The research of sound behavior in materials made possible to design the correct acoustic systems, but experimental studies offers better options than theoretical studies. Thus, a measurements system is necessary to obtain these characteristics that we are looking for. In this paper, we show the construction of an acoustic measurement system of materials and how for obtaining the complex reflection factor at normal sound incidence, based on the impedance tube prototype and the development of an algorithm that relate the signals of two microphones of the system. The method of calculation, construction and measurement are based on the procedure recommended by the ISO10534-2, which reaches a transfer function between two signals of sound pressure measures in an impedance tube. Simulations were performed with fictional materials to prove the functionality of the algorithm and his coherence in the results, and then implement it with real experimental signals in the range between 280Hz and 2465Hz. In addition to showing the results of measurement, we analyze the physical phenomena inside the tube that influences the measurement and determinate the corresponding corrections to make in the algorithm.