Humans discriminate level differences of under half a decibel across a 100 dB range

Evidence

Humans are exceptional at their ability to tell whether two sounds are louder than one another — note that these thresholds are less than ½ of a decibel, particularly in light of the dynamic range of the auditory system being over 100 dB! — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 820