Sound insulation and radiation directivity of plenum windows installed with cylinder arrays

Traffic noise control has long been a challenging task in a congested high-rise city, where residential buildings are built next to busy trunk roads. Many conventional mitigation measures, such as the noise barriers, setbacks and etc, are not solutions because of land shortage and safety issues. Plenum windows are proposed in recent years because of their good noise reduction capacity and their allowance for natural ventilation at the same time. However, there are many situations where the required sound insulation is too high to be provided by the current plenum window alone. Sound absorptions and fins have been proposed to be used together with plenum windows to improve the noise reduction level. However, the improvement so far is quite limited. This study is a preliminary investigation to understand how the installation of cylinder arrays (sonic crystals) inside the plenum windows can help increase the noise reduction level. A two-dimensional finite-element approach is adopted in this study to estimate the difference in the transmitted sound powers across a plenum window with and without a cylinder array. Effects of sound incidence angle on the noise reduction spectrum are also studied. Results show that a 3-by-3 circular cylinder array can provide very good noise reduction, especially at frequencies where the wavelengths are less than the clear separation between the cylinders. The corresponding noise reduction improvements are as high as 10 dB. The directivity index of sound radiation out of the window increases as frequency increases. In the cases without the cylinder arrays, the radiation directivity is reduced with increasing sound incidence angle when the latter exceeds 45° However, the variation is much smaller when cylinder arrays are installed inside the plenum window.