Sound is a longitudinal wave

We’ve all heard that there’s no sound in space, even though tons of movies and TV shows have you hearing everything from ships’ engines to explosions. If there’s sound on Earth, why wouldn’t there be sound in space? The secret is in how sound travels.

Sound travels as waves of energy, but, unlike light, the waves transmit energy by changing the motion of particles. Let’s say you clap your hands together. The molecules of air between your hands are squished out, those particles hit the particles outside your hands and push them outward, those particles hit the particles next to them, and so on until the particles next to your eardrums get hit. You can think of it like a tiny air explosion that travels outward around the thing that made the sound.

Don’t forget that once a moving particle has hit its neighbour, it slows down, or stops. The energy that the particle was using to move was passed to the neighbouring particle, that then passes it to its neighbour and so on. That means that each individual particle is only moving a short distance. It’s the energy from your clap that travels all the way to your ears, not the particles themselves.

It’s also important to remember that the particles are moving in the same direction as the wave is moving. If you drop a marble into a bucket of water, you will be able to see the ripples that are produced making humps and dips in the water. This is because the water particles are moving up and down while the wave travels outward from the spot where the marble hit. In sound waves, the particles move in the same direction as the wave is moving, so you wouldn’t be able to see them like ripples in the air, instead they make areas in the air where the particles are more squished together, and areas where the particles are further apart.

A standing wave is a wave that doesn’t move. You might ask, “isn’t the point of a wave that it moves energy around?” Well, yes, but what if the wave is trapped? When you had a bath when you were young, maybe you moved your body back and forth in the tub and sloshed the water, and it sloshed higher and higher with every pass that you made. The water wave was trapped in the bathtub, bouncing back and forth against the ends of the tub.

The same principle holds when we’re talking about musical wind instruments, for example, a flute. The air is trapped in the tube of the instrument, and starts to slosh back and forth. Usually though, there’s more than one wave in the flute at the same time. They slosh against each other, bounce off one another, and go back the way they came. They keep bouncing back and forth off each other for as long as the person is blowing across the flute’s opening.

For the waves to keep moving back and forth, something needs to push them. In the case of the flute, the person blowing across one end of it pushes the first bit of air, which then pushes the next bit, and so on. The person blowing is providing an area of constant pressure. That means that the particles there don’t get squished together and pulled apart, they move back and forth as a group. The particles on either side of that group do get squished together and pulled apart, as the constant pressure particles move back and forth, but they don’t move back and forth at all. The two kinds of groups of particles alternate all the way down the tube of the flute, so we have some particles that move back and forth, and some particles that squish together and pull apart. Of course, the two groups do have some stragglers in between them that don’t quite know what to do, so they do a bit of both.

We can show sound waves by graphing either how particles move (displacement) or how squished together they are (density). But since density and pressure are related, a pressure vs time graph has the same form as a density vs time graph.

If you’re looking at particle displacement, then your graph will represent how far the particles are from their normal places, and which direction they have moved. In a particle displacement graph, the particles that don’t move at all will always show up on the zero line. These are the same particles that get squished and pulled apart the most.

If you’re looking at pressure, then your graph will show you where the particles are really squished together, and where they are really far apart. In a pressure graph, the particles that never get squished together or pulled apart will always be on the zero line. These are the particles that move back and forth the most.

So you can see from comparing the two graphs that the places where the particle displacement is highest and where it is lowest (in the negative) is where the pressure graph is zero, and vice versa.

Sound waves can only travel in space if there are enough particles around to transmit the energy in the wave from the source to the listener.

If you talk under water, it sounds funny because the water is carrying the sound wave instead of air. Water is a liquid and air is a gas, so water is much denser than air, and the particles are not as free to move as air particles are.

In this video you can see (at around 16 s) an area of high particle density moving through the air looking like a line of cloud. The eruption itself takes place at 12 s in, but the sound doesn’t reach the microphone on the camera and the people until 25 s into the video. The sound took 13 s to travel through the air, one group of particles hitting another, to get to the air particles that were near the spectators so that they could hear the sound.

And just for fun, you can figure out how far away the people on the boat were by multiplying 13 seconds by 340 m/s (the speed of sound). Turns out they were 4.4 km (or 2.7 miles) away from the volcano!