Vibration of air-coupled transverse bulkheads of a submarine

Vibration of a submarine's watertight compartments, lined up in the rigid cylindrical pressure shell, in which adjacent bulkheads (treated as thin plates) are coupled weakly through air cavities, is investigated by theoretical methods. Such air coupling separates the natural frequencies of each plate, and for each set of split modes, the frequency difference increases as the cavity depth is decreased. The system-level modes in which plates in the array vibrate in-or out-of-phase with each other are at closely spaced frequencies. Compared to each mode of a single plate, there exist one in-phase mode and N - 1 combinations of out-of-phase system modes for a N-plate array. A vibration model comprising N classical thin plates that are coupled through the three dimensional acoustic fields established in the circular cavities between the plates is developed, and its results are shown in splitting the natural frequencies. In order to solve for the system modes, plate- and air-alone expansion functions are obtained in advance by considering a clamped circular plate and a rigid cylindrical acoustic cavity, separately.