Background noise is a continually fluctuating sound level measured in decibels. It is made up of all the acoustic events that are co-occurring in a setting. For instance, the acoustic environment of a classroom consists of the sounds that the air conditioner makes, plus the sound of the computer, plus the sound of chairs scooting, plus the voices of children talking, plus the teacher's voice, plus software speech, coughs, sneezes, footsteps, paper, and many other possible sound sources.