Normal incidence sound transmission loss evaluation with a general upstream tube wave decomposition formula

This study presents a method for measuring the normal incidence sound transmission loss of acoustical materials used in classical piping systems from upstream surface complex reflection coefficient measurements only. Based on the standard transfer function method, the wave field of the upstream tube is further decomposed, and the relationship between the complex reflection coefficient and the twice-transmitted wave is deduced. Similar to the so-called upstream surface impedance (USI) method, two microphone locations and air cavities with several depths are required. As a validation of the proposed method, both symmetrical and non-symmetrical materials are investigated. The influence of the air cavity on the experiment result is also discussed. A comparison of the results between the proposed method and existing methods reveals a perfect agreement with the verified USI method.