Question
How does sound travel through solids, liquids, and gases? Compare the speed of sound in each medium and explain why the speed differs.
Solution — Step by Step
Sound is a longitudinal mechanical wave — it travels by causing compressions and rarefactions (alternating regions of higher and lower pressure) in the medium. The particles of the medium vibrate to and fro in the direction of wave propagation.
Because sound requires particles to vibrate and transmit the disturbance, it cannot travel through a vacuum (no particles, no sound). This is why you cannot hear an explosion on the Moon.
In a gas, molecules are far apart with weak intermolecular forces. When one molecule is compressed and pushed into the next, it takes more time to transmit the disturbance because:
- Molecules are separated by large distances
- Intermolecular forces are weak (low restoring force)
Speed in air (at 0°C): approximately 332 m/s Speed in air (at 25°C): approximately 346 m/s
Temperature effect: (where is in Kelvin). Higher temperature → faster molecular motion → faster sound propagation.
Speed formula in ideal gas:
where = adiabatic index, = gas constant, = temperature, = molar mass.
In a liquid, molecules are closer together and intermolecular forces are stronger than in gases. Compressions transmit more quickly. Sound travels faster in liquids than in gases.
Speed in water (at 25°C): approximately 1500 m/s — about 4–5 times faster than in air.
Speed formula in liquids:
where = bulk modulus of elasticity (resistance to compression), = density.
In a solid, atoms/molecules are tightly packed in a rigid lattice with very strong intermolecular forces. A compression at one end is transmitted almost instantly to neighbouring atoms. Sound travels fastest in solids.
Speed in iron: approximately 5100 m/s — about 15 times faster than in air. Speed in steel: approximately 5960 m/s
Speed formula in solids:
where = Young’s modulus (stiffness), = density.
| Medium | Typical Speed | Example |
|---|---|---|
| Air (25°C) | 346 m/s | Sound in air |
| Water (25°C) | ~1500 m/s | Underwater sound |
| Iron | ~5100 m/s | Metal structures |
The speed difference explains why you can hear an approaching train by putting your ear to the rail long before you hear it in the air — sound reaches you through the solid rail much faster.
Why This Works
The speed of a wave depends on two properties of the medium:
- Elasticity (restoring force) — how strongly the medium “snaps back” after being disturbed
- Density (inertia) — how much mass resists the motion
Solids are both stiffer (higher elastic modulus) and denser than gases, but the increase in stiffness outweighs the increase in density, so solids transmit sound faster. Liquids fall between.
Alternative Method
A simple analogy: imagine passing a push through a line of people. If people are far apart (gas), the push takes time to reach the next person. If they’re standing shoulder to shoulder (solid) and the line is rigid, the push transmits almost instantly.
Common Mistake
Students sometimes write “sound travels fastest in vacuum because there’s nothing to slow it down.” This is completely wrong — sound cannot travel in vacuum at all because it needs a medium (particles) to propagate. Electromagnetic waves (light) can travel in vacuum; sound waves cannot.
CBSE Class 9 frequently asks: “Why does sound travel faster in hot air than cold air?” Answer: in hot air, molecules have more kinetic energy and move faster, so they transmit compressions more quickly. Temperature and speed of sound are directly related: .