How do humans perceive sound intensity?
As humans differ in general, people will perceive it differently.
It can be very technical because perception of loudness will vary depending on the frequency. As you may know, as people age, they will not be able to hear higher frequencies. Wikipedia has a technical article on perception of loudness.
However, I prefer a more general explanation of most human perception given by the Weber-Fechner Law. Essentially, we have a logarithmic sense of intensity rather than one that is linear. This actually corresponds with how we measure sound levels because we use decibels which is a logarithmic measure. Note that this law is a general explanation and is not an exact model for sound loudness.
Here's an example with weight because it's easy to replicate. If you take 2 weights, one in each hand: one that is