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The Skinny on Sonic Booms

        Science | Acoustics

The Myth of the Sonic "Boom"

Calling it a "boom" is misleading. No, you don't bounce off the sound barrier, and no, you don't make a boom when you break through it.

"People think when you go through the sound barrier, it makes this sound once," says Robinson. "That's wrong. You're dragging this boom around with you wherever you go."

That's right: If people stood shoulder to shoulder from California to New York, and the Supersonic Transport Concorde flew over their heads, every single person would hear a sonic boom. It's really more of a "sonic bellow." The auditory fact of the matter is, however, that each person hears just one boom as the shock wave reaches his ears. Well, maybe he hears two booms.

"In reality, shocks are generated off all sorts of surfaces, not just the nose of the airplane," says NASA boom specialist Kevin Shepherd. "But often, the big ones swallow up the little ones, and you end up with one that appears to be from the nose, and one that appears to be from the tail."

Well, truth be told, you probably hear no booms.

Because earth-bound spoil-sports didn't like the noise any more than they liked having their windows broken, all but the military are now forbidden to boom over land. Robinson and the other astronauts fly their training jets sedately until they're out over the Gulf of Mexico.

Accidents do happen, Shepherd says. Because the speed of sound (and hence the sound barrier) does vary with air temperature, and because pilots never hear their own "sonic bellow," pilots can be ignorant of their crime until they get the bill for the windows.

Breaking the sound barrier isn't cheap, which is why airliners abstain. As you may know from daily experience, it takes far more energy to push your fist through cheese than to slice it with a knife. Seats on the Concorde are pricey because it takes a lot of jet fuel to plow cheese all the way to Paris.

So why bother going supersonic? There's a brief silence from Robinson's end of the phone, as though he's admiring the naiveté of the question.

"You go faster," he says.


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