Sound is the vibration of air or other matter. Sound intensity refers to the energy of the sound waves, and loudness refers to the interpretation of sound as it reaches the ears. Both intensity and loudness are measured in logarithmic units called decibels (dB). A sound that is barely audible has an intensity of zero decibels. Every 10 decibels indicates a tenfold increase in sound intensity. A sound is ten times louder than threshold at 10 dB and 100 times louder at 20 dB.