You Could Not Shout to Someone Standing on the Moon

Because there is no air there, and sound has nothing to travel through.

Sound is not a thing that flies across a gap. It is a disturbance passed along by the molecules of a material, each nudging the next. Take the molecules away and there is nothing left to pass the nudge — the vibration simply has nowhere to go.

Light faces no such problem, which is why an astronaut on the Moon can see perfectly well and hear nothing at all. This page covers the first part of the ICSE Class 8 Physics chapter on sound: how sound is produced, how it travels, the experiment that proves it needs a medium, and why its speed depends so strongly on the material.

Every sound is produced by a vibrating body. No vibration, no sound — without exception.

The vibration is often too fast or too small to see, but it can always be found, and exam questions frequently ask you to name it.

- Tabla or dholak — the stretched membrane vibrates. Sprinkle a few grains of rice on it and they jump when it is struck.
- Sitar, guitar or veena — the string vibrates. It becomes a blur while sounding.
- Flute or whistle — the air column inside vibrates. Nothing solid is moving.
- Temple bell or a steel plate — the metal body vibrates. Touch a struck bell and you feel it, and the sound stops at once.
- Human voice — the vocal cords in the throat vibrate. Place your fingers on your throat and hum to feel it.
- Loudspeaker — the diaphragm or cone vibrates.
- Mosquito or housefly — the wings vibrate, which is why the buzzing stops the instant it lands.

How to prove the vibration is essential. Strike a tuning fork and dip a prong into water: the water splashes. Touch the vibrating prong with your finger and the sound stops immediately, because the vibration has been damped.

A vibration is a to-and-fro motion about a fixed position. Each back-and-forth movement pushes on the surrounding air, and it is that push that becomes sound.

The boundary case worth knowing. Not every vibration is heard. The human ear responds only to a certain range of vibration rates; slower or faster vibrations are producing sound waves, but ones we cannot detect. So inaudible does not mean no sound — a distinction that matters when animals hear things we do not.

As a longitudinal wave made of alternating compressions and rarefactions.

Follow a vibrating prong of a tuning fork. Each time it swings forward, it squeezes the air just in front of it: the molecules are crowded closer and the pressure and density are higher than normal. That crowded region is a compression.

Each time the prong swings back, it leaves the air in front of it thinned out: the molecules are further apart and the pressure and density are lower than normal. That thinned region is a rarefaction.

As the prong keeps vibrating, compressions and rarefactions are produced one after another and travel outwards, each pushing the next region of air. A sound wave is therefore a travelling pattern of pressure changes.

Why it is called longitudinal. The air molecules vibrate back and forth along the same direction in which the wave is travelling. Compare a wave on a rope, where the rope moves across the direction of travel — that is a transverse wave. Sound in air is never transverse.

What travels and what does not. The energy and the pattern travel from the source to your ear. The air itself does not — each molecule only oscillates about its own position and hands the disturbance on. This is the commonest misconception in the chapter: shouting across a room does not blow air into the listener's ear.

A useful model. A row of people standing shoulder to shoulder: push the first and a shove passes down the line to the last, while every person ends up where they began. That shove is a compression travelling along the row.

And a slinky spring shows both kinds. Push one end along its length and a compression runs down it — that is sound's behaviour. Flick it sideways instead and you get a transverse wave, which sound in air cannot do.

That sound needs a material medium, and cannot travel through vacuum.

The arrangement. An electric bell is hung inside a thick glass bell jar standing on the plate of a vacuum pump, with its wires led out through a sealed opening. The bell can be rung from outside at any time.

What happens as the air is removed.

- With the jar full of air, the ringing is heard clearly.
- As the pump gradually removes the air, the sound grows fainter and fainter.
- When almost all the air is gone, the sound becomes inaudible.
- Yet the hammer is still seen striking the gong throughout, so the bell has never stopped working.
- Let the air back in and the sound returns, growing louder as the jar refills.

Why this is conclusive. Two things could have explained the silence — the bell stopping, or the sound failing to travel. Being able to see the hammer still moving rules out the first. The only remaining explanation is that the sound has no medium to travel through.

Why the sound never quite vanishes in practice. A real pump cannot remove every last molecule, and a little sound also travels through the wires and the stand by conduction through solids. A carefully built version hangs the bell on soft insulating supports to reduce that leak.

Sound travels through solids and liquids too, not just air. Tap one end of a long iron railing and a friend with an ear to the other end hears it clearly. Two stones knocked together under water are plainly heard by a swimmer with their head submerged — which is how many fish and whales communicate.

So the requirement is a material medium of any kind. Only the complete absence of matter silences sound.

Because the closer and more firmly bound the molecules are, the faster each can pass the disturbance to the next.

Approximate speeds of sound, all at ordinary temperatures:

- Air — about $332\text{m/s}$
- Water — about $1500\text{m/s}$
- Steel — about $5000\text{m/s}$

So the order is solids > liquids > gases.

The reason, in terms of the molecular model. In a solid the molecules are tightly packed and strongly bound, so a push is handed on almost immediately. In a liquid they are further apart and less firmly held, so the relay is slower. In a gas they are very far apart and barely interact, so a molecule must travel some distance before it meets another to push — and the wave crawls by comparison.

Worked example — how long sound takes to cross one kilometre.

In air:

$$t=\frac{d}{v}=\frac{1000}{332}\approx 3.0\text{s}$$

In water:

$$t=\frac{1000}{1500}\approx 0.67\text{s}$$

In steel:

$$t=\frac{1000}{5000}=0.20\text{s}$$

So the same kilometre takes about fifteen times longer through air than through steel:

$$\frac{5000}{332}\approx 15$$

Compare that with light. Light covers the same kilometre in

$$t=\frac{1000}{3\times 10^8}\approx 3.3\times 10^{-6}\text{s}$$

which is a few millionths of a second — about a million times quicker than sound in air.

That enormous gap is why lightning is seen well before thunder is heard, even though both begin at the same instant. The flash arrives almost immediately; the sound takes roughly three seconds for every kilometre it has to cover.

And sound travels faster in warm air. Raising the temperature speeds up the air molecules, so they pass the disturbance on sooner. This is why sound carries slightly differently on a hot afternoon than on a cold night — a point that separates sound from light, whose speed in air does not depend on temperature in any way you would notice.