(upbeat music)
- When my wife was pregnant, she had ultrasound scans.
That is not my wife, by the way.
You can't film inside NHS hospitals,
so that is stock footage, which I really like,
because look at how the image of the baby
isn't really changing, and yet, this guy's going crazy.
Look, whoo! Let's look all around the baby!
That guy's not a sonographer.
Anyway, sometimes I'm accused of not being in the moment,
but that's not true, I'm just in a different moment.
Like instead of thinking, wow,
I'm seeing my unborn child for the first time,
I'm thinking, wow, that machine is amazing.
I wonder how it works.
And I actually know a little bit about how
ultrasound scanning machines work now.
What really struck me the last time
was the gel that they squeeze onto your belly
before they rub the ultrasound wand on it.
I remember after the scan,
during a moment of contemplative silence,
Lianne saying, "What are you thinking about?"
And I said, "Well, I just wonder whether that gel is purely
for lubrication or whether it serves some other purpose."
So next time we went in, I snuck a look at the bottle
the sonographer was dispensing the gel from,
and it specifically said ultrasound gel,
so it wasn't just a generic lubricant.
It turns out that ultrasound gel
is specifically formulated to do one job,
and that job is acoustic coupling,
and it's really interesting,
and that's what this video is about.
But first of all, what is an ultrasound scan?
Well, it's like sonar, so you send out a sound waves,
they bounce off various things and you listen out
for the reflected waves coming back to you.
And you can use the timing
to figure out how far away things are,
and you can use the direction
to figure out where things are.
With a sophisticated device like an ultrasound scanner,
you can use that information
to build up quite a good picture.
In the case of a pregnancy scan,
it's the baby that the sound waves are bouncing off,
specifically the baby's bones.
The purpose of the gel is to sonically merge
the probe and the human body
into a single acoustic entity.
And by doing so, it removes unwanted reflections.
Think about this, if you stand in front of a wall
and scream, like I do for relaxation,
you'll hear your scream coming back at you,
because walls reflect sound.
(claps echoing)
But also passes through walls,
like this airplane. (airplane rumbling)
My studio is near City Airport in London,
which is annoying.
So the question is, how much acoustic energy gets reflected
and how much gets transmitted at the interface
between two different substances, and why?
That's what we need to figure out.
And to do that, I've got an analogous wave system.
So sound waves are longitudinal,
which means the vibrations go in the same direction
as the wave is traveling.
These waves are transverse,
so the oscillations are perpendicular
to the direction of wave travel,
but the physics is the same.
So look, if I lock one of these rods in position,
then this is gonna be like a sound wave coming up
against a really solid, unmovable wall,
and you can see that the wave gets
reflected completely, and you can see why.
If you look at the rod that's just next to it,
as the wave comes in, that rod is pulled upwards.
That creates tension in these two cords here,
which pulls the rod back down again.
But it actually travels past the origin
and goes much lower than it,
and that pulls on the rod next to it,
which pulls on the rod next to it,
and so the wave travels back in the direction it came from.
Sound waves don't reflect completely off of walls
because walls have a little bit of give,
and we can simulate that by adding
weights to the wave machine.
So the heavier rods represent the wall,
because like the atoms in the wall,
they're harder to get moving,
in this case, because of inertia.
Look what happens when we send
an impulse up to the boundary.
It's partially reflected and it's partially transmitted.
Notice, by the way That the transmitted wave
is traveling slower than the incoming wave.
That's not true for sound going from air to wall.
Sound actually travels more quickly through the wall.
That's because this analogy isn't perfect.
If I add even more weights,
then even more of the wave is reflected,
and less is transmitted, which makes sense,
because we're getting closer to the situation
where I was physically holding one of the rods in place.
So hopefully you can see
that the more different two substances are,
the more reflection you get
and the less transmission you get.
And when I say "more different,"
I'm actually talking about something
called acoustic impedance,
which is a measure of how much does a substance
resist acoustic motion,
basically the vibration of atoms
and molecules that is sound.
It's a bit like refractive index
when you're dealing with electromagnetic waves,
if you're familiar with that.
The only difference is that refractive index
is the inverse of wave impedance.
The human body has an acoustic impedance
of about 3,000 times that of air.
So if you want to work out how much sound
that is produced outside the human body
ends up inside the human body when it passes
through the boundary between the two,
you put the two acoustic impedances into this equation,
and you get out an answer of about 0.1%,
so hardly any, and by the way,
you get this same problem in reverse.
If you go from high acoustic impedance
to low acoustic impedance, you get reflection as well.
Hopefully you can see that we would expect that intuitively,
if I show you what happens when you go
from some impedance to zero impedance,
you see, if I pinch these two cords together,
the cord is providing no torque to resist
the twisting of that final rod.
This final rod actually travels higher
than it would normally because it's unimpeded on this side.
And so that pulls on the rod next to it
and that pulls on the rod next to it,
and so you end up with a wave traveling backwards,
a reflected wave.
So without ultrasound gel, 0.1% of the acoustic energy
would find its way into your body,
and then 0.1% of that would make its way back out again
for the receiver to pick up.
0.1% of 0.1% is 1/10,000 of a percent,
completely useless.
What ultrasound gel does is remove the boundary
between different acoustic impedances.
It removes the impedance mismatch.
It's formulated to have the same
acoustic impedance as the human body,
so that there's no barrier between the speaker
and the inside of the human body
and the microphone inside the probe.
I don't know if speaker or microphone
are the right words in this context,
but at least you know what I mean.
While we're on the subject of ultrasound scans,
I wanna tell you about a collaboration
between art and science that I think is really interesting.
It's called Ode to the Future,
and it's a collaboration between a musician
and people who are yet to be born,
and it uses the technology of ultrasound scans.
And I just think it's a really interesting
kind of artistic way of looking at how
curiosity-driven innovation will shape the future.
Links in the description.
I hope you enjoyed this video.
If you did, don't forget to hit Subscribe,
and I'll see you next time. (screen whizzes)
(upbeat music)