So 7-volting gives about 11.7dbA drop, 9-volting 6.2dBA below the 12v noise level. With the perfect case, you'll lose 1/12 of your flow for every volt down you go, but with flow resistance it's a fair ...

As a rule of thumb, its going to be a little more then that of the highest fan because anything quieter then the loudest won't be very noticable. If you want the exact forumla its in the C&CF FAQ.