What is quiet?
Forum Overclocking : Cooler and Heatsinks - What is quiet?
0 dB for sound is the "threshold of hearing". Below that, there is no sound that can be detected.
This is what you hear lying in bed in windless night with no animal sounds outside, no ticking clocks, or droning fan/furnace/waterpump/stove motors, and only the sound of your own breathing.
.....actually, 0 dB is what you hear when you hold your breath in this silent room, and are listening for anything at all. In which case you'll probably hear the beats of your own heart, or the blood thumping in your neck as it surges through your own arteries.
For everyday life, 0 decibels is actually a relative sound level since the threshold of hearing will be different for each person.
For a deaf person, the threshold of detection is probably the same point that causes pain and physical damage to the eardrum for a hearing person. (Things become unclear for very loud and very low-frequency sounds which can be "felt" rather than "heard". A deaf person can likely feel them just as well as anyone else.)
Also, the perception of sound for a hearing person is somewhat flexible. There is a small muscle inside the ear that can tighten up the bones that transmit sound to the cochlea. This deadens the incoming sounds and can help protect the sound-detecting cells inside the cochlea from damage. However, this muscle cannot respond quickly enough to instantaneous loud sounds like gunshots so the cochlea can be damaged before this muscle can react. Also, when listening to prolonged loud sounds such as in the front row of a rock concert, this small muscle eventually tires out, so ear sensitivity goes back up, and cochlea damage can then occur.
So, for someone who has regularly blasted their ears out by firing guns at close range without ear protection, or constantly listens to music at maximum volume, their threshold of hearing will be much higher than normal because their cochlea is already damaged, and so they won't notice fairly loud droning sounds that annoy people who still have more sensitive hearing.
This is also why droning sounds may be so annoying. Your ear can temporarily quiet them down, but eventually it gets tired and you have to sit through the full intensity of the loud whirring and whining of your CPU and case fans, your CD drive which seems ready to vibrate apart and explode... AIIGHH, can't stand it anymore. I'm getting outta this room for a while, away from the noise. Gotta give my ears a break. 
Well i dont know much about the scale, but when dealing with casefans of an 80mm variety i know that anything below 24dBa is very quiet. my 21dba Panaflo fans are perfect. Often lost behind the hum of hard drives and other components. anything over 35dBa for me is highly annoying.
<b><font color=purple>[Rik_]</font color=purple> I wonder how many people have made their own phasechange system?
<font color=blue>[LHGPooBaa]</font color=blue> I get phasechange whenever i eat a hot chillie 
</b>
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