CAMBODIA INDONESIA MALAYSIA MALAYSIA EQUATOR 82° F 41° F 36° F THAILAND Indian Ocean Andaman Sea Bay of Bengal 95° East Longitude Currentsearch areas Perth Basin Broken Ridge 1 2 OCEAN SOUND CHANNEL 3 40° 30° 20° 10° 0° 10° Water temperature and depth at 95° East Longitude A history of ocean search areas for Flight 370 DEPTH, MILES How sound travels underwater WARM WATERFASTER SOUND WAVES 1 2 3 Surface ship Beacon The beacon’s high-frequency waves would be detectable in the sound channel, but only from a much shorter distance. Much of the signal would be drowned out by natural high-frequency noise created by waves, rainfall and winds within 12 miles of the beacon. Ions in the seawater would also absorb much of the sound.Why don’t black boxes emit low-frequency sounds that can travel farther? Large, heavy devices would be required to generate such signals, and they would need a lot of power to be heard above the low-frequency noise that fills the ocean. HIGH-PRESSURE WATERFASTER SOUND WAVES Both water temperature and pressure affect the speed of sound waves. Sound travels faster in warm water at the surface and in high-pressure water at the bottom. But at almost a mile down — where water is cooler than at the surface but is under less pressure than at the bottom — sound slows down. This is called the ocean sound channel, where low-frequency sound can be captured and transmitted for thousands of miles. 0 Signals can be blocked byseamounts, anda 37.5 kHz soundloses 6.5 decibelsevery 0.6 milesunderwater. A portion of the signal is guided horizontally along the ocean sound channel. A portion of the signal is guided horizontally along the ocean sound channel. Part of the guided signal can emerge from the sound channel milesfrom the original source. Part of the guided signal can emerge from the sound channel milesfrom the original source. OCEAN SOUND CHANNELSLOWER SOUND WAVES DEPTH, MILES Previoussearch areas

SOURCE: Timothy F. Duda, Ocean Acoustics and Signals Lab, Woods Hole Oceanographic Institution.