TOEFL Listening: ETS-TOEFL听力机经 - BGK42ZH1O8P4531NQ$

Lecture: Underwater Noise: Narrator: Listen to a part of a lecture in a marine ecology class. Professor: When we talk about ocean pollution, we normally think about things like oil spills, um, chemical pollution, and ... and solid waste, like, um, plastic bag that end up in the stomach a feature role. That there's an invisible form of pollution that can also affect marine life, noise – sound waves generated from human activity. Female Student: You mean sonar sound wave? You know, what people used to look for sunken ships and stuff? Professor: Um, yeah. Sonar and other activities like drilling for oil under the sea do contribute. But we believe the major source of underwater noise comes from ships. Large commercial vessels that carry goods throughout the world. We know that underwater background noise produced by ship traffic has been doubling every ten years since the 1950s. Um. You see, the international trade is growing all the time. Also thought what is an amazing conductor of sound waves? Sound travels five times faster underwater than through the air. So it's no surprise that many million animals evolved a new sound for things like communication, navigation, locating food, detecting predators, finding a mate. But unfortunately, the sounds produced by large ship are primarily in the same bandwidth that we mammals use – low frequency sounds which can travel great distances. Male Student: Is there any direct proof that the animals are actually getting hurt by this? Professor: No. No hard evidence yet, but we're still researching this. You do blieve however, that noise pollution does add to the animal stress and potentially affect their behavior. In one study for example, it was found that the song or call of a right whale can be discerned by another right whale swimming twenty kilometers away in a quiet ocean. But in a noisy ocean, the whales would have to be swimming within one or two kilometers to hear each other. Um, imagine if everyone in this class and started talking at once, It'll be a struggle to hone in on just my voice unless you walk right up to me. Right? Male Student: So it's like a whale mating call got found out, it could prevent it from finding a mate. Professor: Sure. Look, right now there's something like 200,000 animal species on the United States endangered species list, virtually all the marine mammals, including whales and many these species. See that low frequency shift noise interferes with mating calls of the species that already endangered by other forms of pollution. If this happens often enough or widely enough, it could conceivably contribute to that species' extinction. Male Student: But it's not like they're gonna stop the ships to save the whales. Professor: No. But there are ways to make ships less noisy. For example, you can insulate the engine. You could also make ships' hull more streamlined, um, the hull of most commercial ships are robin boxes, but even the hull of everything with streamlined it wouldn't fix the biggest source of the noise, a phenomenon known as cavitation. Cavitation refers to the formation and collapse of tiny bubbles that form around the propeller blades as they rotate. Male Student: I can't believe that popping bubbles make so much noise. Professor: You said ordinary bubbles like soap bubbles. The bubbles are actually cavities. Hanging vapor filled cavity that get banged around propeller blades rotating at high velocity. The cavity glass just split seconds before they collapse. That each collapse, send kind of shockwave through the water, in shockwave combined, to produce a steady roar of sound wave. Now, cavitation can be reduced by uh, by redesigning the propellers and changing how water gets funneled through them. Also, there's less cavitation at lower cruising speed. But you can imagine, records that in the world fleet of 100,006 to mitigate sound would be incredibly expensive to shipping companies. On the other hand, it's a widely acknowledged back that noisy chips are also the least efficient ships. Like, it takes less fuel to propel the streamlined hull through the water compared to a box shaped one. Female Student: And propellers that produce lots of cavitation? Professor: They use more fuel too. And the more cavitation and more wear and tear on the propeller's leg. So they need to be replaced more often.