Listen for a ping, and the water may play tricks on you

STORY HIGHLIGHTS

• 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 in 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 questions 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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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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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.

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.

The possibility of such squirrely pathways makes it all the more necessary for searchers to record multiple pings compare the data.

Wave logic

There is some logic to the weird pathways sound can take, and it 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) in air."

In the water, sound waves can get warped. They curve towards water, in which they travel more slowly, NOAA says. Towards colder water or into the shallows of less pressurized water.

Sometimes it bounces back and forth between two areas and gets trapped in a "deep sound channel," which carries it 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 interfere with hearing 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.