托福模拟试题听力 - TPO23 - 讲座3

时间 : 2013-10-25 08:18来源 : VOA官网 收听下载次数 :
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TPO23 Lecture3 Biology (Dolphins)

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. 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. And the puzzle is how does the click sounds get transmitted into the 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 ? OK. You've got a sound wave traveling merrily along through one medium, when suddenly; it hits a different medium, what does gonna happen then? Well, some of the energy is going to be reflected back, and some of it is going to be transmitted into the second medium. And ? and ? and if the two media have really different densities, like air and water, then most of the energy is going to 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 is underwater.

So, how did 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 kind of go bouncing around inside the dolphin's head, which will do it no good at all. If it's going to navigate it, needs those sounds to be broadcast and bounced 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, which 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 um, modify the overcome this mismatch in the density of air and water would be ? if you travels through velocity of the sound wave, make it precisely match the speed at which water. And that's exactly what marine biologists have discovered the melon Note that the bursa, these little projections at the rear of the melon, are right up against the air-filled nasal sacs. And these bursa, it turns out, are what's responsible for transferring sound to the melon. The sound waves are then transmitted by the bursa 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 is found in the dolphin, is in the lower jaw. Turns out that the lower jaw, well, it is made of a specially thin bone. And it is 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, 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.

 

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



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



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



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



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



6  Why does the professor say this [one being the vocalizations you are probably all familiar with, which they emit through their blowholes.

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