What if sound couldn’t travel through air?

Clap your hands. Your friend across the room hears it.

How does the sound get to their ears? And what if the air could not carry it?

When you clap, a friend across the room hears it almost at once. But nothing visible flies from your hands to their ears. So how does the sound get there?

The answer is the air in between. In this lab you can send a clap through air, water, steel, or empty space with no air at all, and see whether the listener hears it and how long it takes to arrive.

Sound needs a medium: it is a mechanical wave, a pattern of compressions passed between neighboring particles. Its speed is set by the medium's stiffness and density, not by the loudness or pitch of the source.

This lab sends a single clap 340 m to a listener through dry air, fresh water, steel or a vacuum, and measures the arrival time. The what-if removes the medium entirely.

When you clap, you squash the air next to your hands.

That squash passes from bit to bit of air, all the way to your friend's ear.

When you clap, your hands squash the air right next to them. That squash pushes on the air beside it, which pushes on the next bit, and so on, like a push passed along a line of people. That traveling push is sound.

In the model, sound in air at 20 °C travels about 343 meters every second. A clap heard 340 meters away arrives in about one second.

In dry air treated as an ideal gas, the model uses c = 331.3 m/s × √(1 + T/273.15 °C), giving 331 m/s at 0 °C and 343 m/s at 20 °C. A clap therefore takes t = d/c ≈ 0.99 s to cover 340 m.

The particles themselves barely move; it is the compression that travels. Pitch does not change the speed; it sets the wavelength, λ = c/f: a 440 Hz note in 20 °C air has λ ≈ 0.78 m.

Now Olly takes all the air away. There is only empty space.

Olly claps. Nothing happens. The friend hears nothing at all!

Now choose empty space: no air, nothing at all between the clapper and the listener. Clap as hard as you like. In the model, the sound never arrives and the listener hears nothing.

But sound isn't only carried by air. Choose water and the clap arrives in about a quarter of a second, more than four times faster than in air. Through steel it arrives in under a tenth of a second, about 17 times faster.

With the vacuum selected, the model has no medium and no wave: speed and arrival time are undefined and the listener hears nothing, however loud the source.

Other media carry sound faster: 1481 m/s in fresh water at 20 °C (4.3 times air), covering 340 m in 0.23 s, and 5960 m/s for longitudinal waves in bulk steel (17.4 times air), arriving in 0.057 s.

Sound is a push that goes from one tiny bit to the next.

Empty space has no tiny bits to push. So the sound cannot go anywhere.

Sound needs something to travel through, because it is a push passed from particle to particle. Empty space has no particles, so there's nothing to carry the push.

Water and steel are much harder to squash than air, so they push back faster and pass the sound along more quickly. Our model only follows the arrival time of one clap; it ignores how loud the sound is, echoes and wind.

The speed of sound is c = √(K/ρ), where K measures the medium's resistance to compression. OpenStax explains that the more rigid, or less compressible, the medium, the faster the speed: liquids and solids are far stiffer than air, so c is larger despite their higher density. With no medium, no mechanical wave exists.

Model limits: straight-line travel with no echoes, wind or bending; loudness and fading are not modeled; seawater would be about 1500–1540 m/s rather than fresh water's 1481; real steel structures also carry slower transverse and surface waves; and particle motion is drawn hugely exaggerated.

Space is almost empty. There is no air to carry sound.

So space is very, very quiet.

Most of space is very nearly empty, with almost no particles in it. That's why scientists say there is no sound in most of space: nothing is there to carry the push.

But not every part of space is empty. Giant groups of galaxies, called galaxy clusters, hold huge amounts of hot gas around their galaxies. Astronomers using NASA's Chandra X-ray Observatory found real sound waves rippling through the gas around a black hole in the Perseus galaxy cluster.

NASA's Chandra team notes that the idea that there is no sound in space comes from the fact that most of space is essentially a vacuum, providing no medium. Galaxy clusters, by contrast, hold enough gas to carry sound waves.

Since 2003 the black hole in the Perseus cluster has been associated with sound: its pressure waves cause ripples in the cluster's hot gas that correspond to a note some 57 octaves below middle C, far below human hearing.