Placeholder Image

字幕表 動画を再生する

  • Humans have been fascinated with speed for ages.

  • The history of human progress is one of ever-increasing velocity,

  • and one of the most important achievements in this historical race

  • was the breaking of the sound barrier.

  • Not long after the first successful airplane flights,

  • pilots were eager to push their planes to go faster and faster.

  • But as they did so, increased turbulence

  • and large forces on the plane prevented them from accelerating further.

  • Some tried to circumvent the problem through risky dives,

  • often with tragic results.

  • Finally, in 1947, design improvements,

  • such as a movable horizontal stabilizer, the all-moving tail,

  • allowed an American military pilot named Chuck Yeager

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

  • becoming the first person to break the sound barrier

  • and travel faster than the speed of sound.

  • The Bell X-1 was the first of many supersonic aircraft to follow,

  • with later designs reaching speeds over Mach 3.

  • Aircraft traveling at supersonic speed create a shock wave

  • with a thunder-like noise known as a sonic boom,

  • which can cause distress to people and animals below

  • or even damage buildings.

  • For this reason,

  • scientists around the world have been looking at sonic booms,

  • trying to predict their path in the atmosphere,

  • where they will land, and how loud they will be.

  • To better understand how scientists study sonic booms,

  • let's start with some basics of sound.

  • Imagine throwing a small stone in a still pond.

  • What do you see?

  • The stone causes waves to travel in the water

  • at the same speed in every direction.

  • These circles that keep growing in radius are called wave fronts.

  • Similarly, even though we cannot see it,

  • a stationary sound source, like a home stereo,

  • creates sound waves traveling outward.

  • The speed of the waves depends on factors

  • like the altitude and temperature of the air they move through.

  • At sea level, sound travels at about 1225 km/h.

  • But instead of circles on a two-dimensional surface,

  • the wave fronts are now concentric spheres,

  • with the sound traveling along rays perpendicular to these waves.

  • Now imagine a moving sound source, such as a train whistle.

  • As the source keeps moving in a certain direction,

  • the successive waves in front of it will become bunched closer together.

  • This greater wave frequency is the cause of the famous Doppler effect,

  • where approaching objects sound higher pitched.

  • But as long as the source is moving slower than the sound waves themselves,

  • they will remain nested within each other.

  • It's when an object goes supersonic, moving faster than the sound it makes,

  • that the picture changes dramatically.

  • As it overtakes sound waves it has emitted,

  • while generating new ones from its current position,

  • the waves are forced together, forming a Mach cone.

  • No sound is heard as it approaches an observer

  • because the object is traveling faster than the sound it produces.

  • Only after the object has passed will the observer hear the sonic boom.

  • Where the Mach cone meets the ground, it forms a hyperbola,

  • leaving a trail known as the boom carpet as it travels forward.

  • This makes it possible to determine the area affected by a sonic boom.

  • What about figuring out how strong a sonic boom will be?

  • This involves solving the famous Navier-Stokes equations

  • to find the variation of pressure in the air

  • due to the supersonic aircraft flying through it.

  • This results in the pressure signature known as the N-wave.

  • What does this shape mean?

  • Well, the sonic boom occurs when there is a sudden change in pressure,

  • and the N-wave involves two booms:

  • one for the initial pressure rise at the aircraft's nose,

  • and another for when the tail passes,

  • and the pressure suddenly returns to normal.

  • This causes a double boom,

  • but it is usually heard as a single boom by human ears.

  • In practice, computer models using these principles

  • can often predict the location and intensity of sonic booms

  • for given atmospheric conditions and flight trajectories,

  • and there is ongoing research to mitigate their effects.

  • In the meantime, supersonic flight over land remains prohibited.

  • So, are sonic booms a recent creation?

  • Not exactly.

  • While we try to find ways to silence them,

  • a few other animals have been using sonic booms to their advantage.

  • The gigantic Diplodocus may have been capable of cracking its tail

  • faster than sound, at over 1200 km/h, possibly to deter predators.

  • Some types of shrimp can also create a similar shock wave underwater,

  • stunning or even killing pray at a distance

  • with just a snap of their oversized claw.

  • So while we humans have made great progress

  • in our relentless pursuit of speed,

  • it turns out that nature was there first.

Humans have been fascinated with speed for ages.

字幕と単語

動画の操作 ここで「動画」の調整と「字幕」の表示を設定することができます

B2 中上級

TED-ED】ソニックブーム問題-カテリーナ・カオウリ (【TED-Ed】The sonic boom problem - Katerina Kaouri)

  • 707 66
    稲葉白兎 に公開 2021 年 01 月 14 日
動画の中の単語