How to see with sound - Jacques S. Abramowicz
In a pitch-black cave,
bats can’t see much.
But even with their eyes shut,
they can navigate rocky topography
at incredible speeds.
This is because a bat’s flight
isn’t just guided by its eyes,
but rather, by its ears.
It may seem impossible to see
with sound,
but bats, naval officers, and doctors
do it all the time,
using the unique properties of ultrasound.
All sound is created when
molecules in the air, water,
or any other medium vibrate
in a pulsing wave.
The distance between each peak determines
the wave’s frequency,
measured as cycles per second, or hertz.
This means that over the same amount
of time,
a high frequency wave will complete
more cycles than a low frequency one.
This is especially true of ultrasound,
which includes any sound wave
exceeding 20,000 cycles per second.
Humans can't hear or produce sounds
with such high frequencies,
but our flying friend can.
When it’s too dark to see, he emits
an ultrasound wave with tall peaks.
Since the wave cycles are
happening so quickly,
wave after wave rapidly bounces
off nearby surfaces.
Each wave’s tall peak hits
every nook and cranny,
producing an echo that carries
a lot of information.
By sensing the nuances
in this chain of echoes,
our bat can create an internal map
of its environment.
This is how bats use sound to see,
and the process inspired humans
to try and do the same.
In World War One, French scientists
sent ultrasound beams into the ocean
to detect nearby enemy submarines.
This early form of SONAR
was a huge success,
in large part because sound waves
travel even faster through mediums
with more tightly packed molecules,
like water.
In the 1950s, medical professionals began
to experiment with this technique
as a non-invasive way to see
inside a patient’s body.
Today, ultrasound imaging is used
to evaluate organ damage,
measure tissue thickness, and detect
gallbladder stones, tumors,
and blood clots.
But to explore how this tool
works in practice,
let’s consider its most well-known use—
the fetal ultrasound.
First, the skin is covered
with conductive gel.
Since sound waves lose speed and clarity
when traveling through air,
this gooey substance ensures
an airtight seal
between the body and the wand
emitting ultrasound waves.
Then the machine operator begins sending
ultrasound beams into the body.
The waves pass through liquids like urine,
blood, and amniotic fluid
without creating any echoes.
But when a wave encounters a solid
structure, it bounces back.
This echo is rendered as a dot
on the imaging screen.
Objects like bones
reflect the most waves,
appearing as tightly packed dots
forming bright white shapes.
Less dense objects appear
in fainter shades of gray,
slowly creating an image
of the fetus’s internal organs.
To get a complete picture,
waves need to reach different
depths in the patient’s body,
bypassing some tissues
while echoing off others.
Since longer, low frequency waves
actually penetrate deeper
than short, high frequency ones,
multiple frequencies
are often used together
and composited into a life-like image.
The operator can then
zoom in and focus on different areas.
And since ultrasound machines send and
receive cascades of waves in real time,
the machine can even visualize movement.
The waves used for medical ultrasound
range from 2 million to 10 million hertz—
over a hundred times higher
than human ears can hear.
These incredibly high frequencies create
detailed images
that allow doctors to diagnose
the smallest developmental deviations
in the brain, heart, spine, and more.
Even outside of pre-natal care,
medical ultrasound has huge advantages
over similar technologies.
Unlike radiation-based imaging
or invasive surgical procedures,
ultrasound has no known negative
side effects when used properly.
At very high levels,
the heat caused by ultrasound waves
can damage sensitive tissues,
but technicians typically use
the lowest levels possible.
And since modern ultrasound machines
can be small and portable,
doctors can use them in the field—
allowing them to see clearly
in any medical emergency.
