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Narrator:Listen to part of a lecture in a marine biology class.
獨白:聽下面一段關于海洋生物學的課程講解。
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.
教授:我們已經教過海洋動物在水下如何定位，也講了它們如何辨別方向。這就引出了一個有趣的謎題，我相信大家對此都很感興趣。
I mean, everybody loves dolphins, right?
我的意思是，大家都愛海豚，對吧？
And dolphins, well, they actually produce two types of sounds.
海豚，實際上他們可以發出兩種聲音。
Uh, one being the vocalizations you are probably all familiar with, which they emit through their blowholes.
一種是我們大家可能都比較熟悉的聲音，就是從它們的噴水口發出的聲音。
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.
但是我們今天所講的聲音是他們用以回聲定位的咔嚓聲，這樣他們就能感知周圍的事物。
These sounds, it has been found, are produced in the air-filled nasal sacs of the dolphin.
研究發現，這種聲音是海豚的充滿空氣的鼻囊里發出的。
And the puzzle is how does the click sounds get transmitted into the water?
謎題就是振動波是如何傳輸到水中的？
It's not as easy as it might seem.
它看起來并不像表面上那么簡單。
You see, the denser the medium, the faster sound travels.
你想，介質的密度越大，傳輸的速度就越快。
So sound travels faster through water than it does through air.
所以聲音在水中的傳輸速度要比在空氣中快。
So what happens when a sound wave um…OK.
而當聲波…？好。
You've got a sound wave traveling merrily along through one medium, when suddenly;it hits a different medium, what does gonna happen then?
當聲波在一種介質里平緩的傳輸，忽然間，它進入了另一種介質，會發生什么呢？
Well, some of the energy is going to be reflected back, and some of it is going to be transmitted into the second medium.
一些能量將會被折射回來，另一部分將會傳入到另一種介質中。
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.
而如果這兩種介質的密度相差較大，就像空氣和水，大部分的能量將會折射回來，只有一小部分會繼續傳播進入到新的介質中。
I mean, just think how little noise from the outside world actually reaches you when your head is underwater.
你可以想想當你的頭在水下時，你能聽到多少外面的聲音。
So, how did the dolphin's clicks get transmitted from its air-filled nasal sacs into the ocean water?
那么，海豚的聲波是如何從她的充氣的鼻囊進入到海水中的呢？
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.
由于鼻腔中空氣和海水的密度的不同，根據我們的分析這些聲音只能在海豚的腦袋里來回反彈，完全沒有任何作用。
If it's going to navigate it, needs those sounds to be broadcast and bounced back from objects in its path.
如果想要用聲音來為海豚辨別方向，就需要浙西而聲音傳播出去并在遇到物體時反射回來。
Well, turns out dolphins have a structure in their foreheads, just in front of their nasal sacs, called a melon.
后來證明海豚的前額有一個構造，就在它的鼻囊前方，叫鼻疣。
Now, the melon is kind of a large sac-like pouch, made up of fat tissue.
鼻疣就像是一個大個兒的囊狀的袋子，是由脂肪組織構成的。
And this fat tissue has some rather fascinating acoustical properties.
這個脂肪組織具有獨特的聲學特征。
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.
動物體內的大部分脂肪都是用來儲存能量的，但是在海豚身上發現的這種脂肪，它只生長在鼻疣和下頜周圍。
This fat is very different, very rich in oil.
十分的獨特，油的含量非常高。
And it turns out it has a very different purpose as well.
而且它的作用也十分的特殊。
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.
那么，一種克服空氣和水的密度差異的方法就是調整聲波的速度，使它與聲音在水中的傳播速度相同。
And that's exactly what marine biologists have discovered the melon.
而海洋生物學家發現這就是鼻疣的作用。
Note that the bursa, these little projections at the rear of the melon, are right up against the air-filled nasal sacs.
請注意這個囊，這些鼻疣后面的突起，他們就在充氣的鼻囊的旁邊。
And these bursa, it turns out, are what's responsible for transferring sound to the melon.
這些囊的作用就是將聲音傳輸到鼻疣中。
The sound waves are then transmitted by the bursa through the melon.
這些囊狀物使聲波在鼻疣中傳播.
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.
先經過一個低速率中心，再經過一個高速率框架，在聲波進入海水之前將聲波的速度提高。
So now the signals can be efficiently transferred into the water, with minimal reflection.
這樣信號就能夠以最小的反射有效地傳輸到水中。
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.
而在海豚身上其他地方，唯一有這種脂肪的就是下頜。
Turns out that the lower jaw, well, it is made of a specially thin bone.
海豚的下頜是由非常薄的骨頭構成的。
And it is very sensitive to vibrations, to sound energy traveling through the seawater.
它對震動和海水中聲音能量的傳遞十分敏感。
It turns out that the jaw is primarily responsible for capturing and transferring returning sound waves to the dolphin's inner ear.
下頜主要是負責捕獲聲音并將聲波傳遞回海豚的內耳。
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.
而這些聲波發出后碰到物體，有可能是這邊的魚群或是那邊是來的船。
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.
就會折射回來，海豚的下頜收到這些返回的聲音，使海豚能夠感受到周圍水域的情況并決定游向哪里。
