KS3 · Physics

Sound vibrations

Every sound you have heard started the same way: something moving back and forth, often too small and fast to see. Change that movement, and you change what you hear.

One vibration, two dials

Setting 1

Setting: 1. complete vibrations in this time: 2

This trace follows one vibrating object over time — how far it is from its resting position at each moment. The height of each swing is the amplitude: how far the object moves from rest. How many swings fit into the same time shows the frequency: the number of vibrations every second, measured in hertz (Hz). Step through settings 1 to 4 — each step changes only one of them. Which one? (The axis numbers are just a scale for comparing, not real measurements.)

Watch out: This is a picture of one object moving back and forth over time. It is not a picture of the sound travelling through the air.

Predict, then check

Every trace above starts with something vibrating. But a humming tuning fork looks perfectly still. Is it really moving?

A tuning fork has just been struck and is humming. A table tennis ball hangs on a thread. You gently touch the ball against one prong of the fork. What happens to the ball?

Where is the vibration hiding?

You can hear a trumpet playing, a friend talking, and two stones being knocked together. But you can't see anything shaking in any of them.

Which is closest to what you think right now?

Some sounds, like a voice, are made without anything vibrating at all.In a trumpet, it's the metal tube that vibrates. The air inside doesn't do anything.Stones are far too hard to vibrate. The click comes just from them hitting.It's the hit that makes a sound, so hitting anything will make a noise.Every one of these sounds comes from something vibrating, even if it's air I can't see.

How sure are you?

I'm sureI think soI'm guessing

Some sounds, like a voice, are made without anything vibrating at all.

This is a common misconception

Your voice does rely on a vibration — you just can't see it. Your voice-box (the larynx) starts the air in your throat, mouth and nose vibrating. Whenever there is a sound, something is vibrating. Sometimes that something is air.

In a trumpet, it's the metal tube that vibrates. The air inside doesn't do anything.

This is a common misconception

It's the other way round. In wind and brass instruments, the air inside the tube vibrates. The thing that looks most solid isn't always the thing making the sound.

Stones are far too hard to vibrate. The click comes just from them hitting.

This is a common misconception

Hard objects vibrate too. When two stones are knocked together they vibrate briefly, and that short vibration is the click you hear.

It's the hit that makes a sound, so hitting anything will make a noise.

This is a common misconception

Try hitting a pile of soft cloth: you'll hear very little. Soft objects like cloth don't vibrate much, so they make little sound. It's the vibration that makes the sound, not the hit itself.

Every one of these sounds comes from something vibrating, even if it's air I can't see.

This matches the evidence

Exactly. In the trumpet, the air in the tube vibrates. When your friend talks, their voice-box sets the air in their throat, mouth and nose vibrating. The knocked stones vibrate briefly. In a loudspeaker, a part called the cone vibrates. The vibrating thing is often hidden, but it is always there.

Whenever there is a sound, something is vibrating. If you can't see it, look for the air.

Explain it like a physicist

A drummer wants to make their drum sound louder. Later, they want a different drum that gives a higher-pitched sound. Explain what they could do in each case, and why it works. Use the words amplitude and frequency. [4 marks]

Your answer

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The mark scheme

To make it louder: hit the drum harder. (1 mark)
This makes the drum skin vibrate with a larger amplitude — it moves further from its resting position — so the sound is louder. (1 mark)
For a higher pitch: use a smaller drum, or one with a tighter skin. (1 mark)
This vibrates at a higher frequency — more vibrations every second — so the pitch is higher. (1 mark)

Model answer

To make the sound louder, the drummer should hit the drum harder. The drum skin then vibrates with a larger amplitude, moving further from its resting position, so the sound is louder. For a higher-pitched sound, they should use a smaller drum or one with a tighter skin. That skin vibrates at a higher frequency, making more vibrations every second, so the pitch is higher.

A strong explanation has three links every time: what you do → what happens to the vibration → what you hear.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every sound starts with something moving back and forth — and two things about that movement decide what you hear.

What you need to know

All sounds are caused by vibrations. To vibrate is to move back and forth very quickly.
Amplitude is the size of a vibration: how far the object moves from its resting position. The greater the amplitude, the louder the sound.
Frequency is the number of vibrations every second, measured in hertz (Hz). The greater the frequency, the higher the pitch.
Volume (how loud or quiet) and pitch (how high or low) are separate: a sound can be loud and low at the same time.

The big picture

Every sound is made by something vibrating — moving back and forth very quickly. The size of the vibration, its amplitude, sets how loud the sound is. How many vibrations happen every second, its frequency (measured in hertz, Hz), sets how high or low the sound is. You can change one without changing the other.

Key points

1Whenever there is a sound, something is vibrating — even when you can't see it, as with the air inside a trumpet.

2Vibrations too small or too fast to see give themselves away by their effects, like a table tennis ball flicked by a humming tuning fork.

3Hitting, plucking, bowing or blowing harder makes larger-amplitude vibrations, so the sound is louder.

4A smaller drum, a tighter drum skin or a shorter column of air vibrates at a higher frequency, so the pitch is higher.

5Soft objects such as cloth don't vibrate much, so they make little sound.

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Worked example

Problem

Two traces show two different vibrating objects, drawn over the same length of time. In trace P, each swing reaches 3 squares from the resting line and 4 complete vibrations fit in. In trace Q, each swing reaches only 1 square from the resting line and 8 complete vibrations fit in. Which sound is louder? Which sound has the higher pitch?

⚠ Watch out

Mixing up "louder" and "higher". Higher describes pitch — how high or low a note is — not volume. Making a vibration bigger makes the sound louder; only making more vibrations every second makes it higher.

🧠

Memory hook

Amplitude is the Amount of swing: it sets how loud. Frequency is how Frequently it swings: it sets how high. And remember the lion's roar — loud but low — to prove they are two separate dials.

✓

Check yourself

Without looking back: what does the amplitude of a vibration tell you, what does its frequency tell you, and which one would you change to make a note higher?

Flashcards

(12)

What causes every sound?

Vibrations — something moving back and forth very quickly. Whenever there is a sound, something is vibrating.

What is the amplitude of a vibration?

Its size: how far the object moves from its resting position.

What does a larger amplitude do to a sound?

It makes the sound louder.

What is the frequency of a vibration?

How often the vibrations happen: the number of vibrations every second.

What unit is frequency measured in?

Hertz (Hz).

What does a higher frequency do to a sound?

It gives the sound a higher pitch.

Volume or pitch: which one is "how high or low" a sound is?

Pitch. Volume is how loud or quiet a sound is.

How can you show that a sounding tuning fork is vibrating?

Touch it against a hanging table tennis ball — the ball is flicked sideways.

In a trumpet, what vibrates to make the sound?

The air inside the tube.

How does your voice make a sound?

Your voice-box (larynx) starts the air in your throat, mouth and nose vibrating.

Why does hitting soft cloth make very little sound?

Soft objects like cloth don't vibrate much.

Give two ways to make a drum sound higher-pitched.

Use a smaller drum, or tighten the drum skin.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Sound vibrations

One vibration, two dials

Where is the vibration hiding?

Louder, or higher?

Explain it like a physicist

More KS3 Physics topics