Nothing actually travels

Start somewhere that looks like a detour and is not.

When you sing, and someone across the room hears you, it feels obvious what happened. Something went from you to them. Air, maybe. Your voice, carried over.

That is not what happens. Nothing goes from you to them.

The air molecules in front of your mouth get shoved forward a tiny bit. They bump the molecules next to them and bounce back to about where they started. Those molecules bump the next ones and bounce back. And so on, all the way across the room. Every molecule ends up roughly where it began. What crosses the room is not the air. It is the pattern of the bumping.

Physics has a name for this kind of wave. Sound is a longitudinal wave (one where the material moves back and forth along the same line the wave is travelling, rather than side to side across it).¹ Shove one end of a stretched-out Slinky and you can watch the whole thing happen slowly enough to follow. A squeeze of tightly packed coils runs down the length of the toy. Every coil just jiggles a little and stays put. The squeeze is the wave. The coils are the room.

Where the molecules crowd together, the pressure goes slightly up. That is a compression. Where they thin out behind the crowd, the pressure goes slightly down. That is a rarefaction. A sound wave is nothing but compressions and rarefactions chasing each other outward. That is why it is also called a pressure wave.¹ Your eardrum does not answer to wind. It answers to a tiny, fast wobble in pressure, riding on top of the plain weight of the air.

You have already seen this idea at a much bigger scale. A wave in the ocean travels for thousands of miles. The water does not. Each parcel of water goes up, forward, down, and back. It traces a little circle and ends up almost right where it started.² That is why a duck sitting on the water bobs up and down as a wave passes instead of getting carried off to Portugal. The wave carries energy across an ocean. It does not carry the ocean.

Now take it somewhere with no water and no air in it at all. That is where the idea gets sharp.

Sit in stopped traffic on a highway. Up ahead, one driver taps the brakes. The car behind brakes a half-second later, then the next, then the next. A band of stopped cars forms. It travels backwards down the road toward you at maybe fifteen miles an hour. Every car in it is either stopped or moving forwards. The jam is real. You can measure how fast it moves. You can work out where it will be in ten minutes. And it is made of nothing. There is no object called a traffic jam. There is only a pattern in the spacing of cars, moving the other way from the cars themselves.

That is the whole trick. It is worth holding onto, because voice is that trick used on a body.

How fast, and what the numbers mean

Sound in air moves at about 343 metres per second at 20 °C. That is roughly a kilometre every three seconds.³ That is why you count between the lightning and the thunder. Light gets to you more or less at once. So the delay is pure sound-travel time. Three seconds of counting is about a kilometre of distance. Fifteen seconds and the storm is a long way off.

The speed depends on the temperature and on what the gas is made of. It does not depend on how loud the sound is. A shout and a whisper arrive at the same moment. This matters more than it sounds like it does. It comes back later. It is the reason a lungful of helium does something odd to your voice.

Two numbers describe any pattern like this. Neither of them is a substance.

Frequency is how many compressions pass a fixed point each second, measured in hertz. That is what you hear as pitch. When you sing an A above middle C, 440 compressions leave your mouth every second.

Amplitude is how big the pressure swings are: how far the crowding and thinning stray from the normal pressure of the room. That is what you hear as loudness.

Both belong to a pattern. Neither belongs to the air. You can take the same air and press a whole new pattern into it. You do it all the time, without adding or removing a thing.

Why this is the load-bearing idea

Almost everything people believe about voice quietly assumes that voice is a substance. That you produce it, that it comes out, that some people were handed more of it, that it can be used up. The language is built that way and it drags the gut sense along behind it.

The physics says something else. The thing that matters is usually a pattern, not a substance. Your voice is not a thing you make and then send. It is a pattern you press into something that was already there, filling the room, doing nothing.

Once you take that in, several things stop being a mystery.

It explains why a voice can be huge without being forceful. You are not throwing air. You are setting up a wobble in air that is already there. How well you set it up matters far more than the effort behind it. Trained singers are not blowing harder than you. Most of them are blowing less hard. They shape the pattern better.

It explains why sound goes around corners and through doors. A substance would need a path. A pattern just needs a medium that keeps touching itself. The air in your hallway does.

It explains why a recording works at all. A microphone does not capture your voice. It captures the pressure pattern and turns it into a matching pattern of voltage. Later a speaker pushes that pattern back into different air, in a different room, years on. Nothing of the original crossed over. The pattern was the whole content. Copying the pattern copied everything.

And it sets up the question the rest of this piece is about. If voice is a pattern rather than a substance, then getting better at voice cannot mean making more of something. It has to mean shaping the pattern more exactly. Which raises the next question: what, physically, does the shaping?

References