...I'll give some extra info! I was thinking of a peculiar feature of logarithmic scales. They can't actually represent the complete absence of something! With sound this would correspond to writing -∞ dB. And infinity isn't a proper number. The decibel scale has been set so that the minimum sound level that the average human can hear is 0dB. There are probably animals that have more sensitive hearing, so they could detect, say, -10dB.
With the decibel scale, for every (approximate) 10dB increase in noise level - humans, on average, would perceive a doubling of the loudness. (Obviously -10dB corresponds to a perceived halving of the volume). This doesn't reflect the physics, since if you precisely double the sound pressure waves this gives a different amount of decibels that you ADD (not multiply).
Therefore, because we subtract a few decibels every time we reduce a sound level by a certain factor, say 1/2, this means that we can never get to an amount of decibels that represents the complete absence of sound! If we keep halving the sound level there will still be some sound remaining, so we can halve forever. Therefore we can also keep subtracting from a decibel amount forever, never reaching pure silence.
Hope this makes sense. I'm writing this in a bit of a rush today. (I remember this stuff from some of my electronics classes!).