Speaker
Description
The two-microphone method, as defined in the standard ANSI/ASA S1.18-2018, is used to determine the acoustic impedance of ground surfaces. To derive the impedance from the sound pressure ratio of the two microphones, the method relies on a specific geometric relationship between the sound source and the two receiving microphones. Therefore, we explore the effect of positioning errors within the measurement setup, i.e. in the alignment between source and receivers.First, an experiment is conducted to characterize the uncertainties in triaxial geometric positioning that occur during manual setup under practical measurement conditions. The resulting probability distribution is used as input for a Monte Carlo (MC) uncertainty propagation across the template method applying the variable porosity model for a sample material. Consequently, the uncertainty is analyzed for the level difference of the two microphone signals, which is a critical intermediate processing step. The microphone signals are computed by adding the geometrical paths of direct and reflected sound emitted by a point source using the Weyl–Van der Pol approximation for spherical waves. The MC simulation yields the resulting uncertainty estimates for the impedance.First results show that uncertainty arising solely from geometric setup variations is relatively small but strongly frequency-dependent. The uncertainty of the impedance is larger at low frequencies than at high frequencies. Positioning errors in height show to have the strongest influence on the output quantities.