Australian navy officials reported they picked up an electronic signal at the same frequency as a flight data recorder, or black box, near the search area for Malaysian Airlines Flight 370. But tracking the location of a sound to its source thousands of feet below the ocean surface won’t be so easy, according to experts in marine acoustics.
“Sound doesn’t travel in straight lines underwater,” said Bruce Newhall, a researcher at Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
Layers of warm and cold water block or divert sound waves, and so does the extreme pressure at the seafloor. High frequency sounds, like the one coming from the black box recorder at 37.5 kilohertz, don’t carry much energy. That means they don’t travel very far.
“You can have several bounces off the surface and the bottom of the ocean before the sound gets to you,” said Newhall, who works on sonar systems for the U.S. Navy. “That would confuse the direction where it’s coming from. Instead of one signal coming in, you might be getting several coming from (different) directions.”
Searchers aboard ships that are looking for the missing flight and its 239 passengers and crew are also crossing acoustic “dead zones” where the black box signal is ping-ponging around the ocean, and the sound waves actually cancel each other out. The same principal is also at work when ripples meet from two rocks thrown into a pond.
“As your ship moves, you can move into a fade or out of a fade,” Newhall explained.
That could explain why the Australian ship heard an electronic signal for two hours on Sunday, it stopped, and then they heard another signal for 13 minutes.