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The acoustical insertion losses of plenum windows installed on a building facade in the presence of a non-parallel line source are studied by using a 1:4 scaled down model in a semi-anechoic chamber in the present investigation. Two types of insertion losses, weighted by the normalized traffic noise spectrum (from the 100 Hz to 5000 Hz one-third octave bands), are defined with different references. The first one is for the case where the orientation of the building facade relative to the line source is fixed. The reference case is the opened window having the same orientation angle as the plenum window. The maximum and minimum insertion losses under this condition across the orientations tested are found to be around 14 dB and 5 dB, respectively. The other is the opposite situation where such orientation is allowed to change because of practical purposes and the reference for this condition is the opened window with its width span parallel to the line source. The corresponding maximum and minimum insertion losses are found to be around 18 dB and 8 dB, respectively. There are evidences showing that the lower order plenum acoustic modes are responsible for the relatively high low frequency insertion loss.

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The acoustical insertion losses produced by a balcony-like structure in front of a window are examined experimentally. The results suggest that the balcony ceiling is the most appropriate location for the installation of artificial sound absorption for the purpose of improving the broadband insertion loss, while the side walls are found to be the second best. Results also indicate that the acoustic modes of the balcony opening and the balcony cavity resonance in a direction normal to the window could have a great impact on the one-third octave band insertion losses. The maximum broadband road traffic noise insertion loss achieved is about 7 dB.

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