Sound is produced when a body vibrates. The vibrations started by the source are transmitted in the medium in the form of waves from that point to the next and so on. Sound is longitudinal pressure waves in the air. Due to propagation of the wave in a medium, the particles of the medium vibrate about their mean positions. The maximum displacement on one side of the mean position of the particle of the medium is called the amplitude of the wave which is a measure of energy.
Different levels of loudness are created by sound waves that have different amplitudes. The more energy a wave has, the higher its amplitude. As amplitude increases, intensity also increases. Intensity is the amount of energy a sound has over an area. The same sound is more intense if you hear it in a smaller area. In general, we call sounds with a higher intensity louder.
The Loudness or Intensity is measured in decibels (abbreviated as dB). Normal conversation is about 60 dB. Anything louder than 120 decibels can cause your ears to hurt.
Other differences between sounds are caused by pitch and tone.