'''[[Infrasound]]''' is an anthropocentric term that refers to sounds containing some or all energy at frequencies lower than the low frequency end of human hearing threshold at 20 Hz. It is known, however, that humans can perceive sounds below this frequency at very high pressure levels.<ref>{{cite journal|last=Yeowart|first=N. S.|author2=M. J. Evans|title=Thresholds of audibility for very low-frequency pure tones|journal=[[J. Acoust. Soc. Am.]]|year=1974|volume=55|pages=814–818|doi=10.1121/1.1914605}}</ref> Infrasound can come from many natural as well as man-made sources, including weather patterns, topographic features, ocean wave activity, thunderstorms, [[magneticgeomagnetic storms]], earthquakes, [[jet streams]], mountain ranges, and rocket launchings.<ref>{{cite journal|last=Cook|first=R. K.|title=Atmospheric sound propagation|journal=Atmospheric exploration by remote probes|year=1969|volume=2|pages=633–669}}</ref><ref>{{cite journal|last=Procunier|first=R. W.|title=Observations of acoustic aurora in the 1-16 Hz range|journal=Geophys. J. R. Astron. Soc.|year=1971|volume=26|pages=183–189}}</ref> Infrasounds are also present in the vocalizations of some animals. Low frequency sounds can travel for long distances with very little attenuation and can be detected hundreds of miles away from their sources.<ref name="Kreithen & Quine 1979">{{cite journal|last=Kreithen|first=M. L.|author2=D. B. Quine|title=Infrasound detection by the homing pigeon: A behavioral audiogram|journal=Journal of Physiology A|year=1979|issue=129|pages=1–4}}</ref><ref name="Langbauer et al 1990">{{cite journal|last=Langbauer|first=W. R.|author2=K. B. Payne |author3=R. A. Charif |author4=E. M. Thomas |title=Responses of captive African elephants to playback of low-frequency calls|journal=Canadian Journal of Zoology|year=1990|volume=67|pages=2604–2607|doi=10.1139/z89-368 }}</ref>
