ABSTRACT
Earlymeasurements of reflection coefficients of outdoor ground surfaces at normal incidence used adaptations of the standing wave or impedance tube technique, employed primarily to obtain the absorbing properties of building materials [1]. The method assumes plane wave incidence and requires the measurement of the ratio of the maximum sound pressure to the level of the first (or subsequent) pressure minimum (as counted from the surface of interest) as well as the distance from the surface to the first minimum at each frequency. The rather different acoustical, physical and biological properties of outdoor grounds result in several problems not encountered often with architectural absorbents. These include the following: (i) the crucial importance of low background noise, probe end and tube absorption corrections [2] in measuring the relatively high impedances associated with many ground surfaces, (ii) the difficulties of establishing the surface plane location (particularly important in determining the phase (imaginary part) of the impedance), (iii) the changing micro-meteorological conditions and the formation of worm casts during lengthy measurements [3], (iv) the unrepresentative nature of the (necessarily) small tube sample in view of the lateral inhomogeneity typical of many grounds and (v) the possibility of destruction of the local ground structure during insertion of the lower end into the ground. If a probe microphone is used in the tube, there is the additional problem of devising a stable system for vertical probe traversal. Nevertheless several useful data sets for impedance as a function of frequency have been obtained with a vertical impedance tube and probe microphone [4, 5]. The impedance tube method has proved particularly successful as a laboratory technique for measuring the acoustical properties of snow [6]. However, this involved use of a horizontal tube with automatic tracking of the probemicrophone andmeasurements both of hard-backed and quarter wavelength (air)-backed finite length samples. By this means it was possible to evaluate the normal surface impedance and the propagation constant; both quantities are
importantwhen describing propagation over such a low flow resistivity, potentially externally reacting surface as snow.
