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 Helmholz resonator, constructed of heavy-walled stainless steel but open at the bottom, is positioned over the surface having the unknown impedance. The resonator chamber measures 0.229 m in diameter by 0.152 m high. 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. 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. Measurements using a five-lobed cam can be made up to 300 Hz; these can be extended by means of a 15-lobed cam, presently under fabrication, to nearly 1000 Hz. Detailed design criteria and results on a soil sample will be presented.