Sound ISN'T a Thing

Subtitles

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!