However, overall loudness is only one metric. What does the amplitude of the noise in dBSPL look like when graphed out as a function of frequency in the audible range? How does the shape and level of the curve defined by that graph compare to the threshold of human hearing as described by the equal loudness contour?
My understanding is that we are talking here about the molecules tapping on the eardrum, i.e. the shot noise - similar to the noise of the rain drops. It is a white noise, i.e. its spectrum has the same amplitude at all frequencies.
On a deeper level however there are at least two characteristic timescales that would limit the width of the spectrum:
- the collision time between the molecule and the wall
- the intermolecular scattering time, which characterizes the density fluctuations of the air
it is also important to note that the air consists of several types of molecules (nitrogen, oxygen, carbon dioxide and some others), each of which is characterized by its own scattering times.