Sound needs a medium

1. A sound wave is a longitudinal wave made when a vibrating object pushes and pulls on the particles around it.
2. Each particle passes the vibration on to its neighbours, so sound can only travel where there are particles to carry it.
3. The material a wave travels through is its medium, and sound can travel through solids, liquids and gases.
4. Sound cannot travel through a vacuum, because there are no particles to pass the vibrations on.
5. An electric bell ringing inside a sealed glass jar goes quiet as the air is pumped out, even though you can still see the hammer striking the bell.

Speed of sound in different media

1. The speed of sound depends on the medium, and it is usually fastest in solids, slower in liquids and slowest in gases.
2. Typical values are about $340m/s$ in air, about $1500m/s$ in water and about $6000m/s$ in steel.
3. In a solid, the particles are packed closely together and held by strong forces, so each vibration is passed to the next particle very quickly.
4. In a gas, the particles are far apart and only pass on a vibration when they collide, so the disturbance travels much more slowly.
5. The speed of sound in a gas also increases with temperature, because faster-moving particles pass the vibration on sooner.

Crossing a boundary

1. When sound passes from one medium into another, its speed changes, because the new medium passes vibrations along at a different rate.
2. The frequency does not change, because the particles at the boundary are forced to vibrate at the same rate as the particles hitting them.
3. Every vibration that reaches the boundary in one second is passed on in that same second, so the number of waves per second is the same on both sides.
4. Since $v=f\lambda$ and $f$ stays fixed, the wavelength must change in the same proportion as the speed.
5. If the sound speeds up as it enters the new medium, its wavelength increases, so the compressions spread further apart.
6. If the sound slows down as it enters the new medium, its wavelength decreases, so the compressions crowd closer together.
7. For the same frequency, $\frac{{\lambda }_2}{{\lambda }_1}=\frac{v_2}{v_1}$, so tripling the speed triples the wavelength.
8. A swimmer under water hears a whistle from the poolside as the same note, because the frequency is unchanged, even though its wavelength in the water is more than four times longer.