Sound Energy What is sound energy? How does it travel? How fast does it move? We'll try to answer these questions here. Sound waves are not like light. Here's how they are different: SOUND Sound waves require some kind of material to travel through. They can't move through a vacuum. Sound waves move much much slower. Their speed in air is about 330 metres per second. Sound waves are like heat conduction. No particles actually travel. LIGHT Light waves don't require a material to travel through. They can move through a vacuum. Light waves move very very fast. The speed of light is about 300,000 kilometres per second. Light waves are like little particles that actually move from place to place. Let's look at some sound waves to make it clear what we mean. Here we have someone who can hear the sound of the alarm clock. The sound waves are travelling from the clock to her ear. They are travelling through air. What is actually travelling here? What is it that actually moves from the clock to her ear? Surprisingly, nothing at all, except energy! No air molecules move very far. They just vibrate. What happens is this:      - the vibrating bell on the clock bumps some air molecules next to it.      - those molecules jostle the ones next to them.      - those molecules bump into their neighbours...      - they bump the ones next to them ...      ... and so on, across the room. If you could see the air molecules in the room between the clock and her ear, this is what you would see: Notice that the molecules aren't moving across the room. They just get bumped, and they bump the ones next to them. It's this bumping energy that moves across the room, not the air molecules! When the molecules next to the girl's ear get bumped, they bump her eardrum, and her brain interprets the pattern of vibrations on the eardrum as 'sound'. (Find out more about how this works on our Ear page). You can see now why sound can't travel through a vacuum. There aren't any particles in a vacuum to do the bumping! This also suggests that sound should travel faster in a solid or liquid, where the particles are closer together and can bump each other quicker. Can you think of an example where you could see that this is true? Sound travelling through air is just like 'the wave' that hockey spectators do in a big arena. The wave of people standing up and sitting down travels quickly around the building, but of course no people actually travel ... just the energy of their wave. You might also like to find out about sound in more detail, or about what causes an echo. Resources