【小站原创】官方真题Official7托福听力Lecture2文本+真题+答案解析

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官方真题Official7托福听力Lecture2原文文本

Narrator: Listen to part of a lecture in a Biology class.

Pro: So, that is how elephants use infrasound. Now, let’s talk about the other end of the acousticalspectrum, 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 they are all blind, bats have to use sound for, you know, to keep from flying into things.

Pro: 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.

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

Pro: Right, on both counts. Avoiding other predators, and locating prey, 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 pitch 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 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 on the echoes. Your reading assignment for the next class, it has a diagram that shows this very clearly.

So, anyway, as I were saying, by analyzing these echoes, the bat can determine, say, if there is wall in a cave that it needs to avoid, and how far away it is. Another thing it uses ultrasound to detect is the size and shape of objects. For example, one echo they quickly identify is the one they associatewith a 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 detectultrasound; this means that when a 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’d 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 assumed that they were filtering a lot out, the way a sophisticatedradar system can ignore echoes from stationary objects 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 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 acoustical 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 a mass of chaotic acoustic reflections, right, not like the echo from a moth.

So, we thought for a long time that bats stop their evaluation at simply that is a tree. Yet, it turns out that bats or at least this particular species, cannot only tell that is a tree, but can also distinguish between, say, a pine tree, and a deciduous tree, like a maple or oak tree, just by their leaves. And when I say, leaves, I mean pine needles too. Any ideas on how it would know that?

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

Pro: You are on the right track — It is actually the echo of all the leaves as whole that matters. Now, think, a pine tree with all those little densely packed needles. Those produce a large number of faint reflections in what’s... what’s called a ... a smooth echo.

The wave form is very 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 with trees, but with any echo that comes in a smooth or rough shape.

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