The QUEST for QUIET, QUICK PLANES

Supersonic Flight Could Be Making a Comeback

- Abha and Deepa Jain 

BA-BAM! Whoa. The deep, loud sounds startle you. You feel a sudden jolt, and the whole house seems to rattle.

Whatever it was seemed to come from above, but what was it? Thunder? Not a chance-the sky couldn't be more cornflower-blue. Then you notice a jet, flying so high it's a mere white speck. You might not believe it, but that tiny blip caused the rattling booms. Soon, however, these shocking airplane noises may become sound relics, thanks to cutting-edge research.

Creating Thunder in the Sky

When an object moves through air, it pushes against the molecules in front of it. This compressed zone of air ripples out in all directions at the speed of sound. The zone is called a pressure wave. While the object travels slower than sound, the ripples of the pressure waves stay smooth. They flow away from the object before it reaches them.

But problems begin once the object zooms faster than sound, or at supersonic speeds. Now the object is traveling faster than its pressure waves. All the ripples in front get squished together, forming a cone-shaped shock wave.

An airplane that goes supersonic produces many shock waves from tip to tail. By the time they reach you on the ground, though, most have merged into two. One shock wave is from all the aircraft's front-end parts. The other is from those at the rear. When each shock wave reaches you, your ears feel a change in pressure. This change-one or two pounds per square foot is actually rather small. It's what you feel when riding an elevator down a couple floors. But because the change occurs in a fraction of second, you end up hearing a thunder-like double clap. These are sonic booms.