In July 2015,an underwater microphone was lowered into the Challenger Deep, the deepest point in the Mariana Trench,a trough on the Pacific Ocean floor to the east of the Philippines, the deepest part of the world’s oceans.
The scientists, from the US National Oceanic and Atmospheric Administration, Oregon State University and US Coast Guard, had expected a deep silence; instead, they heard a cacophony of sounds, both natural and created by humans.
Human-created noise has increased steadily in recent decades, and scientists in the future need data to determine how this might affect marine animals that use sound to communicate, navigate and feed — whales, dolphins and fish. The hydrophone stayed underwater for about 3 weeks; the next mission, in early 2017, will deploy it for longer, and also attach a deep-ocean camera.
Sound travels faster in water than in air; the distance it travels depends upon ocean temperature and pressure. Pressure keeps increasing with depth, but temperature stops falling after a point. The ripple-like sound waves from, say, the call of a whale, slow down as depth increases (and temperature falls), thus causing them to refract downward. Once the waves reach the bottom of the thermocline layer (600 ft-3,300 ft, corresponding to dysphotic and mesopelagic zones; right) their speed reaches its minimum. Below the thermocline, temperature is constant, but pressure continues to increase. This causes the speed of sound to increase and makes the waves refract upward. This “channeling” allows sound waves to travel thousands of miles without the signal losing too much energy. Placed at a proper depth, hydrophones can pick up whale songs and manmade noises like ship propellers many miles away.
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