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Sonic booms created by aircraft flying faster

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than the speed of sound certainly aren't

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known for being faint, but rather for their

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loud, make-you-jump effect.

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(sound of double sonic booms)

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But sonic booms also have a quieter side.

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NASA's Supersonics Project is embarking on

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a new effort to characterize that fainter

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side of sonic booms in the Farfield Investigation

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of No Boom Threshold project, or FaINT.

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Principal investigator Larry Cliatt at

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NASA's Dryden Flight Research Center says

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FaINT is designed to enable engineers to better

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understand evanescent waves, an acoustic

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phenomenon that occurs at the very edges

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of the normal sonic boom envelope.

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Then, you have your evanescent waves which is

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on the other side of that. They tend to be a

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lot quieter, probably about five to 10 times

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quieter than that of your normal N-wave

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sonic boom. And they kind of sound like

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a distant thunder rumble.

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Supersonic shockwaves produced by an aircraft

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flying at a speed of about Mach 1.2 or less,

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and at an altitude above 35,000 feet,

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typically do not reach the ground, so no

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sonic boom is heard. This is because the

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supersonic shockwaves created at higher altitudes

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are refracted, or bent upwards, as they enter

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the warmer air closer to the ground.

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When these shockwaves curve upward, they create

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a series of sonic boom waves that are focused

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along what is called a caustic line.

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Another phenomenon that produces evanescent waves

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is called lateral cutoff. And there you have the

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aircraft flying and the sonic boom carpet that's

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produced on the ground laterally to that.

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you also have another disturbance,

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and those are evanescent waves.

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Linear and spiral microphone arrays laid out

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on the dry lakebed at Edwards Air Force Base,

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along with microphones on a blimp-shaped balloon

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and a motor glider flying above the arrays,

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record the faint sonic booms.

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Project manager Brett Pauer says the overarching

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goal of NASA's sonic boom reduction research is

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to collect data that could help make commercial

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supersonic flight over land practical.

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The purpose of the FaINT project in general is

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just an additional piece of research that we're

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working on for having a supersonic commercial

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transport that can fly over land.

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NASA's always trying to push research,

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push boundaries, and one to the things

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we're trying to do is to bring commercial