Sound, but bitesize.

Part 1A wave carries energy, not stuff

Drop a stone into a still pond. Ripples spread out in circles. It looks like the water is travelling outwards — but it isn't. Put a leaf on the surface and watch: the leaf bobs up and down on the spot. It doesn't get carried to the edge.

That's the big idea. A wave transfers energy from one place to another without transferring matter. The water stays put; only the energy moves through it. The same is true for sound, light, and every other wave you'll meet.

Part 2Transverse waves and their parts

A transverse wave is one where the particles vibrate up and down, at right angles to the direction the wave travels. Water ripples are transverse. So is light. The classic "wiggly line" you draw for a wave is a transverse wave.

Every transverse wave has the same set of parts, and you need to be able to label them:

The rest position is the flat middle line — where the water would sit if nothing was disturbing it. The peaks above the line are crests; the dips below are troughs. The amplitude is the height from the rest position up to a crest (how far the particle moves from the middle). The wavelength is the length of one complete wave — for example, from one crest to the very next crest.

Part 1Three things we measure

From the last topic you already know two of the measurements: amplitude (how tall the wave is, from rest to crest) and wavelength (how long one wave is). There's a third one, and it's the most useful for sound: frequency.

Frequency is the number of complete waves that pass a point every second. If three full waves go past your finger in one second, the frequency is 3 waves per second. The unit for frequency is the hertz (Hz). So 3 waves per second is written as 3 Hz.

That's the whole idea — frequency is just counting. More waves per second means a higher frequency. A buzzing mosquito makes a very high-frequency sound; a deep drum makes a low-frequency one.

Part 2Reading and comparing waves

When two waves are drawn side by side, you compare them by looking at the height and the spacing. The diagram below shows two waves drawn over the same length of time.

The top wave is taller — it has a bigger amplitude. The bottom wave is squashed up — more waves fit into the same space, so it has a shorter wavelength and a higher frequency. Being able to spot these differences at a glance is exactly what exam questions ask for.

Part 1Sound starts with a vibration

Every sound starts with something vibrating. Pluck a guitar string and you can see it shaking. Touch your throat while you hum and you can feel it buzzing. Bang a drum and the skin trembles. No vibration, no sound.

That vibration pushes against the particles next to it. Those particles bump into the ones beyond them, which bump into the next ones, and so on. The vibration gets passed along from particle to particle, spreading out as a wave until it reaches your ear.

This is why sound needs something to travel through. The stuff a wave travels through is called the medium — it could be air, water, a wall, anything made of particles. Without particles, there's nothing to pass the vibration along.

Part 2Solids, liquids, gases — and the vacuum

Sound can travel through all three states of matter, but not equally well. It travels fastest in solids, a bit slower in liquids, and slowest in gases like air. The reason is how close the particles are: in a solid the particles are packed tightly together, so they pass the vibration on very quickly. In a gas the particles are far apart, so it takes longer for the vibration to be handed along.

And here's the crucial one: a vacuum is a space with no particles at all. With nothing to pass the vibration along, sound cannot travel through a vacuum. This is shown by the famous bell jar experiment: a ringing bell is placed inside a glass jar, and the air is slowly pumped out. As the air disappears, the ringing fades to silence — even though you can still see the bell hammer moving.

Part 1Frequency controls pitch

Pitch is how high or low a note sounds. A piccolo plays high notes; a tuba plays low ones. Pitch is controlled by one thing: frequency.

A high frequency (lots of waves per second) makes a high-pitched sound. A low frequency (few waves per second) makes a low-pitched sound. That's the whole rule. When a singer slides up to a higher note, the air is vibrating faster — more waves are reaching your ear each second.

Part 2Amplitude controls loudness

Loudness (also called volume) is how loud or quiet a sound is. This is controlled by a completely different part of the wave: the amplitude.

A big amplitude means a loud sound; a small amplitude means a quiet sound. When you turn up the volume, you're making the speaker vibrate further each time, which pushes the air harder and makes a taller wave. The pitch doesn't change — only the loudness.

Scientists view sound on a screen called an oscilloscope, which draws the wave so you can see it. Below are four traces. Compare them carefully — changing the height (amplitude) changes loudness, and changing the spacing (frequency) changes pitch. They are independent.

Part 1The parts of the ear

Your ear is a machine for turning sound waves in the air into signals your brain can understand. Sound enters from the left and is passed along through several parts, each with its own job.

The pinna is the flap on the outside of your head — it funnels sound into the ear. The sound travels down the ear canal to the eardrum, a thin sheet of skin that vibrates when the sound hits it. Behind the eardrum are three tiny bones, the ossicles, which pick up the vibration and pass it on, making it bigger. They hand it to the cochlea, a coiled, fluid-filled tube that turns the vibration into electrical signals. Finally the auditory nerve carries those signals to the brain, where they are understood as sound.

Part 2The order of events, and what we can hear

It helps to remember hearing as a chain. Each part passes the energy to the next, in order:

pinna → ear canal → eardrum → ossicles → cochlea → auditory nerve → brain

Notice the change at the cochlea: everything before it is a vibration being passed along; everything after it is an electrical signal. The cochlea is where one becomes the other.

Humans can't hear every frequency. The human hearing range is roughly 20 Hz to 20 000 Hz. Below 20 Hz the sound is too low for us; above 20 000 Hz it's too high. Sound that is too high for humans to hear is called ultrasound — some animals, like dogs and bats, can hear well into that range.

Part 1Sound is fast — but light is much faster

Sound travels through air at about 340 metres every second (340 m/s). That's fast — roughly the length of three football pitches every second — but it's nothing compared to light, which travels at about 300 million metres per second. Light is almost a million times faster than sound.

You can see this gap for yourself in a thunderstorm. Lightning and thunder happen at the same moment, but you see the flash first and hear the rumble later. The light reaches you almost instantly; the sound takes time to travel the same distance. The further away the storm, the longer the gap. Counting the seconds between flash and bang tells you roughly how far away it is.

Part 2Working out a speed

To measure a speed, you time how long something takes to travel a known distance, then use:

speed = distance ÷ time

This is how you can measure the speed of sound: stand a measured distance from a big wall, make a sharp sound (a clap), and time how long the echo takes to come back. As always, use the FIFA method to lay the working out cleanly — Formula, Insert, Fix units, Answer.

Part 3Ultrasound and microphones

Ultrasound is sound that is too high-pitched for humans to hear — above 20 000 Hz. We can't hear it, but it's extremely useful because we can send it out and detect the echoes that bounce back.

Sonar uses ultrasound to measure depth and find objects underwater: a ship sends a pulse downwards and times how long the echo takes to return, which tells it how deep the seabed is or where a shoal of fish is. In hospitals, ultrasound scans use the same idea to build a picture of a baby inside the womb, safely and without any harmful radiation.

Finally, a microphone does the reverse of a loudspeaker: it turns sound into an electrical signal. Sound waves make a part inside the microphone vibrate, and those vibrations are changed into a changing electrical signal that can be recorded, amplified, or sent down a wire.