Sound reaches you only if there are particles to hand it along

1. A vibrating source sets up a longitudinal disturbance in whatever material is touching it.
2. Each particle that is pushed forward presses on the particles next to it, those particles press on their own neighbours, and in this way the compression is passed along the material.
   1. The particles only shuffle to and fro about their own positions, so what travels from the source to your ear is energy, not matter.
3. The material that does this passing on is called the medium, and it can be a solid, a liquid or a gas.
4. Sound cannot travel through a vacuum, because there are no particles there to be squeezed together and none to press on a neighbour.
   1. Take the particles away and the chain of pushes has nothing to run along, so the wave is never carried at all.
5. Two astronauts standing on the surface of the Moon cannot hear each other shout, because there is almost no gas around them, so they speak by radio instead.
6. Light and radio waves are electromagnetic waves, which need no medium, and that is why you can see the Sun across empty space but never hear it.

In the bell jar demonstration the ringing fades as the air is pumped out

1. An electric bell is hung on threads inside a thick glass bell jar that is sealed onto a base plate, with its wires taken out through the seal to a supply so that it keeps ringing throughout.
2. While there is air in the jar the bell is clearly heard: the vibrating gong pushes on the air inside, that air passes the vibration to the glass, and the glass passes it to the air outside and on to your ear.
3. A vacuum pump joined to the base plate then slowly draws the air out of the jar.
4. As the number of air particles inside falls there are fewer and fewer particles to hand the vibration on, so the ringing grows steadily quieter and fades away to nothing.
   1. Through the glass you can still see the hammer striking the gong, so the bell has not been switched off and is not broken; what has been removed is the path for the sound.
5. Letting the air back in brings the ringing back at once, which shows that the gas was doing the carrying.
6. The bell is hung on threads or stands on a soft pad so that its vibrations cannot leak out through the base plate as a solid path, which would still let sound reach you after the jar had been emptied.

Sound travels through air at roughly $340m/s$

1. In air the value is approximately $330m/s$ to $350m/s$, and $340m/s$ is the figure normally used in calculations.
   1. A speed of $340m/s$ means the wave has moved $340m$ away from the source after $1s$.
2. A range is quoted rather than one number because the speed rises with the temperature of the air.
   1. Particles in warmer air are already moving faster, so each one reaches its neighbour sooner and the disturbance is handed on more quickly: nearer $350m/s$ on a hot afternoon, nearer $330m/s$ on a cold night.
3. Light travels at $3.0\times 10^{8}m/s$, close to a million times faster than sound in air.
4. That gap is why a distant lightning flash arrives at your eyes with no delay you could notice, while its thunder needs about $3s$ for every kilometre it has to cross.
   1. For the same reason a firework seen from far off flashes silently and bangs a second or two later, even though the flash and the bang began together.

Closer particles with stronger forces between them pass the disturbance on faster

1. The speed of a sound wave is set by how quickly one particle can make the next particle move.
2. The closer together the particles are, and the stronger the forces between them, the sooner a neighbour feels the push and the faster the wave advances.
3. In a gas the particles are far apart and only affect each other when they collide, so a particle travels a long way before it passes anything on, and sound in air manages only about $340m/s$.
4. In a liquid the particles are already touching, with moderately strong forces between them, and sound in water reaches about $1500m/s$, over four times the speed in air.
5. In a solid the particles are packed close and held by strong bonds, so a vibration is handed on almost immediately, and sound in steel reaches around $5000m/s$.
6. In general, then, sound travels faster in solids than in liquids, and faster in liquids than in gases.
   1. The claim is a general one because the exact speed belongs to the particular material, so $1500m/s$ in water and $5000m/s$ in steel are indicative values rather than fixed constants for all liquids and all solids.
7. Press your ear to a long metal railing while a friend taps the far end, and the tap arrives through the metal a moment before the same tap reaches you through the air.

Crossing into a new material changes the speed and the wavelength but not the frequency

1. Wave speed, frequency and wavelength are tied together by the wave equation.

   $$v=f\lambda$$

2. The frequency is fixed by the source, because every particle along the path is driven to and fro once for each vibration the source makes, so the number of vibrations per second cannot change part way along.

3. When a sound crosses from air into water the speed rises from about $340m/s$ to about $1500m/s$, so with $f$ unchanged the wavelength must rise in the same proportion.

   1. A $500Hz$ note has a wavelength of $\frac{340}{500}=0.68m$ in air and $\frac{1500}{500}=3.0m$ in water, while its frequency stays at $500Hz$ in both.

4. The same argument runs the other way, so a sound passing into a slower material has its wavelength squeezed shorter while its frequency holds steady.