Lack of attenuation is a large factor. Attenuation is the reduction of sound energy by the absorption (via trees, buildings etc.) or divergence of sound waves. The amount of attenuation due to absorption varies with the substance through which the sound waves are being transmitted, or propagating. Generally speaking, through air, noise decreases by six decibels for a doubling of distance when sound emanates from a single point in space. In contrast, noise emanating from a line, such as a train or constant road traffic, decreases by only three decibels per doubling of distance.