Reduction of Sound Power Radiated into a Free Field by a Continuous Silencer

研究了辐射到自由场的声功率的被动降低。一种嘴部连续包围一次源的被动源被命名为连续被动源，因为它被认为是有效的获得嘴部面积。本文分析的模型由一个发射纯音的一次源和一个具有环形口的二次连续被动源组成。分析了无源源声阻抗与辐射声功率的关系。推导出连续无源源的最佳声阻抗、最小声功率和最大声功率是连续无源源的波数与半径乘积的函数。分析了连续无源源的辐射特性。得到了声阻抗辐射分量的表达式及其在实际重要频率范围内的近似表达式。结果表明，这种被动控制方法理论上可以实现与主动控制方法相同的声功率最大减小。还发现，这种方法的结果与另一种使用多个被动点源的被动方法的结果一致，这些被动点源在主点源周围排成一圈。以连续无源消声器为例，研究了无限挡板上有圆形环形口和与口厚相同的腔体的圆柱形消声器和环形消声器。讨论了其辐射声功率最小的优化设计及其特性对降低声功率的影响。实验结果表明，本文提出的连续被动源是有效的。

Passive reduction of sound power radiated into a free field has been investigated. A passive source whose mouth surrounds a primary source continuously, which is named a continuous passive source, is proposed because it is thought to be effective to gain the area of the mouth. A model analyzed here consists of a primary source radiating a pure tone and a secondary continuous passive source having a circular ring-shaped mouth. The relation between acoustic impedance of a passive source and sound power radiated was analyzed. The optimal acoustic impedance of a passive source and the minimum and maximum sound power were deduced as functions of a product of a wavenumber and a radius of a continuous passive source. The radiation characteristics of a continuous passive source were also analyzed. The expression of the radiation component of acoustic impedance and their approximation in the practically important range of frequency were found. It was shown that this passive approach can theoretically achieve the same maximum reduction in sound power as an active control approach. It was also found that the result from this approach is consistent with the result from another passive approach using multiple passive point sources, which are arrayed in a circle surrounding a primary point source. A cylindrical silencer and an annular silencer, which have a circular ring-shaped mouth on an infinite baffle and a cavity with the same thickness as the mouth, were studied as examples of a continuous passive source. Its optimal design for the minimization of sound power radiated and the effects of its characteristics on sound power reduction were discussed. The experiments were carried out and it has been verified that the continuous passive source proposed here is effective.