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Humans have been fascinated[br]with speed for ages.

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The history of human progress[br]is one of ever-increasing velocity,

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and one of the most important achievements[br]in this historical race

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was the breaking of the sound barrier.

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Not long after the first[br]successful airplane flights,

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pilots were eager to push[br]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[br]prevented them from accelerating further.

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Some tried to circumvent[br]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,[br]the all-moving tail,

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allowed an American military pilot[br]named Chuck Yeager

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to fly the Bell X-1 aircraft at 1127 km/h,

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becoming the first person[br]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[br]supersonic aircraft to follow,

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with later designs reaching[br]speeds over Mach 3.

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Aircraft traveling at supersonic speed[br]create a shock wave

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with a thunder-like noise[br]known as a sonic boom,

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which can cause distress to people[br]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 world[br]have been looking at sonic booms,

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trying to predict their path[br]in the atmosphere,

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where they will land,[br]and how loud they will be.

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To better understand[br]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[br]in a still pond.

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What do you see?

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The stone causes waves[br]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[br]are called wave fronts.

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Similarly, even though we cannot see it,

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a stationary sound source,[br]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[br]of the air they move through.

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At sea level, sound travels[br]at about 1225 km/h.

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But instead of circles[br]on a two-dimensional surface,

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the wave fronts[br]are now concentric spheres,

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with the sound traveling along rays[br]perpendicular to these waves.

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Now imagine a moving sound source,[br]such as a train whistle.

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As the source keeps moving[br]in a certain direction,

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the successive waves in front of it[br]will become bunched closer together.

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This greater wave frequency is the cause[br]of the famous Doppler effect,

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where approaching objects[br]sound higher pitched.

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But as long as the source is moving[br]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,[br]moving faster than the sound it makes,

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that the picture changes dramatically.

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As it overtakes sound waves[br]it has emitted,

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while generating new ones from[br]its current position,

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the waves are forced together,[br]forming a Mach cone.

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No sound is heard[br]as it approaches an observer

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because the object is traveling faster[br]than the sound it produces.

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Only after the object has passed[br]will the observer hear the sonic boom.

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Where the Mach cone meets the ground,[br]it forms a hyperbola,

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leaving a trail known as the boom carpet[br]as it travels forward.

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This makes it possible to determine[br]the area affected by a sonic boom.

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What about figuring out how strong[br]a sonic boom will be?

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This involves solving the famous[br]Navier-Stokes equations

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to find the variation[br]of pressure in the air

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due to the supersonic aircraft[br]flying through it.

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This results in the pressure signature[br]known as the N-wave.

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What does this shape mean?

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Well, the sonic boom occurs[br]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[br]at the aircraft's nose,

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and another for when the tail passes,

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and the pressure suddenly[br]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[br]by human ears.

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In practice, computer models[br]using these principles

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can often predict the location[br]and intensity of sonic booms

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for given atmospheric conditions[br]and flight trajectories,

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and there is ongoing research[br]to mitigate their effects.

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In the meantime, supersonic flight[br]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[br]using sonic booms to their advantage.

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The gigantic Diplodocus may have been[br]capable of cracking its tail

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faster than sound, at over 1200 km/h,[br]possibly to deter predators.

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Some types of shrimp can also create[br]a similar shock wave underwater,

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stunning or even killing pray[br]at a distance

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with just a snap of their oversized claw.

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So while we humans[br]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.