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Polymer foams are the most commonly used acoustic materials for absorbing the sound energy in noise control engineering, because of their high porosity and lightweight. The acoustic performance of these materials, namely, absorption coefficient and transmission loss can be measured using impedance tube, reverberation room method, and Intensity methods. Among these, the impedance tube method is the most popular method in laboratory conditions. Depending on the plane-wave propagation requirement, the tube is categorized as large and small. The large tube covers the low-frequency range (64 Hz – 1600 Hz) and facilitates the acoustic measurements for 100 mm diameter samples. On the other hand, 30 mm diameter samples are required for the small tube that covers the high-frequency range (800 Hz - 6300 Hz). Sample size is one of the critical parameters that result in accurate measurements using the impedance tube. This paper discusses the effect of the variation in sample sizes on their acoustic measurements. Foam samples are prepared using the resistance wire foam cutter (RWFC). This cutter is developed to cut circular samples accurately. It consists of two major subsystems, one of which is the temperature-generating unit and the other is the sample rotating mechanism. The temperature-generating unit consists of Nichrome wire connected with the DC power supply. It has a provision to control the voltage and current based upon the temperature requirements. The sample-rotating unit consists of a belt-driven gear train with rotating input. The acoustic measurements using the impedance tube can be performed for two different sized samples, namely, 30 mm and 100 mm diameter. These test samples are prepared with developed RWFC. One of the parameters chosen for study is the diameter of a sample for constant sample thickness. The large sized samples are prepared with diameters of 98 mm, 100 mm, and 102 mm. Similarly, for small sized samples, the diameters are 28 mm, 30mm and 32 mm. Samples which have a size less than and greater than reference size is called as undersized and oversized, respectively. Absorption coefficient and transmission loss for large and small samples are measured and compared with the accurate dimensional results. A detailed measured data analysis was done to understand the effect of undersize and oversize foam samples on absorption and transmission loss results as a function of frequency.
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