Measurement of normal‐incidence impedance of outdoor ground surfaces in the frequency range 20–500 Hz

Abstract

The impedance of natural ground surfaces has been measured extensively at frequencies above about 200 Hz. At lower frequencies, however, measurements are scarce and have been inaccurate. We have begun using a two‐microphone, phase‐difference technique similar to that of Nicolas and Legouis [Proc. 2nd Int. Congress on Acoustic Intensity, Senlis (1985)], which promises to overcome some of the difficulties at low frequencies. A point source is suspended above the ground and the sound field is measured with two phase‐matched microphones along the vertical line below the source. The reflection coefficient, and hence the impedance, is obtained from the variation of the phase difference between microphones as a function of height above the ground surface. The use of two closely spaced microphones has the advantage of minimizing the effects of random fluctuations due to turbulence. Preliminary measurements have been made down to 50 Hz over grass covered ground. Further measurements are underway in the range 20–500 Hz.