Narrator: Listen to part of a lecture in a physics class.

旁白：请听一段物理课的讲座。

The professor has been discussing electromagnetic waves.

教授正在讨论电磁波的问题。

Professor: So are there any questions before we continue our discussion of different types of electromagnetic waves?

教授：在我们讨论不同种类电磁波之前，有其他问题吗？

Um, today, we'll focus on radio waves, and specifically, very low frequency radio waves.

今天，我们讨论无线电波，特别是低频率的无线电波。

Yes, Tim.

Tim，什么事？

Student: Are you going to talk at all about the difference between radio waves and sound waves?

学生：你将讨论无线电波和声波的区别吗？

Professor: Uh, Ok. That might be a good place to start actually.

教授：好吧，这或许会是一个不错的出发点。

Sound waves are mechanical in nature, right?

声波本质上是力的作用对吗？

They can only originate and spread in places where there's some dense physical medium, like atmosphere or water.

它只在一些有致密物质媒介的地方产生和传播，比如大气和水。

They result from changes in pressure in that medium, like changes in air pressure.

它们来自于传播媒介中压力的改变，比如，空气中压力的改变。

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.

所以它们不能在真空中传播，真空中没有致密物质媒介，这也是它们不能在星系间传播的原因。

Radio waves, on the other hand, are fundamentally different from sound waves.

无线电波，从另一方面看，本质上不同于声波。

They are electromagnetic.

它们是电磁波。

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.

它们产生于电磁振动，而且不需要物理媒介，所以它们，和其他类型的电磁波一样，几乎可以任意传播，在真空，大气或者让水中（都可以）。

Now, radio waves can be detected.

无线电波是可以被检测到的。

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.

比如，低频无线电波可以通过无线电接收器来接收。这个接收器叫做低频无线电或VLF无线电。可以接收特别低频的无线电，从3到30千赫兹。这个范围是家庭无线电和汽车无线电不能够接收到的。

So VLF radios pick up VLF radio waves and convert them to sounds we can hear.

所以，VLF无线电接收器可以接收VLF无线电波并把它们转换成我们可以听到的声音。

Urn, on Earth, the main source of naturally occurring VLF emissions is lightning, which generates a pulse of radio waves every time it flashes. Yes, Laura.

在地球上，自然产生VLF发射的主要来源是闪电，每次闪光的时候就会产生无线电波。Laura?

Student: Since you almost always get lightning with thunderstorms, we can pick up VLF waves pretty often, right?

学生：既然几乎总是能从雷暴中获得闪电，那么我们可以经常接收到VLF的无线电波对吗？

You just have to wait until there is a thunderstorm.

你只需要等到有雷暴的时候。

Professor: Ah, do you? Have to wait?

教授：啊，是吗？必须要等吗？

VLF receivers are very sensitive and VLF waves travel very far.

VLF 接收机非常灵敏，VLF波传播得非常远。

So we can pick up emissions from lightning that's far away.

我们可以接收到遥远的闪电产生的放射。

So actually, you can pretty much listen to them all the time because lightning strikes Earth constantly, about a hundred times per second.

实际上，你可以一直听到，因为闪电持续不断地袭击地球，几乎每秒一百次。

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.

即使在你所在的地方没有闪电，使用甚低频无线电，你也可以听到来自数千公里之外的风暴的噼里啪啦声。

However, some times of day are better than others for picking up VLF waves.

然而，一天中的有些时间比其他时间更容易接收无线电。

Daytime isn't as good as night time, for example.

比如，白天就没有晚上好。

And what's more, my colleague Denis Gallagher says, and in my opinion, he's right.

而且，我的同事Denis Gallagher说过，在我看来，他是正确的。

He says the best time to listen for them is around sunset or sunrise.

他说听它们最好的时间是日出或日落的时候。

That's when there're natural waveguides in the local atmosphere.

那时，当地大气中存在着天然波导

Student: Did you say waveguide?

学生：你是说波导吗？

Professor: Yes, a waveguide.

教授：是的，波导。

Usually it refers to a device, like a metal conductor that's used to guide and direct waves.

它指的是一个像金属导体的装置，用来指引电波。

But waveguides also occur naturally.

但是，波导也会自然产生。

They make a path for radio waves to follow in our atmosphere.

它可以使无线电波按照一定途径进入我们的大气。

These natural waveguides occur when the Sun is rising or setting, which makes sunrise and sunset good times to pick up VLF emissions.

当日出或日落的时候，这些波导就会产生，这样日出日落时就成了接收VLF排放的好时机。

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.

通过VLF接收器，你可以听到不同种声音。因为当闪电发生的时候，无线电波到达接受器之前，会以不同的方式穿过不同的距离。

Some really interesting ones are called whistlers.

一些有趣的声音叫口哨。

Whistlers come from lightning-generated radio waves that leave earth's atmosphere and travel into earth's magnetosphere before bouncing back down.

它来自于闪电产生的无线电波。这些无线电离开地球的大气层并在反弹回来之前传播到地球的磁气圈。

Not all radio waves do this and the sound they make, well, we call them whistlers because they sound like slowly descending tones.

并不是所有的无线电波都会这样，它们发出的声音，嗯，我们称之为哨声，因为它们听起来像缓慢下降的音调。

And no two whistlers are alike.

没有两种口哨是一样的。

To me, these are the most intriguing.

对我来说，它们都是最迷人的。

Another interesting sound is the tweek.

另一种有趣的声音是吱吱声。

Tweeks are the result of VLF waves that have travelled a long distance through the waveguides.

吱吱声来自于通过波导穿过长距离而来的VLF波。

They produce a chirpy sound because the higher frequency parts of the wave reach the radio receiver before the lower frequency parts.

它们会产生啾啾声因为高频部分的电波会比低频部分的电波先到达接收器。

The entire wave is still considered very low frequency.

这整个的电波仍然被认为是低频的。

It's just that some parts of the wave have lower frequencies than others, ok?

只是因为电波的有些部分比其他部分有更低的频率，明白吗？