http://www.fantech.net/inline_duct.htm
I got the 750 cfm one for my home office/smoking room. On a variable rheostat so under normal circumstances it is very quiet at a lower speed.
these work excellent I have installed numerous fans like this,I have found that metal duct works better,less restriction,also for a direct ceiling mount fan check out the panasonic whisper quiet series they are so quiet you can hardly hear them, for bath fans and such these are great.