Low frequency passive noise control of a piston structure with a weak radiating cell

The concept of a weak sound radiating cell is proposed to reduce the low-fr equency radiated noise from structures. The cell consists of two mechanical ly coupled surfaces such that, when placed on a vibrating structure, the re sponse of the two surfaces are nearly out-of-phase and nearly of the same s trength over a wide frequency range. This structure response leads the cell to behave as an acoustic dipole and thus a poor sound-radiating source. Th e control of low-frequency structurally radiated noise is then achieved by covering the structure with an array of these weak radiating cells, i.e., s urface treatment. Thus the surface treatment essentially transforms the res ponse of the structure to that of a distributed array of dipoles yielding a low sound radiating structure. A theoretical model of a single weak radiat ing cell applied to a simple piston structure was developed and experimenta l verification was performed. Overall sound power level reductions of over 6 dB were experimentally achieved between 400 and 1600 Hz with maximum redu ctions of over 30 dB at discrete frequencies. (C) 1999 Acoustical Society o f America. [S0001-4966(99)02007-X].