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Humans have been fascinated
with speed for ages.
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The history of human progress
is one of ever increasing velocity,
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and one of the most important achievements
in this historical race
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was the breaking of the sound barrier.
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Not long after the first
successful airplane flights,
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pilots were eager to push
their planes to go faster and faster.
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But as they did so, increased turbulence
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and large forces on the plane
prevented them from accelerating further.
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Some tried to circumvent
the problem through risky dives,
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often with tragic results.
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Finally, in 1947, design improvements,
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such as a movable horizontal stabilizer,
the all moving tail,
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allowed an American military pilot
named Chuck Yeager
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to fly the Bell X-1 aircraft
at 1127 km/hr.
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becoming the first person
to break the sound barrier
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and travel faster than the speed of sound.
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The Bell X-1 was the first of many
supersonic aircraft to follow,
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with later designs reaching speeds
over Mach three.
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Aircraft traveling at supersonic speed
create a shockwave
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with a thunder-like nose
known as a sonic boom,
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which can cause distress to people
and animals below,
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or even damage buildings.
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For this reason,
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scientists around the the world
have been looking at sonic booms,
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trying to predict their path
in the atmosphere,
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where they will land,
and how loud they will be.
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To better understand
how scientists study sonic booms,
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let's start with some basics of sound.
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Imagine throwing a small stone
in a still pond.
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What do you see?
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The stone causes waves
to travel in the water
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at the same speed in every direction.
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These circles that keep growing in radius
are called wave fronts.
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Similarly, even though we cannot see it,
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a stationary sound source,
like a home stereo,
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creates sound waves traveling outward.
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The speed of the waves depends on factors
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like the altitude and temperature
of the air they move through.
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At sea level, sound travels
at about 1225 km/hr.
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But instead of circles
on a two-dimensional surface,
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the wave fronts
are now concentric spheres,
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with the sound traveling along rays
perpendicular to these waves.
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Now imagine a moving sound source,
such as a train whistle.
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As the source keeps moving
in a certain direction,
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the successive waves in front of it
will become bunched closer together.
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This greater wave frequency is the cause
of the famous Doppler effect,
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where approaching objects
sound higher pitched.
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But as long as the source is moving
slower than the sound waves themselves,
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they will remain nested within each other.
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It's when an object goes supersonic,
moving faster than the sound it makes,
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that the picture changes dramatically.
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As it overtakes soundwaves it has emitted,
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while generating new ones from
its current position,
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the waves are forced together,
forming a Mach cone.
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No sound is heard
as it approaches an observer
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because the object is traveling faster
than the sound it produces.
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Only after the object has passed
will the observer hear the sonic boom.
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Where the Mach cone meets the ground,
it forms a hyperbola,
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leaving a trail known as the boom carpet
as it travels forward.
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This makes it possible to determine
the area affected by a sonic boom.
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What about figuring out how strong
a sonic boom will be?
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This involves solving the famous
Navier-Stokes equations
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to find the variation
of pressure in the air
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due to the supersonic aircraft
flying through it.
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This results in the pressure signature
knowsn as the N-wave.
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What does this shape mean?
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Well, the sonic boom occurs
when there is a sudden change in pressure,
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and the N-wave involves two booms:
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One for the initial pressure rise
at the aircraft's nose,
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and another for when the tail passes,
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and the pressure suddenly
returns to normal.
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This causes a double boom,
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but it is usually heard as a single boom
by human ears.
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In practice, computer models
using these principles
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can often predict the location
and intensity of sonic booms
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for given atmospheric conditions
and flight trajectories,
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and there is on going research
to mitigate their effects.
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In the meantime, supersonic flight
over land remains prohibited.
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So, are sonic booms a recent creation?
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Not exactly.
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While we try to find ways to silence them,
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a few other animals have been
using sonic booms to their advantage.
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The gigantic Diplodocus may have been
capable of cracking its tail
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faster than sound, at over 1,200 km/hr,
possibly to deter predators.
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Some types of shrimp can also create
a similar shockwave underwater,
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stunning or even killing pray
at a distance
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with just a snap of their oversized claw.
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So while we humans
have made great progress
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in our relentless pursuit of speed,
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it turns out that nature was there first.