A Study Of Effects Of Boundary And Directional Sound Sources On Virtual Sound Barrier Systems

A Virtual Sound Barrier system (VSB) can create a quiet zone by using active control method. Research on VSB is mainly based on the assumption of free field and monopole secondary sources. This thesis investigated VSB systems near a rigid wall or with directional secondary sources. Furthermore, a new driving function for secondary sources in a VSB system is deduced and used in local three dimensional sound reproduction.The effects of a rigid wall on a VSB system are investigated firstly. It is found that when a VSB system is close to a reflective surface, its noise reduction performance fluctuates around the one in free field. The rigid reflective surface makes the noise reduction vary periodically with the increase of the distance and the period length is demonstrated to be half wavelength for normal incidence. When the distance is even and odd times of quarter wavelength, the noise reduction becomes minimum and maximum respectively. This is caused by the similarity of the distribution patterns between the primary sound field and the control sound field. Besides, A hybrid virtual sound barrier system with a nearby reflective surface is proposed and it's performance is discussed.A new driving function for secondary sources in a VSB system is deduced by using the conception of continuous sources. Based on it, a VSB system using fixed-directivity sources and variable-directivity sources is discussed and compared with that using the conventional least square method. It is found that the noise reduction using the directional sources is higher than that using the monopole sources when the least square method is adopted. The noise reduction of continuous sources method is higher than that of least square method at the place near the center of VSB system, while at the far place the noise reduction of continuous sources method is lower. Being compared with the monopole sources, the directional sources can lower the influence on the outer space of VSB system.Finally, based on the continuous sources concept, a new method is proposed in local three dimensional sound reproduction and the feasibility of the method is demonstrated by using numerical simulations. The driving function for sound sources is deduced, which depends on the original sound field, the source locations and the frequency. Being compared with the spherical harmonic expansion method and the wave field synthesis method, the proposed method can avoid the error caused by the matrix inversion and enhance the reproduction accuracy in a small reproduction area containing the center point. Thus, the proposed method has a better balance between reproduction accuracy and reproduction area.