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旁白：请听一段关于海洋生物学的课程讲解。

Narrator: Listen to part of a lecture in a marine biology class.

教授：我们已经教过海洋动物在水下如何定位，也讲了它们如何辨别方向，这就引出了一个有趣的谜题，我相信大家对此都很感兴趣。

Professor: We have been talking about how sea animals find their way underwater, how they navigate, and this brings up an interesting puzzle, and one I'm sure you'll all enjoy.

我的意思是，大家都爱海豚，对吧？

海豚，实际上它们可以发出两种声音。

And dolphins, well, they actually produce two types of sounds.

一种是我们大家可能都比较熟悉的声音，就是从它们的喷水口发出的声音。

Uh, one being the vocalizations you are probably all familiar with, which they emit through their blowholes.

但是我们今天所讲的声音是它们用以回声定位的咔嚓声，这样它们就能感知周围的事物。

But the one we are concerned with today is the rapid clicks that they use for echolocation, so they can sense what is around them.

研究发现，这种声音是海豚的充满空气的鼻囊里发出的。

These sounds, it has been found, are produced in the air-filled nasal sacs of the dolphin.

谜题就是振动波是如何传输到水中的？

And the puzzle is how does the click sounds get transmitted into the water?

它看起来并不像表面上那么简单。

你想，介质的密度越大，传输的速度就越快。

You see, the denser the medium, the faster sound travels.

所以声音在水中的传输速度要比在空气中快。

So sound travels faster through water than it does through air.

So what happens when a sound wave um? OK.

当声波在一种介质里平缓的传输，忽然间，它进入了另一种介质，会发生什么呢？

You've got a sound wave traveling merrily along through one medium, when suddenly, it hits a different medium, what does gonna happen then?

一些能量将会被折射回来，另一部分将会传入到另一种介质中。

Well, some of the energy is going to be reflected back, and some of it is going to be transmitted into the second medium.

而如果这两种介质的密度相差较大，就像空气和水，大部分的能量将会折射回来，只有一小部分会继续传播进入到新的介质中。

And, and , and if the two media have really different densities, like air and water, then most of the energy is going to be reflected back, very little of it will keep going, uh, get transmitted into the new medium.

你可以想想当你的头在水下时，你能听到多少外面的声音。

I mean, just think how little noise from the outside world actually reaches you when your head is underwater.

那么，海豚的声波是如何从它的充气的鼻囊进入到海水中的呢？

So, how did the dolphin's clicks get transmitted from its air-filled nasal sacs into the ocean water?

由于鼻腔中空气和海水的密度的不同，根据我们的分析这些声音只能在海豚的脑袋里来回反弹，完全没有任何作用。

Because given the difference in density between the air in the nasal cavity and the seawater, we'd expect those sounds to just kind of go bouncing around inside the dolphin's head, which will do it no good at all.

如果想要用声音来为海豚辨别方向，就需要这些声音传播出去并在遇到物体时反射回来。

If it's going to navigate it, needs those sounds to be broadcast and bounced back from objects in its path.

后来证明海豚的前额有一个构造，就在它的鼻囊前方，叫鼻疣。

Well, turns out dolphins have a structure in their foreheads, just in front of their nasal sacs, called a melon.

鼻疣就像是一个大个儿的囊状的袋子，是由脂肪组织构成的。

Now, the melon is kind of a large sac-like pouch, made up of fat tissue.

这个脂肪组织具有独特的声学特征。

And this fat tissue has some rather fascinating acoustical properties.

动物体内的大部分脂肪都是用来储存能量的，但是在海豚身上发现的这种脂肪，它只生长在鼻疣和下颌周围。

Most of the fat that you find in an animal's body is used for storing energy, but this fat, which you find in dolphins, and only in the melon and around the lower jaw.

这种脂肪是很独特的，有着非常丰富的油。

This fat is very different, very rich in oil.

And it turns out it has a very different purpose as well.

那么，一种克服空气和水的密度差异的方法就是调整声波的速度。

Now, one way to overcome this mismatch in the density of air and water would be.

使它与声音在水中的传播速度相同。

If you could modify the velocity of the sound wave, make it precisely match the speed at which sound travels through water.

而海洋生物学家发现这就是鼻疣的作用。

And that's exactly what marine biologists have discovered the melon does.

请注意这个囊，这些鼻疣后面的突起，它们就在充气的鼻囊的旁边。

Note that the bursae, these little projections at the rear of the melon, are right up against the air-filled nasal sacs.

这些囊的作用就是将声音传输到鼻疣中。

And these bursa, it turns out, are what's responsible for transferring sound to the melon.

这些囊状物使声波在鼻疣中传播。

The sound waves are then transmitted by the bursa through the melon.

先经过一个低速率中心，再经过一个高速率框架，在声波进入海水之前将声波的速度提高。

First through a low velocity core, and then through a high velocity shell, where their speed is increased before they are transmitted into the surrounding seawater.

这样信号就能够以最小的反射有效地传输到水中。

So now the signals can be efficiently transferred into the water, with minimal reflection.

而在海豚身上其它地方，唯一有这种脂肪的就是下颌。

The only other place, this special fatty tissue, like that in the melon, the only other place is found in the dolphin, is in the lower jaw.

海豚的下颌是由非常薄的骨头构成的。

Turns out that the lower jaw, well, it is made of a specially thin bone.

它对震动和海水中声音能量的传递十分敏感。

And it is very sensitive to vibrations, to sound energy traveling through the seawater.

下颌主要是负责捕获声音并将声波传递回海豚来，海豚的下颌收到这些返回的声音，使海豚能够感受到周围水域的情况并决定游向哪里。

It turns out that the jaw is primarily responsible for capturing and transferring returning sound waves to the dolphin’s inner ear.

所以这些快速的敲击被弹回，也许一组游泳的鱼游过去，一条小船从那边来了。

So these rapid clicks that are sent out bounce off objects, maybe a group of fish swimming over here, a boat coming from over there.

声音弹开它们和下颚捕获返回的声音，从而有可能为海豚感知什么是周围的水，并决定在哪里游泳。

The sounds bounce off them and the lower jaw captures the returning sounds, making it possible for the dolphin to sense what's in the surrounding water and decide where to swim.

Why does the professor discuss the speed at which sound travels?

How does the professor organize the information in the lecture?

Listen again to part of the Lecture. Then answer the question. Why does the professor say this

Narrator: Listen to part of a lecture in a marine biology class. 
Professor: We have been talking about how sea animals find their way underwater, how they navigate, and this brings up an interesting puzzle, and one I'm sure you'll all enjoy. I mean, everybody loves dolphins, right? And dolphins, well, they actually produce two types of sounds. Uh, one being the vocalizations you are probably all familiar with, which they emit through their blowholes. But the one we are concerned with today is the rapid clicks that they use for echolocation, so they can sense what is around them. These sounds, it has been found, are produced in the air-filled nasal sacs of the dolphin. And the puzzle is how the click sounds get transmitted into the water does? 
It's not as easy as it might seem. You see, the denser the medium, the faster sound travels. So sound travels faster through water than it does through air. So what happens when a sound wave um ? OK. You've got a sound wave traveling merrily along through one medium, when suddenly; it hits a different medium, what does gonna happen then? Well, some of the energy is going to be reflected back, and some of it is going to be transmitted into the second medium. And ? and ? And if the two media have really different densities, like air and water, then most of the energy is going to be reflected back, very little of it will keep going, uh, get transmitted into the new medium. I mean, just think how little noise from the outside world actually reaches you when your head is underwater. 
So, how did the dolphin's clicks get transmitted from its air-filled nasal sacs into the ocean water? Because given the difference in density between the air in the nasal cavity and the seawater, we'd expect those sounds to just kind of go bouncing around inside the dolphin's head, which will do it no good at all. If it's going to navigate it, needs those sounds to be broadcast and bounced back from objects in its path. Well, turns out dolphins have a structure in their foreheads, just in front of their nasal sacs, called a melon. Now, the melon is kind of a large sac-like pouch, made up of fat tissue. And this fat tissue has some rather fascinating acoustical properties. Most of the fat that you find in an animal's body is used for storing energy, but this fat, which you find in dolphins, and only in the melon and around the lower jaw. This fat is very different, very rich in oil. And it turns out it has a very different purpose as well. Now, one way to um, modify the overcome this mismatch in the density of air and water would be? If you travels through velocity of the sound wave, make it precisely match the speed at which water. And that's exactly what marine biologists have discovered the melon Note that the bursa, these little projections at the rear of the melon, are right up against the air-filled nasal sacs. And these bursa, it turns out, are what's responsible for transferring sound to the melon. 
The sound waves are then transmitted by the bursa through the melon. First through a low velocity core, and then through a high velocity shell, where their speed is increased before they are transmitted into the surrounding seawater. So now the signals can be efficiently transferred into the water, with minimal reflection. The only other place, this special fatty tissue, like that in the melon, the only other place is found in the dolphin, is in the lower jaw. Turns out that the lower jaw, well, it is made of a specially thin bone. And it is very sensitive to vibrations, to sound energy traveling through the seawater. It turns out that the jaw is primarily responsible for capturing and transferring returning sound waves to the dolphin’s inner ear. So these rapid clicks that are sent out bounce off objects, maybe a group of fish swimming over here, a boat coming from over there. The sounds bounce off them and the lower jaw captures the returning sounds, making it possible for the dolphin to sense what's in the surrounding water and decide where to swim. 
Professor: We have been talking about how sea animals find their way underwater, how they navigate, and this brings up an interesting puzzle, and one I'm sure you'll all enjoy. I mean, everybody loves dolphins, right? And dolphins, well, they actually produce two types of sounds. Uh, one being the vocalizations you are probably all familiar with, which they emit through their blowholes. But the one we are concerned with today is the rapid clicks that they use for echolocation, so they can sense what is around them. These sounds, it has been found, are produced in the air-filled nasal sacs of the dolphin. And the puzzle is how the click sounds get transmitted into the water does? 
It's not as easy as it might seem. You see, the denser the medium, the faster sound travels. So sound travels faster through water than it does through air. So what happens when a sound wave um ? OK. You've got a sound wave traveling merrily along through one medium, when suddenly; it hits a different medium, what does gonna happen then? Well, some of the energy is going to be reflected back, and some of it is going to be transmitted into the second medium. And ? and ? And if the two media have really different densities, like air and water, then most of the energy is going to be reflected back, very little of it will keep going, uh, get transmitted into the new medium. I mean, just think how little noise from the outside world actually reaches you when your head is underwater. 
So, how did the dolphin's clicks get transmitted from its air-filled nasal sacs into the ocean water? Because given the difference in density between the air in the nasal cavity and the seawater, we'd expect those sounds to just kind of go bouncing around inside the dolphin's head, which will do it no good at all. If it's going to navigate it, needs those sounds to be broadcast and bounced back from objects in its path. Well, turns out dolphins have a structure in their foreheads, just in front of their nasal sacs, called a melon. Now, the melon is kind of a large sac-like pouch, made up of fat tissue. And this fat tissue has some rather fascinating acoustical properties. Most of the fat that you find in an animal's body is used for storing energy, but this fat, which you find in dolphins, and only in the melon and around the lower jaw. This fat is very different, very rich in oil. And it turns out it has a very different purpose as well. Now, one way to um, modify the overcome this mismatch in the density of air and water would be? If you travels through velocity of the sound wave, make it precisely match the speed at which water. And that's exactly what marine biologists have discovered the melon Note that the bursa, these little projections at the rear of the melon, are right up against the air-filled nasal sacs. And these bursa, it turns out, are what's responsible for transferring sound to the melon. 
So, how did the dolphin's clicks get transmitted from its air-filled nasal sacs into the ocean water? Because given the difference in density between the air in the nasal cavity and the seawater, we'd expect those sounds to just kind of go bouncing around inside the dolphin's head, which will do it no good at all. If it's going to navigate it, needs those sounds to be broadcast and bounced back from objects in its path. Well, turns out dolphins have a structure in their foreheads, just in front of their nasal sacs, called a melon. Now, the melon is kind of a large sac-like pouch, made up of fat tissue. And this fat tissue has some rather fascinating acoustical properties. Most of the fat that you find in an animal's body is used for storing energy, but this fat, which you find in dolphins, and only in the melon and around the lower jaw. This fat is very different, very rich in oil. And it turns out it has a very different purpose as well. Now, one way to um, modify the overcome this mismatch in the density of air and water would be? If you travels through velocity of the sound wave, make it precisely match the speed at which water. And that's exactly what marine biologists have discovered the melon Note that the bursa, these little projections at the rear of the melon, are right up against the air-filled nasal sacs. And these bursa, it turns out, are what's responsible for transferring sound to the melon. 
And dolphins, well, they actually produce two types of sounds. Uh, one being the vocalizations you are probably all familiar with, which they emit through their blowholes. But the one we are concerned with today is the rapid clicks that they use for echolocation, so they can sense what is around them.