content: The "boom" is a result of the air pressure suddenly increasing and then decreasing as the nose and tail shockwaves pass over the ground.  This airplane uses aerodynamic techniques to move the nose shockwave forward and the tail shockwave further aft so you still hear a noise, but it is much softer and less startling. Edit:  Here's a NASA paper on the topic:  https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050192479.pdf. If you look on page 4 they have charts of pressure vs. time collected during 2 overflights.  The speeds are barely supersonic.