Control of low frequency structurally radiated noise with an array of weakradiating cells: An experimental study

The concept of a weak sound radiating cell to reduce low frequency radiated noise from structures is proposed. The cell consists of two mechanically c oupled surfaces such that, when placed on a vibrating structure, the respon ses 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 thu s a therefore poor sound radiating source. The control of low frequency str ucturally radiated noise is achieved by covering the structure with an arra y 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. Fo r the implementation of the concept one surface of the cell is rigidly link ed to the vibrating structure while the second surface is obtained by attac hing a plate to the structure through a flexible mounting system. A dipole acoustic source is formed by designing the system so that the radiated soun d from the plate is out-of-phase with the radiated sound from the rigidly l inked surface. Experimental verification was performed by applying an array of weak radiating cells to a vibrating rectangular plate excited with whit e noise from 0-1600 Hz. An overall sound power level reduction of 10.2 dB w as achieved between 100-1600 Hz with maximum reductions of 25 dB at discret e frequencies. These results demonstrate the potential of the weak radiatin g cell concept to reduce low frequency structurally radiated noise.