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what's to eat?


Using Sound to ‘Chat’ Underwater
January 1, 2008 (posted January 2, 2008)
by Susan Humphris, edited by Kristen Kusek

Sometimes it feels like your senses will explode on the first few days at sea. The air buzzes with anticipation, and all your sensory systems are firing like crazy.

Your eyes squint in the tropical sun as they try to take in the land masses disappearing in the distance. Very soon, all you see is a kaleidoscope of blue: water, water everywhere.

Your nose crinkles as it inhales everything from the glorious salty-sweet air to the dankest grease used in a routine maintenance job.

And the sounds? The sounds are, well, downright noisy.

It turns out that a ship is a noisy place to be. What you hear most is the constant drone of the engine—something we want to hear, of course, because it means we are making progress and steaming to our study site!

“Sound is the most important sense of all on this expedition,” said Louis Whitcomb from Johns Hopkins University—and in this case he is talking about the sound under the sea rather than on the ship.

Chatting with your friends—even those who are far, far away—is easy on land where you can use a cell phone or wireless Internet. These use radio waves to send and receive information. But “talking” to your autonomous underwater vehicles (AUVs) while they are in the ocean is much more challenging because radio waves do not penetrate through the ocean water. GPS technology, which also uses radio waves, doesn’t work either.

On this cruise, “we will attempt to use sound to ‘talk’ among the ship and the AUVs so that each knows the location of the other,” said Whitcomb. Just like in a three-way phone call, each one hears everything that is “said” by the others, he explained. The ship will transmit its exact position and time to the robots, which will use that information to calculate their positions at an exact time—and then transmit them for all to receive. The ship can then keep track of each AUV roaming around under the sea.

In addition to navigation, we can also use sound waves to send and receive data. We will be hunting for new hydrothermal vent sites on the mid-ocean ridge. Scientists will equip Puma and Jaguar with various sensors to help them “sniff out” hot, chemical-rich fluids discharging from the vents.

“Our long-term goal is to have an AUV transmit the sensor data back to the ship about once every minute,” Whitcomb said. Scientists on the ship will plot the data in near real time so they can to get a better sense of what lurks below them on the seafloor. If something seems interesting, the engineers will send instructions to Puma and Jaguar, telling them to conduct a closer survey to gather even more information.

“We have tested all pieces of the system individually at shallow water depths,” Whitcomb said. “Our goal is to test and use the system in deep water at a site of real scientific interest. During the first test dives, we will also use a conventional seafloor acoustic navigation system to test the accuracy of our new system.”

“The ocean is a noisy place,” Whitcomb said. “The noisiest thing in the neighborhood is the sea surface where waves and wind interact.” To deal with all this racket, scientists will use sounds within a specific frequency, and the detectors on the vehicles and the ship will be tuned to that frequency to pick out the signals over the noise. Puma and Jaguar may be several thousands of meters away from the ship, so the team will use sound waves at a frequency of 8 to12 kilohertz—approximately the same frequency that a bunch of car keys makes if you jingle it.

Stay tuned to see if they ring in the sweet sounds of success!

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