Acoustical properties of irregular and fractal cavities

Acoustical properties of irregular cavities described by fractal shapes are investigated numerically. Geometrical irregularity has three effects. First, the low-frequency modal density is enhanced. Second, many of the modes are found to be localized at the cavity boundary. Third, the acoustical losses, computed in a boundary layer approximation, are increased proportionally to the perimeter area of the resonator and a mathematical fractal cavity should be infinitely damped. We show that localization contributes to increase the losses. The same considerations should apply to acoustical waveguides with irregular cross section. (C) 1997 Acoustical Society of America. [S0001-4966(92)00210-5] PACS numbers: 43.20.Ks [ANN].