Abstract: In this paper the relationship between the maximum efficiency of transducer and their dynamical parameters (electrical and mechanical Q, effective electromechanical coupling coefficient) is described. The loading condition which gives maximum efficiency is analysed. It is pointed out that the dynamical parameter N_eff, which is a function of the transducer material, art of preparation and the mechanical structure of the transducer unit, are the important parameters that control the of transducers. The larger the parameter N_eff is, the higher is the efficiency of the transducer and the smaller is the influence of the load. Proposed methods of measuring parameter N_eff are also given.