Bolts, screws, rivets, and pins can significantly complicate the prediction of damping, stiffness, and resonance frequencies of built-up systems. Because the interaction of interfacial joints is not well understood, current predictive models incorporating fastened joints often require over-designing for safety. To better understand the behavior of fastened joints, several experiments have been performed on a built-up structure with two rectangular 1/4" plates fastened together by two screws on a 1/2" thick flange. The structure was excited via acoustic excitation, and acceleration was measured at multiple locations on the flange and plate during and after ensonifying. While the energy input to the system may be lower for acoustic excitations than other direct contact methods, the results in this paper show that ensonifying the structure has two important advantages over traditional impact hammer and shaker methods. First, resonances of the structure can be excited and analyzed individually in both steady state and free-decay conditions, which is an advantage over impact excitation. Second, the excitation method is non-intrusive and does not change the system properties, which is an advantage over shaker excitation. The proposed method is shown to be an effective way to excite structures for both steady-state and decay measurements.