調整平板振形以改善平板揚聲器之聲壓

The Modification of Plate Vibrating Shape to Enhance Sound Pressure Level of Flat-panel Speaker

官韋辰(Wei-Chen Kuan)

指導教授 ： 金大仁

國立交通大學/工學院/機械工程系所/碩士(2019年)

摘要

本文研究透過對平板揚聲器的平板振膜進行結構設計以調整平板的振形來有效改善揚聲器之聲壓表現。本文在設計分析中使用了有限元素套裝軟體ANSYS來模擬實際揚聲器的振動行為，並為了貼近模擬的真實性，決定以薄殼元素建立全揚聲器各元件的模型，進行簡諧振動分析搭配點聲源陣列總和法來計算得出模擬之聲壓位準曲線。本文藉由駐波的觀點解釋並提出了波速法與慣性法以用作改善設計時的概念，不同於以往前人所應用的分組試誤法，本文再據此概念發展出了一套直接設計用的策略步驟：第一，應用波速法的概念，依照音谷模態從結構正相位處使用肋條附加，使得結構勁度可以有效延伸至結構負相位處以增快肋條軸向傳遞之波速並減少波數，降低音谷處反向聲壓的輻射，而延伸附加的長度則是由節線位置所決定，一般只需延伸到節線上或節線與邊界中間處即可；第二，應用慣性法的概念，依照音谷模態再將質量群附加上結構負相位處，使得慣性的額外增加可以有效抑制負相位區域位移量以拖慢振動時的表面速度，癱瘓反向聲壓的輻射，而額外附加的形狀則是由模態所決定，一般只需覆蓋住結構負相位處位移極值一半的區域即可；第三，視設計需求再經由第一二步驟的交錯應用後，由於近高頻音谷模態複雜化的關係，將出現模態重疊的現象，故此後的設計必須和之前的設計進行比對而做出取捨，一般針對所附加的肋條厚度或是質量數來拿捏斟酌會有較顯著的效果，如此一來才可完成聲壓既高、曲線又平滑之均衡設計。另外，本文亦設計了一款新式的平面彈波，此彈波可適應空間的限制給予音圈額外的支撐力以穩定往復的沿軸心運動，但又不至於壓縮過多振動幅度而削減了低頻之可聽頻寬範圍。最終，經實際製作後的實驗量測結果驗證了本文設計方法的正確性以及彈波應用的可行性；與前人所提出的揚聲器平板振膜設計方法比較後也能夠得知，在應用本文之策略步驟下所設計的平板揚聲器其聲壓可比前人的平均高了2.5至5 dB之間，達到了80 dB以上，而低頻頻寬亦較低可下潛至60 Hz，至於音谷數量在10 kHz以內頻寬1 kHz中聲壓跌幅大於5 dB亦僅有一個且發生位置較靠後高頻處，如此皆顯示了本文所提出之策略步驟具備有比較佳的功能性以用於平板揚聲器聲壓改善的設計。

關鍵字

揚聲器 ； 聲壓 ； 振形 ； 振膜 ； 駐波 ； 有限元素法

並列摘要

In this thesis, the modification of plate vibrating shape is studied to enhance sound pressure level (SPL) response of flat-panel loudspeaker via structural designing of its flat-panel diaphragm. In this study, the ANSYS software is used to simulate the vibration behavior of flat-panel loudspeakers. The shell element is used to model the components of the speaker including the sound radiating plate, surround, and spider in the finite element harmonic analysis. The Rayleigh's first integral together with the displacement response is used in to calculate the SPL. The finite element method is then used to design the mechanical and geometric parameters of the sound radiating plate to reduce the SPL dip size. From the aspect of standing waves, the methods of wave speed and inertia are proposed as concepts to use for improvement designs. Unlike former research that applying try and error method, in this thesis a strategy which based on the concepts has been developed for direct designing: First step, utilize the concept of wave speed method. Follow the vibrating shapes of sound dips to attach additional stiffening ribs from positive phase pattern of the structure and make stiffness be able to extend to negative phase pattern of the structure so that the wave speed along axial direction of ribs can be increased but decreased in wavenumber. Reduce the radiation of anti-directional sound pressure at dips. Generally, the length of extension is supposed to be determined by the position of nodal lines and it’s fine as long as the extension is to the nodal lines or the middle place between nodal lines and boundaries; Second step, utilize the concept of inertia method. Follow the vibrating shapes of sound dips to attach additional mass from positive phase pattern of the structure and make extra inertia be able to suppress displacements of negative phase pattern of the structure so that the surface speed of vibration can be slowed down. Paralyze the radiation of anti-directional sound pressure at dips. Generally, the shape of additional attachment is supposed to be determined by the vibrating shape of plate diaphragm and it’s fine as long as the areas with over half of the extreme displacement of negative phase pattern of the structure are covered; Third step, repeat first step or second step alternatively depending on the design demand. Because of the complication of sound dip’s vibrating shape with approaching the treble field, the phenomenon of overlapping vibrating shapes is gradually occurred. Hence, there is need to compare the later designs with the former designs and make a trade-off. Generally, considering the thickness of attached ribs or attached mass numbers will have better effects so that a harmonious design of flat-panel loudspeaker with high sound pressure and smooth curve can be accomplished. On the other hand, a novel type of flat spider is proposed to adapt the limitation of space well and give the voice coil additional forces to help stabilize the motion of vibration without overcompressing the amplitude response for further reducing audible bandwidth of the bass field. At last, the comparison between the numerical predictions and the experimental data has shown that both the trends of the theoretical and experimental SPL curves are basically the same. It has been found that the approaches proposed in this thesis are indeed valid as well as feasible for SPL enhancement. After well designed, the flat-panel loudspeaker finally has reached over 80 dB of its average sound pressure; dived down to 60 Hz of the bass field; had only 1 sound dip dropping over 5 dB within 1 kHz bandwidth below 10 kHz and these are all superior to the usage of past designs.

並列關鍵字

Speaker ； Sound pressure level ； Vibrating shape ； Diaphragm ； Standing wave ； Finite element method

參考文獻

參考文獻

[1] Morse, P.M.; Ingard, K.U., 1987, Theoretical Acoustics, Princeton University Press, Princeton.

[2] Kinsler, L.E.; Frey, A.R.; Coppens, A.B.; Sanders, J.V., 2000, Fundamentals of Acoustics, John Wiley & Sons, New York.

[3] Beranek, L.L., 1996, Acoustics, Acoustical Society of America, American Institute of Physics, New York.

[4] Williams, E.G., 1999, Fourier Acoustics: Sound Radiation and Nearfield Acoustical Holography, Academic Press, London.