The bending stiffness is one of the parameters that determines the mechanical behaviour of a plate and therefore influences the acoustic performance of building elements. Besides, it appears in the prediction models for calculating the sound reduction index, R, or recently the vibration reduction index, Kij. Its accurate determination is thus essential in the field of acoustics. The Atomic Energy Research Institute of Korea has proposed an interesting measurement technique to determine this parameter in-situ. This technique, which uses the time-frequency analysis, has been successfully applied to thin plates. However, this paper shows that corrections should be used for thick walls. Some practical considerations are given and some measurements results for masonry walls are presented.