The decibel scale is a logarithmic ratio between any two sound levels. The addition or subtraction of decibels is different from normal linear calculations.

Adding 3dB's to an existing noise level doubles it, subtracting 3dB's from an existing noise level halves it. In other words, adding two noise levels of 80dB does not give a noise level of 160dB. It gives a level of 83dB, i.e twice the 80dB noise level.