M: What’s all that noise It sounds as if it’s coming from next door. The Nelsons aren’t back yet, are they
W: I don’t think so. It must be the window-cleaner working upstairs.
M: What’s all that noise It sounds as if it’s coming from next door. The Nelsons aren’t back yet, are they
W: I don’t think so. It must be the window-cleaner working upstairs.
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.
Car makers’ 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. By producing pressure waves going in the opposite direction.
B. By mixing high frequency sound waves with low frequency sound waves.
C. By mixing new sound waves with the noise and sending them out together.
D. By making copies of tire unwanted sound waves and letting them out a little later.
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