Absorption of high-intensity sound by a perforated panel

A study was undertaken of the nonlinear processes of fluid dynamics responsible for the absorption of a strong sound wave as it passes through a perforated panel. The velocity field near an orifice in a rigid panel was investigated using a hot-wire anemometer. From an analysis of oscillograms and spectra of the fluctuating velocity, it is concluded that the fluid-dynamic process in question evolves through five stages as the sound intensity is increased. The loss of sound energy was found to increase when vortices began to break off the edge of the orifice. The fourth stage corresponded to the onset of nonlinear sound absorption. On reaching the fifth stage, the resistance presented by the orifice rose in proportion to the velocity in the orifice. A dimensionless number D is proposed to characterize the geometrical similarity of the process upon transition from linear to nonlinear absorption. In addition to velocity, frequency, and viscosity, it includes the geometric parameters of the panel. From experimental data, the dimensionless number at which a transition occurs from linear to nonlinear orifice impedance was found to be D = 50 +/- 6.