Return of the Concorde?? Well, kinda. A company by the name of Aerion are developing commercial jets that have the ability to fly high enough so that sonic booms will not be felt or heard at ground level. The theory is that once an aircraft surpasses the speed of sound at ~750mph, or Mach 1, a shockwave is produced that propogates toward the Earth as a sonic boom. However, if the aircraft has the abilty to increase its altitude into the colder, less dense air of the stratosphere, the sonic boom's energy both travels slower in the cooler air, and has a greater distance to travel, which means it endures increased attenuation. As the wave propgates toward the surface, it encounters the warmer, and denser air, of which some of the initial energy reflects off the warmer air due to a change of impedance, and the remaining energy refracts toward the surface. This remaining refracted wave no longer has the velocity or energy to create a sonic boom at the surface, thus silent Mach speeds can be reached. Basic wave theory being used to make money?! Wonder if these guys are hiring?!!
To check out the article from Popular Mechanics, click here