Decibels are logarithmic units and can't be added like other numbers. If you have one source producing a sound level of 65dB and you add a second 65dB sound source, you don't have a level of 130dB, you have a sound level of 68dB. This seems easy enough when both values are the same, but is much less obvious when the levels are different from each other. You will note that when the two levels are similar, the change in the combined level is larger than when the two levels are significantly different.
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Mc Squared System Design Group, Inc
323 - 901 West 3rd Street
North Vancouver, BC V7P 3P9
Phone 604 - 986 - 8181