ANSI/ASA S1.18:2018

Outdoor sound close to the ground is influenced by the acoustical properties of the ground. This Standard describes recommended procedures to characterize, and the instruments to measure quantities that may be used to deduce, the acoustical properties of ground surfaces. Although this Standard is intended primarily for outdoor measurements, indoor measurement of undisturbed portions of a ground surface, such as sod, is within its scope also.
The Standard yields the real and imaginary parts of the normalized specific acoustic impedance ratio of ground surfaces in the frequency range between 250 and 4000 Hz for outdoor sound propagation predictions. The Standard uses measurements of the interference between direct and ground-reflected sound to deduce both normalized specific acoustic impedance ratio and impedance model parameters. The impedance-ratio model parameters of effective flow resistivity and a porosity factor, determined from best fits to the templates of calculated level differences, may be used to estimate the normalized specific acoustic impedance ratio at frequencies outside the specified range.
The basic purpose of this Standard is to establish uniform procedures for obtaining the real and imaginary parts of the normalized specific acoustic impedance ratio of ground surfaces outdoors.
The method is applicable to all nominally flat, commonly occurring surfaces including grassland or snowcovered ground.
The method is not applicable to rough grounds where the variation in height is greater than half of the shortest wavelength of interest. For the specified upper frequency of 4 kHz this limits the variation in height to about 5 cm. See also Clause 4.4.