# Directional Suppression of Monotone Noises with A Parametric Array Loudspeaker
## 6 Conclusions and Future Work  
This paper explores and confirms the feasibility of using PALs as the secondary source for directional monotone noise suppression at long distances by proposing a PC-powered ANC system running adaptive noise reduction algorithms. The proposed system is constructed with the conventional PC and generic audio hardware to provide a cost  
Figure 7: Measured noise attenuation levels across a horizontal spatial range of 80 cm at the same distances between the secondary source and the error microphone for the VCL and PAL cases. (**a**) Schematic of the setup. (**b**) Comparison of measured results. effective ANC platform. For the PAL with an aperture of 17 cm working at 40 kHz carrier frequency, the proposed ANC system can achieve around 8 dB suppression of monotone noises 260 cm away from the target location. Experimental measurements and comparisons confirm the advantages of PALs over conventional VCLs in long-range directional ANC by reducing the \(-3\) dB effective beam width by around 64% to enhance the directivity while extending the operation distance by 5.8 times when a similar ANC result is achieved. Future improvements of the proposed directional ANC solution include three aspects such as the PAL array, ANC algorithm, and system development. The future PAL design will consider the optimization of the array geometry to reduce the interference of the grating lobes by using narrower array element spacing or a curved aperture. In addition, considering the harmonic aliasing effects in PALs, more elaborate signal modulation techniques will be explored and integrated into the proposed ANC scheme to further restrain the higher harmonics. The current system employs offline identification of the secondary path, which requires fixed positions of the target area and the PAL as well as quasi-static noise conditions, limiting the system's flexibility. This can be further improved by introducing more elaborate ANC algorithms considering online estimation of the secondary path, also targeting diverse noise types such as white, pink, or pulsed noises. During experiments, we observed that it is crucial to define the proper system buffer size for data streaming and processing in the PC-based algorithm to balance the real-time requirement and convergence of the residual noise. Hence, although more costly, high-speed hardware such as digital signal processors (DSPs) or field-programmable gate arrays (FPGAs) will be considered to improve the real-time characteristics of the current system. Conceptualization, H.W., J.Z. and L.G. ; data curation, [PERSON] and [PERSON]; formal analysis, [PERSON] and [PERSON]; funding acquisition, [PERSON]; investigation, [PERSON], [PERSON] and L.G. ; methodology, [PERSON], [PERSON] and L.G. ; resources, [PERSON] and L.G. ; software, [PERSON] and [PERSON]; supervision, [PERSON] and [PERSON]; visualization, [PERSON] and [PERSON]; writing--original draft, [PERSON] and J.Z. ; writing--review and editing, [PERSON], L.G. and Y.L. All authors have read and agreed to the published version of the manuscript. This work is supported by the Natural Science Foundation of Tianjin (No. 22)CZDJC00270). The data presented in this study are available on request from the corresponding author. The authors declare no conflict of interest. The authors declare no conflict of interest. References  
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