# Suitability of Active Noise Barriers for Construction Sites
## 5 Discussion  
### Target Area: Street Area  
Table 1 shows that the performances of both active and passive noise control mechanisms in the shadow zone of the barrier are of the same order for the range of distance of the noise source from the barrier (\(d_{pr}\)) studied in this investigation. It shows the farther the noise source from the barrier, the lower the attenuation achieved by both passive and active noise control mechanisms. The attenuation of active noise cancellation in the street area is in the range of 9.3 to 16.4 dB when the \(d_{pr}\) changes from 7 to 1 m, respectively. Passive attenuation, however, is more stable and varies from 11.4 to 14.9 dB for the same distances of the noise source. The loss of attenuation with the active mechanism is not very noticeable for \(d_{pr}\) more than 2.5 m. The same results were observed for the attenuation attained in the shadow zone of the facade. These observations imply that the active cancellation is more sensitive than passive noise control to the changes in the distance of the noise from the barrier. Table 1 also shows when the target zone is the street area, the active noise barrier can reduce the noise level at the shadow zone of the facade, but it fails to achieve reductions in the whole facade for \(d_{pr}\) from 4 to 7 m. The possible explanation for this result is the noise source farther from 4 m, the number of observational points on the top parts of the facade, which receive direct wave fronts of the noise source, and diffracted wave fronts from the control sources increase, and the control source diffracted field cannot reduce the direct field of the noise source. This suggests that the noise control mechanism is to cancel the diffracted wave fronts rather than the primary noise radiation. Moreover, the positions for the control sources and error sensors show that they are always located on the incident side and shadow side of the barrier, respectively. However, the locations are different for each position of the noise source, and for a practical active noise barrier in a changing environment such as a construction site, it is mandatory to seek a unique configuration for the whole range of the noise source's position. Figure 4 reveals that the position of the control sources and the error microphones significantly contributes to the performance of the control system. Figure 4-i shows that when the squared pressures are minimized directly at the observational points in the street area, there is a rather wide suitable region in the incident side of the control zone for the control sources to be located. However, there is a narrow region in the incident side of the barrier where the control sources achieve lower attenuation in the street area (Figure 4-i). When the control sources are placed in this narrow region, some observational points receive both direct and diffracted wave fronts, while other observational points just receive diffracted wave fronts. This mixture of control sources' field in the target area likely explains the weak performance of the control system for noise control, as it is shown for example in the work of [PERSON] et al. [18] where the active cancellation of the barrier is basically achieved in the region of diffracted primary sound field. Thus, taking into account the position of the street area, the suitable region for the secondary sources is a relatively broad region below the edge of the barrier at the incident side. Figure 4-ii displays that the active noise barrier is more efficient for those positions of the error sensors in the control zone where they receive the same wave-fronts from the noise source and secondary sources as the target area. Considering the best positions of the control sources presented in Table 1, the suitable region for the error sensors is the shadow side of the barrier and below the noise source-edge line. There is no prior study that considered a whole range of positions for transducers around the barrier, but some previous studies confirm the results of the current investigation.