Sound

Summary

We represent a sound wave by drawing lines like the coils of a slinky spring. But we should remember that, although layers of air oscillate, the individual atoms do not. 

Get the basics

Pressure wave

A sound wave is the propagation of changes in pressure, however it is often represented as displacement of air. This is a fair assumption on a macroscopic scale.

However, it is not the air molecules themselves that are transmitting the longitudinal wave, as molecules move with random motion.

Pressure or displacement

Here a sound wave is represented by layers of air showing how displacement is related to changing pressure. The wavelength is the distance between two compressions.

Reflection

When a sound wave meets a wall, it pushes against the wall and the wall pushes back.

Refraction

The velocity of sound in air is related to temperature. If a sound passes from cold air to warm air, its direction will change.

Diffraction

The wavelength of sound is in the order of metres, so sound waves are diffracted by openings such as doors.

Interference

Two sound waves will interfere to produce loud and quiet areas.

This is the principle involved in noise-cancelling headphones, but can also cause quiet patches at live music venues.

Stretch for 7

Standing waves - closed pipes

When a sound wave meets the end of a closed pipe, it is reflected. The reflected and incident waves interfere to produce a standing wave.

Standing waves - open pipes 

Sound waves are also reflected off an open end.

Test yourself

Use quizzes to practise application of theory. 


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