Propagation of Sound in a Medium
Simple Explanation
Sound is a mechanical wave — a traveling disturbance of pressure — that needs a medium (a solid, liquid, or gas) to travel through, unlike light. It travels fastest through solids, slower through liquids, and slowest through gases, because particles that are packed closer together transmit the disturbance more quickly.
Why Do We Need It?
Understanding that sound needs a medium — and travels at different speeds through different ones — is the foundation for everything else in this chapter, from calculating echo distances to explaining the Doppler effect.
Formula
Speed of Sound in Air
v = 331 + 0.6T
The speed of sound in air increases with temperature — warmer air molecules move faster and transmit the pressure disturbance of a sound wave more quickly.
- v
- — speed of sound in air, in metres per second (m/s)
- T
- — air temperature, in degrees Celsius (°C)
When to use it: Whenever the speed of sound in air at a given temperature needs to be estimated (an approximation valid near everyday temperatures).
Worked Example
Find the speed of sound in air at a given temperature
Find the speed of sound in air at 20°C.
Why Does This Work?
Sound travels by particles bumping into their neighbors, passing the disturbance along — in warmer air, particles move faster on average and collide more often, so the pressure disturbance propagates through the medium more quickly.
Real-Life Example
Why sound is louder and clearer through a solid wall than through air
Pressing an ear directly against a wall lets you hear sounds from the other side of the wall more clearly than through the open air.
Solids have their particles packed tightly together, transmitting sound vibrations far more efficiently and quickly than the widely-spaced particles of air — exactly why solids conduct sound fastest.
Practice
Find the speed of sound in air at 30°C.
MediumCommon mistake
Assuming sound can travel through a vacuum like light does — sound absolutely requires a medium (solid, liquid, or gas) with particles to pass the disturbance along; in a vacuum, there is nothing to carry it.
Quick Review
- v = 331+0.6T for sound in air (T in °C).
- Sound needs a medium — it cannot travel through a vacuum.
- Fastest in solids, slower in liquids, slowest in gases.