Waves and Vibrations

What is a wave?

Drop a stone into a still pond and rings spread outwards. Pluck a guitar string and a note fills the room. Both are waves. A wave is a vibration that carries energy from one place to another.

Here is the surprising part: a wave moves energy, but it does not carry the material along with it. A floating leaf bobs up and down as ripples pass, but it does not race across the pond. The water vibrates in place while the energy travels through it. Waves are one way energy moves — you can explore others in the many forms of energy.

Two kinds of waves

All waves fall into two families, sorted by the direction their vibration points.

Transverse waves

In a transverse wave, the vibration is at right angles to the direction the wave travels. Picture shaking a rope up and down — the rope moves vertically while the wave races horizontally along it.

The high points are called crests and the low points are troughs. Light, water ripples, and waves on strings are all transverse.

Longitudinal waves

In a longitudinal wave, the vibration is along the same direction the wave travels. The material squashes together and stretches apart, making compressions (squashed) and rarefactions (stretched).

A stretched spring pushed and pulled from one end shows this perfectly. Sound is the most important longitudinal wave.

Measuring a wave

Three measurements describe every wave.

• Wavelength — the length of one complete wave, from one crest to the next. Measured in metres (m).
• Frequency — how many waves pass a point each second. Measured in hertz (Hz).
• Amplitude — the height of the wave, measured from the middle to a crest. Bigger amplitude means more energy.

For sound, frequency controls pitch (more waves per second = higher note) and amplitude controls loudness (taller wave = louder sound).

Sound: a wave you can hear

Sound is made when something vibrates — a drum skin, your vocal cords, a speaker cone. The vibration pushes the air particles into compressions and rarefactions that ripple outward to your ear.

Because sound is a vibration of particles, it needs a material to travel through. It moves through solids, liquids, and gases, but not through empty space. An alarm clock ringing inside a vacuum jar makes no sound at all.

Sound is also fairly slow — about 340 metres per second in air. Light, a transverse wave, can travel through a vacuum at about 300,000 kilometres per second.

Worked example: wave speed

The speed of a wave links its frequency and wavelength:

speed = frequency × wavelength

A sound wave in air has a frequency of 170 Hz and a wavelength of 2 metres. How fast does it travel?

speed = 170 Hz × 2 m = 340 metres per second

That matches the everyday speed of sound in air — a good check that our numbers make sense.

Try it yourself! 🧪

See and hear vibrations turn into sound.

1. Stretch a rubber band between two fingers, or around an open box for a sound-box.
2. Pluck it. Watch it blur as it vibrates, and listen to the note.
3. Stretch it tighter and pluck again. It vibrates faster (higher frequency) and the note is higher.
4. Now pluck it harder. It swings with a bigger amplitude and sounds louder, but the pitch stays the same.

You have just controlled frequency and amplitude with your own fingers — the two numbers behind every sound you hear.