Loudness is measured in decibels (named after Alexander Graham Bell and abbreviated dB)

0 dB Mosquito flying in a quiet room, ten feet away from your ears
20 dB A recording studio or a very quiet executive office
35 dB A typical quiet office with the door closed and computers off
50 dB Typical conversation in a room
75 dB Typical comfortable music listening level in headphones
100-105 dB Classical music or opera concert during loud passages
110 dB A jackhammer three feet away
120 dB A jet engine heard on the runway from three hundred feet away
126-130 dB Threshold of pain and damage; a rock concert by the Who (note that 126 dB is four times as loud as 120 dB)
180 dB Space shuttle launch
250-275 dB Center of a tornado; volcanic eruption

-This is Your Brain on Music by Daniel J. Levitin