单选题
Scientists now tend to agree that the noise level for potential hearing loss begins at about 70 decibels. Some of them are very concerned because normal daily life often exposes people to noise levels of about 70 decibels even inside their homes. Cities have always been noisy, but noise is now spreading to areas that were quiet just a few years ago.
Clearly, something must be done or noise will seriously and permanently maim the population. Fortunately, the knowledge and methods to control noise already exist. As a matter of fact, this is one instance where the knowledge of control methods exceeds the knowledge about the effects on human life and on the environment.
There are two common means for control. The first is reducing noise at its source, and the second is changing the sound path by distance or by shielding.
The second approach is being used more often today as people become more aware of the danger of noise. New building codes require better sound insulation in homes and apart ments. More and more towns are passing zoning ordinances that try to segregate noisy fac tories or airports from residential areas. Sound-absorbent materials and construction de signed to block sound paths are slowly coming into use in offices and homes. New highways are being built to redirect traffic noise up and away from nearby areas. Aircraft are increas ingly being required to use reduced power flights around airports.
There are many examples of available noise control methods that are not being used. More flexible building codes would permit the use of quieter kinds of plumbing pipes. Sound-absorbing materials can reduce the noise of motors and engines. Power genera tors can be quieted with baffles, exhaust silencers, and sound absorbers. Truck tires can be made with quieter treads. In many cases, the cost of building quieter machines is the same or only slightly higher than that of the current noisy ones. Even though the new equipment may cost more initially, it can prove more profitable in the long run. The new jumbo jets, for example, are quieter than the older ones, yet they are more powerful and carry twice as many passengers.
All of these methods are only partial measures as noisy levels continue to rise. Most specialists in the field agree that much of the solution must come from eliminating some of the noise at its source, therefore saving through prevention the large costs of hearing loss.

单选题 The noise level for possible bearing loss begins at about______.

单选题 Jet pilots are being advised to land______.

单选题 How many practical means for controlling noise does the author present?

单选题 The reader can assume that airport of the future will______.

单选题 Jumbo jets are mentioned as examples of______.