Practice English Speaking&Listening with: How To Hear Halfway Around The World

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Hi, this is Kate from MinuteEarth.

In 1960, scientists blew up hundreds of pounds of explosives off the coast of Australia;

nearly four hours later, sounds from the blasts reached underwater microphones near Bermuda

- over 19,000 km away. In the air, these sounds would have traveled a few dozen kilometers

at most. And sure, sound generally travels a b it farther underwater because sound waves

dont lose as much energy moving through water as they do through air...but not hundreds

of times farther. These sounds traveled halfway around the world thanks to an underwater sound

superhighway. That superhighway exists because sound travels

at different speeds in different layers of the ocean. The temperature of water - and

to a lesser extent, its pressure - affect how densely packed its molecules are and how

rigidly they are connected to each other - the two factors which determine the speed of sound.

At the surface, where the water is generally warm - sound moves pretty quickly, but with

increasing depth, the temperature plummets, drastically slowing the speed of sound. Around

a thousand meters or so down - depending where on the planet you are - the oceans temperature

bottoms out, and the effect of pressure takes over, causing the speed of sound to increase

again. But heres where it gets weird, because

that layer of the ocean where the speed of sound is slowest is where sounds can travel

the farthest. If youve ever skied, you may get why; say you are speeding along on

a packed trail, then you start drifting into a much slower stretch of powder. The ski in

the powder slows down, turning you and pulling you farther into the fluffy stuff, toward

the slower direction of travel. And if you reach the other side of the powder and hit

another packed trail, your outside ski speeds up, turning you back into the pokey powder.

Sound waves are a lot like skiers; if they enter a layer of water where sound travels

particularly slowly, at just the right angle - or if they start out there in the first

place - they can get stuck in that slow layer, bending up and down and up and down. So, rather

than spreading out and scattering off the surface or getting absorbed by the ocean floor,

sounds in this layer get funneled along, and can go and go and go.

This layer - appropriately called theSOFAR channel” - may be a weird quirk of physics,

but its also really useful. Based on whalesbehavior and calls, scientists think some

whales use the SOFAR channel as a reeeally long-distance telephone. And actually so do

we; monitoring systems in the SOFAR channel can detect sounds all over the ocean, from

the breaking up of ice shelves to supposedly secret nuclear tests. Whats more, using

this infrastructure, we can calculate how fast sounds move through this layer of water;

changes in the speed of sound in the SOFAR channel can help us track changes in the temperature

of the ocean - a critical measure of our planets health. And that is SO FAR out.

The Comprehensive Nuclear-Test-Ban Treaty Organization, or CTBTO, uses a network of

11 hydrophone tripletsanchored from the seabed - into the SOFAR channel to listen

in on our oceans. Its part of their effort to collect and study sounds across the planet

to figure out which sounds are nuclear tests, and which are, for instance, simply the long-distance

calls of lonely whales. This work has been mandated by more than 180 member states, and

the data that is collected worldwide is available to researchers who want to further expand

our knowledge about our planet. If youre interested, please contact ctbto.org. Thanks

to CTBTO for sponsoring this video, and for helping keep the world safe from nuclear weapons.

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