A dynamic model of the middle ear and mastoid cavity will be presented. A variety of precisely controlled volumetric changes were simulated by the model, singularly or in conjunction with pressure changes, mass and friction effects at the level of the drum. The contributions of each factor alone and in controlled combinations to the acoustic impedance were serially tested and recorded. The validity of static compliance and tympanometry in volumetric middle ear and mastoid estimates is questioned. The model demonstrated dependability of impedance tympanometric techniques in measuring intratympanic air fluid contents and pressure changes. The model provided objective evidence on the capability of impedance techniques to differentiate mechanical traction on the drum from traction brought about by negative pressures. Clinical applications of these findings will be discussed.