Seawater itself blocks the transmission of sound, according to Timothy Duda, director of the Ocean Acoustics and Signals Laboratory at the Woods Hole Oceanographic Institution. Magnesium chloride and other salts dissolved in the ocean absorb sound energy. Undersea mountain ranges and canyons can cause acoustic "echoes" just as on land. In fact, the ocean is filled with natural and human-produced noises.
“If the ocean were perfectly quiet, you could hear manmade signals extremely far away,” Duda said. But there are other things that make sounds: crashing waves, bubbles popping, underwater landslides and volcanoes, whales and other marine animals. As well as the sound of passing ships. All that sound is running around the ocean.”
Acoustic devices employed by the search crews try to filter out other sound wavelengths so the beeping black box stands out.
Despite these obstacles, pinger locators like the ones being towed across the water by Australian and Chinese naval crews occasionally do work. The U.S. Navy found the remains of Adam Air Flight 574, which crashed on Jan. 1, 2007, while flying between the Indonesian cities of Surabaya and Manado, killing all 102 people on board, between 5,000 and 6,200 feet.
A year later, a Navy locator managed to find a Marine Corps AV-8B Harrier that crashed in the Gulf of Aden near Oman in 2011 shortly after takeoff, according to Foreign Policy magazine.
Duda says the current search teams are going to have to rely on advanced underwater acoustic technology and a healthy dose of good luck.
“They have to be very close,” Duda said. “You are always going to be receiving something, but you are looking for something that stands out above the other noise.”