## 5 Limits in Current Approach towards Acoustic Comfort in Naturally Vertilated Buildings
### The Lack of a Perceptual Perspective  
A differentiation of acoustic criteria between mechanical and natural ventilated buildings has already been advocated in the literature, starting from an analogy with thermal comfort criteria [104; 105]. Indeed, standards on thermal comfort provide different criteria and models, on the basis of \"different thermal experiences, changes in clothing, availability of control, and shifts in occupant expectations\" occurring in naturally ventilated buildings [111], that have led to the definition of the adaptive thermal comfort model [112; 113; 114]. Similarly, according to [PERSON], a different noise sensitivity could be assumed under NV conditions, resulting in higher noise levels to be tolerated, because of the following: (i) a lower expectation of low noise levels, (ii) the appreciation of non-acoustic benefits and (iii) a different availability of control in comparison to mechanically ventilated buildings [104; 105]. Although it is not in the scope of the document, the AVO guide states as well that \"there are other benefits for occupants from opening windows, such as the connection with the outside, sense of fresh air and perception of draughts when overheating, and sense of control over one's environment\" [79]. Only through a perceptual perspective it would be possible to capture a broader picture over the complex user-building interaction occurring under NV conditions. Because it is currently only supported by anecdotal evidence, the existence of an \"adaptive acoustic comfort\" [115] model is worth further investigation by the research community to define acoustic criteria taking into account the multiple cognitive and behavioral factors impinging on comfort in naturally ventilated residential buildings.  
Currently, acoustic design criteria and design strategies for facade devices reflect a general effort of minimizing noise annoyance by reducing noise exposure and, consequently, sound levels. The literature has already reported numerous criticisms on the effectiveness of decibel-based metrics in fully characterizing perceived sound quality (e.g., pleasantness ratings) [51]. Indeed, reducing noise levels do not always lead straightforwardly to an improved comfort or quality of life, as sounds and even loudness can be desirable in some contexts [47; 51; 116]. Aspects related to the nature of sounds and to their spectral and temporal composition have been found to significantly affect perceived sound quality [51]. The picture is even more complex if factors unrelated to the acoustic environment are considered [51].  
That is to say, the mere use of higher sound level limits according to an \"adaptive acoustic comfort\" model is not enough to define acoustic criteria tailored to users' perception in naturally ventilated buildings. In order to assess the effectiveness of facade strategies to satisfy ventilation and acoustic needs, a perceptual perspective should be undertaken. Subjective assessment methods and objective metrics should consider how building users perceive the acoustic environment in order to understand what sound good to them. This is the goal of soundscape research.