Survey of free-field methods to measure the normal specific acoustic impedance of ground surfaces in situ at low frequencies

Abstract

Several techniques for measuring the normal specific acoustic impedance of the ground in situ and at low frequencies are examined. These techniques include the use of a spherical wave model, plane wave model, boundary conditions method, and the ANSI S1.18-2010 standard. Experiments are performed with both normal and oblique incidence configurations. A new short-time Fourier transform technique is applied to the measured pressure data to reduce the variance of estimates. Credible estimates of the ground impedance are shown for frequencies as low as 30 Hz. Suggestions are made for future experiments.