What is a decibel (dB)?

Sound is measured in decibels, which are a logarithmic scale rather than linear.  This means that every 10dB increase sounds approximately twice as loud, so a 20dB increase sounds approximately four times as loud, so an increase from 60dB to 90dB sounds approximately eight times louder.  If decibels were linear, 40dB would sound twice as loud as 20dB. 

When sound levels are expressed in dB(A) rather than dB, this means that a correction has been made to simulate how loud the sound is to the human hearing mechanism.  Hearing sensitivity varies between individuals, but as a general principle 0dB(A) is the threshold of hearing and 140dB(A) is the threshold of pain, but you should be aware that hearing damage can occur with prolonged exposure at levels below 80dB(A).