In the last , a experiment is carried out , the experiment result denotes that the absorption effect is smart in middle and low frequent and imperfect in high frequent for passive absorption is reigning 最后进行了实验，实验结果表明了在中低频平面声波入射时，采用本方法的吸声效果改进比较明显，但在高频平面声波入射时，采用本方法的吸声效果就不是很明显，此时吸声主要是依靠吸声材料被动吸声。
Radiation modes are dependent on the radiator ' s geometry shape and vibration frequency , but are not relative to the material characteristic and the border condition of the radiator . further more , the radiation efficiency of the radiation modes falls off very rapidly with the increase of modes order at low frequency . the adjoint coefficient of sound radiation modes is proportional to the corresponding sound radiation power 声辐射模态由辐射体的几何形状和振动频率决定，与辐射体本身的材料特性以及边界条件无关；特别是在中低频率时，声辐射模态对应的辐射效率随模态阶数的增加而迅速降低。
- 2 . in order to accomplish the extraction measuremellt of distorted wavefront on specific plane of a laser beam , a crsi ( cyc1ic radial shearing interferometer ) based on relay imaging technique was built . the experiinental result on this crsi shows it can used to test the spatial mid and low frequency wavefront distortion 二、为了实现激光光束在确定平面上的波前畸变的准确检测，采用中继成像技术建立了一套基于空间相位调制的环路径向剪切干涉波前传感硬件系统，实验证明，它可用于中低频段波前畸变的检测。
Secondly , according to the character of this kind of harmonic , the solutions of power filter are deeply analyzed and an optimized topology for series connected hybrid power filter is obtained . by pwm technique , midfrequency transformer and the principle of the magnetomotive force ( mmf ) compensation , the problem to the transfer of the low - frequency harmonic power compensatory signal in the series apf is solved , in technique 其次，针对acem系统特有的“电源谐波”特性，深入分析了滤波器方案，得到了一种适用于acem系统谐波抑制的串联混合型有源电力滤波器优化拓扑结构，并采用了pwm调制技术、中频注入变压器和磁势补偿原理有效地解决了串联apf中低频谐波功率补偿信号传输的难题。
It shows that a disadvantag ekists when a crsi is used to measure a step structure in a wavefront , but it is useful to adjust a crsi system with lcslm . 4 . according to the didriction effect of an aperture , the spatial resolution of our crsi was analyzed and the transform function of our crsi was calibrated by using the psd method of a step wavefront . and some factors which are associated with the accuracy of our crsi were analyzed too 四、从光学系统衍射效应的角度分析了本文建立的环路径向剪切干涉仪的空间分辨率，并利用波前功率谱密度的概念对该干涉仪的空间频率响应特性进行了标定，分析了限制该干涉仪测量精度的主要因素以及径向剪切比与测量精度的关系，并指出了该干涉仪的测量范围，结果显示该干涉仪基本上可用于准确测量中低频段的波前畸变。
Since there are more low - frequency elements in the engine noise , and simply barring cannot guarantee satisfactory depression , a composite impedance structure should be applied in muffling passages so as to control the propagation of low - frequency noises by means of the muffling characteristic of the resistance structure 由于发动机噪声中低频成分更为丰富，单纯阻挡不能达到满意效果，因此消声通道应选用阻抗复合结构，借助抗性结构的消声特性来控制低频噪声的传播。
