Simple: the sound of any object going faster than the speed of sound can be VERY loud even without an engine on the plane running. Anyone who remembers the Space Shuttle flying overhead during the landing phase as the Shuttle approaches the landing strip or more recently close to the landing of the Falcon 9 first stage rocket know the very loud and distinct "double bang" of a sonic boom.
As such, thanks to modern computational fluid dynamic research on supercomputers, we know how to shape an airplane so the pressure wave buildup that causes the sonic boom can be dramatically reduced. If the X-59 and the Boom Supersonic XB-1 lives up to their claims, they should be able to fly at supersonic speeds up to Mach 1.5 at 50,000 feet and the resulting sonic boom would sound more like the dull thud of a automobile door closing.