On the Characteristics of the EA Method to Measure the Acoustic Properties of Ground Surface In-situ

In our previous work, the effects of meteorological elements to the variance of the ground absorption coefficient have been modeled through a long-term in-situ measurement campaign but with remaining unexplained variations (Oshima et al., Inter-Noise 2015). The variations could be attributed to the uncertainty of the employed acoustic measurement technique, the EA method. The method was used because it requires only several portable loudspeakers and two microphones. In this work, the characteristics of the EA method in measuring the ground surface in-situ were systematically investigated by modeling the measurement process of the method. Using the model, the surface impedance and the absorption coefficient were calculated for oblique incidence conditions with varying incident angles and random incidence conditions with varying number of sound sources, averaging times and probability distributions of the incident angles. The results show that for the oblique incidence conditions, the imaginary part of the surface impedance becomes positive as the incident angle becomes larger. For the random incidence conditions, wide variance of the surface impedance is seen at low frequencies and slight variance of the absorption coefficient is seen at high frequencies with varying amount of oblique incident components.