## 1 Introduction  
Traffic noise not only degrades people's quality of life but also increases the risk of cardiovascular and metabolic diseases. Complaints and disputes related to the pollution of noise have increased dramatically in recent years [1; 2]. The increase in traffic volume and speed have led to a growing problem of traffic noise pollution, along with the limited availability of urban land, resulting in main roads being located near residential areas. Therefore, traffic noises are pervasive and difficult to avoid in major cities. To combat noise pollution, mitigation measures can be applied at the noise source, along the noise propagation path, or at the receivers' end [3]. For traffic noise control, the general practice is to use noise barriers [4]. A noise barrier is positioned between the noise source and the receiver; the performance of a barrier can be quantified by insertion loss (IL), which is obtained by measuring the difference in sound pressure levels (SPLs) at the same point with and without a barrier [5]. A barrier can prevent the direct traveling of sound from the source to the receiver, but the noise can still propagate to the receiver by diffraction around the top of the barrier. The performance of the barrier is limited by its height, and noise reduction at locations above the barrier is usually unsatisfactory. In addition, according to the theory of diffraction, the low-frequency noise is less affected by the noise barrier during propagation, resulting in unsatisfactory noise reduction in the low-frequency range. To overcome the shortages mentioned above, many solutions have been suggested, such as optimizing the cross-sectional geometry of the top edge of the barrier, adopting composite periodic structures, and active noise barriers [6; 7; 8; 9; 10]. Among them, active noise barriers have attracted a lot of attention. Active noise barriers are based on active noise control theory and use an electronic system containing loudspeakers and microphones that generate secondary sound waves to eliminate unwanted noise [11]. An active noise barrier prototype developed by [PERSON] et al. was able to operate in a stationary, almost pure tone, low-frequency noise environment effectively, and the prototype could obtain a peak insertion loss of about 15 dB [12]. The resultsof many studies show that the location of the loudspeaker (additional source/secondary source) and the microphone (error sensor) are important factors affecting the performance of the active noise barrier [13; 14; 15; 16; 17], in addition to the number of the controllers [18]. The sound attenuation caused by the active noise control system is only significant when the system is optimally arranged; otherwise, the active control system may be ineffective or even reduce the insertion loss of the barrier. [PERSON] et al. researched the general criteria for the locations of the additional sources and error microphones and suggested that the additional sources should be located on the incident side and below the path that connects the furthest point in the shadow zone to the edge of the barrier, and the error microphones should be located on the shadow side of the barrier where they are under the diffracted field of both the primary and the additional sources [19]. The shadow zone is behind the barrier and below the path that connects the source and the top of the barrier. [PERSON] et al. investigated the effect of the angular orientation on the noise abatement of an active noise barrier; in their research, a linear array of additional sources was placed above the top of a noise barrier. They concluded that highly effective control within the shadow zone is possible when the active noise control system is oriented at an angle where two or more error sensors are in line with the first diffracting edge and the primary noise. To further increase the area of the quiet zone, three or more control sources should be utilized [20]. [PERSON] et al. found that the additional source should be placed in a lower position than the error sensor [21].