## 2 Natural Ventilation-Enabling Facade Noise Control Devices
### Balconies  
A balcony on the facade of a building can provide an extended outdoor area with functionality, which cannot be achieved by indoor living spaces [36], and is very often welcomed by the residents. A balcony blocks the direct line-of-sight of a ground noise source from the receiver, while natural ventilation is possible. It has therefore attracted the attention of many researchers in the past few decades. A description of the various common balcony forms can be found in [PERSON] [27]. These balcony forms are shown in Figure 3, for easy reference. The balcony was once regarded as a green design in terms of noise protection, but it is now understood that its acoustical protection is not obvious. Figure 3: Common forms of high-rise building balconies (extracted from [PERSON] [27]). Copyright Acoustical Society of America, 2005. In an open space with a noise source on the ground, a standalone balcony on a building facade provides significant acoustical protection to the areas beyond it. [PERSON] and [PERSON] [37] carried out a detailed experimental investigation, using a 1:10 scale model, to study the effects of source orientation, balcony form (in terms of depth and with and without parquet), window opening size, and locations, on the acoustical protection. On average, a ~5 dB insertion loss was recorded for a closed form balcony. [PERSON] and [PERSON] [38] have also completed a theoretical analysis on the acoustical protection of a standalone balcony on a building facade. Additionally, [PERSON] and [PERSON] [39] suggested the use of a recessed building form for acoustical protection. The proposed recessed building form consisted of balconies arranged in the form of a terrace, reducing the jeopardizing reflection from the balcony ceiling, onto the windows or openings behind the balcony. The measured traffic noise reduction increased from ~5 dB at a very small effective path difference, to ~14 dB at an effective path difference of 1 m [39]. However, a recessed building form is not practical for a congested city, as it will largely reduce the number of residential units that can theoretically be built. [PERSON] [40] has performed a field measurement within the balconies of a high-rise building located next to a highway. Although a direct measurement of the balcony insertion loss could not be taken, his results show that the reflection from the balcony ceiling, as well as the multiple reflections within the balcony, can largely offset the noise screening effect of the balcony. Sound absorption is thus needed before a balcony on a high-rise building facade can be used as a noise screening device. Conventional courtyards have high solid walls without ceilings, and thus, can offer high acoustical protection. [PERSON] and [PERSON] [41; 42] investigated the resultant acoustical protection when thandners and splitters were applied to the courtyard wall, and the balcony front parquet facing the road traffic. A thnadher is a kind of shaped noise barrier [43]. For the courtyard cases, the use of a thnadher and splitter was shown to not significantly affect the acoustical protection, when compared to a solid wall, except at floor levels close to the ground/source. [PERSON] and [PERSON] [41] showed that the protection of a courtyard with a thnadher ranged from 8 to 26 dB, depending on the depth of the courtyard and the height of the courtyard above the ground. The corresponding protection for the splitter cases, varied between 10 dB and 27 dB. Although the protection appears quite high, these devices have adverse effects on the natural ventilation. The multiple reflections inside a balcony with a ceiling tended to reduce the protection by ~3 dB, as shown by [PERSON] and [PERSON] [42]. Recently, [PERSON] [27; 44] used a 3-storey scale model to study the effects of common high-rise building balcony forms on the acoustical insertion losses. The width-to-depth ratios of the adopted balconies were all 2:1. The source orientation, elevation angle, and source distance, were found to be well correlated to the insertion losses.