My setup is almost silent. I use a moderate speed bow and heavy arrows (214 fps). I use 5" vanes. My first shot of the season was a miss at about 15yds and the deer didn't flinch until the arrow hit the brush behind it(about 30 yards away). I got busted by a second deer who saw me reload and never got the second shot.

There is no doubt in my mind that deer can hear the whoosh of an arrow go by ... if I can hear it, so can they. My theory has always been that deer can hear wind, falling acorns, falling branches, falling rocks, etc. If they are spooky - anything will scare them. If they are relaxed and eating (like mine was) they just go back to their activity.