we're here at dreams studio and media in
Warner Robins where we'll be looking at
the kind of waves we perceive as sound
there's a lot of technology that goes
into creating a good sound there's also
a lot of physics involved and that's
what we're here to check out in this
segment of physics and motion on sound
waves
[Music]
sound as you probably know travels in
waves but there are several types of
waves so let's go over those for a
moment there are mechanical waves and
electromagnetic waves mechanical waves
must have a medium a solid liquid or gas
to travel through but electromagnetic
waves do not both kinds of waves can be
transverse but only mechanical waves can
be longitudinal or transverse what we
call sound is a mechanical longitudinal
wave that creates vibrations in a medium
by the movement of energy sound waves
like other waves are propagations of
energy that create variations in
pressure and travel through a medium
another aspect of longitudinal waves is
that the particles move parallel to the
direction of the wave like this the
slinky vibrates back and forth but
because the vibration of the particles
is equal in both directions the total
displacement of the particles affected
by the wave is zero the medium does its
job transferring energy but then goes
back to its original position unchanged
this is Cornelius a music composer here
to help us analyze what sound really is
and how it works our ability to hear
sound is an amazing feat that involves a
lot of physics let's look at what
happens when we perceive a longitudinal
wave as a sound in the case of this
digital keyboard Cornelius is doing work
on the key when he strikes it producing
mechanical energy that gets converted
into electrical energy by the components
in the keyboard the electrical energy is
converted into sound energy through the
speaker's which makes its way to our
ears in the form of sound waves the
waves become mechanical energy again and
cause the eardrum to vibrate the
vibrations pass through the eardrum to
the middle ear and into the inner ear
that's where the cochlea is located
which has many thousands of cells called
hair cells so-called because they have
little Tufts on them the hair cells act
as receptors and changed the vibrations
into electrical energy
those electrical signals are sent to the
brain the brain also using electricity
tells you that you are hearing a sound
and what that sound is
and the result is here's something
amazing you can actually see sound
happen through a process called
schlieren photography we can see the
actual sound wave this is not the effect
of sound you're looking at but the sound
wave itself moving through the air
creating areas of high and low pressure
now that you've seen what sound looks
like let's look at the architecture of a
longitudinal sound wave the energy
causes variations in pressure that makes
molecules band together in some areas
creating areas of high pressure called
compressions we call the low pressure
areas of the wave where the particles
are farthest apart rarefactions to
measure a wavelength which we do in
meters we measure the distance between
two waves that include one full
compression and one full rarefaction we
call the number of wavelengths that pass
by a given point every second the
frequency which we measure in hertz so
one wavelength in one second would have
a frequency of one Hertz twenty
wavelengths in one second would have a
frequency of 20 Hertz two thousand
compressions would be how many of course
two thousand Hertz sound waves like
other longitudinal waves behave in
certain ways when the velocity of the
wave is constant the wavelength and
frequency are inversely proportional
that means when you increase one you
decrease the other like this the
relationship between these variables is
the velocity of the wave in meters per
second is equal to the frequency in
Hertz times the wavelength in meters now
what about the speed of sound
we often say it travels at 750 miles per
hour but that can actually vary there
are two variables that affect the speed
of sound the medium it's traveling in
and the temperature of that medium let's
talk about how the medium affects speed
first which is the result of the density
of the medium sound travels the slowest
in gases faster and liquids and fastest
in solids it's easier for the sound
waves to go through solids than through
liquids for example because the
molecules and solids are closer together
and can push each other more quickly it
can make a big difference your ears
receive a sound wave four times faster
under water than through the air because
water is more dense if you've ever
noticed that you can't really tell where
sound is coming from when you're
underwater it's because the waves are
reaching your ears so quickly but your
brain can't distinguish where the sound
is coming from so would sound travel
faster through air or a metal rod right
the metal rod because it's much more
dense there's another factor about the
medium that affects speed its elasticity
which is how quickly the molecules
bounce back after a wave has moved
through them so for instance while a
lead rod is more dense than a steel rod
sound travels faster through the steel
because the steel is more elastic the
more quickly the molecules in the medium
return to its original shape the more
quickly the next pressure wave moves
through it now what about temperature
how does that affect the speed of sound
sound waves travel faster in warm air
than cooler air and here's why the
molecules in warm air are moving much
faster than those in cooler air because
they're zooming around so much they will
vibrate more quickly which helps
transmit the wave faster so would you
hear us sound more quickly on a hot
summer day or hiking in the mountains in
the fall right on the hot summer day so
where did that 750 miles per hour come
from that's about how fast sound travels
in dry air at three point three degrees
Celsius which is about 38 degrees
Fahrenheit that number will change as
the temperature changes for instance at
room temperature which is 20 degrees
Celsius sound travels at 343 m/s or 768
miles per hour the speed of sound in air
increases by about 0.6 meters per second
for
one degree Celsius increase in
temperature the equation is the speed of
sound in air equals 331 meters per
second plus 0.6 times the temperature of
the air in Celsius
you've heard about sonic booms right
when an airplane travels faster than
sound the boom is basically a shock wave
created from the change in pressure
caused by the object speeding through
the air did you know this is also a
sonic boom just on a smaller scale but
the end of the whip moves faster than
the speed of sound okay
one more very cool example of how sound
waves affect the medium there is
something called a non-newtonian fluid
which is a material that gets thicker as
you disturb it by doing something like
stirring like when you add cornstarch to
water and heat it here's how sound waves
make this non-newtonian medium I'll call
it by its technical name goop although
most people call it Oobleck the Oobleck
is placed on some saran wrap on top of
the speaker what you are seeing is sound
waves forming they are passing through
the medium with a lot of energy at high
intensity it works best with songs with
a lot of bass that's because bass has a
low frequency and to be heard well is
produced at a higher intensity remember
we defined frequency earlier in this
segment as the number of wavelengths
that pass by a given point every second
but how does this translate into what we
hear we humans perceive frequency as
pitch let's listen to some different
frequencies and hear what we mean here's
a high frequency
here's a low-frequency humans can hear
frequencies that are roughly between 20
Hertz and 20,000 Hertz sounds with a
lower frequency than humans can hear are
called infrasonic and those higher than
what we can here are called ultrasonic
no what animal has the best hearing you
think dog maybe how about a dolphin
nope it's actually the moth it can hear
a hundred and fifty times more
frequencies than we can another quality
of a sound wave is its amplitude which
is a measure of the amount the wave
compresses the medium it's traveling
through higher amplitude waves have more
compact compressions while in lower
amplitude waves
the particles in the compressions are
less dense humans perceive amplitude as
the loudness of a sound changes made to
frequency or amplitude are independent
of one another you can have sounds that
have a low frequency and low amplitude
which we'll hear as low and soft
high frequency and high amplitude we
hear as high and loud or other
combinations of these characteristics
amplitude is proportional to the
intensity of a sound wave which is a
measure of how powerful the wave is over
a given area we perceive the intensity
of the wave as it moves through the
medium as the volume of a sound how loud
it is we measure sound wave intensity in
decibels which tells us the loudness of
a wave as we perceive it decibels are
the logarithmic ratio between the sound
waves highest air pressure when it's
loudest and the pressure at its lowest
and most quiet a general rule of thumb
for loudness is that the power must be
increased by about a factor of 10 to
sound twice as loud Cornelius can you
help us out give us something at 70
decibels
okay can you cut that into 35 decibels
and to give you even more of a sense of
what decibels mean in the real world a
normal conversation between two people
standing three feet apart is about 60
decibels a subway train coming into the
station is 95 decibels and a loud rock
concert can be a hundred and fifteen
decibels you can develop hearing loss
from listening to sounds over 90
decibels for a long time and anything
over a hundred and twenty five decibels
is literally painful so we won't try
that here near that that's a simple
sound wave that we call a periodic wave
or sine wave spelled this way periodic
or sine waves stay the same there are no
changes in their characteristics like
amplitude frequency or wavelength most
real-world sounds are much more complex
they're made up of many periodic waves
that combine to make one complex wave
and there's another characteristic of
sound
it's called Tambor Tambor is the quality
of a sound what makes one sound
different than another say a trumpet
sound different from a clarinet sound
Thank You Cornelius different timbres
result from the way the sound waves
bounce off the inside of the instrument
and recombine with the original sound in
a process called reverberation we've
learned a lot about sound waves how to
analyze them what they look like and how
they work that's it for this segment of
physics in motion and we'll see you guys
next time for more practice problems lab
activities and note taking guides check
out the physics and motion toolkit