The speed of sound is about 1235
kilometers per hour or 768 miles
per hour . And, no, I'm not going
to delay the audio of the entire
video like that. But that is what
you would have experienced if
you were just 150 meters or 500
feet from this video unless, of
course, you are in an acoustic
shadow zone. But anyway, the reason
I say the speed of sound is "about"
1235 kilometers per hour is because
the speed of sound is not absolute,
partially because sound...isn't
really "a thing".
Sound is often compared with
light because they are really
the two largest building blocks
that give us our perception of reality.
But, unlike light, sound can't
exist on it's own. Light can travel
through space all the way from
the Sun to Earth without anything
there in space. But sound could not.
You see, while light was created
as electromagnetic radiation and
is therefore propagated by electromagnetic
waves, sound is composed of mechanical waves.
Putting that into simpler terms,
light is it's own thing, electric
and magnetic fields that are made
of photons. It can therefore travel
on it's own without anything else
around it. But sound is not it's
own thing, it requires a medium
with which to pass through, otherwise...
it simply doesn't exist. As a
mechanical wave, sound requires
physical interaction between the
medium's molecules or their bonds.
The best way to visualize sound
is with a grid. The grid is the
medium (or substance) through
which the sound is traveling.
The points on the grid are the
material's molecules. The lines
connecting the dots are the bonds
connecting the molecules together.
In elastic (or springy) materials,
the only materials through which
sound can travel, these bonds
are compressible. This is what
allows sound to propagate so we'll
visualize these lines as springs.
For simplicity's sake, we'll only
visualize one sound wave at a
time and at a much slower speed,
otherwise...it'd be a confusing mess.
There are two types of sound
waves, shear waves and compression waves.
Shear waves only exist in solids.
Liquids and gases don't have shear
waves, so for simplicity's sake
we'll focus on compression waves.
If a sound is created on this
side of the material, the sound
propagates through the material
like this. The amount the wave
compresses and stretches the material
is the amplitude (or volume) of
the sound and the speed of the
wave is the speed of sound. This
speed varies greatly depending
on the material and the material's
temperature. This is why there
is really no absolute speed of sound.
There are a fair number of factors
that play into what the speed
of sound is for different materials.
For solids and liquids the speed
of sound is most affected by the
material's stiffness and density.
For gases, however, it's primarily
the temperature and molecular
structure of the gas that determines
the speed of sound. Air is basically
an ideal gas, so the speed of
sound in air is primarily affected
by the air's temperature. The
colder the temperature, the slower
the speed of sound, and the hotter
the temperature, the faster the
speed of sound. At a typical 20
degrees Celsius (or 68 degrees
Fahrenheit) the speed of sound
through air is about 1235 kilometers
per hour or 768 miles per hour
or about 0.0001% the speed of light.
A common measurement in various
fields, such as aerodynamics and
ballistics, is Mach number, named
after physicist Ernst Mach who
captured the first photograph
of a bow shockwave on an object
traveling faster than the speed
of sound. Mach number is not a
specific speed but instead the
ratio of the object's airspeed
to the speed of sound. Mach 1
is always equal to the speed of
sound, but, of course, the speed
of sound is not always the same.
For example 100 kilometers per
hour could be Mach 0.081  if the
temperature is 20 degrees Celsius
or 100 kilometers per hour could
be Mach 0.084  if the temperature
is 0 degrees Celsius. Anything
with a Mach number higher than
Mach 1 is supersonic. The fastest
manned aircraft ever was the X-15
which traveled at a top speed
of 7274 kilometers per hour or
4520 miles per hour which (at
an altitude of about 31 kilometers)
is Mach 6.7; two times faster
than the renowned SR-71 Blackbird!
Because of the incredible speed,
the surface temperature of the
X-15 on some of these missions
reached temperatures of over 700
degrees Celsius or 1300 degrees
Fahrenheit simply due to friction
with the air!
The average temperature of Earth's
atmosphere varies greatly depending
on the altitude. What's interesting
about this is that, up to about
11 kilometers, the temperature
of the air decreases which causes
the speed of sound to decrease as well.
This causes an interesting phenomenon
where sound is actually refracted
upward and thus causes an acoustic
shadow some distance from the
source of the sound. Wind gradients
can also decrease or increase
this effect to such an extent
that in some cases the sound of
war can't even be heard just 3
kilometers away! In fact, on one
occasion, during World War 1,
a bombardment of German cannon
fire could not be heard 50 to
100 kilometers from the source.
The sound was heard past the 100
kilometer radius, but anyone inside
the 50 to 100 kilometer radius
could not hear the explosions.
This means that air temperatures
and wind gradients refracted the
sound first up but then down.
These temperature and wind gradients
are the reason why some times
you will see a flash of lightning,
but never hear it. It's not necessarily
because you're too far to hear
it, it could just be because you're
in an acoustic shadow zone.
One advantage sound has over
light is that, unlike light, sound
can travel through almost anything,
so long as there is something.
We generally think of sound traveling
through air, but sound travels
through much more than just air.
In fact, the speed of sound is
generally faster through solid
objects than it is through air.
Even though the speed of sound
through air is generally a fairly
slow 1200 kilometers per hour,
the speed of sound in steel is
21,600 kilometers per hour or
13,422 miles per hour! The speed
of sound through water is also
faster than through air at about
5400 kilometers per hour or 3355
miles per hour. Although in steel
and water the speed of sound is
faster than through air, the speed
of sound through rubber is slower
than the speed of sound through
air at a quite slow 216 kilometers
per hour or 134 miles per hour.
Diamond compensates for whatever
rubber lacks though. Since diamond
is one of the stiffest substances
we know of, the speed of sound
through diamond is one of the
fastest possible under normal
circumstances. The speed of sound
through diamond is an absolutely
mind-boggling 43,200 kilometers
per hour or 26,843 miles per hour!
But this absolutely mind-boggling
speed is only 0.004% the speed of light!