  Due to their unique electromechanical coupling properties, soft
electro-active (SEA) resonators are actively tunable, extremely suitable, and
practically important for designing the next-generation acoustic and vibration
treatment devices. In this paper, we investigate the electrostatically tunable
axisymmetric vibrations of SEA tubes with different geometric sizes. We
consider both axisymmetric torsional and longitudinal vibrations for an
incompressible SEA cylindrical tube under inhomogeneous biasing fields induced
by radial electric voltage and axial pre-stretch. We then employ the
state-space method, which combines the state-space formalism in cylindrical
coordinates with the approximate laminate technique, to derive the frequency
equations for two separate classes of axisymmetric vibration of the tube
subjected to appropriate boundary conditions. We perform numerical calculations
to validate the convergence and accuracy of the state-space method and to
illuminate that the axisymmetric vibration characteristics of SEA tubes may be
tuned significantly by adjusting the electromechanical biasing fields as well
as altering the tube geometry. The reported results provide a solid guidance
for the proper design of tunable resonant devices composed of SEA tubes
