行動裝置電聲系統之分析與最佳化

Abstract

近年行動裝置的快速成長，多媒體的使用經驗扮演著重要的角色，其中微型揚聲器的系統響應，受限於外觀設計的影響不易於評估與優化。本研究的主要目標是研究三個主題: (1) 手持微型揚聲器系統的聲學性能分析 (2) 手持微型揚聲器系統在行動裝置上的低音延伸方法 (3) 手持微型揚聲器系統低音延伸的優化。 第一主題為利用兩種模型研究手持微型揚聲器在行動裝置不同外殼參數中的聲學性能。 第一模型是微型揚聲器的電機聲模型以及聲腔效應的分佈模型; 第二模型為計算微型揚聲器系統的分佈模型時並且考慮了膜片的參數，並且應用於行動裝置的微型揚聲器聲學性能分析，兩種模型均顯示模擬與實驗的結果一致，其中分佈模型的模擬結果比類比集中參數模型的高頻響應更為精確。 第二主題有關微型揚聲器系統的低音延伸一直是行動裝置音訊設計的挑戰， 這裡是主要關注的焦點。 本研究為行動裝置的微型揚聲器系統中低音延伸提供一個解決方案，其中涉及使用前腔和矩形長管形埠的共振組合。 微型揚聲器系統與該結構的聲學負載有效共振耦合，使微型揚聲器在適度的振膜位移下，達到較高聲壓水準 (SPL) 和低音擴展，讓系統的低頻響應能低於揚聲器在自由空氣中的共振並且達到最小化的結構總尺寸。 採用電機聲與有限元素結合的方法， 確定行動裝置的聲壓位準(Sound Pressure Level, dBSPL) 和低音延伸水準，模擬結果顯示與實驗結果吻合。 本結果實現在10吋平板電腦上，應用了一個合適的低頻擴展的揚聲器系統，第一共振的音訊頻率響應可以從630Hz降低到 300 Hz， 以達到最大的音量擴展。 最後針對五個案例進行聲學性能研究分別為(1)前腔容積 (2)後腔容積 (3)矩形管形口長度 (4)橫截面積 (5)出音孔等參數設置進行了研究，研究結果可用於優化合適的低頻擴展範圍之微型揚聲器系統。 第三主題利用微型揚聲器單體集中參數結合分佈參數模型分析揚聲器的前腔容積、聲孔的大小、波導聲管長度及其橫截面積，對微型揚聲器上低音擴展的聲學設計參數進行了模擬與最佳化，利用田口法中的直交表L_9 計算最佳參數三位準與四個因子 (3^4)，因此藉由此方法來設計一個微型揚聲器系統低頻延伸的優化參數。 最後實驗驗證可以對三個主題的結果進行設計，藉由這些參數設計讓 3C 行業的音頻工程師可以在執行設計前優化其揚聲器系統設計概念的 SPL 性能與頻率頻寬。 這些結果可以在執行工業設計和機構放置過程前直接預測微揚聲器系統的聲學性能。

Microspeakers are key components of mobile devices in current consumer products. Furthermore, the user experience of multimedia products has gained much importance. In this light, the present study investigated three topics: (1) acoustical performance analysis of a microspeaker system; (2) bass extension of a microspeaker system on a mobile device and (3) optimization for bass extension of a microspeaker system. In the first topic, the acoustic performances of microspeakers with different enclosure parameters are studied for mobile devices. Two methodologies were used for this purpose: (1) an electro-mechano-acoustical model (EMA) of microspeaker along with distribution model of enclosure effect, and (2) the details of diaphragms were considered when calculating the distribution models of microspeaker with enclosure parameters. These methodologies were combined and applied to determine microspeaker performance levels for mobile devices. They showed acceptable agreement with the experimental results. Furthermore, the distribution model could simulate the high frequency response well compared with the lumped model of microspeaker system. In the second topic, because the bass extension of microspeaker system remains a challenge in audio design for mobile devices, we provided a solution for the bass by using a resonant combination of a front chamber and a rectangular long pipe-shaped port. The efficient resonant coupling of the microspeaker system to the acoustic load in this structure enables a microspeaker with modest cone displacement to achieve a high sound pressure level (SPL) and bass extension below the resonance of the microspeaker in free air, while the total dimensions of the structure are minimized. A combination of electro-mechanic-acoustic and finite element methods was applied to determine the SPL and bass extension levels for mobile devices. The simulation results showed acceptable agreement with the experimental results. A suitable extended-range microspeaker system was applied in a 10' tablet. The audio frequency response could be extended from 630 to 300 Hz with the maximum loudness. Finally, five cases of parameter settings for the front chamber volume, rear chamber volume, rectangular pipe-shaped port, cross-sectional area, and opening area were studied. The results can be applied to optimizing a suitable extended-range microspeaker system. Finally, the acoustical design parameters of bass extension for a microspeaker were simualted by using distribution model and change the enclosure volume of the front speaker chamber, size of the sound hole, length of the sound pipe tube, and the cross-sectional area of the sound pipe to determine the optimal parameter design through an orthogonal array of table L_9 (3^4) using the “Taguchi Method”. Based on the result, optimization of bass extension of microspeaker system can be achieved. The obtained results validated the accuracy of the simulation. By investigating each of these parameters, engineers in the computer, communication, and consumer (3C) industry can optimize the SPL performance and frequency bandwidth of their basic microspeaker design concepts before executing designs. Those results can predict micro-speaker system performance before industrial design and mechanism placement processes are executed.