A modal summation method based on far-field sound intensity is used to study the average radiation efficiency and the corresponding radiation power of a point-excited, simply-supported, rectangular sandwich plate containing a tunable electrorheological fluid (ERF) core, and set in an infinite rigid baffle. In addition, a classical analytical procedure based on the Rayleigh integral equation method is adopted to investigate the sound transmission characteristics (TL) of the adaptive plate insonified by plane pressure waves at an arbitrary angle of incidence, or excited by a perfectly diffuse sound field with a Gaussian directional distribution of energy. Numerical results reveal the imperative influence of an applied electric field strength (0-3.5 kV/mm) on controlling acoustic radiation from (or sound transmission through) the smart panel in a wide frequency range. In addition, an effort is made to find the optimal electric field which yields improved sound radiation and transmission characteristics for each excitation frequency. Limiting cases are considered and good agreements with the solutions available in the literature used in this study are obtained.