An important subject of the acoustics is to suppress noise by using sound-absorbing materials. Depending on use there are many different methods to determine acoustical parameters, which creates the opportunity of an efficient material usage. The presented Tamura-method allows a determination of the material characteristics in term of the complex reflection coefficient for a wide range of angles of incidence. With the help of a maximum length sequence it is possible to determine the impulse response along two measurement lines above the sample surface. This distribution can be spatial Fourier-transformed into a domain of the wave vector component. Thus, it is possible to use the pane wave equations to separate the incoming and reflecting sound components analytically. The quotient of these components can be used to calculate the reflection coefficient immediately. Beside the characterization of different sound sources and the measurement system, numerical calculations are considered in advance. On one hand, these calculations serve as possibility to find out practical geometrical dimensions and verify the measurement setup, on the other hand to identify systematic errors. The numerical results are based on an empirical Komatsu-model for porous absorbers. Finally, the measurement system is verified not just with a perfect absorbing material out of air, but also with rock wool. Here the same empirical model is used as a comparative value.