Through the Ocean Noise

Sound is something you might not think about when it comes to the ocean. But sound plays a key role in understanding the ocean, and the diverse life that call it home. Marine animals use sound to navigate, communicate, find food, find mates and even avoid predators.  

Sound is important in the ocean, which is why researchers at East Carolina University are so excited about the discoveries they could potentially make with Blackbeard the wave glider. The new tool can measure conditions above and below the ocean's surface, as well as record a soundscape of the ocean.  

But anthropogenic, or human-generated sound, is drowning out a lot of that marine mammal sound. Those human sounds range from the whirr of engines and propellers of ships, to the whine of jet skies, sonar pings and even anchors dragging along the ocean floor, just to name a few. In fact, researchers say all of that man-made noise poses a serious threat to ocean ecosystems and the survival of many marine animals. 

To better assess the situation, the question then is, how far does sound travel in the ocean? 

It turns out, sound travels much faster in the ocean (4,921 feet/second) than in the air (1,115 feet/second). But how far sound waves travel in the ocean depends on the ocean temperature and pressure. And while pressure increases the deeper you dive in the ocean, the temperature only drops up to a certain point after which it stabilizes. 

Imagine a whale that's trying to find other whale friends, perhaps the whales in its pod. The whale sends out sound in waves (think ripples in the water). Then the waves slow down as they travel deeper in the ocean because the temperature is dropping, and that means the sound waves are refracted downward. 

The waves continue diving deeper in the ocean until they reach a dramatic temperature change. It’s called the thermocline layer and it’s a region of the ocean characterized by rapid change in temperature and pressure. It occurs at different depths all around the world. 

Below the thermocline layer, the water temperature stays pretty constant, but because of the deeper depth, the water pressure increases. That causes the sound waves to refract upward. 

That means the whale call bounces up and down between the different layers as it travels through the ocean. The region where sound travels is called the sound channel. And scientists say sound waves, like the whale call, can travel through the sound channel for dozens of miles. 

Now imagine that whale calling out, looking for its friends, and that call essentially competing with other natural sounds plus all of the man-made sounds listed above.

The ocean is a pretty noisy place. And all that extra noise can make it difficult for ocean animals to communicate with other members of their species. With tools like the Blackbeard wave glider, we can better understand these ocean dynamics, and how we can better help the species of the ocean. 

— Frank Graff 

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!  


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