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Using a tube apparatus, sound transmission loss measurements can be performed per ASTM E2611-09. Usually the intent of these measurements is to determine the sound transmission of an infinite sheet of material (i.e. the one-dimensional (1D) absorption) of a specific thickness. These results are used in simple room acoustics models and in finite element models. The tube measures the sound transmission loss of a round sample constrained within a tube which, due to edge constraint, can be quite different from the 1D transmission loss. This same issue is seen when measuring the sound absorption of material samples. The edge constraint effect is examined for automotive materials (sheet metal and vinyl barrier material) measured in a tube configured to measure transmission loss, and alternate mounting methods are investigated. The results are compared to theoretical results.