Absorption coefficient is a well known characteristic for different passive noise control applications, usually given in octave bands. Instead of having a diffuse field absorption coefficient as the only specification for an absorbing surface, one might be interested in having more information to go on. According to the references, using an angle dependent absorption coefficient in room acoustic modelling can yield a better agreement with the real life situation, up to 15%. During this work the absorption coefficients in different angles of incidence were studied for two materials – the plates of mineral wool and for perforated plasterboards. The set-up with different absorbing materials and an air gap of 20cm behind them is measured. Most of the measurements were conducted in the free field environment, using the MLSSA system. In addition the traditional Kundt’s tube measurements were conducted in comparison. Furthermore, in the analysis part, the subtraction technique is employed. The results are compared with each other and with an analytical model. The materials under observation show an adequate agreement with the expectations and the analytical model.