Speed of sound Video Lecture | Science Class 9

- If we let a speaker
make one short burst,
we'll create a pulse wave.
We can find the speed of sound
by looking at the speed
of this compressed region
as it travels through the medium.
In non-humid air at 20 degrees Celsius,
the speed of sound is about
343 meters per second
or 767 miles per hour.
We can also watch the speed of sound
of a repeating simple harmonic wave.
The speed of the wave
can again be determined
by the speed of the compressed regions
as they travel through the medium.
Note that the speed of sound does not mean
the speed of the air molecules
as they move back and forth.
The air molecules are
moving with the speed,
but by the speed of sound,
we mean the speed of the disturbance
as it moves through the air molecules.
We call sound a longitudinal wave
because the wave is traveling parallel
to the line traced out by the
oscillations of the medium.
The other type of wave
is a transverse wave.
Transverse waves happen
when the wave velocity
points perpendicular to the
oscillations of the medium.
Waves on a string or waves
on the surface of water
are examples of transverse waves.
If we look at a graph
of the air displacement
versus position of the air,
we can see that as the wave travels
the shape of this wave
travels to the right.
So, the speed of a sound wave can be found
by finding the speed of the peaks
or the speed of the valleys
or the speed of any single
point on the wave shape.
To figure out a formula for
the velocity of a sound wave,
let's look closely at
what's happening here.
Watch one of the air molecules.
It takes one period for this molecule
to move back and forth
through a full cycle.
During this time, the wave
shape has moved forward
one complete wavelength.
This is because the wave has to overlap
with its initial shape after one period,
because the molecule has
to be back where it started
after one period.
Now, since speed is defined
to be the distance per time,
the speed of a sound wave has to be
the wavelength of the wave
divided by the period of the wave.
Since the wave is traveling forwards
one wavelength per period,
or since the frequency is defined
to be one over the period,
we can rewrite this formula as
speed equals wavelength times frequency.
This formula is accurate
for all kinds of waves,
not just sound waves,
because a wave has to move one
wavelength for every period.
Be careful.
When looking at this equation,
you might think that if you adjust
the setting on your speaker
and increase the frequency
you'd also be increasing
the speed of the sound wave,
but that's not what happens.
If you increase the frequency,
the wavelength will decrease
by that same factor,
and the speed of the sound
wave will remain the same.
In fact, there's nothing
you can do to the speaker
that would increase the speed of sound.
So, how can we change the speed of sound?
Well, the only way to
change the speed of sound
is to change the medium or
the properties of the medium
that the sound wave is traveling in.
So, to change the speed of sound in air,
you can change things like
the temperature of the air
or the humidity of the air
or the density of the air,
or you can swap out the air
entirely for another material,
like water or helium or a metal.
All of these changes to the medium
would affect the speed of sound.
People often think that
changing the amplitude
will change the speed of a
sound wave, but it won't.
If we create a sound pulse
with a large amplitude,
it won't travel any
faster than a sound pulse
with a small amplitude in the same medium.
It will just be louder.
In other words,
yelling won't cause
anyone to hear you faster,
they'll just hear a louder sound
when the sound wave
arrives at their location.
So remember, the speed of a sound wave
is determined entirely by
the properties of the medium
through which it's traveling.