The science of hearing - Douglas L. Oliver

譯者: Lilian Chiu 審譯者: Helen Chang

You hear the gentle lap of waves,

你聽見海浪溫柔拍打的聲音，

the distant cawing of a seagull.

聽見遠方的海鷗叫。

But then an annoying whine interrupts the peace,

但接著，惱人的聲音打斷了寧靜，

getting closer, and closer, and closer.

這聲音越來越近，越來越近。

Until...whack!

直到……狠狠一打！

You dispatch the offending mosquito, and calm is restored.

你迅速了結進攻的蚊子， 再度恢復平靜。

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

你是如何遠遠就偵測到那噪音的？ 又如何準確找到噪音來源？

The ability to recognize sounds and identify their location

你能辨識聲音並找出它們的所在

is possible thanks to the auditory system.

可能要感謝聽覺系統。

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

這系統主要由兩部分構成： 耳朵和大腦。

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

耳朵的工作是要把聲能 轉變成神經訊號；

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

大腦的工作是要接收 和處理那些訊號所含的資訊。

To understand how that works,

為了要了解這系統如何運作，

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

我們跟聲音進入耳朵的旅程走一回。

The source of a sound creates vibrations

聲音的來源會創造出震動，

that travel as waves of pressure through particles in air,

以壓力波的形式透過粒子在空中、

liquids,

水中，

or solids.

或固體中傳遞。

But our inner ear, called the cochlea,

但我們的內耳，也就是耳蝸，

is actually filled with saltwater-like fluids.

其實充滿了像鹽水一樣的液體。

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

所以，第一個要解決的問題 是如何轉換那些聲波，

wherever they’re coming from,

不論它們來自何處，

into waves in the fluid.

都要能轉為液體中的波。

The solution is the eardrum, or tympanic membrane,

解決方案就是鼓膜，也就是耳膜，

and the tiny bones of the middle ear.

以及中耳的小骨頭。

Those convert the large movements of the eardrum

它們將鼓膜的大震動轉換為壓力波，

into pressure waves in the fluid of the cochlea.

傳入耳蝸的液體中。

When sound enters the ear canal,

當聲音進入耳道時，

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

它會撞擊到鼓膜，使之震動， 就像鼓的鼓皮一樣。

The vibrating eardrum jerks a bone called the hammer,

震動的鼓膜會猛力推動 一塊骨頭，稱為錘骨，

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

它會撞擊到砧骨， 動到第三塊骨頭，叫做鐙骨。

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

它的活動就會推動 耳蝸長室當中的液體。

Once there,

一旦這些都完成了，

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

聲音震動就被轉換為液體的震動，

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

之後它們就像波一般， 從耳蝸的一端傳導到另一端。

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

有片和耳蝸一樣長的表面， 叫做基底膜，

It’s lined with hair cells that have specialized components

它的上面排滿了毛細胞， 毛細胞有專門的組成成分，

called stereocilia,

叫做纖毛，

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

纖毛會隨著耳蝸液體 和基底膜的震動而動。

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

震動會觸發訊號，通過毛細胞傳輸，

into the auditory nerve,

傳到聽覺神經，

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

進而傳到大腦， 大腦將它轉譯為特定的聲音。

When a sound makes the basilar membrane vibrate,

當一個聲音讓基底膜震動時，

not every hair cell moves -

並非所有的毛細胞都會跟著動，

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

只有被選中的毛細胞才會動， 根據聲音的頻率來選。

This comes down to some fine engineering.

這就涉及了非常精良的工程。

At one end, the basilar membrane is stiff,

基底膜的一端是硬的，

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

只有在遇到短波長、 高頻率的聲音時才會震動。

The other is more flexible,

另一端比較有彈性，

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

遇到較長的波長 和低頻率的聲音時才會震動。

So, the noises made by the seagull and mosquito

所以，海鷗和蚊子造成的噪音，

vibrate different locations on the basilar membrane,

會導致基底膜上 不同的位置產生震動，

like playing different keys on a piano.

就像按下鋼琴上不同的琴鍵一樣。

But that’s not all that’s going on.

但還不只如此而已。

The brain still has another important task to fulfill:

大腦還有另一項重要的工作要完成：

identifying where a sound is coming from.

找出聲音是從何而來。

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

為了這個目的，它會比較 來自兩隻耳朵的聲音，

to locate the source in space.

來判定聲音源頭在空間中的所在。

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 會同時抵達你的兩耳。

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

兩耳聽到的強度也會一樣。

However, a low-frequency sound coming from one side

然而，來自單邊的低頻聲音，

will reach the near ear microseconds before the far one.

抵達較近的耳朵和較遠的耳朵之間 會有幾微秒的時間差。

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

高頻的聲音，用近的耳朵 聽起來會比較強，

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

因為你的頭會擋住它們， 使之無法接觸較遠的耳朵。

These strands of information reach special parts of the brainstem

這一股一股的資訊 會到達腦幹中的特殊部位，

that analyze time and intensity differences between your ears.

這些部位會分析兩耳 聽到的時間差和強度差。

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

它們會把分析結果送到聽覺皮質區。

Now, the brain has all the information it needs:

現在，大腦有了所有需要的資訊：

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

活動的模式說明聲音是什麼，

and information about where it is in space.

還有關於在空間中所在位置的資訊。

Not everyone has normal hearing.

並不是每個人都有正常的聽覺。

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

聽力損失是世界上 排名第三的常見慢性疾病。

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

暴露在大聲的噪音或某些藥物中， 都可能殺死毛細胞，

preventing signals from traveling from the ear to the brain.

讓訊號無法從耳朵傳到大腦。

Diseases like osteosclerosis freeze the tiny bones in the ear

像骨質硬化這類的疾病， 會讓耳朵中的小骨頭僵固，

so they no longer vibrate.

它們就無法再震動。

And with tinnitus,

至於耳鳴，

the brain does strange things

大腦會做出很奇怪的事

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

在沒有聲音的時候， 讓我們認為有聲音存在。

But when it does work,

但在運作良好時，

our hearing is an incredible, elegant system.

我們的聽覺是個很了不起、 很精緻的系統。

Our ears enclose a fine-tuned piece of biological machinery

我們的耳朵內部 有很精確的生物機械裝置，

that converts the cacophony of vibrations in the air around us

會把我們周邊空氣中的震動雜音轉換

into precisely tuned electrical impulses

成為精準的電脈衝，

that distinguish claps, taps, sighs, and flies.

因此能區別出拍手、水龍頭滴水、 嘆氣，和蒼蠅的聲音。