[Music]
have you ever heard of 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
amplified 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 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
Cornelia's 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
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 wavelength 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 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 these wavelengths
are so long and they can bend around
objects like trees so well but elephants
can hear them up to 6 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 sound tracking 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 if the two sound
waves undergo constructive interference
their amplitude increases and the sound
gets louder there's another way two
waves can interfere with each other
that's when they're a hundred and eighty
degrees out of phase with each other
that's called destructive interference
so what happens to the sound of the
waves undergo destructive interference
the sound waves cancel each other out
and the result is no sound 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
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 that 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 2 Hertz 450 Hertz
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
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
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