A loud rock concert has a sound pressure level of 115-120 decibels. A gain of 10 decibels sounds twice as loud to the human ear. When a motorcycle has a sound pressure level of 100 decibels, a rock concert at 120 decibels, 20 decibels higher, sounds four times as loud.
Sound pressure level is given by 20 times the logarithm in base 10 of the ratio of the effective sound pressure, to a reference sound pressure of 20 micropascals, which is at the threshold of human hearing.
A quiet room is 8 times as loud as this threshold at about 30 decibels. A normal conversation is 8 times as loud as a quiet room at about 60 decibels. A rock concert is about 64 times as loud as a normal conversation at about 120 decibels.