具附加質量長形平面揚聲器之最佳化設計

Abstract

長形揚聲板的聲壓曲線在中音域處附近易有一聲壓落差，會影響揚聲器之表現。因此為了改善聲壓曲線之中音谷問題，本文研究利用附加對稱之均布質量負載來改變揚聲板的節線位置與振形模態，並探討附加質量對聲壓曲線的影響。利用有限單元軟體ANSYS分析揚聲板於簡諧激振時各節點之振幅與相角，然後將這些資料輸入由聲壓公式，以計算聲壓並繪製出聲壓曲線。實際製作並測量出複合材料揚聲器之聲壓曲線，藉比較理論與實驗結果，研究含附加點質量揚聲板之振動行為，並探討附加質量大小與位置對振形及聲壓曲線的影響，同時觀察其節線的變化和中音谷落差之關係。最後藉由最佳化設計方法找出適合之附加質量位置與大小以減緩中音谷之落差，並獲得較平滑之聲壓曲線。另依分析的結果實際製作出揚聲器並量測其聲壓曲線，結果顯示其確有較平滑的聲壓曲線表現。

Long rectangular radiating plates may have a major dip in mid frequency range on the sound pressure level (SPL) curve. In general, such dip can be harmful to the sound quality of the loudspeaker. In order to improve the SPL curve, symmetrically distributed masses can be placed on the panel to change its nodal line and modal shape. Use the experimental results to study the effects of distributed masses on acoustic radiation behavior of the plate. The finite element model of the loudspeaker is constructed through the software ANSYS. Substituting the amplitudes and phase angles obtained in the finite element analysis into the equation of sound pressure, we can determine the theoretical SPL curve of the plate. It has been shown that the theoretical and experimental SPL curves are in good agreement. The effects of magnitude and locations of the masses on the changes of modal shapes and the SPL curve are investigated. Observing the relationship between mass locations and SPL curve dip, we use an optimization technique to find the appropriate locations and magnitude of the masses that can yield smoother SPL curve. An appropriate mass is determined to suppress the detrimental modes of the radiating plate while at the same time, we can make the SPL curve smoother.