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2. Speed of Sound and Density
Sound waves can travel through other materials besides air. The speed
in which sound travels in a given highly depends on the density of the
medium. In fact, sound is usually fastest in solids where its atoms or
molecules are closest together, and slowest in gases where molecules are
farthest apart.

3. Speed and Elasticity
The elasticity of a material also affects the speed of sound. The elastic
property of a material refers to its tendency to maintain its shape and not
deform when a force is applied to it. Rigid materials such as steel will
experience a smaller deformation than rubber when a force is applied to it
because rubber is more flexible.
Elastic materials vibrate at lower speeds because their particles return to
their original positions at lower rates. Thus, sound travels very slowly in elastic
materials than rigid materials. In fact, these types of materials, such as rubber,
can be used for soundproofing.

4. Speed and Temperature
Sound travels faster if it is hotter. As a substance heats up, its molecules
move faster, so they collide more frequently. The more frequent the collisions are
the faster the speed of sound is in material.
The speed of sound in an ideal gas v, for example, in a given temperature
T in Kelvin is given by:
v = vo
𝑇
𝑇𝑜
(Equation 1)
where vo is the speed of sound at absolute temperature To.

5. For air, the speed of sound in air can be approximated using the
equation:
v = 331 m/s + (0.606
𝑚/𝑠
°𝐶
) Tc (Equation 2)
where Tc is the temperature of the air in °C. For every 1 °C rise in
temperature, the speed of the air increases by 0.606 m/s.
Other factors such as humidity and pressure, can also affect the
speed of sound in air.

6. Example 1.
What is the speed of sound in helium at 20°C? The speed of
sound in helium at 0°C is 972 m/s.
Given:
Tc = 20.0°C
Solution:
Using the speed of sound in helium at 0°C, you have:
v = vo
𝑇
𝑇𝑜
= = 972 m/s
20+273
0+273
= 1,010 m/s
Answer: Sound travels in helium at 20°C at a speed of 1, 010 m/s.

7. Example 2.
A thunder was heard by Frandon exactly 5.00 seconds after a
lightning flashed. Assuming that the temperature of the air was equal to
26.0°C, how far was the lightning?
Given:
Tc = 26.0°C
T = 5.00 s
Solution:
To determine how far the source of sound is, you need to get first
the speed of sound at 26.0°C.

8. v = 331 m/s + 0.606
𝑚/𝑠
°𝐶
Tc
v = 331 m/s + 0.606
𝑚/𝑠
°𝐶
26.0°C
v = 331 m/s + 15.756 m/s
= 347 m/s
Thus, the distance of the lightning from Frandon is:
d = vt
= 347 m/s x 5.00 s
= 1,740 m
Answer: Frandon is 1,740 m away from the lightning.

9. Thank You!