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Studying FaINT Sound
November 14, 2012

Dryden's F/A-18B mission support aircraft No. 852 flies near the Tehachapi Mountains. Dryden's F/A-18B mission support aircraft No. 852 flies near the Tehachapi Mountains. The aircraft flew a series of low-supersonic, high-altitude flight profiles during the Farfield Investigation of No Boom Threshold, or FaINT, flight research project at Dryden. (NASA/Jim Ross) › View Larger Image

NASA's Supersonics Project embarked on its latest effort to soften sonic booms when a NASA F/A-18 aircraft took to the air for a project called Farfield Investigation of No Boom Threshold, or FaINT, in late October.

The latest in a continuing progression of NASA supersonics research projects aimed at reducing or mitigating the effect of sonic booms, FaINT is designed to enable engineers to better understand evanescent waves. Evanescent waves are 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 FaINT.

Cessna researchers prepare to launch a blimp that carries several microphones used to record sonic booms for the FaINT project. Cessna researchers prepare to launch a blimp that carries several microphones used to record sonic booms for the FaINT project. (NASA/Tom Tschida) › View Larger Image

"It's exciting to help lead a new area in sonic boom flight research," said Larry Cliatt, principal investigator for the FaINT flight project at Dryden. "We are investigating supersonic technology and research that is relatively raw in the modern sense. When overland supersonic commercial travel is commonplace, it will be efforts like this that helped get us there."

The evanescent wave flights were over Edwards. Recording them on the ground were special microphone arrays placed on the southern portion of Rogers Dry Lake that are Dryden researchers' sensors of choice.

For the FaINT flight project, capturing the fleeting sounds of evanescent waves coming off sonic boom shockwaves was a challenge. Similar to the shadow the sun creates behind a building, if some light were to still leak around the edges it would not get completely dark, but it would get darker the further you move away from the edge. Certain conditions and refractions create a similar "shadow side" of a sonic boom where evanescent waves are generated, sounding similar to distant thunder. These waves quickly fade and disappear, as supersonic shockwaves act similar to boat wakes on water, decreasing with distance.

"The FaINT team has been working hard on the development and design of the FaINT project for the last six months," said Brett Pauer, FaINT deputy project manager at Dryden. "NASA and its seven industry and university partners are ready to collect data and expand our collective knowledge of sonic boom propagation effects near the shadow side of them."

Characterizing the effects of both normal and loud sonic booms in order to provide the data necessary for engineers to design future low-boom supersonic aircraft has required an amazing amount of work and tenacity by NASA engineers from the agency's Dryden and Langley research centers and industry partners.

Recent and related sonic boom research preceding FaINT included the Superboom Caustic Analysis and Measurement Program, which produced and measured amped-up, super-loud sonic booms, and the Waveforms and Sonic boom Perception and Response project, which gathered data from a select group of volunteer Edwards Air Force Base residents on their individual perceptions of sonic booms produced by aircraft in supersonic flight over Edwards.

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.

NASA's Aeronautics Research Mission Directorate at NASA Headquarters in Washington, D.C., funded the research.

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