Understand that sound waves require a material medium (solid, liquid or gas) to travel, and that they cannot propagate in a vacuum.
Why a Medium Is Required
Sound is a mechanical longitudinal wave. It consists of alternating compressions and rarefactions of particles in a material. The particles themselves do not travel with the wave; they only oscillate about their equilibrium positions, passing the disturbance from one particle to the next.
Compression – particles are pushed together, increasing pressure.
Rarefaction – particles are pulled apart, decreasing pressure.
Because the disturbance is transferred by particle interaction, a medium must be present.
What Happens in a \cdot acuum?
In a vacuum there are no particles to interact, so no compressions or rarefactions can be produced. Consequently, sound cannot travel.
Examples of Media
Solids – particles are tightly packed; sound travels fastest.
Liquids – particles are less tightly packed than in solids; speed is lower than in solids but higher than in gases.
Gases – particles are far apart; sound travels slowest.
Speed of Sound in Different Media
Medium
Typical Speed of Sound
Air (at 20 °C)
≈ \$340\ \text{m s}^{-1}\$
Water (at 20 °C)
≈ \$1480\ \text{m s}^{-1}\$
Steel
≈ \$5000\ \text{m s}^{-1}\$
Key Points to Remember
Sound is a mechanical wave → needs a material medium.
It cannot travel through a vacuum (e.g., space).
The speed of sound depends on the medium’s elasticity and density; generally faster in solids, slower in gases.
All everyday sounds we hear on Earth travel through air, but some sounds (e.g., sonar) use water, and some engineering applications use solids.
Suggested diagram: Longitudinal sound wave showing compressions and rarefactions in (a) a solid rod, (b) water, and (c) air, with an illustration of a vacuum where no wave propagates.