- [Voiceover] To change the speed of sound you have to change the
properties of the medium that sound wave is traveling through. There's two main factors about a medium that will determine the
speed of the sound wave through that medium. One is the stiffness of the
medium or how rigid it is. The stiffer the medium
the faster the sound waves will travel through it. This is because in a stiff material, each molecule is more interconnected to the other molecules around it. So any disturbance gets
transmitted faster down the line. The other factor that determines
the speed of a sound wave is the density of the medium. The more dense the medium, the slower the sound wave
will travel through it. This makes sense because if
a material is more massive it has more inertia and
therefore it's more sluggish to changes in movement or oscillations. These two factors are taken
into account with this formula. V is the speed of sound. Capital B is called the bulk
modulus of the material. The bulk modulus is the
official way physicists measure how stiff a material is. The bulk modulus has units of pascals because it's measuring how
much pressure is required to compress the material
by a certain amount. Stiff, rigid materials
like metal would have a large bulk modulus. More compressible
materials like marshmallows would have a smaller bulk modulus. Row is the density of the material since density is the mass per volume, the density gives you
an idea of how massive a certain portion of
the material would be. For example, let's
consider a metal like iron. Iron is definitely more
rigid and stiff than air so it has a much larger
bulk modulus than air. This would tend to make
sound waves travel faster through iron than it does through air. But iron also has a much
higher density than air, which would tend to make sound waves travel slower through it. So which is it? Does sound travel faster
though iron or slower? Well it turns out that the
higher stiffness of iron more than compensates
for the increased density and the speed of sound through iron is about 14 times faster than through air. This means that if you were to place one ear on a railroad
track and someone far away struck the same railroad
track with a hammer, you should hear the noise 14 times faster in the ear placed on the track compared to the ear just listening through the air. In fact, the larger bulk
modulus of more rigid materials usually compensates for
any larger densities. Because of this fact, the
speed of sound is almost always faster through solids
than it is through liquids and faster through liquids
than it is through gases because solids are more rigid than liquids and liquids are more rigid than gases. Density is important in
some aspects too though. For instance, if you heat
up the air that a sound wave is travelling through, the
density of the air decreases. This explains why sound travels
faster through hotter air compared to colder air. The speed of sound at 20 degrees Celsius is about 343 meters per second, but the speed of sound
at zero degrees Celsius is only about 331 meters per second. Remember, the only way to
change the speed of sound is to change the properties of
the medium it's travelling in and the speed of sound is
typically faster through solids than it is through liquids and faster through liquids
than it is through gases.