All audible sound is the result of variations in air pressure that produce vibration. In vibration, the pressure in a particular place (for example, inside the ear) becomes alternately higher and lower. This is usually described in terms of wave motion, though sound waves do not move up and down like waves on the sea; they are more like shock waves that travel outwards from an explosion. We can diagram the pattern of a particular sort of vibration by displaying its waveform. If the vibration happens rapidly, we say it has a high frequency, and if it happens less rapidly, we say it has a lower frequency. If the vibration is regular, repeating its pattern over and over, we call the sound periodic, while a pattern of vibration which does not have such a pattern of regular vibration is called aperiodic. If the sound contains a large amount of energy, we say that it has high amplitude.