An improved direct impedance deduction technique for locally reacting ground surfaces

A standard acoustical impedance deduction technique for in-situ ground characterization is described in ANSI/ASA S1.18-2010. The direct method relies on using both the magnitude and phase of the measured sound field at the prescribed source-to-receiver geometries. Indoor measurements have been conducted using a pair of phase matched microphones. However, in outdoor environments, measurement uncertainties exist in both the magnitude and the phase of the acoustic transfer function. The phase transfer function is especially susceptible to metrological effects. An improved technique, which relies only on the magnitude information, is proposed. A minimum of two measurements at two source/receiver configurations are needed to determine the acoustic impedance. Even in the absence of measurement uncertainties, a more careful analysis suggests that a third independent measurement is often needed to eliminate rogue solutions. In the presence of experimental uncertainties, a selection of optimal geometries becomes necessary to reduce the sensitivity of the deduced impedance to small variations in the measured data. A graphical method is provided which offers greater insight into the impedance deduction process. Multiple case studies using both indoor and outdoor measurements are presented. The proposed method can greatly simplify the data acquisition process since only a single microphone is needed.