Difference between 36 and 42db of noise isolation?
Every 3db is double the power of the signal (amplitude), however this is also the point at which most humans report hearing a noticeable difference.  Most humans will perceive the volume to double around 9-10dB.

So if you found one door that offered 30dB of noise isolation, and another that offered 40dB, you would say that the 40dB is "twice as quiet as the 30dB", approximately. 

In reality, your sensitivity to sound is not linear so it really depends on the frequency of the noise.  You are less sensitive at the high/low extremes and most sensitive in the center of the range.  Likewise, as you get older, you tend to lose sensitivity to the extreme frequencies first.