The free‐field two‐microphone method for the measurement of the acoustic impedance of the ground

A new method for measuring the acoustic impedance of the ground has been developed. The free‐field two‐microphone method, as it is called, consists of measuring the sound pressure field, radiating from a loud‐speaker source and propagating parallel to the ground, by two microphones located on the same vertical axis. The transfer function between these two microphone signals is shown to be, for total coherence, equal to the ratio of velocity potentials, predicted by the appropriate point‐to‐point propagation theory over a plane boundary. A numerical search with a digital computer provides the value of ground impedance which satisfies the transfer function relationship stated above. Two ground models have been tested: a locally reacting and an extended reacting model. From the close agreement of local and extended reaction models in predicting ground impedance, it is concluded that the grass‐covered surfaces tested here can be considered as locally reacting, without significant error. The free‐field two‐mic...