as someone above me stated, anything sub 20db is extremely quiet. Even my ACF12's which are rated at 24db's i cant hear them more then 2 feet away. Anyhow, theres a lot of factors when determining how quiet something is. If you have something called constructive interference, sound waves traveling at the same phase, (IE, 2 identical fans next to each other) they will add to the db raiting. So two fans at 19db will produce combine more then 19, somewhere near or close to double *IN THEORY* but will most likely total to about 25+ and not actually 38db. So dont think adding 5 fans which are 19db will give your system an overall of 19db because that just wont happen.