As I remember from a brief exposure to electronics in the late 60s a dB is a measure of ratio. 6dB ="double", so 12db=4 times as much, 18db = 8 times as much, 24db = 16 times as much, etc.

So if you ordered +15dB relative to a pint of beer you'd get about 6 pints, but relative to a gallon you'd get 6 gallons.