Abstract

The Silent Sea. Made famous by perhaps the most widely-known and respected marine environmentalist of our time, Jacques Cousteau, in his book and award-winning film of the same title. But the sea is far from a silent world. To those who live there, the sea is a seething torrent of sound, rich with information about the geography, local inhabitants and perhaps the latest gossip from neighbours. In air, sound propagates rather poorly compared to underwater. Even very loud sounds, such as jet engines on aircraft, can only be heard for O(103)m. A similar acoustic power, if injected into the deep sound axis channel of the ocean, can travel full global distances, O(106)m. Light, by contrast, propagates extremely poorly, travelling only a few tens of meters before being absorbed. It is thus that sound and hearing replaces light and vision as the primary sensory system for marine mammals. Consequently, this group of animals has evolved extremely sophisticated and sensitive hearing, employing both passive and active acoustics to navigate through their environment, find their food and communicate. We must therefore closely examine the acoustic environment of the oceans if we are to understand the possible impact of manÂ's activities. Human hearing ranges from about 50 Hz to perhaps 16 kHz. Marine mammal hearing ranges from as low as 18 Hz (Blue and Fin whales) to 160 kHz (Dolphins). Underwater noise is characterised by an extreme geographical and temporal variability over five decades of frequency. In this astonishing range, over ten times that of human hearing, there are many players.