♪♪
Have you ever heard
a recording of your voice?
Didn't it sound different
than you expected?
You know why?
When we talk,
we're hearing sound waves
transmitted through
the bones in our head
and through the air
into our ear canal,
two different mediums.
The mechanical properties
of your head
amplify deeper, lower
frequency vibrations.
When I hear my voice
in a recording,
I'm only hearing the waves
that were transmitted
through the air
with none of the waves
that I would
normally be hearing
transmitted
through my head.
(recording)
...transmitted
through my head.
Do I really
sound like that?
Okay, here's something
else about waves.
Another sound phenomenon
we experience every day
is called diffraction.
It's the bending of waves
around a barrier.
Let me show you.
Cornelius,
can you hear me?
Yes, I sure can.
So even when
I'm in another room,
Cornelius can hear me
because sound waves
bend around objects,
or diffract.
Just like waves in water
bend around a boat,
sound waves are doing
the same thing.
They're making
a great escape.
Some sound waves
will hit the entryway
and get absorbed,
but others make it
through the entryway,
and transfer the energy
to particles
on the other side.
That's diffraction.
The reason Cornelius
hears me is because
the sound waves
of my voice
are bending around
the edges
of the entryway
and walls.
How well sound waves do this
depends on their wavelength.
Longer wavelength,
lower frequency waves
diffract more as they travel
through and around an entryway.
Because they bend so much,
it's more difficult
to figure out
where they're coming from.
They are less directional.
Shorter wavelength,
higher frequency waves,
on the other hand,
don't bend as much,
so they are more directional.
(thunder rumbling)
Think about
when you hear thunder.
The thunder from
a bolt of lightning
that strikes
close to you
will sound
like a sharp crack,
indicating the presence
of shorter wave length,
high frequency sound.
The thunder from
a distant lightning strike
will sound like
a low rumble
since the long wavelengths
will be the ones
that bend around the objects
to get to you.
The high frequency parts
of the sound
can't bend
around barriers,
so they get absorbed
by the surroundings
before they hit your ear.
The ability to hear low
and high frequencies
depends on who
does the hearing.
The audible range
of hearing for humans
is about 20
to 20,000 hertz,
or 20 kilohertz.
Anything lower than
20 hertz
is known as infrasonic,
and anything above
20,000 hertz
is ultrasonic.
(elephant trumpeting)
Different animals hear
and produce
different frequencies.
Elephants, for example,
can be miles apart
and still communicate
with each other
using frequencies that
are infrasonic to humans,
the sound waves
with frequencies
lower than 20 hertz.
(elephant trumpeting)
These wavelengths
are so long,
and they can bend
around objects
like trees so well
that elephants can hear them
up to six miles away.
Bats are very different.
They don't need
their sounds to diffract
and bend around objects
like the longer
wavelengths do.
They need to pinpoint
objects precisely
so they can target prey
like bugs
and navigate obstacles
in their path.
They emit
short wavelengths
that reflect off
objects close by
and back to
the bat.
These pulses of
high frequency sound
can be as high
as 50 kilohertz,
more than two and
a half times higher
than the highest sound
humans can hear.
These frequencies
are ultrasonic to humans,
but not to bats.
They hear these sounds.
So when I yell,
"Hey, you!"
the sound leaving my mouth
sends energy
that compresses
air molecules together
in some places and
leaves gaps in others.
The compressed areas
are high pressure,
called compressions.
The areas of lower pressure
are called rarefactions.
Okay, back to
soundtracking.
That's what they call it
when they record my voice.
So think of sound waves
as waves of energy,
and sometimes we have
the ability to manipulate them.
I'm in a special sound room
that does just that.
The sound waves from my voice
can't leave the room
because they're absorbed
by the padding.
That's very different
than if I was
in the studio hallway,
where sound waves
bounce off walls
at all different angles.
The reflection of
many sound waves
compounded one upon another
within a space
is called reverberation.
Okay, here's another
characteristic
of sound waves called
sound interference.
Interference happens
when two waves
occupy the same space
at the same time.
Constructive interference
occurs when both waves
are identical in frequency
and their compressions
and rarefactions
are aligned 100%.
When this happens,
we say they're in phase
with each other.
Basically, they
add to each other.
So what do you think
happens to the sound?
Does it get louder
or softer?
(tone playing)
If the two
sound waves undergo
constructive interference,
their amplitude increases
and the sound gets louder.
(tone playing)
There's another way
two waves
can interfere
with each other.
That's when they're
180 degrees
out of phase
with each other.
That's called
destructive interference.
So what happens
to the sound
if the waves undergo
destructive interference?
The sound waves
cancel each other out,
and the result
is no sound.
(tone playing)
The fact that
waves line up like this
is called the principle
of superposition.
The principle of
superposition tells us
when waves in space
interfere with one another,
they combine to form
bigger or smaller waves.
But even when
this happens,
each wave maintains
its individuality.
Ultimately, they pass through
each other, unchanged.
The net displacement
of the medium,
that is,
all the compressions
and rarefactions
at any point,
is simply the sum
of the individual waves.
In fact, all waves,
from sound to light,
follow this same principle,
as you'll see
in another segment.
So constructive
and destructive
seem like they're
opposites, right?
But sometimes
you can get both.
Let's say you have
a loud sound source
in a room with two
slightly open windows
that are close together.
Maybe someone is playing
their favorite song
full volume.
If you're in the room,
you're overwhelmed,
and if you're standing
outside the open windows,
it can seem
pretty loud, too.
Why is so much sound
coming through
those two narrow windows?
This is an example of
double slit diffraction.
Sound is traveling
through two openings,
like windows,
rather than
a single opening,
like a door,
and the window doesn't
have to be wide open.
This even works with
two small slits.
When sound travels through
two separate openings,
some of the waves
leaving one window
will match up to the waves
leaving the other window.
You have compressions
on compressions,
and rarefactions
on rarefactions.
That's called
constructive interference.
The black spaces
on the right
show where the waves
don't match up.
That means the compressions
of the waves
from one window line up
with the rarefactions
of the waves
from the other window,
and they cancel
each other out.
That's destructive
interference.
If I were standing in
the destructive
interference zone,
the music will be
much more quiet.
If I were standing
in a constructive
interference zone,
the music
will sound loud.
When sound waves with
almost identical frequencies
interfere with
one another,
you get a rhythmic change
in amplitude,
loud to soft,
which we hear as beats.
The beat frequency
is the absolute value
of the difference between
the high and the low frequency.
So if we play a tone
at 450 hertz
and one at 452 hertz,
the beat frequency
will be two hertz.
450 hertz.
(tone playing)
(tone playing)
452 hertz.
Do you hear it?
We're just scratching
the surface
of how we can
manipulate sound.
Everything from creating
echoes
to combining sound waves
in a way
that they
actually disappear.
(playing keyboard)
You'll certainly appreciate
sound effects
in movies more
knowing the
physics of sound.
And if you ever thought about
being a sound engineer,
or even an architect,
getting this physics down
will help you design
better sound environments
and technologies,
like speaker systems,
where it's important
that sound waves ring clear
and don't appear
to disappear.
That's it for this segment
of "Physics in Motion."
We'll see you guys
next time.
What you got, Cornelius?
(playing piano)
