Baseline correction of transfer matrix-based standing wave tube measurements

Material property characterization by audible normal incidence acoustic waves has been under development since the early 1980s. Initially, reflection-based measurements were investigated by the General Motors Corporation, later termed impedance tube testing. In the late 1990s, standing wave tube testing was developed using the transfer-matrix method and a commercially available impedance tube setup. Since then, this method has been standardized in ASTM E2611-17. The standing wave tube evaluation can give measurements of effective material or acoustic properties of samples or acoustic elements placed at the sample position. This method has been applied widely since its creation to great success; however, the experimental apparatus assumes that all segments are ideal (i.e. no broken seals, walls are rigid, etc.). In this talk, a method for baseline correction of a standing wave tube test, containing non-ideal segments, is presented. The procedure is based on the transfer matrix method and is, thus, compatible with standard standing wave tube theory. Specifically, effective transfer matrices are experimentally determined by additional measurements of the empty tube (i.e., the baseline system) and used to compensate for the errors that appear in the test derived measurements (e.g., transmission loss, density, and transmission/reflection coefficients).