The sonic boom problem - Katerina Kaouri

譯者: Lilian Chiu 審譯者: Helen Chang

多年來，人類一直都對速度很著迷。

Humans have been fascinated with speed for ages.

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

人類的進步史也是 一段不斷加速的歷史，

and one of the most important achievements in this historical race

而在這場歷史賽跑中 最重要的成就之一，

was the breaking of the sound barrier.

就是突破音障。

Not long after the first successful airplane flights,

在飛機最早成功飛行之後不久，

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

飛行員們就渴望 不斷刷新飛行速度記錄。

But as they did so, increased turbulence

但是當他們試著加速時，

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

亂流增加以及空氣阻力， 迫使他們無法再加速。

Some tried to circumvent the problem through risky dives,

有些人試圖用危險的俯衝 來避開這些障礙，

often with tragic results.

結果通常都很不幸。

終於，1947年，設計上的改良，

Finally, in 1947, design improvements,

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

比如可動式水平安定面， 也叫全動尾翼，

allowed an American military pilot named Chuck Yeager

讓一位名叫查克耶格的 美國軍方飛行員

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

駕駛貝爾 X-1 飛機的速度

達到每小時 1127 公里。

becoming the first person to break the sound barrier

成為第一個突破音障的人，

and travel faster than the speed of sound.

用比聲音更快的速度飛行。

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

在貝爾 X-1 之後， 又出現了許多超音速飛機，

with later designs reaching speeds over Mach 3.

後來的設計可以超過三馬赫。 （註：三倍音速）

Aircraft traveling at supersonic speed create a shock wave

以超音速飛行的飛機， 會產生衝擊波，

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

會發出雷聲般的噪音， 就是所謂的音爆，

which can cause distress to people and animals below

會對底下的人類和動物有不良影響，

or even damage buildings.

或甚至會損壞建築物。

For this reason,

因此世界各地的科學家 一直在研究音爆，

scientists around the world have been looking at sonic booms,

trying to predict their path in the atmosphere,

試圖預測它們在大氣中的路徑、

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

會落在哪裡，以及會多大聲。

為更了解科學家如何研究音爆，

To better understand how scientists study sonic booms,

let's start with some basics of sound.

咱們先從一些聲音的基本知識談起。

Imagine throwing a small stone in a still pond.

想像把一顆小石頭丟入一池靜水中。

What do you see?

你會看到什麼？

The stone causes waves to travel in the water

石頭引起的水波在水中移動，

at the same speed in every direction.

朝每個方向前進的速度都相同。

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

這些半徑不斷增加的圓圈稱為波前。

Similarly, even though we cannot see it,

同樣的，即使肉眼無法看見，

a stationary sound source, like a home stereo,

一個固定聲源，比如家庭音響，

creates sound waves traveling outward.

會產生向外傳播的聲波。

The speed of the waves depends on factors

聲波的速度取決於多種因素，

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

像高度和它們穿越之空氣的氣溫。

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

在海平面上，聲速約為 每小時 1225 公里。

But instead of circles on a two-dimensional surface,

但這裡的波前 不是二維平面的同心圓，

the wave fronts are now concentric spheres,

而是三維空間的同心球，

with the sound traveling along rays perpendicular to these waves.

聲音會沿著與這些球面 垂直的路徑傳播。

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

現在想像一個移動的聲源， 如火車的汽笛。

隨著聲源不斷朝某個方向移動，

As the source keeps moving in a certain direction,

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

火車前方的連續聲波會被擠在一起。

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

波頻因而增加， 這就是著名的都卜勒效應，

where approaching objects sound higher pitched.

接近中的物體聽起來的音調比較高。

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

但只要聲源移動的速度 比聲波的本身慢，

they will remain nested within each other.

聲波就能保持一個個套疊的狀態。

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

當物體以超音速前進， 移動得比它發出的聲音更快，

that the picture changes dramatically.

情況就會截然不同。

As it overtakes sound waves it has emitted,

因為它會追上它自己放出的聲波，

while generating new ones from its current position,

同時會在當下的位置 產生新的聲波，

the waves are forced together, forming a Mach cone.

聲波會被迫疊在一起， 形成一個馬赫錐。

No sound is heard as it approaches an observer

當它靠近觀察者時， 不會聽見任何聲音，

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

因為該物體移動的速度 比它產生的聲音更快。

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

只有在該物體已經經過之後， 觀測者才會聽到音爆。

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

馬赫錐接觸地面時會形成雙曲線，

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

當它向前移動時，會拖曳出 一條軌跡，稱為音爆接地區域。

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

這樣便可判斷受音爆影響的區域。

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

那要如何得知音爆的強度呢？

This involves solving the famous Navier-Stokes equations

這就涉及如何解出著名的 納維爾－斯托克斯方程，

to find the variation of pressure in the air

來算出因為超音速飛機通過空氣，

due to the supersonic aircraft flying through it.

而在空氣中造成的壓力變化。

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

這導致壓力特徵「N 波」。

What does this shape mean?

這形狀有什麼意義？

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

當壓力突然改變時，就會發生音爆，

and the N-wave involves two booms:

而 N 波和兩次音爆有關：

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

第一次發生在飛機 機鼻的初始壓力上升，

and another for when the tail passes,

另一次是在機尾穿過的時候，

and the pressure suddenly returns to normal.

然後壓力就會突然返回正常狀態。

這會導致雙重音爆，

This causes a double boom,

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

但人耳通常只能聽到一次音爆聲。

In practice, computer models using these principles

在實做上，利用這些 原則建立的電腦模型

can often predict the location and intensity of sonic booms

通當可以針對給定的 大氣條件和飛行軌跡，

for given atmospheric conditions and flight trajectories,

來預測音爆的位置和強度，

and there is ongoing research to mitigate their effects.

目前還有關於減低 音爆影響的研究在進行中。

In the meantime, supersonic flight over land remains prohibited.

在這之前，仍然禁止 以超音速飛過陸地上空。

So, are sonic booms a recent creation?

所以，音爆是近期 才創造出來的產物嗎？

Not exactly.

不完全是。

While we try to find ways to silence them,

雖然我們試著尋找 讓音爆靜音的方式，

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

其他少數幾種動物已經懂得 運用音爆的好處了。

The gigantic Diplodocus may have been capable of cracking its tail

巨大的梁龍也許能 猛力揮擊牠的尾巴，

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

速度比音速還快， 達每小時 1200 公里，

目的可能是為了威懾掠食者。

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

某些類型的蝦子可以 在水中製造類似的衝擊波，

stunning or even killing pray at a distance

只需要彈一下巨型的鉗子，

就能驚嚇甚至殺死遠處的獵物。

with just a snap of their oversized claw.

So while we humans have made great progress

所以，雖然人類對速度的持續追求

in our relentless pursuit of speed,

已經有巨大的進展，

it turns out that nature was there first.

結果發現，大自然早就搶先了一步。