Important transducer characteristics such as resonance frequency, mechanical quality factor, characteristic impedance and electromechanical coupling coefficient will be discussed in more detail in this chapter. One reason for more discussion is that some aspects of transducer theory are not standardized. For example, several different definitions of electromechanical coupling coefficient and of mechanical quality factor are in use. In particular it is important to extend the discussion of the electromechanical coupling coefficient in Section 1.41 to include other definitions, properties and interpretations. Another reason for more discussion of transducer characteristics is the need to present certain practical considerations that have not been fully developed in the previous chapters. In Chapters 2 and 3, we pointed out that transducer performance is determined by an effective coupling coefficient that is usually less than the material coupling coefficient of the active material used in the transducer. There are numerous causes of this reduced effective coupling that occur in all transducers, such as inactive transducer components and dynamic operating conditions, and eddy currents that occur in magnetostrictive transducers. We will present practical methods of determining the effective coupling coefficients that result from many of these causes, with some specific examples.