How Sound Is Made and Travels: Quick Revision

A brief note on How Sound Is Made and Travels. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Every sound is made by a vibration.
• Sound needs a medium: it travels through solids, liquids and gases, but not through a vacuum.
• A sound wave is a train of compressions (high pressure) and rarefactions (low pressure).
• Sound is longitudinal: the particles vibrate along the direction of travel; only the disturbance moves on.
• Speed $v = \dfrac{d}{t}$: about $340\ \text{m/s}$ in air, $1500\ \text{m/s}$ in water, $5100\ \text{m/s}$ in steel.
• Warmer air carries sound faster: $v \approx (331 + 0.6T)\ \text{m/s}$.

The facts at a glance

A worked example

Two friends stand at the ends of an iron pipe $765\ \text{m}$ long. One taps the pipe. The other hears two taps. Sound goes at $5100\ \text{m/s}$ in the iron and $340\ \text{m/s}$ in the air. Find the gap between the taps.

1. Iron. $t = \dfrac{765\ \text{m}}{5100\ \text{m/s}} = 0.15\ \text{s}$.
2. Air. $t = \dfrac{765\ \text{m}}{340\ \text{m/s}} = 2.25\ \text{s}$.
3. Gap. $2.25\ \text{s} - 0.15\ \text{s} = 2.1\ \text{s}$.

Practice with new numbers

A program to try

The program works out the two taps of the worked example.

Mistakes to avoid

• 'Sound can travel through empty space.' It needs a medium.
• 'The air travels from the source to the ear.' Only the disturbance travels.
• Halving the thunder distance. Thunder comes one way: $d = vt$. Only an echo is halved.
• Using $340\ \text{m/s}$ in water or steel. Each medium has its own speed.

Check yourself

1. Can sound travel through a vacuum?

2. Thunder is heard $9.5\ \text{s}$ after the flash. How far is the storm?

3. Is the pressure higher at a compression or at a rarefaction?

4. Why do we hear a train sooner by putting an ear to the rail?