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Today in our busy world, one of the key prerequisites for many people in personal and business life is speed.

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This is especially true when it comes to aviation.

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Although air travel is almost always the fastest means of travel, many would like it to become even faster.

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Though the technology exists for aircraft to fly at speeds faster than the speed of sound,

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today's aircraft don't because of the problem with sonic booms.

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To help lessen the impact of these booms,

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NASA researchers are attempting to find a way to help aircraft move faster without causing disruptions on the ground.

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Our own Johnny Alonzo spoke with researcher Dr. Kevin Shepard at NASA Langley Research Center

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to learn what a sonic boom is and find out how it works.

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In the early days of flight, having an aircraft that could fly even as fast as 30 miles per hour seemed revolutionary.

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But a goal that pushed virtually every aircraft designer, engineer, and pilot at that time

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was to find a way to increase the speeds of their aircraft.

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As new designs began to emerge, aircraft were continually getting stronger, safer, and above all, faster.

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By the mid-1940s, aircraft technology had advanced to the point that breaking the sound barrier was finally in sight.

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After numerous attempts and failures, the world's first sonic boom was heard on October 14, 1947,

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when Chuck Yeager flew the X-1 aircraft into history over the desert near Edwards, California.

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From that point on, military and civilian test pilots were regularly breaking the sound barrier

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in fighter aircraft and in specialized test vehicles like the X-15.

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But it wasn't until 1976 that civilian passengers finally got their chance to fly supersonically,

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with the introduction of the famed Concorde.

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The Concorde had the ability to fly at over 11 miles high, 1,350 miles per hour,

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and travel from Paris to New York in only three and a half hours.

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Unfortunately, one of the major drawbacks from the Concorde's incredible speed was the amount of noise it produced.

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Not only was it noisy when taking off and landing, but once it reached supersonic speeds,

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it created a very loud sonic boom.

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Sonic booms are so disconcerting to most people on the ground

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that commercial aircraft have only been given the clearance to break the sound barrier over water.

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So, are we just relegated to flying below the speed of sound?

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Maybe not.

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To help us understand what causes a sonic boom, and if there's anything we can do to lessen its impact,

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I spoke with Dr. Kevin Shepard at NASA Langley Research Center to find out how it works.

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Any vehicle traveling faster than the speed of sound creates a sonic boom.

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What actually happens is shock waves, which are pressure rises, develop near the airplane.

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And as those travel to the ground, what we perceive as a noise, in fact, is this sudden pressure jump.

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Much like a rifle crack or a balloon popping.

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In fact, what you hear are two booms closely separated in time.

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Boom, boom.

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And you could visualize it as two rifle cracks or as two claps of thunder.

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Sure.

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Closely spaced in time.

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What is the speed of sound?

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And how do you measure the speed of sound?

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We like to say Mach 1 is supersonic.

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Everyone knows that expression.

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Mach 2 is twice the speed of sound.

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Mach 3, three times, and so forth.

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The actual speed depends on the atmospheric conditions.

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So if you're near the surface where it's typically quite warm, speed of sound is 700, 750 miles an hour.

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When you're at altitude where airplanes fly, it's a little lower, maybe 600 miles an hour.

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So, for example, Concorde traveled at Mach 2, 1200 miles an hour is roughly the speed it traveled at.

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A common misconception about the sound barrier is once it has been broken, there is just one quick noise.

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And then the noise dissipates.

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One reason this misconception is so prevalent is that most people hear a sonic boom

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when they're standing in a stationary position on the ground.

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What actually happens is when the aircraft breaks the sound barrier,

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it continues to break it as long as it's flying supersonically.

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Any observer on the ground hears the airplane go by.

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If you picture a boat in the middle of a creek and the bow wave from the boat,

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you watch the boat go by.

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A little while later, that bow wave passes you on the riverbank.

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People further down the riverbank have the exact same experience.

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So what's happening is, in the case of the airplane,

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it's dragging this boom carpet behind it all the way across the country.

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Depending on weather and altitude, the sonic boom created by the aircraft

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can be heard in a path of about 60 miles wide for the entire distance of the flight.

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So, if an aircraft is flying from New York to Los Angeles,

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the sonic boom will be heard consistently across the country in a 60-mile-wide path.

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This is the foremost reason supersonic flights are not allowed to fly over land in the United States.

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Yeah, most people find the sonic boom unacceptable.

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There's the too loud sounds. They're startling. They're annoying.

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They tend to shake buildings, rattle windows.

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And so, based on experience with Concorde, for example, it just doesn't happen.

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There is no commercial overland supersonic flight.

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But revolutionary steps now being taken by NASA may change that in the future.

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So, Dr. Shepard, are we stuck with the fact that we'll never be able to fly over land at supersonic speed?

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We're hopeful that's not the case.

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The current programs we're working on are aimed at allowing supersonic overland flight.

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The hope we have is based on a recent flight test,

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which demonstrated that we can, in fact, shape the airplane in such a way that we can shape the sonic boom

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and it sound different, sound more acceptable.

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This has been known in theory for 40-plus years,

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but it was only demonstrated in the last couple of years with a real flight vehicle.

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Now, that's part of the story.

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The real issue is can we get the boom low enough for people to find it acceptable?

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We think we can reduce it. Can we reduce it enough?

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We're hopeful, and we're hoping we'll have a flight demonstrator within the next few years.

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So, Dr. Shepard, how do you test sonic booms?

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I mean, is it always in flight, or can you also test it on land?

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We'd love to do it in flight.

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But building vehicles, as you can imagine, is very expensive, and you don't get to do it very often.

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So if you've got a theory that this kind of vehicle will make a different kind of boom than this,

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yeah, we'd like to build the vehicles, but that's not going to happen.

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So in terms of figuring out what people might find acceptable,

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we simulate the sonic booms using ground-based simulators,

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which are basically loudspeaker systems where we can produce the sounds

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that would be developed by certain vehicle types.

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The simulators that we have here at Langley, they're being used for that

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because we hope that will guide the design of the airplanes to ultimately lead to an acceptable sonic boom.

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Can you give me some examples of what you test in these simulators?

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These simulators are basically loudspeaker-based systems, so we can make sounds,

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and we can design them to make sounds that sound very much like real sonic booms.

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We bring in human test subjects, members of the public, and in essence they give us their opinion.

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We have a sonic boom versus another, which actually corresponds to one airplane versus another

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because we're trying to design airplanes to give us the right sonic boom.

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So the characteristics of the boom is what they're assessing with their ears.

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If we can solve the sonic boom problem, then we can have supersonic flight over land.

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People and goods can get from place to place quicker

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because our overall aim here is to make the air transportation system more efficient, safer,

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in this case faster, but also environmentally acceptable.

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That way we save time, we save money, we have a more efficient system.

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That's it for this edition of NASA's Destination Tomorrow.

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I'm Kara O'Brien.

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For all of us here at NASA, we'll see you next time.

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NASA Jet Propulsion Laboratory, California Institute of Technology

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NASA Jet Propulsion Laboratory, California Institute of Technology

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NASA Jet Propulsion Laboratory, California Institute of Technology