Intensity or Loudness
A sound wave's amplitude is the change in pressure as the sound wave passes by. If you decrease the amplitude, you are making the sound softer. Characteristics that scientists use to describe sound, such as amplitude, can be related to a picture of a wave.
The amplitude of a wave is related to the amount of energy it carries. A high amplitude wave carries a large amount of energy; a low amplitude wave carries a small amount of energy. The average amount of energy passing through a unit area per unit of time in a specified direction is called the intensity of the wave. As the amplitude of the sound wave increases, the intensity of the sound increases. Sounds with higher intensities are perceived to be louder. Relative sound intensities are often given in units named decibels (dB).