The science of hearing - Douglas L. Oliver

翻译人员: Riley WANG 校对人员: Xiaoyu Ye

You hear the gentle lap of waves,

你听到海浪温柔的拍打声，

the distant cawing of a seagull.

听到远处海鸥的叫声，

But then an annoying whine interrupts the peace,

但一阵嗡嗡声扰乱了平静，令人恼火。

getting closer, and closer, and closer.

声音越来越近，越来越近。

Until...whack!

然后，啪的一声！

You dispatch the offending mosquito, and calm is restored.

你拍死了烦人的蚊子， 世界终于重回安静。

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

你是如何听到远处的声音， 并如此准确地锁定声音来源的呢？

The ability to recognize sounds and identify their location

我们之所以能够 识别声音并判断出其所在位置

is possible thanks to the auditory system.

很大程度是因为听觉系统。

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

它由大脑和耳朵这两部分构成。

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

耳朵的任务是将声音转换成为神经信号；

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

大脑则是接受并处理这些信号中的信息。

To understand how that works,

为了更好地理解它的工作原理，

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

我们跟随声音进行一场耳朵之旅。

The source of a sound creates vibrations

声源会产生振动，

that travel as waves of pressure through particles in air,

这会以压力波的形式 通过空气中的微粒传播，

liquids,

或是在液体

or solids.

和固体中传播。

But our inner ear, called the cochlea,

但我们的内耳，也叫耳蜗，

is actually filled with saltwater-like fluids.

实际上充满了盐水一样的液体。

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

因此，第一个要解决的问题是

wherever they’re coming from,

如何将这些四面八方传来的声波

into waves in the fluid.

转化成液体的波动。

The solution is the eardrum, or tympanic membrane,

答案在于鼓膜。

and the tiny bones of the middle ear.

以及中耳处一些微小的骨头。

Those convert the large movements of the eardrum

它们将鼓膜较大的震动

into pressure waves in the fluid of the cochlea.

转化成耳蜗液体的压力波。

When sound enters the ear canal,

当声音进入耳道，

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

它首先使鼓膜震动， 就像敲打一面鼓。

The vibrating eardrum jerks a bone called the hammer,

震动的鼓膜使 一块叫做锤骨的骨头发生震动。

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

锤骨击打砧骨， 并继续震动第三块骨头镫骨。

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

这些动作推动耳蜗狭长腔室中的液体流动。

Once there,

至此，

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

声音的震动终于转化成为液体的震动。

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

之后他们如波浪般从耳蜗的一头传至另一头。

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

一种叫做基底膜的膜状结构遍布耳蜗。

It’s lined with hair cells that have specialized components

基底膜上覆盖拥有特殊结构的毛细胞，

called stereocilia,

称为纤毛。

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

它会随耳蜗液体和基底膜而震动，

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

这种动作会沿毛细胞触发信号，

into the auditory nerve,

信号会进入听觉神经，

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

之后进入大脑， 由大脑解析识别出特定的声音。

When a sound makes the basilar membrane vibrate,

声音让基底膜震动时，

not every hair cell moves -

并非所有毛细胞都会随之震动，

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

依据声音的频率不同， 只有特定的毛细胞会摆动。

This comes down to some fine engineering.

这与一些精密工程学相关。

At one end, the basilar membrane is stiff,

基底膜一段较为坚硬，

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

只会对波长短、频率高的声音 做出震动反应。

The other is more flexible,

另一端则更为灵活，

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

只对波长长、频率低的声音 做出反应。

So, the noises made by the seagull and mosquito

因此海鸥和蚊子的声音

vibrate different locations on the basilar membrane,

其实会引起基底膜不同区域的震动，

like playing different keys on a piano.

就像是在钢琴上弹不同的琴键。

But that’s not all that’s going on.

但这并不是全部过程。

The brain still has another important task to fulfill:

大脑仍然有另一个重要的任务：

identifying where a sound is coming from.

它需要识别声音来自何处。

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

为此，大脑会比照进入两个耳朵的声音，

to locate the source in space.

以此在空间中定位声源。

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

直接来自你面前的声音 会同时到达双耳，

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

每个耳朵听到的声音强度也相同。

However, a low-frequency sound coming from one side

但是来自其他方向的低频声音

will reach the near ear microseconds before the far one.

会以微秒之差先进入靠近声源一侧的耳朵。

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

靠近声源的耳朵 会感受到高频声音更大的强度，

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

因为头部会阻挡 声音到达另一边的耳朵。

These strands of information reach special parts of the brainstem

这部分滞后的信息 会到达脑干的特殊位置，

that analyze time and intensity differences between your ears.

以此分析双耳信号的时间和强度差异。

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

它们将分析结果 传输至大脑皮层的听觉中枢。

Now, the brain has all the information it needs:

现在，大脑拥有了全部所需的信息：

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

可以断定是哪种声音的活动模式，

and information about where it is in space.

以及声音在空间中的位置信息。

Not everyone has normal hearing.

并非所有人都拥有正常听力。

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

失聪是世界上第三大最普遍的慢性疾病。

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

接触巨大噪音和一些药物会杀死毛细胞，

preventing signals from traveling from the ear to the brain.

从而阻止信号从耳朵传送至大脑。

Diseases like osteosclerosis freeze the tiny bones in the ear

骨硬化等疾病会使耳朵中的小骨硬化，

so they no longer vibrate.

使其无法震动。

And with tinnitus,

至于耳鸣，

the brain does strange things

大脑会做出奇怪的反应，

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

使我们听到并不存在的声音。

But when it does work,

但是当听觉系统正常工作时，

our hearing is an incredible, elegant system.

它是一个精妙无比的系统。

Our ears enclose a fine-tuned piece of biological machinery

我们的耳朵包含了 精密协调的生物构造，

that converts the cacophony of vibrations in the air around us

将周边空气中的刺耳震动

into precisely tuned electrical impulses

转化成为精确校准的电脉冲，

that distinguish claps, taps, sighs, and flies.

使我们能够分辨掌声， 敲打声，叹息和苍蝇的嗡嗡声。