Estimation of vehicle speed using wayside sound pressure onset rate

Transportation-related environmental noise studies, particularly those associated with rail traffic, often require long term measurements near existing rights of way. In addition to the sound produced by passing vehicles, it is also useful to know their speed. Previously, speed measurements have generally required an additional piece of instrumentation, such as a second microphone, video camera, or a radar gun--resulting in added measurement cost and complexity. The present study shows how estimations of vehicle speed can be obtained using a single wayside microphone. The method is based on the rate of pressure rise as the vehicle approaches, specifically the maximum onset rate. This paper shows how the maximum onset rate depends only on the vehicle speed, the microphone distance, and the speed of sound. In order to demonstrate the feasibility of the approach field test data are presented for train passages ranging in speed from 33.5 to 55.6 m/s, as measured by a microphone located 28.65 m from the tracks. Speed estimates derived from the onset rate were generally accurate to within 8% of the independently measured speed of the train.