Waves and sound · Contrast
Sound is longitudinal
Every drawing of sound you have ever seen was a wavy line. Almost none of them meant that the air goes up and down, and this lesson is about the difference.
Start here
One slinky. Two completely different waves.
A long slinky stretched across the floor with someone holding each end. Flick your end sideways and a hump runs down it. Instead, shove your end sharply towards your partner and pull it straight back, and something runs down it again — but this time there is no hump at all. What travels is a squashed-up patch of coils.
In that second wave, which way does each individual coil move as the disturbance goes past it?
Each coil shuffles a little way along the slinky and back, in the same line the wave is travelling, and finishes where it started. Nothing goes to the far end except the pattern of crowding and spreading. A wave whose material moves along the line of travel is longitudinal, and sound is one.
Both of those are waves: both carry energy from one end to the other, and neither one moves any coil permanently along the slinky. What separates them is the direction the material moves compared with the direction the wave travels.
In a transverse wave the material moves across the line of travel, at right angles to it. A shaken rope and a wave on water both do this, and both show crests and troughs. In a longitudinal wave the material moves along the same line the wave is travelling, backwards and forwards. There are no crests: instead there are places where the material is bunched together, called compressions, and places where it is pulled apart, called rarefactions.
Sound is longitudinal. A vibrating surface pushes the air in front of it into a compression, then pulls back and leaves a rarefaction, and that pattern travels away. Each patch of air shuffles a tiny distance to and fro along the line the sound is going, and finishes where it began.
At the bench · one slinky, 1.20 m long, driven at the left-hand end
Drive it two ways. Watch one coil.
Change a control to begin
The same slinky, driven at the same size of movement — 60 mm — and with the same 300 mm from one repeat to the next. Choose how it is driven, then choose which coil to mark.
Commit first. A loudspeaker cone moves forwards and backwards along the direction the sound is going. Which way does the air next to it move?
How the end is driven
25%
Which way the wave travels
Along the slinky, to the right
Which way the marked coil has moved
—
—
Where the marked coil sits
—
Kind of wave
—
The figure
Same job, two directions
Transverse
The material moves across the line of travel. Crests and troughs. Waves on water, a shaken rope, light.
Longitudinal
The material moves along the line of travel. Compressions and rarefactions. Sound, in air, in water and in solids.
What the two share: both carry energy from one place to another, both leave the material where they found it, both have an amplitude and a wavelength, and both reflect off a barrier. The direction of the material's movement is the only thing that separates them.
Key fact
In a transverse wave the material moves at right angles to the direction of travel, giving crests and troughs. In a longitudinal wave it moves back and forth along the direction of travel, giving compressions where the material is bunched and rarefactions where it is pulled apart. Sound is longitudinal, in air, in liquids and in solids.
Think again
“Sound must be transverse — it is always drawn as a wavy line.”
That wavy line is a graph, not a picture. What is plotted up the page is how squeezed the air is, or how far each patch of air has shuffled from its rest place; what runs across the page is distance along the sound, or time. Nothing in the air is going up and down. Reading a graph of a longitudinal wave as a photograph of it is the single commonest mistake in this topic, and it is worth checking what the axes of a wave diagram actually say before believing your eyes.
“In a compression, air travels from the loudspeaker to your ear.”
A compression is a place where the air is momentarily squeezed, and the place travels; the air does not go with it. Each patch of air shuffles a fraction of a millimetre forwards, then the same distance back, and stays where it was — exactly like the coil you marked on the slinky, and exactly like a cork on a pond. If air really did travel from the speaker to your ear, a loud concert would leave a vacuum on stage and a gale at the back of the hall.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Classify
Which of these describes a longitudinal wave?
Rung 2 · The one that catches people
A textbook draws a sound wave as a wavy line. A student says this proves the air moves up and down as sound goes past. Which statement is right?
Rung 3 · Explain
A loudspeaker cone moves forwards and backwards. Explain how that makes a longitudinal wave in the air, using the words compression and rarefaction.
Rung 4 · Take it somewhere new
A long queue of people is standing still. The person at the back steps forward and bumps the next, who bumps the next, and a bunching-up travels all the way to the front. Say which kind of wave this is like and why, then give one way the queue is not a good model of sound.
Key note
A transverse wave moves its material at right angles to the direction of travel and shows crests and troughs; a longitudinal wave moves its material backwards and forwards along the direction of travel and shows compressions, where the material is bunched, and rarefactions, where it is pulled apart. Sound is longitudinal. Each patch of air shuffles to and fro along the line the sound is going and finishes where it started, so no air travels from the source to the listener.
Going further
The distinction earns its keep in seismology. An earthquake sends out two kinds of wave through the rock at once: P waves, which are longitudinal, and S waves, which are transverse. P waves are faster, so they always arrive first — that is what the P is for. The useful part is that a transverse wave needs the material to resist being sheared sideways, and a liquid does not, so S waves cannot cross a liquid at all. Seismometers all over the world record P waves arriving from an earthquake on the far side of the planet and no S waves at all, and the size of that S wave shadow is the main evidence that the Earth's outer core is liquid. Nobody has been anywhere near it.
Water waves are the awkward case. On the surface of deep water each patch travels round a small near-circle rather than straight up and down, which makes it partly transverse and partly longitudinal at once. The transverse description is close enough for the crest-and-trough picture and it is the one used at this stage, but the honest version is that a surface wave is not purely either.
Before this lesson
Connects to
At GCSE this becomes
- Longitudinal and transverse waves compared in detail, wave speed in different materials, and the P wave and S wave evidence for the structure of the Earth.
Where to next
Ask Mr Badmus AI
Got a wave you cannot decide the kind of?
The bench is a teaching model and both drives are shown at the same 60 mm of movement and the same 300 mm from one repeat to the next, so the two pictures can be compared directly; a real slinky driven those two ways would not match so neatly. Everything is drawn to one scale of about 0.73 pixels per millimetre in both directions. The pictures are frozen snapshots rather than animations, so the marked coil shows where it has been displaced to at one instant and not how fast it is going. The mark moves along the slinky in fixed steps rather than to any point you choose, so it samples the wave rather than sitting on the exact top of a crest: the largest reading the slider can be set to is 57 mm, not the full 60 mm. Air is drawn as evenly spaced columns for clarity; real air is not in rows, and the distance a patch of air actually shuffles in ordinary sound is a small fraction of a millimetre.
Lesson content © MrBadmusAI.