Often, you will see the line “when turning on three channels, the total output volume increases.” There is science in them thar words!
The first thing you must understand is how sound volumes are added.
Two sound sources at 96 dB and 104 dB do not equal 200 dB. That’s pretty obvious; otherwise, mixing music would be a nightmare. The answer lies in the math classes, like algebra, you took in high school. “Why do I need to understand logarithmic equations” you might have asked yourself as you tossed your math book in your locker. Would it have mattered if you would have known that sound addition requires logarithmic equations?
Let’s say you have something playing in one channel at 62 dB and something else in another channel at 73 dB. Here is how you add them together;
N = 10 x log ( 10^(62/10) + 10^(73/10) )
N = 73.3 dB
Additionally, if you have two different uncorrelated sound sources, the new combined level cannot be more than 3 dB above the highest level. If the sounds are correlated (“phase related“), there can be up to a 6 dB increase.
This is the reason, that when adding channels together, the overall volume will go up. It’s easy to have a full band with 8 channels and you realize you have to lower your overall levels because the new combined volume is greater than you originally expected.