補償駐極體揚聲器低頻響應之柔性結構板設計與分析

Enhancing Low-Frequency Response of the Electret Loudspeaker by Flexible Structure Plate Design and Analysis

廖旭清(Hsu-Ching Liao)

指導教授 ： 李世光

國立臺灣大學/工學院/工程科學及海洋工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.01626

摘要

近年來，可撓的駐極體靜電式制動器被廣泛的討論，它具有輕薄可撓、低耗能、可任意剪裁及極佳的聲音品質…等特性，非常符合未來之應用趨勢，因此也發展出了駐極體揚聲器，然而駐極體揚聲器在低頻的表現卻不甚理想，其受到材料選用和結構設計所影響。
本論文將針對結構設計方面來作低頻響應的改進，駐極體揚聲器天生為陣列的小單體結構，使其有較佳的指向性並在中高頻率範圍有非常好的表現，但薄膜極小的振幅無法提供足夠的衝程來產生低頻的聲音。因此我們透過一個複合式的結構設計來補償低頻的不足，將一制動器放置於固有的背板上，藉由驅動制動器來推動板振動，進而得到較大的振幅來發出低頻的聲音，駐極體揚聲器黏貼於背板上負責中高頻段的聲音，如此即可形成一寬頻域的平面揚聲器。
在複合式揚聲器的設計上，將探討不同懸邊長度的振動板、不同的驅動位置以及振動板之材料等對頻率響應的影響，藉由有限元素分析軟體COMSOL進行板振動分析，並量測其聲壓位準(SPL)與頻率的關係來評判該設計的優劣。
最後，本論文成功研發出複合式平面揚聲器，從中了解結構、驅動位置和頻率響應的關係，材料特性對聲阻匹配與阻尼的關係，並利用等化器的調整，得到相當平滑的頻率響應曲線，於180 Hz達66 dB，在1 kHz至20 kHz平均有75 dB以上的表現。

關鍵字

駐極體 ； 平面揚聲器 ； 複合結構 ； 柔性結構板 ； 致動器

並列摘要

Recently, the development of flexible electret based electrostatic actuator has been widely discussed. The device was known to be light weight, flexible, energy saving, can be cut to any shape and have high sound quality. These characteristics were in line with many potential future applications, which propel the development of electret loudspeakers. However, the performance of the electret loudspeaker in low frequency is poor due to material selection and structure design.
In the thesis, the structure design was used to improve the response in low frequency. Electret loudspeakers inherently are array of cell structure, so they have excellent directivity and good responses in mid to high frequency range. Nevertheless, the vibration amplitude of electret film is too small to supply enough strokes to generate the sound in low frequency. The design of composite structure was used to compensate for the lack of low frequency response. We added an actuator on the back plate to drive the plate to enhance the audible frequency range performance. That is, it is expected that the integration of the low frequency sound generated by the plate vibration and the mid to high frequency sound radiated by electret loudspeakers can lead to high performance wide-band flat loudspeaker.
In this paper, the choice of the driving position, different length of suspension and material of vibration plate were discussed. By using finite element analysis software COMSOL, the vibration of the plate was simulated and analyzed. Then, sound pressure level (SPL) was used to evaluate the performance of loudspeakers.
Finally, the composite flat loudspeaker was developed in this thesis. We learned about the relationship among the driving positions, the structure relationship and the frequency response. The relationship among the material, impedance matching and damping were also explored. Using the equalizer, smooth frequency response curve can be obtained. The sound pressure of 66 dB was achieved at 180 Hz and the average sound pressure was maintained at higher than 75 dB from 1 kHz to 20 kHz.

並列關鍵字

electret ； flat loudspeaker ； composite structure ； flexible structure plate ； actuator

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