audible range, that is, below about 16Hz

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infrasound [′in·frə‚sau̇nd]

(acoustics)

Vibrations of the air at frequencies too low to be perceived as sound by the human ear, below about 15 hertz.

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Infrasound

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