Sound reductions of plenum windows installed with cylindrical scatterer arrays in the presence of a linear sound source

• Sound transmission across plenum window installed with scatterer arrays was studied. • Source orientation ranges for weak sound reduction without scatterers are determined. • Gap-wise standing waves within the window inlet cavity are important in shaping the insertion loss landscape. • Results indicate the variation of insertion loss within a bandgap. • A higher density of bandgaps does not lead to higher insertion loss. A scale model experiment was carried out in the present study to understand the sound transmission losses across acoustic plenum windows installed with the ‘sonic crystal like’ cylindrical scatterer arrays with square and rhombic lattices in the presence of an incoherent linear sound source. The effects of the band structures of these arrays, the plenum window dimensions and the sound source orientation angle on the sound transmission losses were also examined. The results illustrated that the traffic noise transmission loss increases with increasing filling fraction, but meaningful noise reduction enhancement is only possible at filling fractions greater than 0.033. A larger window gap width results in improved angular uniformity of noise reduction. Stronger sound transmission loss is achieved with those scatterer array configurations having a half scatterer attached to the window panes. The arrays with square lattice and half scatterers on the window panes gives the best sound reduction. It is also evident that the band structures of the arrays have no effect on the spectral characteristics of the sound transmission when the sound source is nearly parallel to the window. Strong influences of the standing waves formed within the window inlet cavity are observed under such circumstance. The bandgaps of the scatterer arrays in general run over regions of high spectral sound transmission loss or of strongly increasing sound transmission loss. However, the sound transmission losses vary within each bandgap. Besides, it is found that the density of the bandgaps does not have direct relationship with the magnitude of the sound transmission loss. The length-wise acoustic modes of the window cavity only have limited effect of the sound transmission loss even at frequencies below the lowest bandgap eigenfrequency.