One can already tell you some factors deductively (and verifiedly): it's not the density, it's the presence. Mass is phonoabsorbent; there where you have buildings, you have both noise from the inhabitants and shielding from the constructions.

In the open spaces, one single source can pollute for even ten kilometers or five miles of distance, and more. The added effects of natural acoustics can be worsening - barkings bouncing in the echo from valley to valley, starting in front and returning behind etc.