It's like a "desensitization process"..part of our adaptive abilities "survival"... Everything is well said, and very interesting.... I will add 2 very basic and yet powerful forces in shaping our already fine hearing abilities.... 1) is Body Chemistry, what we are , or have been lately, putting in our bodies ie. nutrients or lack thereof, too much coffee, booze, on a bad diet kick, lack of exercise, fatigue and on and on... (I am sure Jim already included these in the above Physiology category)... 2) the often overlooked and never mentioned amount of Earwax a person CAN have.... I for one, usually have to go to an audiologist from time to time and get it removed.. When, I leave it for 2 years or so, My ear drum is completely covered over.. I can hear very well, but, the funny thing is, just how much it robs my hearing of the lower midrange and makes me much more sensitive to high frequencies.. Many things sound a tad brighter to me and more annoying.. When, I came back from my appointment the first time, I played my system, expecting maybe nothing but a little clearer sound, but, BOY, the FULL, rich midrange that was filling the room that wasnt there before...I thought my speaks were just a little weak in that area, but NO, it was my ears....I never hear people discussing this, I just assume it must be me only..However, it surely doesnt hurt to make an appointment, If you dont do it already to an audiologist and make sure your lower ear canal is pretty wax free.... Maybe, this is a dumb post from me, but I am just making sure if anybody hasnt thought of this and has relatively plugged ears and doesnt know it... It is a CHEAP and simple upgrade to your TOTAL audio system experience.... just sayin..