Standing or stationary waves

These can be called either Standing or Stationary waves, both names are used for the same thing. You might find it easier to watch the video at the bottom of this page first.

If two lots of waves meet they will interfere with each other. If the wavelength and the frequency are the same, they have a similar amplitude and they occur at the same time then this is called coherence. A recognisable interference pattern will be formed.

Interference pattern caused by two sources in a ripple tank
Interference pattern in a ripple tank

Where the two waves meet crest to crest or trough to trough, then we get constructive interference - a large wave is formed.

Constructive and destructive interference between two waves
Constructive and destructive interference

If the waves meet trough to crest then they cancel out completely. This is destructive interference.

If we shake a rope which is fixed at one end, then the waves travel down the rope and reflect from the fixed end. The reflected wave then meets the wave going down and the two interfere. If the wavelength of the waves is a simple fraction or multiple of the length of the rope then the interference pattern is fixed in one place. The waves reflect up and down the rope easily. This is a condition known as resonance and the interference pattern, which looks fixed, is called a standing wave.

Fundamental and harmonics of standing waves in a rope
Standing waves in a rope

We can create standing waves in a large number of situations. The two that are the most common examples in student examinations and practical work are vibrations in a string/rope and sound in a tube. These are the simplified versions of the conditions that exist in most musical instruments.

Graphical image of standing waves in a tube
Graphical image of waves in a tube

Note: These diagrams are a graphical representation of the amplitude of the vibration of air molecules in the tube. The direction of vibration is up and down the tube.

A PDF sheet of notes on an introduction to standing waves can be downloaded here:  Introducing standing waves

The video lesson below explains standing waves with the help of practical demonstrations.

There are two pages of notes each with video lessons to extend your knowledge of standing waves here: