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  • You hear the gentle lap of waves,

  • the distant cawing of a seagull.

  • But then an annoying whine interrupts the peace,

  • getting closer, and closer, and closer.

  • Until...whack!

  • You dispatch the offending mosquito, and calm is restored.

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

  • The ability to recognize sounds and identify their location

  • is possible thanks to the auditory system.

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

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

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

  • To understand how that works,

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

  • The source of a sound creates vibrations

  • that travel as waves of pressure through particles in air,

  • liquids,

  • or solids.

  • But our inner ear, called the cochlea,

  • is actually filled with saltwater-like fluids.

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

  • wherever they're coming from,

  • into waves in the fluid.

  • The solution is the eardrum, or tympanic membrane,

  • and the tiny bones of the middle ear.

  • Those convert the large movements of the eardrum

  • into pressure waves in the fluid of the cochlea.

  • When sound enters the ear canal,

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

  • The vibrating eardrum jerks a bone called the hammer,

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

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

  • Once there,

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

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

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

  • It's lined with hair cells that have specialized components

  • called stereocilia,

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

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

  • into the auditory nerve,

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

  • When a sound makes the basilar membrane vibrate,

  • not every hair cell moves -

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

  • This comes down to some fine engineering.

  • At one end, the basilar membrane is stiff,

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

  • The other is more flexible,

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

  • So, the noises made by the seagull and mosquito

  • vibrate different locations on the basilar membrane,

  • like playing different keys on a piano.

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

  • The brain still has another important task to fulfill:

  • identifying where a sound is coming from.

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

  • to locate the source in space.

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

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

  • However, a low-frequency sound coming from one side

  • will reach the near ear microseconds before the far one.

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

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

  • These strands of information reach special parts of the brainstem

  • that analyze time and intensity differences between your ears.

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

  • Now, the brain has all the information it needs:

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

  • and information about where it is in space.

  • Not everyone has normal hearing.

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

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

  • preventing signals from traveling from the ear to the brain.

  • Diseases like osteosclerosis freeze the tiny bones in the ear

  • so they no longer vibrate.

  • And with tinnitus,

  • the brain does strange things

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

  • But when it does work,

  • our hearing is an incredible, elegant system.

  • Our ears enclose a fine-tuned piece of biological machinery

  • that converts the cacophony of vibrations in the air around us

  • into precisely tuned electrical impulses

  • that distinguish claps, taps, sighs, and flies.

You hear the gentle lap of waves,

Subtitles and keywords

B2 H-INT US sound membrane cochlea frequency vibrate fluid

The science of hearing - Douglas L. Oliver

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    April Lu   posted on 2018/06/27
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