Decibels are only ever a relative measurement on a logarithmic scale. In this case, we aren't even talking about sound (these are dB of power), but this is true independent of base unit. For sound, to represent silence, you actually need -∞ dB. The fact that this is a relative scale is also why your mixer/home theatre receiver represents full intensity as 0 dB and any volume adjustments as some negative number of dB (some mixers may allow you to go a little bit over 0 dB but of course this risks clipping when a full-strength signal comes in).

Fun fact though: the sound scale is capped on the upper extreme depending on your environment. You know how recordings of launches of rockets sound like the sound is clipping? It's not hardware limitations. The sound is actually clipping as the pressure hits vacuum. See https://en.wikipedia.org/wiki/Sound_pressure#Sound_pressure_... .