Acoustic ground impedance meter

Abstract

A compact, portable instrument has been developed to measure the acoustic impedance of the ground, or other surfaces, by direct pressure-volume velocity measurement. A Helmholtz resonator, constructed of heavy-walled stainless steel but open at the bottom, is positioned over the surface having the unknown impedance. The sound source, a cam-driven piston of known stroke and thus known volume velocity, is located in the neck of the resonator. The cam speed is variable up to a maximum 3600 rpm, and since the cam has five lobes, the maximum acoustic frequency is 300 Hz. The sound pressure at the test surface is measured by means of a microphone flush mounted in the wall of the chamber. An optical monitor of the piston displacement permits measurement of the phase angle between the volume velocity and the sound pressure, from which the real and imaginary parts of the impedance can be evaluated. The prototype instrument can measure specific ground impedance at normal incidence up to 50 times the specific impedance of air. Detailed design criteria and results of measurements on an uncultivated grass field are presented.