Study on Loosely-Supported Technique for Controlling Elastic Behavior of Test Samples in an Impedance Tube Measurement

The efficacy of loosely-supported technique for supporting test samples in the transmission loss tube is considered. This sample mounting technique can suppress the elastic behavior produced by edge constraint of the specimen by supporting the specimen loosely. The improved performance by achieved by using a loosely-supported holder of which the inner diameter of the holder is several millimeters larger than an inner diameter of the impedance tube and to cut test specimens 1 mm smaller diameter than loosely-supported holder. However, since an inner diameter changes within an impedance tube from using loosely-supported holder, the measurement results will be changed compared with the results of standard impedance tube. To account for this issue, the specific impedance can be corrected by using cross-sectional ratio between the loosely-supported holder and the impedance tube. Thus the other properties calculated from specific impedance are also corrected. In this paper, agreement of results measured by loosely-supported technique using Brüel & Kjær's Type 4206 large tube (100 mm diameter tube) and small tube (29 mm diameter tube) are explained.