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9.26早听写
http://blog.sina.com.cn/s/blog_86290bae0100wz4h.html
Professor:
So that's how elephants use infrasound. Now let's talk about the other end of acoustic 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?
Carol:
Well, bats, since the are all blind, bats have to use sound for you know to keep them from flying into things.
Professor:
That's echolocation. Echolocation is pretty self-explanatory: using echoes reflected sound waves to located things. As Carol said that's used for navigation and orientation. And what else? Mike?
Mike:
Well finding food is always important, and I guess not becoming food for other animals.
Professor:
Right, on both accounts. Avoiding other predators and locating prey, typically in sense of flying around it at night. Now, before I go on, let me just respond something Carol was saying, this idea that bats are blind. Actually, there're 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 how echolocation works. The bat emits the ultrasonic pulses very high pitch sound waves that we can't hear. And then they analyze the echoes how the waves bounce back. Here, um, let me finish the style diagram I started before the class. So the bat sends out the pulses very focused verses of sound, and echo bounce back. You know I don't think I need to draw the echoes, your reading assignment for the next class, it has a diagram shows this very clearly. Anyway as I was saying, by analyzing these echoes, the bat can determine, say, if there is a 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 the shape of objects. For example, one echo they quickly identified is one way associated with moth, which is common prey for a bat, particularly a moth beating its wings. However, moth happened to have a major advantage over most other insects.
They can detect ultrasound, this means that when the bat approaches, the moth can detect about 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... or a leave or some other object. Now we've tended to underestimate just how sophisticated the ability of animals that use ultrasound are. In fact, we kind of assume that they were filtering a lot out. Um, the ways are sophisticated radar on our system can ignore the echo on the stationary object on the ground. Radar does this to remove ground clutter, information about 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 we were wrong. Recently, there was this experiment with trees and a specific species of bat. A bat called the lesser spear-nosed bat. Now a tree should be a huge and acoustic challenge for a bat, right? I mean it's got all kinds of surfaces with different shapes and angles. So, well, the echoes from a tree are going to be massive and chaotic acoustic reflections right? Not like the echo from a moth. So we thought for a long time that the bats stop their valuation as simply "that's a tree". Yes, it turns out that the bats or at least this particular species cannot 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. And when I say leaves I mean pine needles too. Any ideas on how we would know that?
Mike:
Well, like with the moth, could it be their shape?
Professor:
You are on the right track-it's actually the echo of all the leaves as a whole, that matters. Now, think, a pine trees with all those little densely packed needles. Those produced a large number of fain reflection in which what's called as: a smooth echo. The wave forms were even but an oak which has fewer but bigger leaves with stronger reflections produces a jagged wave form, or what we called a rough echo. And these bats can distinguish between the two and not just was trees , but with any echo come in smooth and rough shape.
infrasound n. 次声(风暴产生的低频音波)
ultrasound n. 超声; 超声波 ultrasonic adj. (声波)超声的
acoustic adj. 声音的, 听觉的
spectrum n. 光谱
echolocation n. 回声定位能力,回声定位法
self-explanatory adj. 不解自明的;明显的
orientation n. 方向, 目标
account n. （思想、理论、过程的）解释；说明；叙述
predator n. 食肉动物
prey n. 被捕食的动物, 猎物, 牺牲品
count on 依赖, 依靠;期望, 指望
verse n. 诗, 韵文；诗节, 歌曲的一段
echo n. 回声; 共鸣
moth n. 蛾
approach vt. & vi. 接近, 走近, 靠近
motionless adj. 不动的,静止的
underestimate vt. 对…估计不足, 低估
sophisticated adj. 老练的; 老于世故的
filter vt. & vi. 透过, 过滤
radar n. 雷达
clutter n. 杂物, 零乱的东西
spear n. 矛, 枪, 鱼叉 vt. 用矛刺, 用鱼叉捉
chaotic adj. 混沌的;一片混乱的;一团糟的
valuation n. 估价;估定的价格;定价
deciduous adj. (指树木)每年落叶的
maple n. 槭树, 枫树
oak n. 栎树, 橡树
track n. 踪迹, 痕迹, 足迹
densely adv.浓密地,稠密地;密集地
packed adj. 异常拥挤的;挤满人的 有大量…的;…极多的；紧密地压在一起
fain adj. 乐意,不得不,只得…
even adj.均匀的; 有规律的; 稳定的
jagged adj. (边缘)粗糙的,有缺口的;参差不齐的
kind of 〈口〉稍微, 有点儿, 有几分
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发表于 2011-9-19 19:20:11
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