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  3. To fight a noise by making more noise sounds strange, but that’ s exactly what motor engineers are doing in Germany and some oth

To fight a noise by making more noise sounds strange, but that’ s exactly what motor engineers are doing in Germany and some oth

admin2012-10-16  95
问题     To fight a noise by making more noise sounds strange, but that’ s exactly what motor engineers are doing in Germany and some other countries.
    Carmaker’ s research and development laboratories have already proved that mixing more noise with the help of loudspeakers can reduce the unwanted noise. Physicists have known about the technique for a long time. Sound is made up of pressure waves in the air. If two sound waves of the same frequency(频率) mix so that the highest point of one wave happens at the same time as the lowest point of the other wave, the result is no sound. Therefore, by producing a perfect copy of the noise and delaying it by half a wave cycle (周期) ,we can kill the unwanted noise. Using this technique many carmakers are racing to develop noise killing systems (系统)both inside and outside the cars.
    Another good thing about the use of noise killing systems is that it saves the need for a silencer; this not only reduces the weight of a car, but also makes the motor burn less fuel and work better.
    Some engineers believe that the noise killing system will be used in most cars within a few years. But the carmakers haven’t decided if they will carry it into production because it would add several hundred dollars to the cost of their cars.
Besides its main use the noise killing system can______.
选项 A、make a car lighter
B、make a car quieter
C、reduce the cost of a silencer
D、improve the performance of a silencer
答案A
解析 判断推理题。通过文章第三段：Another good thing about the use of noise killing systems is that it saves the need for a silencer；this not only reduces the weight of a car…可知，应用了这种设备，还可以减少汽车的重量。所以正确的答案为A。
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