## 4 Facade Noise Control Strategies Enabling NV
### Passive Strategies  
External protrusions can be coupled with windows to shield from road traffic noise while allowing window opening for ventilation purposes. Solutions include horizontal lintels, vertical fins, eaves, louvers, and solar shadings (see Figure 2a-d for a schematic representation). Their efficacy is generally limited and depends on the view angle of the road seen by the openings and on the presence of absorption material on them [20]. Indeed, reflections by protrusions can even worsen the acoustic conditions in the building facade, by screening some areas while directing reflections on others. Louvers are most effective at high frequencies, because of diffraction effects around the blades at low frequencies. Similar considerations apply to balconies (Figure 2i), whose acoustical protection effectiveness mainly depends on the application of sound absorption material on the ceiling and on the application of ceiling reflectors, designed and oriented on the basis of the elevation angle, source orientation, and distance. Parapet inclination and balcony depth are further factors that improve the acoustic attenuation. As observed by [PERSON], studies on facade protrusions usually have not taken city reverberation into account and performance is supposed to be reduced in the presence of street canyon effects [20]. The application of reactive elements, such as quarter wave resonators around ventilation openings, has been investigated in the literature (Figure 2e). The strategy is efficient around the frequency to which the resonator is tuned, with limitations at low frequencies where the resonator would be impractically large. The main limitation of such a solution is the fact that to attenuate noise over a broad spectrum, multiple rows of resonators tuned at different frequency bands are required, resulting in a bulky device. Recently, [PERSON] et al. studied a ventilation window designed by using resonant-chamber unit cells, made of transparent acrylic panels, with apertures on the front and rear surfaces, arranged  
Figure 2: Schematic representation of passive strategies enabling natural ventilation (NV): (**a**) Horizontal lintels; (**b**) vertical fins; (**c**) eaves; (**d**) louvers or solar shadings; (**e**) resonators around ventilation opening; (**f**) trickle vents; (**g**) plenum window, horizontal type; (**h**) plenum window, vertical type; (**i**) balconies; and (**j**) double skin facades. in a plane to form an acoustic metasurface [89; 90]. The transmission loss of the partition with the installed window prototype is reported to be above 20 dB in the 800 to 3150 Hz frequency range [90]. Trickle vents are openings usually integrated into the window frame and used to provide background ventilation in combination with other forms of ventilation (see Figure 2f for a schematic representation). Recently [PERSON] et al. reviewed the performances of such devices and reported that their acoustic insulation can reach values of around 40 dB (D\({}_{n,e}\)) [91]. Notably, it must be observed that the ventilation performance depends on their correct usage by building occupants. The occupant survey conducted in Scotland by [PERSON] et al. revealed that little occupant interaction occurred with such devices (i.e., trickle vents were in many cases always left closed) and window opening remained in many cases the preferred method for ventilation provision [92]. Plenum windows are double glazed structures with staggered air inlet and outlet, forming an air passage between the two window glass panes. Depending on the opening orientation, plenum windows can be of horizontal or vertical type (Figure 2g,h). Performance depend on the overlapping length of the two glazed layers, the opening sizes, the gap width, the sound incidence angle, and the relative orientation between the window and the sound source [20].