Abstract
The concept of a weak sound radiating cell to reduce low frequency radiated noise from structures is proposed. The cell consists of two mechanically coupled surfaces such that, when placed on a vibrating structure, the responses of the two surfaces are nearly out-of-phase and of equal strength over a wide frequency range. The cell thus behaves as an acoustic dipole and thus a therefore poor sound radiating source. The control of low frequency structurally radiated noise is achieved by covering the structure with an array of these weak radiating cells, i.e., surface treatment. Thus, the surface treatment essentially transforms the response of the structure to that of a distributed array of dipoles yielding a low sound radiating structure. For the implementation of the concept one surface of the cell is rigidly linked to the vibrating structure while the second surface is obtained by attaching a plate to the structure through a flexible mounting system. A dipole acoustic source is formed by designing the system so that the radiated sound from the plate is out-of-phase with the radiated sound from the rigidly linked surface. Experimental verification was performed by applying an array of weak radiating cells to a vibrating rectangular plate excited with white noise from 0-1600 Hz. An overall sound power level reduction of 10.2 dB was achieved between 100-1600 Hz with maximum reductions of 25 dB at discrete frequencies. These results demonstrate the potential of the weak radiating cell concept to reduce low frequency structurally radiated noise.