科学与数学nautil.us
大到绕地球四圈的巨响
The Sound So Loud That It Circled the Earth Four Times
2014 — 2020，它一次次回到首页
黑点是上首页（≥ 50 分）的那次投稿，空心点是没冲上去的投稿，点越大分数越高。
导读
这篇文章讲述1883年喀拉喀托火山喷发产生的巨响——它如此之响，以至于声波绕地球传播了四圈。文中探讨了声音强度的极限、声波传播与衰减，以及这次喷发释放的巨大能量和全球影响。
为什么经久不衰：它把一次极端自然灾害变成理解声学、次声监测和地球系统风险的入口，数字与画面感俱佳，至今仍让人震撼。
HN 评论精选
解释声强按1/r而非1/r²衰减，并推测地球对面17小时后仍很响。
有趣的是，声强会按1/r衰减，其中r是到声源的距离。（我假设声能水平传播时守恒，即不与大气和地形相互作用造成损失）。与1/r²定律相比，比如距离电荷Q为r处的电场强度E(r)=kQ/r²。两种情况都是总量（声能或电场线）必须分散到所有可能方向。总电场必须分散到越来越大球体的表面积上，因此是1/r²行为（精确地说是1/4πr²）。声能均匀分散在周长为C=2πr的圆上，因此在短距离内是1/r衰减。距离更远时，地球曲率会起作用。回想起来，在喀拉喀托正对面某处，火山喷发17小时后一定非常响……
ivan_ah查看英文原文
Interestingly, the sound intensity will decay as a 1/r law, where r is the distance from the source. (I'm assuming conservation of sound energy as it travels horizontally, i.e., no loss to interactions with atmosphere and terrain). Compare with the 1/r² laws like the strength of an electric field at a distance r from charge Q: E(r)=kQ/r². In both cases there is a total of something (sound energy or electric field lines) and that total must be split over all possible directions. The total electric field must be split over the surface area of increasingly larger and larger spheres hence the 1/r² behaviour (1/4πr² to be precise). The sound energy is spread uniformly over a circle with circumference C=2πr, hence giving a 1/r decay, over short distances. For longer distances, the curvature of the earth will play a role. Come to think of it, it must have been really loud somewhere diametrically opposite to Krakatoa, 17 hours after the eruption...
能量相当于200兆吨、全球降温，并警示火山威胁被低估。
背景对比：喀拉喀托喷发释放的能量大约相当于200兆吨炸弹，约为‘沙皇炸弹’的四倍，后者是迄今引爆的最大核武器。由于SO2增加地球反照率，喷发后全球平均温度下降了1.2°C。喀拉喀托的火山爆发指数（VEI）为6，估计大约百年一遇。想想看，我们生活在极薄的地壳上，它形成于一个熔融翻滚的岩石金属球表面。人们奇怪地痴迷于小行星撞击威胁，而我们脚下可能呈现更大危险，且我们对它几乎一无所知！
beloch查看英文原文
To put this in context, the Krakatoa eruption released energy roughly equivalent to a 200 megaton bomb, which is roughly four times larger than the "Tsar Bomba", an H-Bomb which is the largest nuclear weapon detonated to date. The globe's average temperature fell 1.2 C after the eruption due to SO2 increasing the Earth's albedo. The Volcanic Explosivity Index (VEI) of the Krakatoa eruption was 6, and it's estimated to be a roughly once in a century eruption. When you think about it, we're living on a extremely thin crust that's formed on the surface of a molten, roiling ball of rock and metal. People are strangely obsessed with the threat of asteroid impacts when what's beneath our feet presents a perhaps even greater danger, one which we know next to nothing about!
喀拉喀托促使发现次声，今用于监测核试验。
这一事件导致发现了‘次声’，即传播很远距离的极低频声音。目前它仍被用于探测（地上）核爆炸，作为《全面禁止核试验条约》（CTBT）的一部分。南极洲麦克默多站附近‘无风地带’有一个站点，每年有几位技术人员去挖出来维护。我相信全球大约有30个。
rexarex查看英文原文
This event led to the discovery of ‘infrasound’ or very low frequency sound that travels very long distances. It’s currently still used to detect (above ground) nuclear explosions as part of the Comprehensive Nuclear Test Ban Treaty (CTBT). There is a site in the ‘Windless Blight’ in Antarctica near McMurdo Station that a couple of techs go dig out and maintain every year. I believe there are about 30 around the world. https://www.ctbto.org/verification-regime/monitoring-technologies-how-they-work/infrasound-monitoring/
有学者认为喀拉喀托爆发启发了《呐喊》。
一些学者认为喀拉喀托爆炸是爱德华·蒙克画作《呐喊》的灵感来源。
bitcurious查看英文原文
The explosion of Krakatoa is believed by some scholars to be the inspiration for Edvard Munch's painting "The Scream." https://www.skyandtelescope.com/press-releases/astronomical-sleuths-link-krakatoa-to-edvard-munchs-painting-the-scream/
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