When we talk about sound being loud or quiet in terms of decibels, that's another example of referencing off some fixed quantity; there, a measurement of the quietest sound most people can hear. A sound that's 90 dB loud is then 10^(90 / 10) = 10⁹ = a billion times as loud as that reference sound.
Strictly speaking, we might should write dBI to indicate that there's an intensity reference, but in practice no one does that — confusingly, the reference is just left off for sound intensity.