Theory of steady-state urban noise for an ideal homogeneous city.

Abstract

The city is treated as a plane surface with many identical sound sources (motor vehicles) randomly distributed over its area. The mean energy density at any point in the plane is expressed in terms of the individual source strength, the average number of sources per unit area N, the atmospheric absorption constant α, and a shielding factor F associated primarily with obstacles in the transmission path. To obtain the steady-state (median) energy density, a central cell containing a single discrete “local” vehicle is identified and treated separately from the rest of the distribution. Graphs and tables of steady-state level and energy density as functions of N and α are given for the homogeneous infinite city, the city of finite size, and the traffic-free zone within a city. The theory indicates that the spreading of urban noise is determined by a characteristic distance with a typical value of 0.25 km. The observed octave-band sound-pressure levels from 31.5 to 4000 Hz at one location in Ottawa are compared with calculated levels based on statistical data for vehicle source strength, estimates of vehicle density, and known atmospheric absorption constants. The differences are consistent with a shielding factor of 15 dB which is in good agreement with measurements of sound transmission in urban areas reported by others. The shielding factor has an effective value substantially independent of frequency.