题目内容
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[单选题]
若信噪比S/N的值为100，则对应的分贝值为（）dB。
A.20
B. 1000
C. 100
D. 30
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A.分贝,简写为dB;
B.秒,简写为s;
C.赫兹,简写为Hz;
D.贝尔,简写为Bell;
E.克,简写为g。
更多“若信噪比S/N的值为100，则对应的分贝值为（）dB。”相关的问题
第5题
Unwanted sound, or noise, such as that produced by airplanes, traffic, or industrial machinery, is considered a form. of pollution. Noise pollution is at its worst in densely populated areas. It can cause hearing loss, stress, high blood pressure, sleep loss, distraction (注意力分散), and lost productivity!
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Sounds are produced, by objects that vibrate (振动), at a rate that the ear can detect. This rate is called frequency and is measured in hertz (赫兹), or vibrations per second. Most humans can hear sounds between 20 and 20,000 hertz, while dogs can hear high pitched sounds (高频声音) up to 50,000 hertz. While high frequency sounds tend to be more hazardous (危险的) and more annoying to heating than low frequency sounds, most noise pollution damage is related to the intensity of the sound. Measured in decibels (分贝), noise intensity can range from zero, the quietest sound the human ear can detect, to over 60 decibels. Conversation takes place at around 40 decibels, a subway train is about 80 decibels, and a rock concert is from 80 to 100 decibels. The intensity of a nearby jet taking off is about 110 decibels. 120 decibels axe the limit at which human beings will be aware of pain, and suffer from potential heating loss. Long lasting, high intensity sounds are the most damaging to heating and produce the most stress in humans.
Solutions to noise pollution include adding insulation and sound proofing (隔音) to doors, walls, and ceilings; using ear protection, particularly in industrial working areas; planting vegetation to absorb and screen out noise pollution; and zoning urban areas to maintain a separation between residential areas and zones of excessive noise.
Noise pollution can cause ______.
A．lower productivity
B．lower blood pressure
C．sound sleep
D．less stress
第7题
Section B
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Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
How would you design an aircraft if your main aim were to keep its roar from waking up the entire neighbor hood during takeoffs and landings? A group of 40 engineers from the Massachusetts Institute of Technology and England's University of Cambridge took on that challenge and, three years later, has unveiled designs for the SAX 40. Thanks to a host of noise-silencing innovations, the 215-seat airplane should be virtually inaudible outside the airport. Designers estimate that the aircraft will emit just 63 decibels(分贝) at takeoff, about as loud as an average conversation, while Today's airliners hit up to 150 decibels. However, when landing, it still is noisier than at take off, about 100 decibels.
To achieve such relative silence, researchers needed to rethink' nearly every aspect of the typical airliner. "There's no one thing you can do," says Will Graham, who headed up the Cambridge airframe-design team. "You have to treat all the noise sources."
What aspects should be considered? First, the long exhaust tubes have acoustic liners to get rid of noise, and the size of the exhaust nozzles can be made smaller during takeoff to reduce noise or larger while cruising for fuel efficiency. Second, instead of noisy flaps, a sloped wing and steep landing angle do the braking. Engines embedded in the frame, with air intakes an top, reduce drag and noise. Last, three radial-blade(径向叶片) fans per turbine move large volumes of air at low,quiet speeds. Precisely tuned turbine blades don't vibrate nonmoving parts.
Moreover, chief among the aspects is the airframe. The faster a plane flies, the louder it mars. So Graham and his group designed the SAX-40 with a blended-wing body, a concept that discard the tube-and-wing approach for a tailless, triangular structure. That shape provides extra lift, enabling the plane to fly more slowly during takeoffs and landings. It also saves a significant amount of fuel, getting some 35 percent more passenger-miles per gallon than a Boeing 777.
A number of other researchers are working on similar concepts. But challenges remain. An aircraft without a stabilizing tail would be far more difficult to fly. And then there is the question of how to make its flattened fuselage(机身) as structurally strong as a cylinder. But perhaps the biggest difficulty is the airline industry's financial in vestment in conservative design. Betting on such a radical plane, Graham says," would be a huge step for manufacturers."
What will be achieved by the design of SAX 40?
A．The noise SAX-40 made cannot be heard at take off.
B．The noise SAX-40 made cannot be heard during landings.
C．The noise SAX-40 made during landings will decrease sharply.
D．The noise SAX-40 made at takeoff will decrease sharply.
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