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______ were made on the text before and after it was presented to the Congress.
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听力原文： As we all know, when machines work they give off a lot of noise, and the noise can sometimes be very unpleasant, or annoying. As a result, people have been trying to find ways to reduce noise. Although it may sound a bit strange to you all, one of the best ways to make machines quieter, in cars for example, may be to make them noisier. The source of this paradox is electronic anti-noise which creates sound waves to cancel out unwanted noise, such as rattles, blare, etc.
Now although the idea dates back to the 1930s, it's only recently that advances in computer technology have made anti-noise a commercial possibility. Take France for example. Here industry spends a fortune to get rid of noise. We all know how unpleasant it is. Both people working in factories and ordinary people at home enjoying their leisure can be affected by noise. Delicate machines are affected by noise as well. The government has passed a lot of regulations concerning acceptable levels of noise.
Noise is costly to industry. In the first place, just following a single regulation can cost 1 to 2 billion US dollars in the textile industry, they say.
Then secondly, of course, vibration can cause damage to machinery. Even very small vibrations can cause parts to wear out and equipment to fail. And naturally enough, this gets added on to the price of the products. About 5% to 15% of the price of a product comes from noise and vibration costs, it's estimated.
At present, methods used to dampen down noise and vibrations rely on techniques that are 30 or 40 years old. They usually involve wrapping the noisy or vibrating component in anything from cotton to concrete. But this is often expensive and inefficient.
The modern electronic anti-noise devices don't reduce sound. Instead, sound is used to attack sound. The trick is to hit these sound waves with other waves in a carefully controlled way. It may not be possible to eliminate noise completely, but engineers can build systems to eliminate specific kinds of noise and vibration. The new systems can deal with repetitive noise. This unfortunately means that there is not much that can be done about one-off noise, like someone trying to learn to play a trumpet. But they can handle fairly regular things like engine noise.
A French company has developed a technique which uses a microphone and a microprocessor. The processor measures the sound and directs a speaker to broadcast sound waves that are out of phase with the engine noise. The company claimed that it will make a car engine quieter, if not completely 100% silent. There are a number of other areas of application in the noisy, industrial environment of today. One system which the company is developing, aims to minimize the noise of aircraft engines and helicopter vibrations. Now anti-noise systems would be able to reduce noise in the cabin of an airplane to mere acceptable levels.
Another area which affects ordinary households nowadays concerns the noise which electricity and gas suppliers create. The electricity companies spend a lot of money each year, cutting the harm of transformers, trying to quiet the noise and maintaining equipment that is constantly affected by vibration. If they can get rid of or even stop the vibrations, manufacturers can increase production speeds. But, of course, people working in noisy workplaces are perhaps more affected than anything by the effects of noise. Anti-noise can also create "zones of quiet" in noisy workplaces. To create such a zone, you do two things.
First, microphones are suspended around the workplace. Then speakers that produce out-of- phase sound waves can then be put close to the worker under the desk or the machine. A company working in this area has tested this system and says that it cuts noise levels enough for somebody inside the zone to hear a conversation from anoth
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