## 2 Natural Ventilation-Enabling Facade Noise Control Devices
### Plenum Windows/Double-Wall Structures with Staggered Air Inlet and Outlet
Figure 5 illustrates the dimensions of the two plenum windows tested. The insertion losses of both the individual plenum windows and their combined version, were measured. The results, after correcting for the room reverberation effect, show that there was an additional 7 to 9 dB traffic noise reduction when adopting plenum windows. The gap widths of these windows were small, at 175 mm, but the computed natural ventilation rate met the local statutory requirement of 1.5 air-change per hour. Furthermore, there have been proposals for improving the insertion loss of a plenum window. [PERSON] and [PERSON] [60] conducted a detailed experimental investigation on the benefit of using micro-perforated absorbers in acoustic chambers. Many different ways of installing the micro-perforated absorber were included. Their results suggest that the application of micro-perforated absorbers inside a plenum window air gap, could increase the average sound level difference (arithmetic average from 500 to 8000 Hz) across a plenum window, by approximately 2 to 6 dB. The extent of this improvement increased with an increasing gap width, and the average sound level differences ranged from \(\sim\)29 to 33 dB. The spectral average sound level differences roughly varied between 15 to 40 dB, within the 125 Hz to 8 kHz one-third octave bands. Their results also indicate  
Figure 4: Basic plenum window schematics and nomenclature. (**a**) Horizontal type; (**b**) vertical type. Red lines: boundaries of window openings; grey shaded areas: glass panes. that an air speed of 2 m/s inside the air passage did not affect the noise reduction capacity of the plenum windows. Another example has been produced by [PERSON] and [PERSON] [65], who carried out finite-element simulations in an attempt to further understand the sound transmission characteristics of plenum windows. Apart from the effect of sound absorption lining on the glass panes, discussed in [PERSON] and [PERSON] [60], it was found that sound absorptive louvers and a hood, hung at the outdoor opening, could significantly improve the noise reduction performance of the windows. Their computed spectral sound level differences roughly varied between 22 to 50 dB, within the 125 Hz to 1 kHz one-third octave bands, in the presence of absorptive louvers. The hood resulted in spectral sound level differences which fluctuated between 8 to 40 dB. In general, the longer the vertical hood length was, the higher the sound level difference was. However, the resonance significantly affected the shapes of the sound level difference spectra. Site measurement, involving a vertical plenum window, was pursued at a student hostel in Hong Kong, in order to investigate the acoustical benefit that could be achieved by lining sound absorption over the different internal surfaces of a plenum window [66]. The dimensions of the tested window were similar to those of [PERSON] and [PERSON] [60]. The sound absorption materials used were two-inch thick high density fibreglass (NRC 1.00), and micro-perforated absorbers with a 50 mm air gap between them and the glass panes (NRC 0.45). Without any additional sound absorption inside  
Figure 5: Dimensions and configurations of the plenum windows and casement windows tested previously by the author (extracted from [64]). (**a**) Plenum windows; (**b**) casement windows. Copyright Elsevier, 2015.  
the window, the difference between the average indoor sound level and that at 1 m from the outer glass pane, was 19 dB. The maximum improvement in noise reduction was 23 dB, which was achieved with all of the concrete/tile surfaces, when the outer surface of the indoor window glass pane was lined with the fibreglass and micro-perforated absorbers. [PERSON] [67] further studied how the noise reduction of the plenum window of [PERSON] [66] would be affected by tilting the indoor glass pane, using a 1:4 scale model. Owing to the limited gap width, the tilt angles could only be varied from \(-5^{\circ}\) to \(5^{\circ}\).