Daryo Dec 6, 2015:

it basically means that what humans perceive as "loudness" has no direct correlation with the energy of the sound wave - outside the range of hearing the sound is simply not perceived at all, whatever is its energy (unless you literally get knocked down by the pressure wave - you definitely notice it but you still don't "hear" it)
and even within the range of hearing it's a bit more complicated than just the general way human senses work (on a logarithmic scale, not on a liner scale)

chris collister Dec 4, 2015:

Philippe is correct. Since the ear is highly non-linear, roughly ten times the power is required to register a twofold increase in the perceived sound.

Philippe Etienne Dec 4, 2015:

ressenti It means that it is not linked to what the ear feels in terms of sound. For instance, a power twice as large doesn't mean a sound twice as loud for the ear. Or something like that.