Buzz a cord at just the right rate and it splits into still points and big loops.

What you'll discover

• Waves bouncing back along a cord can combine into a pattern that seems to stand still.
• Points that never move are called nodes; the big swinging parts are antinodes.
• Only certain buzzing rates make neat patterns, which is how strings play particular notes.

The science in plain words

When you shake one end of a cord, a wave runs along it, hits the fixed far end and reflects back. Now two waves travel along the cord in opposite directions, overlapping all the time.

At most shaking rates, the outgoing and returning waves are muddled and the cord just jiggles. At certain rates, though, they fit the cord exactly. Then, at some points, the two waves always cancel out, so the cord there stays still. These are nodes. Halfway between nodes, the waves always add up, giving big loops called antinodes. The wave pattern no longer appears to travel, so it is called a standing wave.

The simplest pattern is one loop, with a node at each end. Double the shaking rate and you get two loops; triple it and you get three. These special rates are called harmonics. Only whole numbers of half-wavelengths fit along the cord.

This is how stringed instruments work. A guitar or sitar string, fixed at both ends, vibrates mainly in these patterns, which sets its note. Pressing a string shortens it, so higher harmonics fit and the note rises. Wind instruments use standing waves in air.

Where it fits

• India, NCERT: Standing waves go beyond the NCERT syllabus for classes 6 to 10; the lab builds on vibration and frequency in Class 9 Chapter 11: Sound.
• United States: Grades 9 to 12

Common questions

What is a standing wave?

A standing wave is a pattern made when two identical waves travel in opposite directions along the same string or tube. Some points stay still and others swing widely, so the pattern seems not to move.

What are nodes and antinodes?

Nodes are points on a standing wave that do not move, because the two waves always cancel there. Antinodes are the points in between where the waves add up and the motion is largest.

How do standing waves make musical notes?

A string fixed at both ends can only vibrate in patterns that fit its length. Those patterns set the frequencies it can play, which we hear as its note and its overtones.

Related labs

• Wave Motion: Shake one end of a cord. How fast does the wave run, and how long is each wave?
• Resonance: Shake three toys on springs gently. Why does only one go wild?
• Beat Frequency: Two nearly equal notes together go loud, soft, loud, soft. How fast is the wobble?

See all physics labs or every Taz science lab.

Try it yourself

The Standing Waves lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.

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