在火星上，嗓门可得大一点！

研究显示火星大气的吸音效果特好，是地球大气的100倍。

声音依靠介质粒子碰撞传递能量，声波的传播速率与衰减决定于介质的状态与种类。火星地表的大气压仅有地球的0.7％，大气成分95％是二氧化碳、再来是2.7％的氮气、1.6％的氩气，而平均温度是零下63℃。可以预期，声音在火星上可能衰减的很快，但是细节却不甚清楚。

多数模拟声音传播的数值模式，把介质视为连续体来处理：不去考虑分子个别行为的影响，只以整体的平均性质来做计算。这样的观点，适用于地球表面的厚重大气，但不合用于稀薄的火星大气。宾州州立大学的研究生Amanda Hanford与物理学家Lyle Long则建立一个分子模式来模拟声音在火星的传播：他们模拟被局限在虚拟盒子内一千万个二氧化碳分子的运动：当声波出现在盒子的一侧后，以每奈秒为时间间隔计算二氧化碳分子的碰撞与移动，来看声波如何传递到盒子另一侧。模式结果已发表在六月初的声学会议（the Acoustical Society of America）上。

模式结果显示，火星大气的吸音效果特好，是地球大气的100倍，这表示原本在地球上会吵到数公里远的噪音，在火星上在数百公尺的距离就已经听不见。所以，未来在火星上，我们若想要”呼”朋引伴，除非有其他装置的协助，不然嗓门可得大一点。而怕吵的人，火星会是个好地方，或者月球也行！

On Mars, No One Can Hear You Scream

By Kim Krieger
ScienceNOW Daily News
12 June 2006

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 doesn't go far--even a lawn mower's 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 ambulance's 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 example, 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 dioxide and only 0.7% as dense as Earth's 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 Earth's, 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 "box" filled with about 10 million carbon dioxide molecules floating about randomly, at the same density as the Martian atmosphere. A sound wave then appeared on one side of the box, and the model calculated its progress across to the other side, computing nanosecond by nanosecond exactly how the carbon dioxide 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 Oxford, notes that if people ever go to Mars and want to communicate audibly, they'll need to design devices that can work with the lower frequencies transmitted by the Martian atmosphere.