Infrasound is sound that is lower in frequency than 20 Hz (Hertz) orcycles per second, the normal limit of human hearing. Hearing becomesgradually less sensitive as frequency decreases, so for humans toperceive infrasound, the sound pressure must be sufficiently high. Theear is the primary organ for sensing infrasound, but at higher levels it ispossible to feel infrasound vibrations in various parts of the body.
The study of such sound waves is sometimes referred to as infrasonics,covering sounds beneath 20 Hz down to 0.001 Hz. This frequencyrange is utilized for monitoring earthquakes, charting rock andpetroleum formations below the earth, and also inballistocardiography and seismocardiography to study the mechanicsof the heart. Infrasound is characterized by an ability to cover longdistances and get around obstacles with little dissipation.
Infrasound was used by Allied forces in World War I to locate artillery. Oneof the pioneers in infrasonic research was French scientist VladimirGavreau, born in Russia as Vladimir Gavronsky. His interest in infrasonicwaves first came about in his lab during the 1960s, when he and his labassistants experienced pain in the ear drums and shaking lab equipment, butno audible sound was picked up on his microphones. He concluded it wasinfrasound and soon got to work preparing tests in the labs. One of hisexperiments was an infrasonic whistle.
Infrasound sometimes results naturally from severeweather, surf, leewaves, avalanches, earthquakes, volcanoes, bolides, waterfalls, calving of icebergs, aurora, lightning and upper-atmosphericlightning. Nonlinear ocean wave interactions in ocean stormsproduce pervasive infrasound vibrations around 0.2 Hz, knownas microbaroms. Infrasound can also be generated by man-madeprocesses such as sonic booms and explosions (both chemicaland nuclear), by machinery such as diesel engines and older designsof down tower wind turbines and by specially designed mechanicaltransducers (industrial vibration tables) and large-scale subwoofer loudspeakers. The Comprehensive Nuclear-Test-Ban Treaty Organization uses infrasound as one of its monitoringtechnologies (along with seismic, hydroacoustic, and atmosphericradionuclide monitoring).
Whales, elephants, hippopotamuses, rhinoceros, giraffes, okapi, and alligators are known to use infrasound to communicate overdistances—up to hundreds of miles in the case of whales. It hasalso been suggested that migrating birds use naturally generatedinfrasound, from sources such as turbulent airflowover mountain ranges, as a navigational aid. Elephants, inparticular, produce infrasound waves that travel through solidground and are sensed by other herds using their feet, althoughthey may be separated by hundreds of kilometres.
•Infrasound station. The stations detect and give theazimuth of the nuclear explosions. The location is given bythe intersection of different azimuth
Animals have been known to perceive the infrasonic wavesgoing through the earth by natural disasters and can use theseas an early warning. A recent example of this is the 2004 IndianOcean earthquake and tsunami. Animals were reported to fleethe area long before the actual tsunami hit the shores of Asia.Itis not known for sure if this is the exact reason, as some havesuggested that it was the influence of electromagneticwaves, and not of infrasonic waves, that prompted theseanimals to flee.
An elephant generates and probablydetects infrasound.A cassowary might pick up ultrasonicsignals with its casque, a mysteriousstructure on top of its head. No one is yetsure what this structure is for.
20 Hz is considered the normal low frequency limit of human hearing.When pure sine waves are reproduced under ideal conditions and at veryhigh volume, a human listener will be able to identify tones as low as12 Hz. Below 10 Hz it is possible to perceive the single cycles of thesound, along with a sensation of pressure at the eardrums.The dynamic range of the auditory system decreases with decreasingfrequency. This compression can be seen in the equal-loudness-levelcontours, and it implies that a slight increase in level can change theperceived loudness from barely audible to loud. Combined with thenatural spread in thresholds within a population, it may have the effectthat a very low frequency sound which is inaudible to some people maybe loud to others.
Infrasound has been known to cause feelings of awe or fear inhumans.Since it is not consciously perceived, it can makepeople feel vaguely that supernatural events are taking place.Some film soundtracks make use of infrasound to produceunease or disorientation in the audience. Irréversible is onesuch movie.The infrasound and low-frequency noise produced bysome wind turbines is believed to cause certain breathing anddigestive problems in humans and other animals in closeproximity to the turbines.