考研英语作文：在火星可以大声(At Mars can be loud)
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Sound dies quickly in the cold, thin air of mars. researchers have modeled a sound wave traveling through the martian atmosphere and report that it doesnt go far--even a lawn mowers roar dies after a hundred meters or so. the model presents an unusually detailed picture of how sound travels in an alien atmosphere and hints at what it would take to communicate on the red planet.
The shriek of a baby, an ambulances siren, or a violin sonata are all essentially the same thing: waves of pressure traveling through the air. sound can also travel through water, or a solid like the ground, but because molecules must bump into each other to propagate the pressure wave, the denser the medium the better. hoofbeats or footsteps travel farther through the ground than through the air, for eample, because the molecules in air have to travel further to bump into one another than those in soil, thus losing energy more quickly.the martian atmosphere is mostly carbon dioide and only 0.7% as dense as earths is, so sound should fade more quickly. but the details of how sound waves travel in the martian atmosphere were unclear and could be important to future mars missions.now, a computer model has given a molecule-by-molecule map of how sound moves on mars. graduate student amanda hanford and physicist lyle long of pennsylvania state university in state college presented the model last week at a meeting of the acoustical society of america meeting in providence, rhode island. the model is unusual in its molecular approach; most acoustical models of sound treat the medium it travels through as a continuous block with average properties. such models are fine for dense atmospheres like earths, but treating the air like a loose bunch of freewheeling molecules is more realistic for mars rarefied atmosphere, say the researchers.
Hanford and long first set up a virtual "bo" filled with about 10 million carbon dioide molecules floating about randomly, at the same density as the martian atmosphere. a sound wave then appeared on one side of the bo, and the model calculated its progress across to the other side, computing nanosecond by nanosecond eactly how the carbon dioide molecules bumped and moved. the results show that a noise that would travel several kilometers on earth would die after a few tens of meters on mars. quieter sounds would travel far shorter distances, making eavesdropping on a quiet conversation nearly impossible.
Henry bass, a physicist at the university of mississippi in oford, notes that if people ever go to mars and want to communicate audibly, theyll need to design devices that can work with the lower frequencies transmitted by the martian atmosphere.
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