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

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.