Underwater sound breaks the surface
Oct 24, 2006
Low frequency sound can travel almost unimpeded from water into air, claims a theoretical physicist in the US. The results are in stark contrast to the conventional view that the underwater world is largely silent to those above water and could have important implications for marine biology, climatology and geophysics (Phys Rev Lett 97 164301).
The simple ray theory of acoustics predicts that any sound produced underwater will be reflected at the surface, rather than transmitted into the air. As a result scientists had assumed that sound from the oceans are not transmitted into the air above.
Oleg Godin of the University of Colorado's Cooperative Institute for Research in Environmental Sciences (CIRES) has discovered that ray theory breaks down when used to describe low-frequency noises that are produced near to the surface of water. Or more precisely, when the wavelength of the sound is comparable to (or longer than) the depth of the source. By applying a more sophisticated theory of acoustics, Godin has shown that under these conditions sound is almost entirely transmitted to air.
Godin has proposed that water-to-air transmission at low frequencies involves two independent mechanisms. The first involves evanescent sound waves, which are generated by the source in addition to the more familiar plane waves of ray theory. The intensity of evanescent waves decreases exponentially with distance from the source and are normally extremely weak upon reaching the surface. However, for a shallow source, at a depth of a fraction of wavelength, the evanescent waves do not attenuate strongly and are transmitted to air. According to Godin, the sound is then transmitted as a plane wave in air because of the refraction that occurs at the water/air interface.
The second mechanism involves interference between the incident and reflected plane waves in the water, which occurs as the source nears the surface. According to Godin, the nature of this interference ensures that almost all of the sound is transmitted into the air.
Three research groups - two in the US and one in Europe – could soon be verifying Godin’s predictions in laboratory experiments, using acoustic waves in the kilohertz range.
According to Godin, his theory could be used to develop techniques for the detection of low-frequency sounds associated with the testing of nuclear weapons. It could also make marine biologists take a fresh look at how marine birds locate underwater prey and avoid underwater predators.
About the author
Darius Nikbin is a freelance science writer based in the UK