Ecotiere D., Guntzer F., Dutilleux G.

Equivalent acoustic source height is an important parameter for accurate predictions of the acoustic impact of new transportation infrastructures. Recent research has shown that this parameter can be obtained from a two-microphone technique in the case of a rectilinear distribution of point sources which are parallel to a flat homogeneous ground. As an homogeneous ground is not quite common in the field, the present paper introduces a generalization of the procedure to a ground with impedance discontinuity. The latter is taken into account by the replacement of Rudnick's propagation model by Rassmussen's model. Both surfaces are modeled as Delany-Bazley absorbers. It is demonstrated that the two-microphone method is relatively insensitive to ground parameters. Actually measuring the ground impedance when performing a source height determination in the field is not necessary. Qualitative assessment of ground parameters is sufficient to obtain an accurate source height determination. The effect of source misalignment due to the fact that the method takes a sound photograph of the moving sources is shown to be negligible for the speeds of interest. The reliability of the method is established by experiments with controled noise sources. The method is then applied in situ to a real-world vehicle.