THE MEASUREMENT OF NOISE FROM MOTOR VEHICLES ON THE HIGHWAY

Motor vehicles comprise the most widespread source of noise problems in urban areas today. This is particularly true of those vehicles producing noise levels that stand out from the average stream of traffic on highways--the large diesel truck, the noisy passenger or sports car, and many motorcycles. In an attempt to control the increase in noise levels produced by motor vehicles, the California Legislature adopted Motor Vehicle Codes for limiting the noise levels produced by new vehicles, and by existing vehicles on the highway. The responsibility for conducting the necessary highway noise measurements to enforce the Vehicle Code Noise limits was given to the California Highway Patrol (CHP). The method of measuring vehicle noise on the highway, particularly the selection of suitable measurement sites, was adopted by the CHP from the recommendation proposed by the Society of Automotive Engineers, which was designed for the testing of new motor vehicles. Included in this recommendation is the requirement that the noise measurements should be conducted at a distance of 50 feet from the vehicle path, and that there should be no reflecting obstacles within a 100 foot radius of the microphone or the vehicle at its closest point to the microphone. It soon became apparent that the constraints on the choice of suitable measurement locations imposed by this standard made it almost impossible to take measurements in most urban and residential areas. As a result, a research program was undertaken to provide more substantial evidence on the feasibility of enlarging the number of sites available for highway noise measurements by relaxing site restrictions.