If you listen as a fighter jet flies overhead at subsonic speed, you can hear the roar from the jet engines and even the turbulence over its body as it passes through the air. Because the fighter is traveling slower than the speed of sound, all of that raw sound reaches your ears ahead of the fighter. As the jet speeds up and goes “transonic” (the realm of speed very near the speed of sound), sound waves and the air around the airplane start to “pile up” in front of the fighter. The forces pulling the air around the fast-moving jet at this point are comparable to the actual air pressure around it, creating more drag and more turbulence than in deeply subsonic flight, with noise levels increasing many decibels above what they would be at, say, two-thirds the speed of sound.
Once our theoretical fighter jet reaches the speed of sound, its bow wave of pressure becomes a shock wave — the disturbance from air passing over the wings and around the fighter’s body travels faster than sound itself. At this point, new air that comes in contact with the fighter receives no physical information about the jet’s arrival until the shock wave passes over it. Behind this shock wave is air that is traveling with the jet. Sound waves travel forward and backward along the shock. A good fraction of this sound energy from the airplane arrives at our ears pretty much all at once, making the “snap-boom” sound we hear.
Taming the Sonic Boom
For readily understandable reasons, international governing bodies prohibit aircraft from traveling at supersonic speed over populated landmasses. But what if technology could mitigate the sound from an airplane traveling above Mach 1 by essentially turning down the volume of a sonic boom? If we’re ever going to realize the benefits of flying people and cargo over land at super-fast speeds, that’s exactly what today’s research needs to accomplish, experts say.
How do we get there? One leading concept centers on the idea of attaching a long spike to the nose of our supersonic airplane; the spike is intended to keep the shock being created at the front of the aircraft separate from the shock at the rear so they don’t combine to create a big boom. The thinking is that smaller booms will weaken as they travel down to the ground, and if the airplane is flying high enough, by the time these small booms reach the ground, they’ll sound like almost nothing at all.
Study into quieting sonic booms started in earnest at NASA about 15 years ago. The first practical demonstrations of boom-mitigating technology occurred during test flights of a Northrup F-5E with a modified nose and belly that some say gave the airplane the shape of a duck. NASA and Northrup Grumman measured about 1,300 recordings during the so-called shaped-sonic-boom demonstration program in 2003, with encouraging results — the quirky shape of the customized F-5 cut its boom by about a third.
More recently, Gulfstream partnered with NASA’s Dryden Flight Research Center in California to test a concept known as the “Quiet Spike.” The project centered on testing a telescoping, 24-foot, lance-like spike mounted in the nose of a NASA F-15B research test airplane. The composite spike was designed to create three small shock waves that would travel parallel to each other all the way to the surface, thereby producing less noise than the typical shock waves that build up at the nose of a supersonic airplane.