# Impact and Control of Reflected Noise from an Overpass Bottom
## 1 Introduction  
In the recent years, many efforts have been spent by the scientific community to study and propose new mitigation systems for the main sources of noise: Road traffic [1; 2], railway traffic [3; 4], airport [5; 6], and wind turbines [7; 8]. Unfortunately, noise pollution continues to be a major health problem around the world, with millions of people exposed to noise levels leading to health effects studied with sufficient evidence: Sleep disorders with awakenings [9], learning impairment [10; 11; 12], hypertension ischemic heart disease [13; 14; 15], and noise annoyance [16]. The traffic noise is an environmental problem that typically accompanies urbanization. As the network of roads has expanded, traffic noise has become increasingly annoying to the public. Even if is not the most annoying type of noise, road traffic noise is the most widespread, especially in urban areas, indeed it is considered as a reference in limits assessment [17]. The generation of road traffic noise mainly attributes to the interaction between the tire and the road when vehicles are operated at medium/high speeds. The tyre/road noise is considered to be proportional to the vehicle speed [18]. Noise barriers (NBs), indeed, are the most commonly used approach to mitigate noise and not even the best in a correct action plan phase [19; 20]. The use of pavements with low acoustical emission profiles is one of the most applied alternatives to mitigate noise levels [21; 22]. Additionally, the fabrication of sound absorption materials is also an important approach to reduce noise. Sound absorption materials decrease noise by disseminating energy and turning it into heat [23]. Porous materials suchas absorption foams and fibers are commonly used to reduce noise [24]. The best solutions to be chosen are those mitigating the wider number of citizens, which for road noise can be changing flux, reducing heavy vehicles or changing pavements towards ecological and recycled rubberized asphalts [25]. Reflected noise may occur when acoustic barriers exist on either side of a road, when buildings line the roadside, or when vehicles move through a culvert, an underground passage, or on a double-decker road [26; 27; 28]. Generally, reflected noise is most evident within a road when acoustic barriers exist on either side of a road [29]. Other than road width and the material used in the design of acoustic barriers, the noise reflected by acoustic barriers is also affected by natural conditions such as wind speed and wind direction. The volume of reflected noise increases when the distance between roadside acoustic barriers and a vehicle decreases, primarily because when a vehicle is close to acoustic barriers, the area on acoustic barriers available for sound reflection increases [30; 31]. Buildings on either side of a road are also an important source of reflected noise. When building facades have large, flat, and smooth surfaces, the noise from vehicles is reflected easily, increasing the noise level at and around roadsides [32]. Urban geometry also plays an important role in traffic noise reflection and propagation. [PERSON] et al. [28] evaluated the relationship between traffic noise and urban geometry using the sky view factor (the degree of sky obstructed by buildings). The authors indicated that the noise level was higher in areas with low sky view factor (greater sky obstruction caused by the buildings) because of higher noise reflection and propagation. Traffic noise levels may change at different heights of receivers. [PERSON] et al. [33] evaluated vertical distribution of traffic noise levels at different floor levels of a 20-storey residential building in Hong Kong using measurement and the Calculation of Road Traffic Noise (CRTN) model prediction. The results showed that traffic noise level decreased with increasing floor level. Recently, densely populated urban areas have been the target for overpass construction in Taiwan. When an overpass project is complete, the old road remains in use under the overpass.