No, it may be addative, but we got to add the correct amounts. With dB's, if you have two noise sources that are of the same loudness, you add 3dB. I other words your 2 fans at 15dB, are now creating 18dB worth of noise, not 30dB. Also note it takes 10dB worth of a volume increase to be twice as loud, where 3dB is just noticable. But with fans there are a few problems with the dB measurements that we all throw around. One the manufacturers dB claim is not a claim with the fan mounted in a case. Two the manufacturer does not make its claim with grills, bezels, heatsink fins/pins or anything else obsructing airflow. You can almost easily add on 10-18dB+ to any fan measurement to get where your real volume is. Now with that said, it doesn't take a brain surgeon to tell ya, all else being equal, a 20dB fan is most likely gonna be quiter than a 30dB fan. 🙂 One last thing to consider, not all manufacturers are gonna use the same test set up for measuring fan dB's, so even the manufacturer ratings are gonna varry ever so slightly one manufactuer to the next.
