## 1 Introduction  
Excessive exposure to noise is hazardous to human health. [PERSON] et al. [1] stated that, \"prolonged exposure to noise, even at a level lower than statutory limits, can cause serious physiological and psychological symptoms\". A recent report of the World Health Organization illustrates that noise has become the second most important environmental cause of death and disability in Western Europe [2]. Although no similar survey has been conducted outside of Europe, to the knowledge of the author, it is believed that noise pollution has become a worldwide problem. The major source of noise pollution in a densely populated high-rise city, is ground traffic. However, controlling traffic noise exposure in such a city is never an easy task. It cannot be solved by simply migrating citizens, so that they live in sub-urban or even relatively rural areas, as the transportation network necessary for this migration will significantly disturb the originally quiet environment. Noise attenuation devices which are stronger than those used in the urban areas, may be required to reduce the annoying effect of the intrusive noises emitted by the relatively intermittent vehicles, in a low background noise environment [3]. Greenery views and sea views are able to moderate human noise annoyance responses [4; 5; 6]. It has also been found that the visibility of noise sources can affect human noise annoyance (for instance, [PERSON] et al. [7] and [PERSON] et al. [8]). However, such views cannot help when the traffic noise levels that have to be dealt with, can easily exceed 80 dBA. Urban re-development, with the aim of creating residential spaces, is essential for a densely populated city with limited land that is suitable for residential purposes. However, such development is also making the, already very challenging noise control, even more challenging. This is because the targeted areas were not categorized for residential land use in the past and are mostly uninhabited, because of unacceptable noise levels. Since more and more people will be living in cities in the future [9], urban noise mitigation has become increasingly important in town planning. Effective noise mitigation measures urgently need to be developed. Roadside noise barriers and enclosures are very common, as well as traditional noise mitigation measures [10]. They reduce the citizens' exposure to traffic noise by obstructing the direct-line-of-sight between the traffic and the residential units in their shadow zones. There have been extensive studies on their applications and performances (for instance [11; 12]). However, these massive structures occupy relatively large pieces of land, in order to accommodate their foundations. Also, it is undesirable to build residential units close to a barrier because of the risk of an adverse standing acoustic wave setup [13], and air movement restrictions. A noise enclosure tends to trap noise energy, and thus, the areas in the proximity of its exits/entrances are not suitable for residential purposes, due to the strong noise radiation which is emitted from the enclosure [14]. Furthermore, the high construction and land costs make them very cost ineffective for attenuating noise. Besides, the barriers and enclosures may have adverse visual impacts, so they are not always welcomed by the citizens. Moreover, they cannot be adopted in urban re-development projects, due to the lack of space in these very congested areas. Setbacks and extended podia [15] can help reduce noise exposure, but again, these measures tend to sacrifice buildable land areas and spaces, reducing the number of residential units that can theoretically be created. In a congested city, noise barriers, enclosures, setbacks, and extended podia, are therefore not desirable noise pollution solutions. The use of noise insensitive buildings as noise barriers, and a more careful street canyon design, have been proposed [15; 16; 17], but these approaches are also not easy to implement in a densely built environment, again due to the shortage of land.