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Sound absorption coefficient (SAC) and noise reduction coefficient (NRC) are the major parameters of a porous material which states its potential to be used as a noise absorbing material. The experimental set up for its estimation involves the application of a reverberation chamber which gives the SAC under random incidence. A reverberation chamber needs a separate infrastructure which may not always be economical, hence an impedance tube is used which comes with all its components as one single unit and gives the value of SAC under normal incidence. It can also be assembled with the various instruments used in noise control laboratory an hence in this paper the steps and precautions involved in the development of a two-microphone impedance tube by transfer function method as per ASTM E1050-12 has been discussed. The components involve 2 seamless tubes, 2 microphones, speakers, power amplifier, a rigid backing, data acquisition and analysis systems. These components are available with different specifications and proper selection is needed for accurate results. After proper selection, each component is checked individually for their accuracy. The dimensions involved like length and diameter of the pipe, the spacing between microphones, distance from microphone to the absorbing material is decided by referring to the standard and desired working frequency range of the tube. Their final assembly needs several machining operations on lathe, drilling and grinding machine. The concerns which need to be addressed in these steps have been properly highlighted. A LabVIEW block is formed for data acquisition and the steps and precautions involved in it has been thoroughly explained. A MATLAB code has been developed to process the acquired data and implement the equations of correction factor by swapping the position of microphones, transfer function between the two signals and SAC. Finally, to smoothen the trend, noise needs to be removed from the signal during the end stage, which needs the application of a digital filter. The results obtained from this experimental set up has been compared with a calibrated sample of melamine foam of thickness 20 mm, diameter 100mm and density 9 kg/m3. The result obtained showed a good match and thus the developed setup is validated.
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