Ceiling Exhaust Fan -How loud is .5 sones?

Nasaman:
I will offer my thoughts from a different perspective. I have had very extensive experience with Panasonic fans. We currently have five of them in our home. Bottom line, they are 'spooky quiet'.

The largest unit that we have, the FV20VQ3, which is rated at 190cfm and requires 6-inch ducting, is in-audible from ~10 few away, and is barely audible standing directly underneath it.

Sorry I cant answer your question directly, but this is real world experience. I have chosen to install all of them with timers as it is SO VERY easy to mistakenly leave them on because they are so quiet.

I installed the panasonic whisper stink fans in my bathrooms and put a 150cfm one in the ceiling of my HT room just behind my head.
I often will run it to evacuate smells coming from the kitchen while watching TV. Its barely noticable. The panasonic whispers are worth the extra money.

I also purchase large fans for hvac air scrubbers I dersign and have a sound calculation tool in front of me from LorenCook fan suppliers.
Sones .3-.5 = 14.5-15.5 dbs

Im sorry I misread the tool,
.3 sones = 12.6 db,
.5 sones = 17.5 db.
But that is at the source,
figure your ears 5 feet away will reduce that by maybe 5dbs

Mumbles,
Excellent numbers, greatly appreciated. After carefully consider all possibilities, I've decided to go back to my first plan.
2chnlben,
Great to see you, Shadome, and some others have the technical ability and taking this hobby seriously.

As others have pointed out, what is quiet for me may be loud and obnoxious to you, especialy at 2am in quiet mode.

There are some great ideas here for decoupling the mechanical noise and remoting the fan farther up into the attic.
Might look at picking up a dayton fan thru grainger and mounting it up in the attic farther, rather than on top of the sheetrock.