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- [Voiceover] The human
ear can hear frequencies

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from around 20 hertz to about ...

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20 hertz is a very low base ...

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to up to around 20,000 hertz.

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This is way up there.

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If it's a frequency above this,

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this is the range we can hear,

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if the frequency lies above this range,

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we give it a special name.

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We call it ultrasound, or ultrasonic.

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This does more than just annoy animals.

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This has a practical purpose.

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If you wanted to do some medical imaging

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or figure out what's going
on in the human body ...

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So, say, here's a
portion of the human body

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and there's maybe a vital
organ or some tissue here,

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or some tissue over here,

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you're worried something's going wrong,

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you got to figure out
what's going on inside,

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you can operate, but that obviously sucks.

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You want to try to avoid
an operation if possible.

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You can do x-ray, but too much exposure

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to radiation is bad, too,

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so a very good option
is usually ultrasound.

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We can take what's called a transducer.

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We put that transducer up to the skin.

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This transducer takes electrical energy,

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you plug it into the wall,

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turns it into sound energy.

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You send out sound waves.

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You send out a pulse.

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This transducer sends out a pulse.

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This pulse travels
toward anything in here,

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and it turns out it'll reflect.

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It'll reflect any time there's
a difference in the medium,

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so any time there's an
interface between the two media,

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which, in this case, we'll make it simple.

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Let's just say there's tissue
from blood or other things,

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or, sorry, tissue from organs,

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and then the red will represent the blood.

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This is going to keep traveling here,

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it keeps traveling.

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Once there's an interface,
here, between blood and tissue,

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it will reflect, comes back.

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This transducer's always timing.

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It knows when it sent out the pulse

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and it knows when that
pulse got reflected back.

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It also knows the speed of sound,

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so it can calculate, all
right, if it took that long

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to get back, it must have
reflected this far away.

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Something's at this point.

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That's done yet, though.

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Some of this wave is going to travel on.

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In fact, most of this wave
travels through, keeps going.

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Here's another interface
between tissue and blood,

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so it's going to reflect again.

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This reflects back.

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We'll get another pulse.

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This is at some later time.

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The transducer knows, all
right, took that long,

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now there must have been
something else there.

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My one pulse got reflected two times.

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So there's something here,
and here's the end of it.

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But that's not done either.

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This keeps on going.

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It'll reflect against this interface

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between blood and tissue.

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I'm drawing these sound waves crooked

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just so you can see them.

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They'd really be right
on top of each other

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along this line.

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That takes another amount of time.

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It keeps doing this and it knows

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that you'll have points right here,

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difference between interfaces right here,

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an interface between
two different tissues.

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You can get an image of
this whole cross section.

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If you just have a
transducer that sends out

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pulses along this whole
face of the transducer

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you can image this whole region.

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So you can start imaging all these points.

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You can figure out what is inside of here,

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what's the shape of it,

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what are any particular lesions or lumps

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going on inside of here.

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That's ultrasound.

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That's one way it's useful.

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It actually uses ultrasound frequencies.

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You might wonder why.

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Why would we have to use ultrasound?

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One reason why is if
you took this transducer

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and you were using audible frequencies,

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you take this noisy thing,
you hold it up to a patient,

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that patient's going to be like,

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"Uh, are you sure that's
okay to hold up to me, doc?"

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That might be upsetting.

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Another more practical
reason for using ultrasound

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is high frequencies, and that
is to say low wavelengths,

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and these two are the same

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because, remember, speed of a wave is

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wavelength times frequency,

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so if the frequency's high,
the wavelength is low,

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because the speed's not
determined by either of those.

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The speed is determined
by the medium itself.

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It turns out, for high
frequency, low wavelength,

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you get less diffraction.

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Diffraction is an enemy
of making clear pictures

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because what diffraction is,

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is this is a spreading out of waves.

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If I had my wave coming in here,

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wave coming in, and there
was some sort of barrier,

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let's say this barrier is right here,

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and I've got a small hole in it,

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waves spread out.

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But if it's a high frequency wave,

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it won't spread out much.

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It's going to enter through this hole

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and it'll spread out a little bit.

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It's going to get a little bit wider.

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But, if it were a low frequency,

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maybe audible region high wavelength,

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the spreading would be bigger

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and this would be a problem
because if it spreads out,

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think about it, if this
wave was coming in here

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and this wave is coming in here,

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and then it curves around corners.

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Another thing diffraction
does is it causes waves

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to curve around corners,
the spreading happens,

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now you've got all this
bending of sound waves,

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sound waves reflecting off of things

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confusing the transducer,
you get a blurry image.

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That's why we want to
use high frequencies.

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There's less diffraction,
you get a clearer image.

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This is one application of
ultrasound for medical imaging.