Official 23 Set 5

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Dolphin's Navigation

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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 under water

  • D. The meanings of different signals used by dolphins

显示答案 正确答案: A

我的笔记 编辑笔记

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

    We've 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,[implying that this point will not be discussed] 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 that 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.

  • 旁白:听下面一段关于海洋生物学的课程讲解。

    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,我的意思是,大家都爱海豚,对吧?海豚,实际上他们可以发出两种声音,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。但是我们今天所讲的声音是他们用以回声定位的咔嚓声,这样他们就能感知周围的事物,研究发现,这种声音是在海豚的充满空气的鼻囊里发出的。谜题就是:振动波是如何传输到水中的?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。所以声音在水中的传输速度要比在空气中快。而当声波…好,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?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。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。你可以想想当你的头在水下时,你能听到多少外面的声音。

    那么,海豚的声波是如何从充气的鼻囊进入到海水中的呢?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。如果海豚想要导航,完全没有任何作用。它需要那些声音被传播出去并在遇到物体时从它的路径反弹回来。原来海豚的前额有一个构造,就在它的鼻囊前方,叫鼻疣。

    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。这个脂肪组织具有独特的声学特征。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,在鼻疣和下颌周围这些脂肪是非常不同的,油的含量非常高,而且它的作用也十分的特殊。

    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,使它与声音在水中的传播速度相同。而海洋生物学家发现这就是鼻疣的作用。值得注意的是这个囊状物,这些鼻疣后面的突起,他们就在充气的鼻囊的旁边,这些囊的作用就是将声音传输到鼻疣中。这些囊状物使声波在鼻疣中传播,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。这样信号就能够以最小的反射有效地传输到水中。

    而在海豚身上其他地方,唯一有这种脂肪的就是下颌。这个下颌是由非常薄的骨头构成的,它对震动和海水中声音能量的传递十分敏感。下颌主要是负责捕获声音并将声波传递回海豚的内耳。而这些声波发出后碰到物体,有可能是这边的鱼群或是那边是来的船,声音就会折射回来,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。

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    题型分析:主旨题

    原文定位

    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. 

    选项分析

    在文章开头部分先介绍相关的背景知识,然后分别详细讲解了两种navigate 的具体方式,一种是vocalization 一种是bursa。最后讲解了这些navigate的方式在海豚身体里是如何运作的

    A选项即为整篇文章的全文归纳总结,是对全文主旨最准确的概括

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