The sound transmission loss of multilayered structures is commonly used in the building industry to quantify the acoustical performances of walls and rooms, but also in the industry to design car and aircraft components. Multilayered panels are a class of widely diffused components so that prediction models for their acoustical properties are of great importance for designers, especially to limit the number of laboratory tests which may be long, difficult and expensive to accomplish. In practice, however, between leaves in the multilayered components, finite size structural connections (points or lines) normally exist and these are not simple to be considered. Aim of this paper is to determine the equivalent characteristic impedance of a layer and a stud to be used in the classical formulation of the progressive impedance method. Predictions of the proposed method are quite satisfactorily compared with experimental results.