Official 07 Set 3

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置顶

Bats' Use of Ultrasound

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What is the lecture mainly about?
  • A. How animals emit ultrasonic pulses

  • B. How bats use acoustical signals

  • C. A comparison of echolocation and radar

  • D. Variations among bats in the use of ultrasound

显示答案 正确答案: B

我的笔记 编辑笔记

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

    FEMALE PROFESSOR:So, that is how elephants use infrasound.Now, let's talk about the other end of the acoustical spectrum—sound that is too high for humans to hear--Ultrasound

    Ultrasound is used by many animals that detect—and, some of them, send out—very high-frequency sounds.So, what's a good example? Yes, Carol.

    FEMALE STUDENT:[matter-of-factly, stating the obvious] Well, bats—since they are all blind, bats have to use sound for, you know, to keep from flying into things.

    FEMALE PROFESSOR:That's echolocation.Echolocation is pretty self-explanatory: Using echoes—reflected sound waves—to locate things…As Carol said, bats use it for navigation and orientation… and what else? Mike?

    MALE STUDENT:Well, finding food is always important—and I guess not becoming food for other animals.

    FEMALE PROFESSOR: [light laugh]Right, on both counts.Avoiding other predators—and locating prey—uh, typically insects that fly around at night.Now, before I go on, let me just respond to something Carol was saying—this idea that bats are blind…Actually, there are some species of bats, the ones that don't use echolocation that do rely on their vision for navigation, but it is true that for many bats,their vision is too weak to count on.

    OK, so quick summary of how echolocation works.The bat emits these ultrasonic pulses—very high-pitched sound waves that we can’t hear—and then: they analyze the echoes—how the waves bounce back.Here, let me finish this diagram I started before the class.So the bat sends out these pulses, very focused bursts of sound, and echoes bounce back.You know, I don't think I need to draw in the echoes, your reading assignment for the next class—it has diagram shows this very clearly—so anyway as I was saying…

    By analyzing these echoes, the bat can determine, say, if there is wall in a cave that needs to avoid, and how far away it is.Another thing it uses the ultrasound to detect, is the size and shape of objects.For example, one echo they quickly identified is one they associate with moth, which is common prey for a bat, particularly a moth beating its wings.However, moth happened to have major advantage over most other insects: they can detect ultrasound; this means that when the bat approaches, the moth can detect the bat's presence.So, it has time to escape to safety, or else they can just remain motionless.Since, when they stop beating their wings, they will be much harder for the bat to distinguish from, oh a leaf or some other object.

    Now, we have tended to underestimate just how sophisticated the abilities of animals that use ultrasound are.In fact, we kind of assume that they were filtering a lot out—uh, the way a sophisticated radar system can ignore echoes from stationary objects on the ground.Radar does this to remove “ground clutter”—information about, um, hills or buildings that it doesn’t need…But bats, we thought they were filtering out this kind of information, because they simply couldn't analyze it.But, it looks as if we were wrong.Recently, there was this experiment with trees and a specific species of bats—a bat called the lesser spear-nosed bat.

    Now, a tree should be a huge and acoustical challenge for bat, right? I mean it got all kinds of surfaces with different shapes and angles.So, well, the echoes from a tree are going to be a mass of chaotic acoustic reflections, right? Not like the echo from a moth.So, we thought, for a long time, that bats stopped their evaluation at simply “that’s a tree.”Yet, it turns out that-that bats, or at least this particular species, can not only tell that it’s a tree, but can also distinguish between, say, a pine tree and a deciduous tree—like, a maple, or an oak tree: just by their leaves—an-and when I say “leaves,” I mean pine needles, too.Any ideas on how it would know that?

    MALE STUDENT:Well, like with the moth, could it be their shape?

    FEMALE PROFESSOR:You are on the right track.It’s actually the echo off all the leaves—as a whole—that matters.Now, think: A pine tree—with all those little, densely packed needles… those produce a large number of faint reflections in wh-what’s called a-a “smooth” echo—the waveform is very even …But an oak which has fewer but bigger leaves with stronger reflections, produces jagged wave form, or what we called a rough echo.And these bats can distinguish between the two, and not just with trees, but with any echo that comes in a smooth or rough shape.

  • 旁白:听一段生物学课程讲座。

    教授:好了,这就是大象使用次声波的方式。现在我们来讲讲声谱的另一端,一种极高的乃至人类听不见的声音——超声波。

    超声波是许多动物用来侦查,并且有些动物还会发出的非常高频的声音。那么,有什么好的例子吗?嗯,卡洛尔。

    学生:呃,蝙蝠。因为蝙蝠都没有视觉,所以它们不得不用声音来辨别方向,你知道的,以免飞行的时候撞到东西。

    教授:那是回声定位。回声定位不言而喻,就是用回声—反射声波—来定位事物。正如卡洛尔说的,蝙蝠用回声定位来导航和定位。还有别的么?麦克。

    学生:呃,觅食是很重要的,我猜是不想成为其他动物的猎物。

    教授:是的。两个方面都对。躲避捕猎者,定位猎物,昆虫基本都在夜间飞行。在我继续往下讲之前,我先回答一下刚刚卡洛尔说的问题,就是说蝙蝠都是没有视觉的。其实,蝙蝠的一些物种是不依赖回声定位,而是用视觉进行导航的。但是确实,很多蝙蝠的视力太差了,基本起不到什么作用。

    好了,我们快速说一下回声定位是如何起作用的。蝙蝠发出超声波脉冲,很高分贝的声波以至于我们听不到,然后,它们分析回声,声波是如何反弹回来的。在这里,让我完成这个图解,我在上课之前就开始了。那么蝙蝠发出脉冲,就是很集中的爆破声,然后回声反射回来。我想我不用画声波图了,你们下节课的阅读作业中有图表给你们很清晰地展示了这一点,就像我说的...

    通过分析回声,蝙蝠就能知道,比如说, 如果洞中有墙需要躲避,以及墙的距离有多远。使用超声波探测还有一个用途就是探测物体的大小和形状。比如,它们迅速鉴别的一种回声方式和飞蛾有关,飞蛾是蝙蝠的常见的一种食物,尤其是飞蛾在拍打它的翅膀。然后,飞蛾碰巧也比其他昆虫占有优势:它们能够探察到超声波,这就意味着当蝙蝠靠近的时候,它们会知道蝙蝠的存在。因为,它们有时间逃到安全地带,或者它们会纹丝不动。因为一旦它们不再煽动翅膀,蝙蝠就很难将它们从树叶或者其它物体辨别开来。

    我们倾向于低估使用超声波的动物的能力是多么的复杂。事实上,我们推断蝙蝠在分析回声时过滤掉了很多因素,一个复杂的雷达系统可以忽略地面上静止物体的回声。雷达这样做是为了消除“地面杂乱”——关于它不需要的山丘或建筑物的信息。而蝙蝠,我们认为它们过滤掉了这类信息,仅仅是因为它们不能够分析到这些因素,但是,看起来我们错了。最近有一次关于树木和特定物种的蝙蝠的实验,这是一种小矛鼻蝙蝠。

    那么,树木对于蝙蝠来说应该是一个巨大的声学挑战,对吧?我的意思是,树木的表面形状角度各不相同。所以,树的回声将会是一团混乱的声波反射,对吧?不像飞蛾的回声。我们一直以为蝙蝠对树的概念仅仅是树木而已。然而,结果证明蝙蝠,至少是这个种类的蝙蝠不仅能感知到树木,还能仅通过树叶就会区分开松树、落叶树,比如说枫树或者橡树,我所谓的树叶,还包括松针。关于蝙蝠是如何知道这一点有什么想法吗?

    学生:呃,比如飞蛾,可能是它们的形状?

    教授:思路很对。实际上是所有树叶组成整体的一个回声。现在大家想象一棵有着小的,密集的松针的松树,它们产生了大量的微弱的反射,我们称作“平滑回声”,这种声波非常的平均。但是橡树这种树叶少但树叶大的树木就会有更强的回声,会产生参差不齐的声波,或者我们称之为“粗糙回声”。而蝙蝠能够区别树木,不只是因为树木本身,而是以平滑或粗糙的形状出现的回声。

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    题型分析:主旨题

    原文定位

    Ultrasound is used by many animals that detect—and, some of them, send out—very high-frequency sounds. So, what’s a good example? Yes, Carol?

    选项分析

    文章中教授说“Ultrasound is used by many animals that detect”,然后让学生举例,学生说了bats,整篇主要内容是在讲蝙蝠如何利用超声波进行定位等一系列行为。因此选B。

    选项A:信息未提及;

    选项C:对比信息非lecture的主线,排除;

    选项D:variations在lecture未体现,只提及有些bats可以靠视力定位,而大多数蝙蝠只能靠超声波。



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