the distant cawing of a seagull.

和遠處海鷗的叫聲

But then an annoying whine interrupts the peace,

但是突然一陣哀鳴聲擾亂了平靜

getting closer, and closer, and closer.

聲音越來越近，越來越近

Until...whack!

直到 ...... 啪！

You dispatch the offending mosquito, and calm is restored.

你處理了那隻惱人的蚊子，重拾了寧靜

How did you detect that noise from afar and target its maker with such precision?

你是如何在遠處就偵測到噪音，並且精準瞄準到噪音來源的呢？

The ability to recognize sounds and identify their location

成功識別並且定位聲音來源

is possible thanks to the auditory system.

都多虧於聽覺系統

That's comprised of two main parts: the ear and the brain.

系統有兩大組成：耳朵和大腦

The ear's task is to convert sound energy into neural signals;

耳朵負責把聲能轉換為神經訊號

the brain's is to receive and process the information those signals contain.

大腦則負責接收並處理這些訊號涵蓋的訊息

To understand how that works,

為了理解運作過程

we can follow a sound on its journey into the ear.

我們可以跟隨聲音進到耳朵的旅程

The source of a sound creates vibrations

聲音的來源製造出震動

that travel as waves of pressure through particles in air,

形成聲波穿透空氣中的粒子

liquids,

或是液體

or solids.

和固體

But our inner ear, called the cochlea,

但是我們的內耳，稱為耳蝸

is actually filled with saltwater-like fluids.

其實填滿了像是鹹水的液體

So, the first problem to solve is how to convert those sound waves,

所以第一個要解決的問題，是如何把這些聲波

wherever they're coming from,

轉換為液體中的波動

into waves in the fluid.

無論他們來自哪裡

The solution is the eardrum, or tympanic membrane,

解決方式是藉由中耳，或稱為鼓膜

and the tiny bones of the middle ear.

以及中耳的細小骨頭

Those convert the large movements of the eardrum

這些構造將鼓膜的大動作轉換成

into pressure waves in the fluid of the cochlea.

耳蝸中液體的壓力波

When sound enters the ear canal,

當聲音進到耳道

it hits the eardrum and makes it vibrate like the head of a drum.

它會拍擊鼓膜，使之如同鼓面般震動

The vibrating eardrum jerks a bone called the hammer,

震動的鼓膜拉扯一支稱為錘子的骨頭

which hits the anvil and moves the third bone called the stapes.

它再敲擊砧骨，移動第三個稱作鐙骨的骨頭

Its motion pushes the fluid within the long chambers of the cochlea.

它的動作會推動耳蝸長道中的液體

Once there,

一旦抵達

the sound vibrations have finally been converted into vibrations of a fluid,

聲音的震動終於被轉換成了液體中的震動

and they travel like a wave from one end of the cochlea to the other.

他們像是波浪般在耳蝸中來回

A surface called the basilar membrane runs the length of the cochlea.

一個稱作基底膜的表層位在耳蝸的底部

It's lined with hair cells that have specialized components

內部排列有許多毛髮細胞，其中有稱為硬纖毛的

called stereocilia,

特有構造

which move with the vibrations of the cochlear fluid and the basilar membrane.

會隨著耳蝸液體和基底膜的震動而移動

This movement triggers a signal that travels through the hair cell,

這項動作會觸發訊號，透過毛髮細胞

into the auditory nerve,

遊走到聽覺神經

then onward to the brain, which interprets it as a specific sound.

然後上達大腦，才會將之翻譯成一種特定聲音

When a sound makes the basilar membrane vibrate,

當一種聲音使基底膜震動時

not every hair cell moves -

並非所有毛髮細胞都會移動

only selected ones, depending on the frequency of the sound.

取決於聲音的頻率，只有幾個特定的會移動

This comes down to some fine engineering.

這其中牽涉了一些精細的操作

At one end, the basilar membrane is stiff,

在一端，基底層是堅硬的

vibrating only in response to short wavelength, high-frequency sounds.

只會對短波長、高頻率的聲音做出震動反應

The other is more flexible,

另一端則較為彈性

vibrating only in the presence of longer wavelength, low-frequency sounds.

只對長波長、低頻率的聲音有反應

So, the noises made by the seagull and mosquito

因此，海鷗和蚊子製造出來的聲音

vibrate different locations on the basilar membrane,

會震動基底層不同的位置

like playing different keys on a piano.

就好比按下鋼琴不同的按鍵

But that's not all that's going on.

但旅程還沒結束

The brain still has another important task to fulfill:

大腦還有另一項重要的任務要達成：

identifying where a sound is coming from.

識別聲音的來源

For that, it compares the sounds coming into the two ears

為了做到這點，它會比較進到兩耳的聲音

to locate the source in space.

來定位空間中的聲音來源

A sound from directly in front of you will reach both your ears at the same time.

來自你正前方的聲音會同時到達你的雙耳

You'll also hear it at the same intensity in each ear.

你的兩耳聽見聲音的強度也會相同

However, a low-frequency sound coming from one side

然而，一個來自側邊的低頻聲音

will reach the near ear microseconds before the far one.

會早一百萬分之一秒抵達較近的耳朵，才到較遠的

And high-frequency sounds will sound more intense to the near ear

高頻的聲音在近耳中聽起來會強度較高

because they're blocked from the far ear by your head.

因為他們和遠耳之間有你的頭隔絕

These strands of information reach special parts of the brainstem

這些一條條訊息抵達腦幹中特別的部位

that analyze time and intensity differences between your ears.

那區負責分析兩耳間的時間和強度差異

They send the results of their analysis up to the auditory cortex.

他們將分析出的結果傳送到聽覺外皮

Now, the brain has all the information it needs:

現在，大腦有所有需要的訊息了：

the patterns of activity that tell us what the sound is,

告知我們聲音為何的運動軌跡

and information about where it is in space.

以及它在空間中的位置資訊

Not everyone has normal hearing.

並非所有人都有正常的聽力

Hearing loss is the third most common chronic disease in the world.

聽力喪失是世界上第三常見的慢性疾病

Exposure to loud noises and some drugs can kill hair cells,

曝露於大噪音和一些藥物皆會殺死毛髮細胞

preventing signals from traveling from the ear to the brain.

使得訊號無法從耳朵傳達到大腦

Diseases like osteosclerosis freeze the tiny bones in the ear

如骨硬化病這種疾病會硬化耳中的小骨頭

so they no longer vibrate.

讓他們不再能夠震動

And with tinnitus,

至於耳鳴

the brain does strange things

大腦做出奇怪的事

to make us think there's a sound when there isn't one.

讓我們在沒有聲音的時候以為有聲音

But when it does work,

但是當一切正常運作時

our hearing is an incredible, elegant system.

我們的聽力是個驚奇、優雅的系統

Our ears enclose a fine-tuned piece of biological machinery

我們的耳朵具有精確的生理結構

that converts the cacophony of vibrations in the air around us

能夠將我們周遭空氣中的一切雜音震動

into precisely tuned electrical impulses

轉換為精確的電子脈衝

that distinguish claps, taps, sighs, and flies.

區別出拍手聲、水龍頭、嘆息聲和蒼蠅