Abstract: The compound bar transducer occupies an important position in sonar systems because of its simple structure, reliable performance, etc. With the emergence and development of new functional materials and new theories of the transducers, various longitudinal transducers derived by the compound bar transducers are developed continuously, and widely used in scientific research and military fields. Understanding and mastering the working principles of these transducers is not without benefit to the relevant workers. This article describes the basic structures, working patterns, research and design methods, broadband realization ideas and the corresponding working characters of longitudinal transducers. The diverse designs of longitudinal transducers not only extend operating frequency ranges of the compound bar transducer, but also meet the many different application purposes.