[pllbnk]

I think that it's because noise is regulated in terms of decibels while humans only perceive noise by the sound's frequency and profile. For example, a fairly low frequency constant 30 dB low-frequency sound might be perceived very differently than the sound of the same parameters but following a footstep pattern, and I am speaking from my own anecdotal experience here. So, what we can measure is sound, but we don't have a good standard for measuring noise.