Measurement of source height on moving highway vehicles

A measurement method for the equivalent noise source height of an arbitrary distribution of moving noise sources is developed to investigate the highway vehicle equivalent source heights which are currently used by the FHWA for noise barrier design. The study is intended to provide information required to improve this noise barrier design code. The equivalent point source position is defined for an arbitrary distribution of acoustic sources above a reflecting plane and a method for its measurement using a microphone array is developed. The normalized errors of the measured equivalent source heights are defined including the effects of background noise, the geometric near field, source size, and source directionality. Normalized errors of the measured source heights obtained by a numerical simulation for each parameter lead to optimization of the microphone spacing and to the design of an array of microphones which give the equivalent source height as a function of frequency. The method is then applied to the measurement of the equivalent source height of stationary loudspeakers and is shown to give results which are consistent with theoretical predictions. The effect of the Doppler frequency shift of moving vehicles is investigated using a loudspeaker mounted on the roof of a moving car. If the speed of a moving noise source is less than 97 kph, the error due to the Doppler frequency shift was found to be negligible. Results of measurement on highway vehicles show that the noise source heights for heavy trucks and for automobiles are different from those used currently by the FHWA.