400-820-0859Dolphin's navigation托福听力原文翻译及问题答案
一、Dolphin's navigation托福听力原文：
NARRATOR:Listen to part of a lecture in a marine biology class.
FEMALE 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,um,one being the vocalizations you're probably all familiar with,which they emit through their blowholes.But the one we're concerned with today is the rapid clicks that they use for echolocation,so they can sense what is around them—these sounds,it's been found,are produced in the air-filled nasal sacs of the dolphin.
And the puzzle is:How do the click sounds get transmitted into water?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,okay,
you've got a sound wave traveling merrily along through one medium,when suddenly it hits a different medium.What's gonna happen then?Well,some of the energy is gonna be reflected back,and some of it's gonna be transmitted into the second medium.
An-an-and if the two media have really different densities—like air and water—then most of the energy is gonna 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's underwater.So,how do 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 kinda go bouncing around inside the dolphin's head!Which would do it no good at all?if it's going to navigate,it needs those sounds to be broadcast and bounce 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—what 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 overcome this mismatch in the density of air and water would be? if you could,um,modify the velocity of the sound wave,make it precisely match the speed at which sound travels through water. And that's exactly what marine biologists have discovered the melon does.Note that the bursae,these little projections at the rear of the melon,are right up against the air-filled nasal sacs—and these bursae,it turns out,are what's responsible for transferring sound to the melon.
The sound waves are transmitted by the bursae 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 it's found in the dolphin is in the lower jaw. Turns out that the lower jaw—well,it's made of especially thin bone, and it's 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—m-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.
二、Dolphin's navigation托福听力中文翻译：
旁白：在海洋生物课上听一节课的一部分。
女教授：我们一直在讨论海洋动物如何在水下找到它们的路，它们如何导航，这带来了一个有趣的谜题，我相信你们都会喜欢的。我是说，每个人都喜欢海豚，对吗？
海豚实际上会发出两种声音，一种是你们可能都熟悉的声音，它们通过气孔发出。但我们今天所关心的是它们用来进行回声定位的快速咔哒声，这样它们就可以感知周围的东西。我们发现，这些声音是在海豚充满空气的鼻囊中产生的。
难题是：咔哒声是如何传播到水中的？这不像看起来那么容易？你看，介质密度越大，声音传播的速度越快。所以声音在水中传播的速度比在空气中传播的速度快&nbsp；那么当声波出现时会发生什么呢？嗯好吧，
声波在一种介质中欢快地传播，突然它碰到另一种介质。那会发生什么？一些能量会被反射回来，一些能量会被传输到第二种介质中。
如果这两种介质的密度真的不同，比如空气和水，那么大部分能量都会反射回来；只有很少一部分会继续传播到新媒体中。我的意思是，想想看，当你的头在水下时，外界发出的噪音是多么的小。那么，海豚的咔哒声是如何从充满空气的鼻囊传播到海水中的呢；因为考虑到鼻腔中的空气和海水的密度不同，我们预计这些声音会在海豚的脑袋里弹来弹去！那根本没用？如果要导航，它需要广播这些声音，并从其路径中的对象中反弹回来。
海豚的额头上有一个结构，就在鼻囊的前面，叫做瓜。现在甜瓜是一种由脂肪组织组成的大的囊状袋。这种脂肪组织有一些非常迷人的声学特性&nbsp；你在动物体内发现的大部分脂肪都是用来储存能量的，但这种脂肪与你在海豚体内发现的脂肪非常不同，只有在瓜和下颌周围，这种脂肪非常富含油脂，结果证明它也有非常不同的用途。
现在，克服空气和水密度不匹配的一种方法是；如果你能，嗯，修改声波的速度，使其与声音在水中传播的速度精确匹配；这正是海洋生物学家发现甜瓜的作用。请注意，耳囊，即甜瓜后部的这些小突起，正对着充满空气的鼻囊，而这些耳囊，事实证明，是负责将声音传递给甜瓜的。
声波由囊通过瓜传播，首先通过一个低速的核心，然后通过一个高速的外壳，在那里它们的速度增加，然后再传播到周围的海水中。因此，现在信号可以有效地传输到水中，反射最小。这种特殊的脂肪组织，像甜瓜中的脂肪组织，在海豚身上发现的唯一其他地方是下颌；事实证明，下颚是由特别薄的骨头组成的；它对振动非常敏感，对穿过海水的声能非常敏感。结果表明，下巴主要负责捕捉并将返回的声波传输到海豚的内耳。
因此，这些发出的快速咔哒声会从物体上反弹——m——可能是一群鱼在这里游泳，一艘船从那里驶来——声音会从物体上反弹回来，而下颌骨会捕捉到返回的声音，使海豚能够感知周围水里的东西，并决定在哪里游泳。
三、Dolphin's navigation托福听力问题：
Q1:1.What is the lecture mainly about?
A.Parts of the dolphin's anatomy that allow it to navigate.
B.Two different types of communication used by dolphins.
C.The way that dolphins store air while swimming underwater.
D.The meanings of different signals used by dolphins.
Q2:2.Why does the professor discuss the speed at which sound travels?
A.To describe why sounds made under water can travel long distances
B.To show why a person cannot hear a dolphin well when it is under water
C.To compare the speed of two different sounds made by dolphins
D.To explain how sound waves behave when crossing from one medium into another
Q3:3.What is the dolphin's melon?
A.An oval-shaped bone that lets the dolphin hear sounds
B.An organ made of fat tissue that helps a dolphin send sound waves
C.An air-filled cavity that lets the dolphin breathe underwater
D.An organ filled with water that helps the dolphin measure depth
Q4:4.What is the dolphin's jaw able to do?
A.Send rapid clicking sounds into water
B.Increase the speed of sounds
C.Receive sound waves that have reflected off objects
D.Force water through the nasal sacs and out the blowhole
Q5:5.How does the professor organize the information in the lecture?
A.By describing a phenomenon and the physical structures that make it possible
B.By describing several of the dolphin's senses and their relative usefulness
C.By contrasting how the dolphin makes two different types of sounds
D.By describing an old theory and then a new theory
Q6:6.Why does the professor say this:
A.To find out whether students are familiar with the lecture topic.
B.To mention a related topic that will not be discussed in detail.
C.To mention a common misconception about dolphin vocalizations.
D.To point out a primary function of dolphin blowholes.
四、Dolphin's navigation托福听力答案：
A1:正确答案：A
A2:正确答案：D
A3:正确答案：B
A4:正确答案：C
A5:正确答案：A
A6:正确答案：B
