All substances are made up of very small particles called molecules. In some substances, the molecules are close together; in other substances, the molecules are far apart. When the molecules of a substance are sent into motion in waves, vibration of the substance occurs.
In materials where the molecules are closer together (for instance, in solids such as steel), the waves travel faster than in substances where the molecules are farther apart (for instance, in liquids such as water.) In liquids, however, the waves move faster than in air; for in air, which is made up of gases, the molecules are still farther apart. In the air, vibrations strike the ear as sounds, although the human ear can only hear those vibrations of sound waves in a range from 20 to 20,000 per second. Thus these waves vary in length, though all are extremely short.
An entire mass, such as a pendulum, can vibrate, although vibration of an entire mass is generally undesirable. In certain instances, however, vibrating machines are designed for industrial use.
