ACTIVELY CONTROLLED SURFACE IMPEDANCE OF THE UPPER EDGE OF NOISE SCREENS AND ACTIVE MODAL CONTROL OF THE SOUND FIELD

초록

In this paper analytical models with secondary sound sources at the upper edge of a semi-infinite screen is introduced to study the effects of the actively controlled impedance of the upper edge on the primary sound energy transportation mechanism and on creating acoustical shadow zone in noise barriers. To investigate the sound field with locally minimized impedance in the adjacency of the upper edge, a rigid cylinder with integrated secondary sources is selected, as a top section of the screen. For an active modal control of the circumferential mode around the screen, modal sources in the immediate vicinity of the top edge are considered. The improvement of the shielding effect of the screen due to the local active control is compared to the improvement due to the global active control which means the sound field minimization behind the barrier in the far field. On the basis of the theoretical and numerical study, practice-oriented local active control method is tested with model measurements in an anechoic room. The results are shown and discussed.