## 2 Natural Ventilation-Enabling Facade Noise Control Devices
### Plenum Windows/Double-Wall Structures with Staggered Air Inlet and Outlet  
Although double glazing windows do not allow for natural ventilation, a partially opened one will allow for some degree of air movement across it, while the sound transmission loss may still be acceptable. [PERSON] and [PERSON] [59] carried out a series of tests on the sound transmission losses across slightly opened double glazing windows. The two window openings in their experiment were staggered, and the air gap between the two window glass panes formed an air passage. A window design with a width of 2.4 m, a window frame separation of 200 mm, and an opening size of 30 mm, could provide a 9 dB higher traffic noise reduction than an opened single window. Although the 30 mm opening width of [PERSON] and [PERSON] [59] is far too small for meaningful natural ventilation, this window type has recently attracted attention. It is now commonly referred to as a ventilation window or plenum window [60; 61] (hereinafter referred to as plenum window). This device is a double-walled (usually glass panes) plenum structure, with a staggered air inlet and outlet. The plenum window can be categorized into the horizontal and vertical type, according to the orientation of the openings, as shown in Figure 4. However, the corresponding noise reduction mechanisms are basically the same. In order to provide meaningful air movement across the window for acceptable natural ventilation, the sizes of the openings have to be increased. [PERSON] and [PERSON] [62] conducted both laboratory and site tests on an air supply window in Denmark. Their window type was of the vertical plenum window type, without sound absorption material. The opening sizes of their vents, though larger than those of [PERSON] and [PERSON] [59], were still small compared to the physical length of the window, probably because of the Scandinavian climate. The corresponding noise reduction \(R_{w}+C_{tr}\) ranged from 16 to 24 dB. However, such a window design is not suitable for use in tropical or sub-tropical regions. [PERSON] et al. [63] showed that their double-facade system, which was basically a plenum window setup, could offer a noise reduction, \(R_{w}\), of between 18 to 26 dB, when no sound absorption material was applied. A parametric study on the sound insertion loss of a horizontal type plenum window was carried out by [PERSON] and [PERSON] [61], using 1:4 scale model experiments. The effect of the window orientation, relative to the sound source, \(\theta\), was also tested. Their results show that, for a fixed overall window width \(L\), the insertion loss of a plenum window generally increased with increasing overlapping length \(G\), but decreased with increasing window opening sizes (\(W_{1}\) and \(W_{2}\)), or gap width \(D\). In their study, the overall window width, \(L\), equals \(G+W_{1}+W_{2}\) and \(W_{1}=W_{2}=W\). The maximum and minimum traffic noise insertion loss increases, achieved by replacing an opened casement window with a plenum window of the same overall window size, where \(0.1\leq G/L\leq 0.2\), \(0.1\leq W/L\leq 0.5\) and \(-90^{\circ}\leq\theta\leq 90^{\circ}\) were 15 dB and 5 dB, respectively. The maximum insertion loss was recorded within \(-30^{\circ}\leq\theta\leq 30^{\circ}\), while the minimum wasrecorded at \(\theta=-90^{\circ}\) or \(90^{\circ}\). The effect of the window height was not examined, as this height should be more or less fixed in practice. The insertion losses of the horizontal type plenum windows were further tested using a site mockup, whose facade was next to and parallel to a busy trunk road [64]. In this mockup, two side-by-side standard residential units of a Hong Kong public housing estate, were constructed. One of them was equipped with plenum windows, while the other represented normal provision (side-hung casement windows). Measurements inside and outside these units were conducted simultaneously, such that the acoustical benefit of replacing the side-hung casement windows with plenum windows, could be estimated in-situ.