How do decibels add up???

[FireChicken]

Let say you have 2 fans running side by side, each rated at 30dB, what would be the db rating? Dosent seem like they would add up and be 60 dB but when i have a set up like this it also seems louder than just 30 dB.

Is there a way to calculate or even estimate how loud all the fans in your system are together?

EDIT: I found this tool decibel calculator to see how loud your whole system is

I still have 4 antec tricools on low in my case and with the 2 fans on my rad im calculating like 38.4 dB. I think i can get it quieter

 

[smopoim86]

i thinnk it's around +3 per additional fan, but i'm not completely sure

 

[Bill Kunert]

I believe each +3db is a doubling of sound level. decibels are a logarithmic type scale.