WEBVTT

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You hear the gentle lap of waves,

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the distant cawing of a seagull.

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But then an annoying whine
interrupts the peace,

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getting closer, and closer, and closer.

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Until...Whack!

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You dispatch the offending mosquito, 
and calm is restored.

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

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The ability to recognize sounds 
and identify their location

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is possible thanks to the auditory system.

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

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

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

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To understand how that works,

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

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The source of a sound creates vibrations

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that travel as waves of pressure 
through particles in air,

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liquids,

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or solids.

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But, our inner ear, called the cochlea,

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is actually filled 
with saltwater-like fluids.

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

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wherever they’re coming from,

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into waves in the fluid.

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The solution is the ear drum, 
or tympanic membrane,

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and the tiny bones of the middle ear.

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Those convert the large movements 
of the ear drum

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into pressure waves 
in the fluid of the cochlea.

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When sound enters the ear canal,

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

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The vibrating eardrum jerks a bone 
called the hammer,

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

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

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Once there,

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

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

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

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It’s lined with hair cells that have 
specialized components

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called stereocillia,

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

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

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into the auditory nerve

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

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When a sound makes 
the basilar membrane vibrate,

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not every hair cell moves -

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

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This comes down to some fine engineering.

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At one end, 
the basilar membrane is stiff,

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

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The other is more flexible,

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

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So, the noises made by the seagull 
and mosquito

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vibrate different locations 
on the basilar membrane,

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like playing different keys on a piano.

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But that’s not all that’s going on.

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The brain still has another 
important task to fulfill:

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identifying where a sound is coming from.

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

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to locate the source in space.

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

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

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However, a low-frequency sound 
coming from one side

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will reach the near ear microseconds 
before the far one.

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

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

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These strands of information 
reach special parts of the brainstem

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that analyze time and
intensity differences between your ears.

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

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Now the brain has 
all the information it needs:

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

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and information about 
where it is in space.

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Not everyone has normal hearing.

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

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

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preventing signals from travelling 
from the ear to the brain.

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Diseases like osteosclerosis freeze 
the tiny bones in the ear

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so they no longer vibrate.

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And with tinnitus,

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the brain does strange things

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

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But when it does work,

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our hearing is an incredible, 
elegant system.

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Our ears enclose a fine-tuned piece 
of biological machinery

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that converts the cacophony of vibrations 
in the air around us

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into precisely tuned electrical impulses

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that distinguish claps, taps, 
sighs, and flies.