Near‐ground sound fields and surfaces of finite impedance

Abstract

Studies over the past 30 years have elucidated the effects and interactions of many mechanisms involved in sound propagation outdoors. Interactions with the ground surface are especially important for many applications to practical problems. It is particularly significant that ground surfaces are porous, and thus have a finite complex acoustic impedance. The history of sound field measurements to deduce values of impedance, and its direct measurement, is reviewed. Such data prompted, and were then used to validate, models for ground impedance, first in terms of a single parameter (flow resistivity of the ground surface) and later in terms of one to three additional parameters. Impedance values for many grounds at low frequencies are difficult to measure accurately and several new techniques have been developed. Nonisotropic and nonhomogeneous ground has been modeled as a layered medium or as a fluid‐filled porous matrix. These efforts have increased our understanding of the acoustical properties of the gr...