[deepspace]

0db is usually defined as the loudest sound that the audio system can produce. Hence, everything else must be negative.

[rdbl27]

More specifically, 0 dB is the loudest sound the audio system is rated to produce without distortion. It's common to be able to actually drive systems harder than their specified engineering limits, which is why meters have a short positive dB section marked in red.

[mitthrowaway2]

Of course, typical of the wonderful ambiguity of decibels, 0 dB is also usually defined as the quietest sound that the human ear can perceive.

https://en.wikipedia.org/wiki/Absolute_threshold_of_hearing

[ianburrell]

That's why important to give the scale. dBfs is full scale level, and db SPL is sound pressure level.