What is Infrasound?
Infrasound is sound which extends below the range of human hearing (from 20 Hz down to 0.001 Hz), and it eminates from many natural and man made sources.
For example, some animals, such as whales, elephants and giraffes communicate using infrasound over long distances. Avalanches, volcanoes, earthquakes, ocean waves, water falls and meteors generate infrasonic waves. Some sources of man-made infrasound are nuclear and chemical explosions, engines, machinery and airplanes (Figure 1). Infrasonic waves propagate with very little attenuation and hence are capable of propagating over great distances.
The infrasound spectrum is filled with energetic sources and distinguishing between these sources is important since their identification influences the decisions regarding the response. Figure 1 illustrates the many sources of infrasonic waves and figure 2 shows the characteristics periods and wavelength scale for each type of infrasonic wave source.
Figure 2: The characteristics of different sources of infrasound (after Beer, 1974).