Interpretation of sample wave speed measured in an impedance tube

Abstract

The basic measurement in an impedance tube is a complex reflection coefficient, from which the propagation speed and attenuation are computed. This speed is often identified with the dilatational wave speed in the sample material. This is only approximately correct; it depends on the ratio of sample radius to inner radius of the tube. Thus a correction to the measured wave speed is in order. Moreover, by varying the ratio of the radii one may derive two elastic moduli for the sample material from the measurements, as is shown by an experimental example for a Lucite sample.