In a previous project, it was found that the reverberation time of a real room with a perforated suspended ceiling was shorter than expected, based on its absorption coefficient in the reverberation chamber. We hypothesise that the short reverberation time is caused by high absorptive property of a perforated suspended ceiling under high-oblique angle and eventually grazing incidence. It is possible to measure the insertion loss of suspended ceilings under grazing incidence in the laboratory test stand for ducted silencers. Compared to a reference situation, the insertion loss of different suspended ceiling constructions are measured and analysed aiming at the derivation of sound absorption coefficients for gracing incidence. Additionally, numerical simulation of the oblique incidence absorption coefficient of the suspended ceiling was also performed by a three-dimensional model in FEM. The experimental and numerical results are compared and should lead to a better understanding of the absorptive properties of suspended ceilings with perforated plates and the causes of the short reverberation time in a room. In this contribution results of the investigation are presented.