题目内容
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______.
查看答案
搜索结果不匹配？点我反馈
更多问题
若x和y都是buble型变量，且x的初值为3.0，y的初值为2.0，则表达式pow(y,fabs(x))的值为 【7】 。
Why did Mr. Blue buy a camera
The way we feel about our mother’s behavior rather than her behavior may cause us distress.
Every year landslides (滑坡) cause 25 to 50 deaths and $1.5 billion in damage in the United States. They account for 15 percent of the deaths from natural disasters in Europe. And in December, a single event killed more than 200 people in the Philippines. Sending workers to stabilize mountainsides using steel bars and cement (水泥) can help prevent disaster, but it introduces new difficulties. Shaking drills produce harmful dust and loosen heavy, dangerous debris (岩屑). "It’s quite a risky job, "says Giorgio Pezzuto of D’Appolonia, an engineering company in Italy. D’Appolonia, working with eight other companies, may have an answer: a three-ton robot called Roboclimber. "The idea is to operate a machine far away that can drill without a human being on board," says Pezzuto, the manager for the project, which is supported by the European Commission. Engineers claim that the machine will be faster and cheaper than manual labor. The robot, a large radio-controlled four-legged mechanical spider, has cost at least $2 million so far. The final product should be able to climb unstable mountainsides, drill holes, insert bars and cement, and collect data on the stability. Testing should begin in May. What is the purpose in writing this text
微信支付 
支付宝支付