Here the measurement of the normal incidence sound transmission loss (NI-STL) using an impedance tube based on ASTM E2611-09 (e.g., Bruel & Kjaer Type 4206-T) is studied. Generally, it is difficult to measure the NI-STL of a thin test sample (e.g., a thin rubber sheet) which cannot easily be installed in the tube. Also, it is difficult to measure the NI-STL of a test sample that is cut slightly smaller than the inner diameter of the tube since a sound leak is created by the gap between the tube and test sample in that case. In both these cases, a sample holding tool can be used to make it possible to measure the NI-STL, but it is important that the tool not affect the measurement results. In this study, it was found that thin acrylic rings having a rubber-filled gap (to provide a spring effect that holds the rings in the tube) had only a smallest influence on the measurement results. In this paper, use of the rings, a validation method and a validation results are described.
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