Why didn't everyone hear the sonic boom over DC Sunday?

WASHINGTON — The North American Aerospace Defense command (NORAD) deployed two F-16 fighter jets from Joint Base Andrews in Prince George's County Sunday to investigate a private Cessna aircraft as it trespassed into restricted airspace over D.C. The jets were able to intercept the Cessna Citation about 20 miles northeast of Reagan National Airport (DCA), according to NORAD. 

In order to reach the Cessna as fast as possible, the F-16s were authorized to travel at "supersonic speeds," creating a noise loud enough to be heard by people all across the DMV, from Annapolis to the National Mall and parts of Northern Virginia. WUSA9 received dozens of videos recording reactions to the jarring noise. 

Yet other viewers said they didn’t hear a thing, and we wanted to understand why. 

"If the supersonic jet is 30,000 feet up, the sonic boom carpet can be 30 miles wide," Ed Haering, Aerospace Engineer at the NASA Armstrong Flight Research Center, said. "Without having the ground track of the supersonic jet as well as its Mach number and altitude, it would be speculation to guess where it would be heard. A common misconception is that the boom is only generated at the single instance when it exceeded Mach 1.0, which is false. It generates a boom the entire time it is supersonic, and it throws shockwaves forward, even past where it is no longer supersonic.”

If a jet is flying below MACH 1, or 767.3 miles per hour, that’s known as the Doppler Effect, which is what you would hear watching planes take off from Gravelly Point at DCA.  When fighter jets go above MACH 1, like they did Sunday, the sound waves merge into a shock wave that trail the aircraft, creating a “sonic boom.”

According to an Air Force website, as an aircraft flies at supersonic speeds it is continually generating shock waves, dropping sonic booms along its flight path, similar to someone dropping objects from a moving vehicle.

From the perspective of the aircraft, the boom appears to be swept backwards as it travels away from the aircraft, in this case shaking some homes in Annapolis.

But if the plane makes a sharp turn or pulls up, the boom will hit the ground in front of the aircraft, which is why people from D.C. to Virginia reported hearing the boom, too.

The U.S. Geological Survey said aircraft actually produce two booms, but they usually arrive so close together that they're indistinguishable. And that most sonic booms aren't felt on land because most supersonic training flights are out over the ocean.