Waves and Sound · Grade 9

Why does a ringing bell fall silent when the air is pumped out of the jar?

Because there is nothing left for the sound to move through. The hammer still strikes and the bell still vibrates — you can see it — but with no air molecules there is nothing to be squashed and stretched, so no wave forms at all. Let the air back in and the sound returns. This is the experiment that proves sound needs a medium.

Learning objectives

From source to ear

Every sound begins with something vibrating: a string, a drum skin, the vocal folds in a throat, the cone of a loudspeaker. There is no sound without vibration, and that is the starting point of the whole topic.

As the cone moves forward it squashes the air molecules ahead of it, making a region of slightly raised pressure — a compression. As it moves back it leaves a thinner region behind it — a rarefaction.

Compressions and rarefactions travel outwards one after another and reach the eardrum, which begins to vibrate at exactly the same frequency. The air itself barely goes anywhere: each molecule nudges its neighbour and returns to where it started.

The speed depends on the medium

In air at room temperature sound travels at about 340 metres per second. In water it manages about 1,500, and in steel about 5,000 — nearly fifteen times the speed in air.

That order is not an accident. The more strongly and closely the particles are bound to each other, the faster a disturbance passes between them. In a solid the particles are touching; in a gas they are far apart and the handover is slow.

Temperature matters too. In warm air the molecules move faster and pass the disturbance on more quickly, so the speed of sound on a summer day in Israel is several tens of metres per second higher than on a winter night.

Lightning and thunder

Light travels at about 300 million metres per second, so the flash reaches you instantly for any practical purpose. Sound manages only about 340 metres per second.

The gap between the two is a measuring instrument. Count the seconds from the flash to the thunder and multiply by 340 to get the distance in metres.

A convenient rule of thumb: every three seconds is roughly one kilometre. A nine-second gap means a storm about three kilometres away, and a gap that keeps shrinking means it is coming towards you.

Worked examples

  1. Six seconds pass between a lightning flash and the thunder. How far away was it?

    1. The speed of sound is about 340 metres per second
    2. Distance = speed times time
    3. 340 times 6

    Answer: About 2,040 metres, roughly two kilometres

  2. Put air, water and steel in order of the speed of sound in them.

    1. Sound is faster when particles are close and strongly bound
    2. In a gas the particles are far apart
    3. In a solid they are touching and bound

    Answer: Air slowest, then water, with steel fastest

  3. What do you see and hear as air is pumped from a jar containing a ringing bell?

    1. The hammer keeps striking and the vibration stays visible
    2. The number of air molecules falls
    3. There are no particles left to form compressions and rarefactions

    Answer: You still see the bell vibrating while the sound fades to nothing

Common mistakes

Thinking the air travels from the speaker to the ear
Each molecule nudges its neighbour and returns to its place. What crosses the room is a pattern of pressure, not a flow of air — otherwise every concert would feel like standing in a wind.
Assuming sound is faster in a gas than in a solid
It is the other way round. In a solid the particles are in contact and pass the disturbance on immediately, which is why sound in steel is about fifteen times faster than in air.
Allowing for the travel time of the light
Light covers a kilometre in about three millionths of a second. It can be ignored completely, and trying to include it only complicates the working without changing the answer.

What to remember

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