Sound is a vibration

Where does a sound come from?

Touch your throat while you speak: you feel it tremble. Touch a ringing bell: it shakes. Every sound starts with something that vibrates, moving quickly to and fro about one place.

• A drum: the skin vibrates.
• A guitar or a sitar: the strings vibrate.
• A flute: the column of air inside it vibrates.
• Your voice: the vocal cords in your throat vibrate.

A tuning fork

Strike a tuning fork on a rubber pad, and it hums. Its prongs vibrate too fast to see. Touch them gently to the surface of water, and the water splashes. Hold them, and the sound stops: no vibration, no sound.

How does a vibration become a sound?

A vibrating prong pushes the air next to it, then pulls back. That air pushes the air beyond it, and so on. The disturbance spreads out, and when it reaches your ear, your eardrum starts to vibrate too.

Faster and bigger

• The number of vibrations in one second is the frequency, in hertz ($\text{Hz}$). A faster vibration gives a higher sound.
• How far the object swings is the amplitude. A bigger swing gives a louder sound.
• We hear vibrations from about $20\ \text{Hz}$ to $20\,000\ \text{Hz}$.

You should find that a short ruler buzzes at hundreds of hertz, while a long one swings slowly. Below $20\ \text{Hz}$ we see it move but hear nothing. A harder twang makes a bigger swing, but the frequency stays the same.

Watch out. A sound is not moving air like a wind. Each bit of air only shakes about its own place; the disturbance is what travels.

A worked example

A mosquito's wings beat $1200$ times in $2\ \text{s}$. A housefly's beat $380$ times in $2\ \text{s}$. Which makes the higher hum?

1. Mosquito. $f = \dfrac{1200}{2\ \text{s}} = 600\ \text{Hz}$.
2. Housefly. $f = \dfrac{380}{2\ \text{s}} = 190\ \text{Hz}$.
3. So the mosquito's hum is higher: its wings vibrate faster.

Practise with new numbers

Try it in Python

The program finds the frequency of several vibrating things.

Remember these

• Every sound is made by a vibration.
• Frequency (vibrations each second, in $\text{Hz}$) decides how high the sound is.
• Amplitude (how big the swing is) decides how loud it is.
• We hear from about $20\ \text{Hz}$ to $20\,000\ \text{Hz}$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. You grip a ringing bell with your hand. What happens to the sound, and why?

2. Which part vibrates in a sitar? In a flute?

3. A string vibrates $660$ times in $3\ \text{s}$. What is its frequency?

4. A boy hits a drum softly, then hard. What changes?