Uncertainty of an in situ method for measuring ground acoustic impedance
Résumé
The ground acoustic impedance is a key parameter for outdoor sound propagation. Laboratory approaches for measuring the impedance of outdoor ground are often less interesting than in situ approaches because these kinds of ground often require non destructive techniques. The chosen in situ method consists in fitting the transfer function between the frequency spectra measured at two different microphones above the ground with a theoretical model. The theoretical model is based on the Rudnick theory together with an impedance model adapted to the ground studied (the Miki model or the Zwicker and Kosten model are used here).
The uncertainty of the method has been estimated by organizing an inter laboratory experimental campaign and by estimating the repeatability and the reproducibility according to the ISO 5725. Five independent laboratories have tested five different types of outdoor grounds (synthetic lawn, lawn, grass, slightly compacted soil, asphalt). It is shown that the uncertainty for estimating the flow resistivity of the impedance model is increasing with the flow resistivity value : the relative standard uncertainty for the flow resistivity estimation is smaller than 20% for flow resistivity smaller than 5 000 kNsm-4 but can be much higher for very reflective material.