The science of hearing Douglas L. Oliver
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.