Marine acousticsMarine acoustics use sound to investigate the underwater world. Marine acoustics are either active or passive. Active acoustic techniquesActive acoustic techniques allow researchers to "see" underwater, where light does not penetrate, to depths of several hundred metres. Echo-sounders emit sounds that travel through water and encounter obstacles including living organisms. Researchers can determine the position and properties of such obstacles through the echo that is retransmitted. They may then reconstruct an image of the distribution of objects encountered or determine the relief of the seabed. Using echo-sounders, scientists can distinguish fish echoes from zooplankton echoes at different frequencies. For example, the echo from krill is stronger at 120 kHz than it is at 38 kHz. At 38 kHz capelin has a stronger echo. Zooplankton that is smaller than krill has a stronger echo at frequencies above 120 kHz. Here is an echogram of a cloud of krill (blue-green) and of a school of capelin (small red dots) detected at the head of the Laurentian Channel using an echo-sounder operating at 120 kHz (© Fisheries and Oceans Canada chair in applied marine acoustics for research into resources and the ecosystem at ISMER-UQAR).
The study of sound propagation in water can also be used to determine characteristics such as average water temperature. Powerful, low-frequency sounds can be used to find modern submarines over great distances or even to explore the geological composition of rock formations below the seabed. This technology is particularly useful for underwater oil exploration. Passive acoustic techniquesDid you know that sound travels four times faster in water than in air? And that it travels much further? Passive acoustic techniques are used listen in on underwater sounds. To capture these sounds, researchers place hydrophones—waterproof microphones—under water. These then become their ears beneath the waves! However, whales can emit sounds that are inaudible to the human ear, known as infrasounds and ultrasounds. Hydrophones can detect these sounds, but for us to hear sounds below 60 Hz and above 16 000 Hz they must be transformed. They are either accelerated or slowed down. Here is what a low-frequency fin whale vocalization sounds like when accelerated 20 times its original frequency.
Researchers can even determine the position of a sound source—such as a whale—using a network of hydrophones placed underwater. Finally, this technique can also be used to study naturally occurring ambient noise (such as earthquakes) and noise generated by humans (such as boat noise). Thus researchers are able to evaluate noise pollution levels and their effects on different marine species.
Discover projects that use the marine acoustics technique:Can the great whales of the Estuary tell us where they are? What influences the accumulation of whale food in the Estuary? |
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