平板受橢圓激震器激震之振動研究

Abstract

中文摘要

本文使用多層一階剪變形理論及有限元素分析軟體ANSYS來模擬複合材料三明治振動板，並使用振動實驗來證明分析的正確性，來研究複合材料三明治振動板以不同橢圓形激振器激振的行為，並探討其聲傳行為。

由分析中發現，在長寬比大於1.5的振動板，其聲壓頻譜在中頻部分可能會產生中音谷，中音谷都是由第一個彎曲模態所造成。藉由適當地選擇激振器的參數，就可有效改變中音谷。特別是當激振器的長軸愈長，產生的中音谷落差值會愈小，中音谷的出現頻率愈向高頻移動，同時因為磁鐵變大的影響，激振器的出力也會比較大。

討論在振動板厚度、材料常數固定不變的情形下，藉由改變振動板長□比及激振器尺寸，來找出第一個彎曲模態圖中節線到振動板中心距離與中音谷聲壓落差值、位置之間的關係。從這些關係中可以得到激振器長軸是在自由邊界條件下，節線到振動板中心距離的2.5倍時，中音谷會得到比較平緩的結果。

In this thesis, the finite element constructed on the basis of the theory of Multi-layer First Order Shear Deformation Theory is applied to simulate the vibration behavior of composite sandwich plates. Experiments on vibration testing of composite sandwich plates are performed to verify the correctness of the analysis. Vibration and sound radiation of the composite sandwich plates excited by different elliptical transducers are studied using the finite element method. The parameters that can affect the sound radiation behavior of the composite sandwich plate are identified through the analysis. It has been found that the first bending mode of the plates with aspect ratio larger than 1.5 may cause the first dip in mid frequency range in the SPL spectrum of the plate. By choosing the proper diameter of the transducer, the depth of the first dip in mid frequency range of SPL spectrum can be greatly reduced. When the major axis of the elliptical transducer gets longer, the depth of the first dip in mid frequency range of SPL spectrum gets smaller and may move to higher frequency. At the same time, the force exerted by transducer to the plate will become larger due to the increase in magnet size and the sensitivity of the sound radiated from the plate will become higher.

For plates with same thickness and material constants but different aspect ratios, we have studied how the distance from the nodal line of the first bending mode to plate center and the size of transducer affect the depths and locations of the first dips in mid frequency ranges of the plates’ SPL spectra. In the study, we have found that when the long axis of an elliptical transducer is 2.5 times of the distance between nodal line and the plate center under free boundary condition, the depth of the first dip in mid frequency range of SPL spectrum can be greatly reduced.