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The effect of evanescent modes on low-frequency sound field in rectangular rooms

Meissner Mirosław

Vibrations in Physical Systems

2023

Vol. 34, nr 2

art. no. 2023205

The paper investigates the sound field excited by a boundary pure-tone source in rigid-walled rectangular rooms. This approach is applicable in the low-frequency range, where sound absorption by wall surfaces can be considered negligible. The sound pressure was theoretically determined by applying the Green's function based on the cut-on and evanescent modes expansion instead of the usual normal mode expansion. The theoretical model was used to predict the spatial distribution of the sound pressure level at different source frequencies. The calculation results have shown that for audible frequencies below the cut-off frequency, the plane wave mode and evanescent modes strongly interfere which results in an interference pattern with large dips in the pressure level forming a continuous curve. A shape of this curve is highly dependent on the excitation frequency. These dips have been found to occur when the sound associated with the plane wave mode is cancelled by the sound produced by evanescent modes.

Sound Scattering by an Elastic Spherical Shell and its Cancellation using a Multi-pole Approach

Avital E. J., Bholah N. D., Giovanelli G. C., Miloh T.

Archives of Acoustics

2017

Vol. 42, No. 4

697--705

The scattering and transmission of sound by an elastic spherical shell is considered when it is subject to an incoming monochromatic planar wave. It is aimed to cancel the sound scattering using combinations of multi-pole sources located at the centre of a shell filled with compressible fluid. Assuming linear acoustics and structural dynamics, exact solutions are derived for total elimination of the sound scattering for three cases: a free-space, near a hard ground or near a free-surface, where in the last two cases it is assumed that the incoming wave propagates normal to the interface to maximize sound reflection back unto the source of the incoming wave. An elastic spherical shell of 1 m radius embedded in water and filled with air or oil is analysed to show the dominance of low-mode numbers for frequencies of less than 10 kHz and thus demonstrate the ability of this approach to damp acoustic scattering by means of low-order multi-poles inside the shell. Contour and mode distribution plots are also given and analysed.