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第一段
1 .Listen to part of a lecture in a physics class.
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2 .The professor has been discussing electromagnetic waves.
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第二段
1 .So are there any questions before we continue our discussion of different types of electromagnetic waves?
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2 .Um, today, we'll focus on radio waves, and specifically, very low frequency radio waves. Yes, Tim.
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第三段
1 .Are you going to talk at all about the difference between radio waves and sound waves?
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第四段
1 .Uh, Ok. That might be a good place to start actually.
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2 .Sound waves are mechanical in nature, right? They can only originate and spread in places where there's some dense physical medium, like atmosphere or water.
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3 .They result from changes in pressure in that medium, like changes in air pressure.
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4 .So they can't travel through a vacuum, where there is no dense physical medium, which is why they can't travel through interplanetary space.
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第五段
1 .Radio waves, on the other hand, are fundamentally different from sound waves.
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2 .They are electromagnetic.
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3 .They result from oscillations of the electromagnetic field and don't need a physical medium, so they, like other types of electromagnetic wave, can travel basically anywhere, through a vacuum, or through atmosphere or water.
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第六段
1 .Now, radio waves can be detected.
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2 .For example, very low frequency radio waves can be detected with a special type of radio receiver called a very low frequency radio or VLF radio, which can pick up radio waves with very low frequencies, from 3 to 30 kilohertz, which aren't really picked up by a regular household or car radio.
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3 .So VLF radios pick up VLF radio waves and convert them to sounds we can hear.
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第七段
1 .Um, on Earth the main source of naturally occurring VLF emissions is lightning, which generates a pulse of radio waves every time it flashes.
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2 .Yes, Laura.
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第八段
1 .Since you almost always get lightning with thunderstorms. We can pick up VLF waves pretty often, right?
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2 .You just have to wait until there is a thunderstorm.
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第九段
1 .Ah, do you? Have to wait? VLF receivers are very sensitive and VLF waves travel very far.
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2 .So we can pick up emissions from lightning that's far away. So actually, you can pretty much listen to them all the time because lightning strikes Earth constantly, about a hundred times per second.
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3 .Even if there is no lightning where you are, with a VLF radio, you can hear the crackling from storms that are thousands of kilometers away.
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第十段
1 .However, some times of day are better than others for picking up VLF waves.
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2 .Daytime isn't as good as night time, for example.
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3 .And what's more, my colleague Denis Gallagher says, and in my opinion, he's right.
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4 .He says the best time to listen for them is around sunset or sunrise.
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5 .That's when there're natural waveguides in the local atmosphere.
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第十一段
1 .Did you say waveguide?
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第十二段
1 .Yes, a waveguide.
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2 .Usually it refers to a device, like a metal conductor that's used to guide and direct waves.
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3 .But waveguides also occur naturally.
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4 .They make a path for radio waves to follow in our atmosphere.
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5 .These natural waveguides occur when the Sun is rising or setting, which makes sunrise and sunset good times to pick up VLF emissions.
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第十三段
1 .Now, there are a few different sounds that you can hear on a VLF receiver, because when lightning strikes the radio waves travel different distances and in different ways before they reach the receiver.
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第十四段
1 .Some really interesting ones are called whistlers. Whistlers come from lightning-generated radio waves that leave earth's atmosphere and travel into earth's magnetosphere before bouncing back down.
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2 .Not all radio waves do this and the sound they make, well, we call them whistlers because they sound like slowly descending tones.
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3 .And no two whistlers are alike.
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4 .To me, they're the most intriguing.
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第十五段
1 .Another interesting sound is the tweek.
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2 .Tweeks are the result of VLF waves that have travelled a long distance through the waveguides.
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3 .They produce a chirpy sound because the higher frequency parts of the wave reach the radio receiver before the lower frequency parts.
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4 .The entire wave is still considered very low frequency.
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5 .It's just that some parts of the wave have lower frequencies than others, OK?
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