NASA’s Supersonics Project is embarking on its latest effort to characterize or define that fainter side of sonic booms as a NASA F/A-18 aircraft takes to the air in a project called Farfield Investigation of No Boom Threshold, or FaINT.
As the latest in a continuing progression of NASA supersonics research projects aimed at reducing sonic boom levels, FaINT is designed to enable engineers to better understand evanescent waves, an acoustic phenomenon that occurs at the very edges or just outside of the normal sonic boom envelope.
For an aircraft flying at a supersonic speed of about Mach 1.2 or less at an altitude above 35,000 feet, the shockwaves being produced typically do not reach the ground, so no sonic boom is heard. This is because shockwaves from an aircraft flying supersonically at higher altitudes are refracted, or bent upwards, as they enter warmer air closer to the ground, due to the fact that the speed of sound increases with air temperature.
But when sonic booms curve upward they create a series of sonic boom waves that are focused along a line. This line is called a caustic line. The side of the caustic line opposite of the sonic boom waves is called the "shadow side," where the evanescent waves are generated. This is the area that NASA researchers are studying during the FaINT project to learn more about how to reduce the level of sonic booms.
The planned evanescent wave flights will occur over Edwards Air Force Base, Calif., where special microphone arrays placed on the southern portion of Rogers Dry Lake will again be the NASA Dryden researcher’s sensor of choice.
The overarching goal of NASA's sonic boom reduction research is to shrink the sonic boom "footprint" in order to make commercial supersonic flight over land practical.
