Water makes 'pings' tricky'

Sound behaves differently in water than it does in air
Sound can can take curved paths through water
It can travel for miles, even the distance of a small country
Noise pollution in the oceans makes it harder to hear faint sounds







(CNN) -- Put an ear to the water and listen carefully.
Then ask yourself: Do you hear a ping or not, and if so, where is it coming from?
If you're a searcher in pursuit of the pinger locators of missing Malaysia Airlines Flight 370, the answers to those questio are crucial.
They will greatly affect the chances of succeeding at the next formidable task: Find a small box that may have sunk into mushy silt on the pitch dark floor of the ocean, under nearly three miles of water where the pressure is so great that it can crush a submarine.

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How are underwater pings triangulated?


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Could MH370 have landed intact on water?


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Flight 370: Malaysia's missteps


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The Austrialian-based agency coordinating the search for Malaysia Airlines Flight 370 details efforts for the hunt of the missing jet in a map provided Friday, April 11. Searchers are combing thousands of square miles of the southern Indian Ocean off Australia's west coast for signs of Flight 370, which disappeared March 8.
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Chinese navy personnel head out on a boat to the Royal Australian Navy ship HMAS Success on Wednesday, April 9, as part of the search for the missing plane.
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A Royal Australian Air Force AP-3C Orion flies past the Australian vessel Ocean Shield on April 9 on a mission to drop sonar buoys to assist in the search.
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A relative of a passenger on the missing flight cries at a vigil in Beijing on Tuesday, April 8.
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A member of the Royal Australian Air Force walks toward a plane that has just arrived in Perth, Australia, on April 8.
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Australian Defense Force divers from the Ocean Shield vessel scan the water for debris in the southern Indian Ocean on Monday, April 7.
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A towed pinger locator is readied to be deployed on April 7 off the deck of the Australian vessel Ocean Shield in the search for the missing jet's flight data recorder and cockpit voice recorder.
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Capt. Mark Matthews of the U.S. Navy talks to reporters in Perth on April 7 about the search.
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A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean on Friday, April 4, during search operations.
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Members of the Royal New Zealand Air Force monitor data April 4 on board an aircraft during search operations.
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A relative of a Flight 370 passenger watches television in a Beijing hotel as he awaits new information about the missing plane on Thursday, April 3.
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Another relative of a Flight 370 passenger waits for updates in Beijing on Wednesday, April 2. Many families have criticized the Malaysian government's handling of information in the plane's disappearance.
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A member of the Japanese coast guard points to a flight position data screen while searching for debris from the missing jet on Tuesday, April 1.
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Kojiro Tanaka, head of the Japanese coast guard search mission, explains the efforts en route to the search zone April 1.
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A woman prepares for an event in honor of those aboard Flight 370 on Sunday, March 30, in Kuala Lumpur, Malaysia.
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An underwater search-surveying vehicle sits on the wharf in Perth, Australia, ready to be fitted to a ship to aid in the search for the jet.
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A girl in Kuala Lumpur writes a note during a ceremony for the missing passengers on March 30.
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A teary-eyed woman listens from the back as other relatives of Flight 370 passengers speak to reporters March 30 in Subang Jaya, Malaysia. Dozens of anguished Chinese relatives demanded that Malaysia provide answers to the fate of those on board.
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An object floating in the southern Indian Ocean is seen from a Royal New Zealand Air Force P-3K2 Orion aircraft searching for the missing jet on Saturday, March 29. Ships participating in the search retrieved new debris Saturday, but no objects linked to the missing plane, according to Australian authorities.
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A Royal New Zealand Air Force member launches a GPS marker buoy over the southern Indian Ocean on March 29.
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The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on Friday, March 28, after other relatives left en masse to protest the Malaysian government's response to their questions.
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A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on Thursday, March 27.
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Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27.
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People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27.
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Crew members of the Chinese icebreaking ship Xuelong scan the Indian Ocean during a search for the missing jet on Wednesday, March 26.
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People work at a console at the British satellite company Inmarsat on Tuesday, March 25, in London.
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The mother of a passenger who was on Malaysia Airlines Flight 370 cries at her home in Medan, Indonesia, on March 25.
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Australian Defense Minister David Johnston speaks to the media March 25 about the search for the missing jet.
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A family member of a missing passenger reacts after hearing the latest news March 25 in Kuala Lumpur.
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Angry relatives of those aboard Malaysia Airlines Flight 370 react in Beijing on Monday, March 24, after hearing that the plane went down over the southern Indian Ocean, according to analysis of satellite data.
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Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight March 24 in Kuala Lumpur. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived."
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Relatives of the missing passengers hold a candlelight vigil in Beijing on March 24.
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A member of the Royal Australian Air Force looks out an aircraft during a search for the missing jet March 24.
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A woman reads messages for missing passengers at a shopping mall in Kuala Lumpur on March 24.
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Flight Lt. Josh Williams of the Royal Australian Air Force operates the controls of an AP-3C Orion on Sunday, March 23, after searching the southern Indian Ocean.
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Ground crew members wave to a Japanese Maritime Defense Force patrol plane as it leaves the Royal Malaysian Air Force base in Subang, Malaysia, on Sunday, March 23. The plane was heading to Australia to join a search-and-rescue operation.
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A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on Saturday, March 22.
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A Chinese satellite captured this image, released on March 22, of a floating object in the Indian Ocean, according to China's State Administration of Science. It is a possible lead in the search for the missing plane. Surveillance planes are looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia.
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A member of the Royal Australian Air Force looks down at the Norwegian merchant ship Hoegh St. Petersburg, which took part in search operations Friday, March 21.
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The Royal Australian Air Force's Neville Dawson, left, goes over the search area with Brittany Sharpe aboard an AP-3C Orion some 2,500 kilometers (about 1,500 miles) southwest of Perth, Australia, over the Indian Ocean on March 21.
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Satellite imagery provided by the Australian Maritime Safety Authority on Thursday, March 20, shows debris in the southern Indian Ocean that could be from Flight 370. The announcement by Australian officials that they had spotted something raised hopes of a breakthrough in the frustrating search.

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Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight.
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A closer look at the satellite shot of possible debris.
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The Australian Maritime Safety Authority's John Young speaks to the media in Canberra, Australia, on March 20 about satellite imagery.
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A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on Wednesday, March 19.
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A relative of missing passengers waits for a news briefing by officials in Beijing on Tuesday, March 18.
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A relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet.
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Relatives of missing passengers watch a news program about the missing plane as they await information at a hotel ballroom in Beijing on March 17.
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Malaysian Transportation Minister Hishamuddin Hussein, center, shows maps of the search area at a hotel next to the Kuala Lumpur International Airport on March 17.
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U.S. Navy crew members assist in search-and-rescue operations Sunday, March 16, in the Indian Ocean.
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Indonesian personnel watch over high seas during a search operation in the Andaman Sea on Saturday, March 15.
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A foam plane, which has personalized messages for the missing flight's passengers, is seen at a viewing gallery March 15 at Kuala Lumpur International Airport.
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Elementary school students pray for the missing passengers during class in Medan, Indonesia, on March 15.
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Col. Vu Duc Long of the Vietnam air force fields reporters' questions at an air base in Ho Chi Minh City, Vietnam, after a search operation on Friday, March 14.
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Members of the Chinese navy continue search operations on Thursday, March 13. The search area for Flight 370 has grown wider. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, efforts are expanding west into the Indian Ocean.
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A Vietnamese military official looks out an aircraft window during search operations March 13.
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A member of the Indonesian National Search and Rescue Agency scans the horizon in the Strait of Malacca on Wednesday, March 12.
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Journalists raise their hands to ask questions during a news conference in Sepang on March 12.
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Iranians Pouri Nourmohammadi, second left, and Delavar Seyed Mohammad Reza, far right, were identified by Interpol as the two men who used stolen passports to board the flight. But there's no evidence to suggest either was connected to any terrorist organizations, according to Malaysian investigators. Malaysian police believe Nourmohammadi was trying to emigrate to Germany using the stolen Austrian passport.
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Vietnam air force Col. Le Huu Hanh is reflected on the navigation control panel of a plane that is part of the search operation over the South China Sea on March 10.
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Relatives of the missing flight's passengers wait in a Beijing hotel room on March 10.
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A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews before returning to search for the missing plane Sunday, March 9, in the Gulf of Thailand.
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Members of the Fo Guang Shan rescue team offer a special prayer March 9 at Kuala Lumpur International Airport.
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A handout picture provided by the Malaysian Maritime Enforcement Agency shows personnel checking a radar screen during search-and-rescue operations March 9.
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Italian tourist Luigi Maraldi, who reported his passport stolen in August, shows his current passport during a news conference at a police station in Phuket island, Thailand, on March 9. Two passengers on the missing Malaysia Airlines flight were reportedly traveling on stolen passports belonging to Maraldi and an Austrian citizen whose papers were stolen two years ago.
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Hugh Dunleavy, commercial director of Malaysia Airlines, speaks to journalists March 9 at a Beijing hotel where relatives and friends of the missing flight's passengers are staying.
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Vietnamese air force crew stand in front of a plane at Tan Son Nhat airport in Ho Chi Minh City on March 9 before heading out to the area between Vietnam and Malaysia where the airliner vanished.
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Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9.
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The Chinese navy warship Jinggangshan prepares to leave Zhanjiang Port early on March 9 to assist in search-and-rescue operations for the missing Malaysia Airlines flight. The Jinggangshan, an amphibious landing ship, is loaded with lifesaving equipment, underwater detection devices and supplies of oil, water and food.
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Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9. The vessel is carrying 12 divers and will rendezvous with another rescue vessel on its way to the area where contact was lost with Malaysia Airlines Flight 370.
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The rescue vessel sets out from Sanya in the South China Sea.
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A family member of missing passengers is mobbed by journalists at Kuala Lumpur International Airport on Saturday, March 8.
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A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported March 8. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10.
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Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8.
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Malaysia Airlines official Joshua Law Kok Hwa, center, speaks to reporters in Beijing on March 8.
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A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8.
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Wang Yue, director of marketing of Malaysia Airlines in China, reads a company statement during a news conference at the Metro Park Lido Hotel in Beijing on March 8.
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Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8.
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A woman asks a staff member at the Beijing airport for more information on the missing flight.
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A Malaysian man who says he has relatives on board the missing plane talks to journalists at the Beijing airport on March 8.
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Passengers walk past a Malaysia Airlines sign on March 8 at Kuala Lumpur International Airport.
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Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference on March 8 at a hotel in Sepang. "We deeply regret that we have lost all contacts" with the jet, he said.
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Photos: The search for Malaysia Airlines Flight 370



Once the search goes underwater, the area you can cover will narrow severely, so you need to listen well for those pings, while you can.
But while you're trying to zero in their source, beware.
The water may be playing tricks on you -- because of the way sound behaves in it.
Titanic doubts
Paul-Henry Nargeolet will tell you as much.
He's seen the Titanic close up where it rests 12,500 feet under the sea. And he has taken an underwater expedition to search for wreckage of Air France Flight 447, which plunged into the Atlantic Ocean.
"I don't trust very much the acoustic," he says.
The retired French navy commander has undertaken dozens of dives in submersible vehicles to the Titanic to secure artifacts.
After putting them in baskets, his crew affixed locator pingers to them, so he could swing back around to pick them up later, he told CNN's Don Lemon.
"Most of the time we never heard them, and we knew where they were," he said. He was only 1,000 - 2,000 feet away.
Signals believed to come from a pinger in the Indian Ocean are about 14,000 feet away from the ships listening for them.
Nargeolet isn't willing to believe that any part of MH 370 has been located until someone sees wreckage with their own eyes.
He is even suspicious of the four signals Chinese and Australian searchers have picked up that are consistent with the sound emitted from a black box.
"It's really hard to find this kind of pinger," he said.
Water deafness

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Replicating dramatic altitude drop


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CNN joins MH370 search over Indian Ocean


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Sources: Plane flew as low as 4,000 feet

Anyone who has bobbed up and down in a swimming pool has surely noticed that the way things sound in the water is obviously different from how they sound in the air.
Above all, to us humans it's a lot quieter underwater -- nearly silent.
The main reason for this is that humans hear sound mainly through the air, according to acoustic researchers from the National Oceanic and Atmospheric Administration.
Take away the air, and you take away much of what we hear.
On top of it, when your ears fill with water, it keeps your eardrums from functioning.
But we also hear through our bones, principally our skulls, which prevents us from being completely deaf in the water.
"Bone conductivity is used to hear under water, but it is 40 percent less effective than air conductivity," NOAA says.
If it's so hard to hear underwater, you might think that sound travels faster and farther in air than in water.
The opposite is true, NOAA says.
Sound waves zip through water at nearly 5,000 feet per second, more than four times the speed that they do in air, which is at just over 1,100 feet per second.
And researchers have picked up sounds that have traversed a distance through water that would equal a trip from one end of a small country to another, NOAA says.
Some scientists believe that whales can talk to each other through thousands of miles of water.
Underwater curve balls
Naval search crews and oceanographers, of course, don't stick their heads underwater to conduct sound searches.
Their listening devices -- underwater microphones called hydrophones, sono-buoys dropped from airplanes and acoustic detectors dragged behind ships -- are technically adept at hearing even faint sounds beneath the waves.

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Manufacturer: MH370's pinger has 40 days


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Parents of MH370 passengers keep hoping


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CNN joins MH370 search over Indian Ocean

And they can extract mind-boggling information from water sounds.
Researchers listening to the ocean have been able to record the sounds of earthquakes an ocean away or monitor climate change based on sounds, NOAA says. The U.S. military can monitor the strategic movements of other countries' navy submarines.
But the water still throws nasty sound curve balls.
Australian authorities have warned about this to temper enthusiasm that a detected a ping might point to where it came from.
Sound under water often does not travel in a straight line.
"Sound through the water is greatly affected by temperature, pressure and salinity," explained Peter Leavy, commander of the military task force conducting the search. "And that has the effect of attenuating, bending -- sometimes through 90 degrees -- sound waves."
Sound can travel long distances sideways through the ocean without coming up near the surface to be detected.
Such squirrely pathways makes it all the more necessary for searchers to record multiple pings and compare the data.
Wave logic
The is logic behind the twists lies in how water affects the mechanics of its waves, NOAA says.
For example: "A 20 Hz sound wave is 75 m long (246 feet) in the water...whereas a 20 Hz sound wave in air is only 17 m long (56 feet)."
Underwater, sound waves can get warped. They curve towards the kinds of water that make them travel more slowly, NOAA says. Towards colder water or into the shallows of less pressurized water.
Sometimes sound waves bounce back and forth between two areas and get trapped in a "deep sound channel," which carries them for long distances, NOAA says.
Low-pitched sounds travel farthest underwater -- the rumblings of quakes or low whale songs.
High-pitched sounds are less likely to travel far without losing intensity.
A plane's locator pingers give off very high pitched sounds in a range over 30 kHz (30,000 Hz). To put that into perspective, the human ear hears sounds up to around 20,000 Hz, NOAA says.
Because of their higher frequency, it would seem pinger sounds from MH 370 would have less of a chance of traveling the kinds of distances whale songs do.
Noise pollution
Then there is the possibility that other sounds could drown out pings from a black box.
Researchers studying whale calls have complained that it is getting harder to hear them, because the ocean is filling up with sound.
There is a "rising tide of noise from an increasingly urbanizing marine environment, the collective noises from shipping traffic, oil and gas exploration and production, and recreational traffic," researchers from Cornell University have said.
"And every decade the amount of noise is doubling."
The "ocean smog" was strong enough already in 2005 that scientists feared it would keep female whales from hearing the mating calls of male whales and prevent them from breeding.
The ocean is so full of sound, they wrote, that the area where whales can listen clearly "has shriveled down to a small fraction of what it was less than a century ago."
Australian authorities have worked to reduce ship traffic in the search area to be better able to detect those vital pings.
READ: How deep is deep? Imagining the MH370 search underwater
READ: 7 ways air travel changed after disasters
READ: Flight 370: A wealth of info, a welter of questions
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