/ 



a plane flies through the 

 air, the solidly attached molecules of its 

 nose part the gaseous molecules ahead 

 of it, creating a huge ripple of pressure. 

 The ripple passes from the layer of mol- 

 ecules closest to the plane, to the layer 

 just ahead, and then to the layer just 

 ahead of that, all at the speed of sound. 

 Same thing happens ahead of the air- 

 plane's tail. As each layer feels the rip- 

 ple, it bumps into the next layer, an- 

 nouncing the news of the oncoming 

 plane. Meanwhile, the plane cruises 

 through the air without incident. 



But what happens if the plane flies 

 faster than the time it takes for each 

 layer to bump into the layer in front 

 of it — faster, in other words, than the 

 speed of sound itself? (At the frigid air 

 temperatures at cruising altitude, the 

 speed of sound is about 670 miles an 

 hour.) If the plane is suitably designed 

 and powered, it just busts through the 

 medium's hapless molecules. All the 

 pressure waves — including the ones 

 created by the noise of the plane's en- 

 gines — now pile atop one another, 

 greatly amplifying the resulting sound. 



Meet the sonic boom. 



A sonic boom is the audio track of 

 a shock wave. Anyone who happens 

 to be nearby will hear it loud and clear. 

 Amplify a plane's shock wave by a fac- 

 tor of ten, a hundred, a thousand, and 

 you're on your way to simulating the 

 conditions of some common happen- 

 ings in outer space. 



Every syllable you utter sends its 

 own sound wave — its own wave 

 of pressure — rippling through the air. 

 When you stand in one spot and talk 

 nonstop, each wave you generate forms 

 a sphere that is centered on your mouth 



and expands at the speed of sound. 



But let's say you're both chatty and 

 speedy. If, for instance, you start at the 

 base of the Washington Monument and 

 talk while you walk north toward the 

 White House, each new sound you 

 make comes from a new expanding 

 sphere that rides closer than normal to 

 the leading edge of the preceding 

 sphere and farther than normal from its 

 trailing edge. Of course, the faster you 

 walk, the closer together are the lead- 

 ing edges of successive sound waves. 



Now suppose you walk so fast that 

 the sound of your current syllable 

 catches up with the sound of your pre- 

 vious syllable. If you keep walking and 

 talking at that speed — the speed of 

 sound — all your syllables will pile up 

 together as you lay one track after an- 

 other on the same leading edge. That 

 would be your own personal shock 

 wave. At that speed, about 770 miles an 

 hour on a fall day in D.C. (and leaving 

 aside the fact that the ferocious air re- 

 sistance would dismember your body), 

 you would arrive at the back door of 

 the White House about three seconds 

 after leaving the foot of the Washing- 

 ton Monument. 



When physicists refer to the speed 

 of an object in a medium, they almost 

 always invoke "Mach" numbers. This 

 unit is named for the nineteenth-cen- 

 tury Austrian physicist and philosopher 

 Ernst Mach. By definition, an object 

 that moves at Mach 1 moves at the 

 speed of sound. But don't ask, "How 

 fast is that?" unless you're prepared to 

 answer three questions: "What is the 

 temperature of the medium? What 

 kinds of molecules comprise it? How 



compressible is 

 it?" Those questions arise 

 because, unlike the speed of light in a 

 vacuum, which is the same anywhere 

 in the cosmos, the speed that corre- 

 sponds to Mach 1 is strictly local. 



Nowadays, encounters with Mach 1 

 are not rare. The snap of a damp towel 

 against your friend's butt is a mini 

 sonic boom. So is the rapid inflation of 

 your car's airbag. Want bigger booms? 

 Try Mach 2 (the recently retired Con- 

 corde commercial jet liner) or Mach 3 

 (the SR-71 Blackbird, a U.S. Air Force 

 spy plane). How about Mach 25 (the 

 space shuttle re-entering Earth's atmo- 

 sphere)? And by the way, no matter the 

 medium or the speed of sound within 

 it, reaching similar Mach numbers cre- 

 ates similar physical phenomena. 



Ever experience a dish-rattling 

 sonic boom? Most likely it came from 

 a small, high-flying military aircraft. 

 But if the plane is very large or flies su- 

 personically at a low altitude, the boom 

 won't be so innocent. Flown low 

 enough, even an ordinary fighter jet can 

 lay down a carpet of sonic booms that 

 not only rupture eardrums but also 

 break windows and cause nosebleeds. 

 As it re-enters Earth's atmosphere, the 

 returning space-shuttle orbiter makes 

 two ferocious booms, one from the 

 nose and one from the tail. Fortunately, 

 though, the orbiter slows to subsonic 

 speeds before descending low enough 

 for its booms to shatter your skull. 



The solar system is no stranger to 

 shock waves, though earthlings 

 generally remain oblivious to them. 

 Take a garden-variety, pebble-size 

 meteor hurtling through Earth's up- 



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NATURAL HISTORY September 2006 



