The factor of 20 in SPL makes one decibel equal a just-noticeable loudness step

Evidence

The multiplier of 20 has a twofold purpose. The first is to make the result a number in which an integer change is approximately equal to the smallest change that can be perceived by the human ear. The second is to provide some equivalence to intensity measures of sound level as follows. — David Howard, Jamie Angus, coustics and Psychoacoustics, 184

Categorization

• DEFINE: Clarifies why dB SPL uses 20 as its multiplier—tying integer dB steps to human perceptual resolution.