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Infrasound can result from both natural and man-made sources: Natural events: infrasonic sound sometimes results naturally from severe weather, surf, [6] lee waves, avalanches, earthquakes, volcanoes, [7] [8] bolides, [9] waterfalls, calving of icebergs, aurorae, meteors, lightning and upper-atmospheric lightning. [10]
Infrasonic vocalizations have been recorded from captive elephants in many different situations. The structure of the calls varies greatly but most of them range in frequency from 14 to 24 Hz, with durations of 10–15 seconds. When the nearest elephant is 5 m from the microphone, the recorded sound pressure levels can be 85 to 90 dB SPL. [9]
Several animal species can hear frequencies well beyond the human hearing range. Some dolphins and bats, for example, can hear frequencies over 100 kHz. Elephants can hear sounds at 16 Hz–12 kHz, while some whales can hear infrasonic sounds as low as 7 Hz.
When the sound source is to the right or left of the head, localization is restricted to within about 15 degrees. Additionally, localizing a noise rather than a pure tone sound is much easier.
The study authors predicted the migrant birds listened to infrasound — sound at frequencies too low for humans to hear — associated with the storms and heeded it as a warning sign.
Infrasound is sound waves with frequencies lower than 20 Hz. Although sounds of such low frequency are too low for humans to hear as a pitch, these sound are heard as discrete pulses (like the 'popping' sound of an idling motorcycle). Whales, elephants and other animals can detect infrasound and use it to communicate.
A 1973 report cites a university study of fifty cases of people complaining about a "low throbbing background noise" that others were unable to hear. The sound, always peaking between 30 and 40 Hz (hertz), was found to only be heard during cool weather with a light breeze, and often early in the morning. These noises were often confined to a 10 ...
Both of these popular methods are used to analyze sound and better understand the acoustic phenomenon. The entire spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. The audio range falls between 20 Hz and 20,000 Hz. This range is important because its frequencies can be detected by the human ear.