Infrasound – also known as subsonic sound. These are sound waves (acoustic) with a frequency too low to be heard by the human ear. These sounds occur below the frequency threshold of 20 Hz, which is still audible to humans, although researchers also indicate a threshold of 16 Hz. Infrasound that we cannot hear can be perceived and used for communication by animals, such as whales and elephants.
Currently, there are two standards for defining infrasound:
PN–01338:2010 (Polish standard after amendments) – according to this, infrasound is defined as acoustic waves with a frequency between 1 Hz and 20 Hz;
ISO 7196 – similar to the above standard, infrasound noise is defined within the range of 1 Hz to 20 Hz.
A characteristic feature of infrasound is the large wavelength of its acoustic waves, which exceeds 17 meters. Consequently, as long as they are poorly attenuated, they can propagate over long distances. Due to their specificity, acoustic screens and other structures are not very effective against them.
Natural sources of infrasound include:
– bolides;
– ocean waves;
– large waterfalls;
– strong winds;
– avalanches;
– tornadoes;
– lightning;
– auroras;
– seismic waves (earthquakes);
– volcanoes;
– cracking glaciers;
– animals (including whales, elephants, alligators, hippos, and giraffes)
Artificial sources of infrasound generation include:
– explosions;
– loudspeakers;
– thermonuclear and atomic explosions;
– vibrations of bridges;
– heavy motor vehicles;
– helicopters and jets;
– pipelines;
– drilling towers;
– refrigerators and air conditioners;
– wind turbines, forge hammers, vacuum pumps, piston compressors, etc.
