平板受長條狀音圈激振之振動與聲傳研究

Abstract

本文探討狹長形平面揚聲器之激振分佈對揚聲行為之影響。而狹長形揚聲器在圓形激振下卻有聲壓感度低與中音域聲壓落差問題，為改善上述圓形激振之問題，本文選擇使用長圓形激振器激振，藉由改變長圓形音圈之長度與揚聲板之楊氏係數，使激振力增加，揚聲系統之剛性加強，期望達到減緩中音谷落差與提升整體聲壓感度之目的。本文研究可為二部分，第一部分將利用有限元素法分析軟體ANSYS，分析激振器氣隙磁通密度分佈，探討磁通密度在激振器長度增減後之變化趨勢，再將其整理成一磁通資料庫，做為研究中聲壓曲線分析之根據，第二部分則以“Rayleigh-Ritz Method”與“有限元素法”分析理論聲壓曲線，相互討論兩分析方法之結果，研究取得較佳之系統參數。

This thesis studies the vibration and sound radiation behaviors of strip-shaped flat speakers. A strip-type sound radiation panel when excited at the center by a point load will produce a significant sound pressure level (SPL) drop in the mid-frequency range of the panel’s SPL curve. The purpose of this study is to reduce such SPL drop and make the SPL curve smoother by using a long voice coil to excite the panel. The long voice coil can stiffen the sound radiation panel and enlarge the excitation location. A Rayleigh-Ritz is first established to analyze the vibration behavior of the panel. In the mathematical model of the sound radiation panel, the vibration of the panel is formulated on the basis of the first-order shear deformation theory, the flexible surround is treated as spring, and the voice coil is modeled using Timoshenko beams. The accuracy of the proposed method is verified by the finite element code ANSYS. The length change of the voice coil will affect the magnetic flux in the exciter. The actual excitation force of the exciter is then computed using the magnetic module of ANSYS. The vibration response of the panel is used in the Rayleigh first integral to construct the SPL curve of the panel. The appropriate length of the voice coil is determined to minimize the SPL drop in the mid-frequency range of the panel’s SPL curve. The effects of the material constants of the panel’s SPL curve are also studied. It has been shown that the increase in the span-wise stiffness of the panel can raise the frequency of the major SPL drop and make the SPL curve smoother. The results obtained in this thesis should be of value to the audio industry.