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.
> The trend is higher sound levels in wetter areas with more vegetation. This is due to the sounds of wind blowing through vegetation, flowing water, and more animals (especially birds and frogs) vocalizing in more fertile locations.
I find those sorts of natural sounds to be way less annoying and offensive than machine-originated sounds, even at equal decibel levels. Perhaps it’s because of a learned/cultural associations, but I’d bet there’s something inherently annoying to most manmade sounds, probably due to the periodic nature of motors, fans, etc. I’d be interested in the above suggested diff map showing (approximately) machine-originated sound levels.
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.