商業用孔洞駐極體應用於揚聲器系統之初步設計與開發

Abstract

孔洞駐極體材料為一種具可撓性、輕薄之聚丙烯薄膜，從截面觀察可發現許多扁平之孔洞結構，使其具有穩定的儲存電荷能力，常被用於各式致動器與感測器上。而在致動器部分，其電荷分布平均、形狀設計容易、高效率及低聲學阻抗使其常被用在聲學相關之應用，而由於其共振頻位置位於300 kHz左右之高頻區域，多用於超聲波相關之應用。相形之下，此種薄膜十分輕薄，因此其低頻響應不佳，若作為揚聲器振動源，難以在低頻擁有較大之振動幅度。本研究透過提升薄膜張力與改變孔洞結構，影響薄膜之聲壓頻譜輸出，透過施加張力與產生負壓的方式於薄膜產生應變，使內部之孔洞結構趨於扁平，藉此影響其聲壓頻譜響應。本研究並將此概念應用於陣列式結構，利用抽氣施加負壓使其中每個單元薄膜之孔洞的扁平度下降，並經紅外線加熱維持薄膜形狀。由實驗證明提升孔洞扁平度約53.3 %，並可提高聲壓輸出並使聲壓響應往低頻延伸。本研究並研究振動單元之形狀大小對薄膜之聲壓頻譜輸出之影響，並將雙層薄膜結構引入，研究其對聲壓的影響。在陣列單元的變因中，單元之特徵長度為影響聲壓頻譜響應之主要因素，若增加單元尺寸，聲壓響應隨之增大並且有往低頻移動之趨勢，其中，利用陣列單元為直徑12.5 mm圓形之4×4之蜂巢式陣列，聲壓平均整體較抽氣施加負壓前提升9.9 dB，特別是在1 kHz到13 kHz的範圍內，有47 %的提升，而在1 kHz到10 kHz之範圍內，擁有相對平坦、聲壓平均值為48 dB之曲線，而陣列單元為直徑15 mm圓形之4×4之蜂巢式陣列相較於單一矩形薄膜聲壓在1 kHz到12 kHz提升了34.3 %，相較所有裝置有較良好之聲壓頻譜響應。本研究驗證提升孔洞的扁平度及增加陣列單元之特徵長度可提升聲壓及低頻響應之研究目標。透過這些研究，找出更適合應用於揚聲器之薄膜參數，希望在未來製作出頻域廣、擁有低頻輸出之可撓式駐極體揚聲器。

Porous PP film has superior mechanical properties due to its micro voids, including lightweight, flexible, and a high sensitivity. The porous structure allows it to store a large number of space charges and to have piezoelectric effect. Porous-electret has been applied in applications of sensors and actuators. For the application of actuators, it can provide a uniform distribution of space charges, a high energy efficiency, and low acoustic impedance. The resonance frequency of porous-electret is around 300 kHz, It makes it well suited to ultrasonic transducers. But, its low frequency response is poor due to its high flexibility and thin-thickness. In this thesis, we study the influence of porous structure to the acoustic performance of the porous electret film. Mechanical tension was applied to alter the flatness of voids This process can make the voids flatter, and it results in a wider frequency response and can extend to lower frequency region. Using this concept, a suction force was applied on an array of porous electret to change its curvature and to increase the flatness of voids. An infrared emitter was used to maintain the final curvature of each electret film. Scanning electron micrographs shows that this method can reduce ratio of porous diameter and height by 53.3 %. Experimental results also show that the acoustic response can be enhanced by using electret films with flatter voids. We also introduce bimorph structure to study its contributions to sound pressure. In the properties of the array device, the characteristic length of the cell was the main factor that effected on frequency response. If the cell size was increased, the sound pressure would increase and there would be better low frequency response. Experimental studies show that a 4 by 4 array with 12.5 mm in diameter circular cells can increase its sound pressure by an average of 9.9 dB after applying suction force. In the range between 1 kHz to 13 kHz, the sound pressure even enhanced 47 %. Moreover, a flatten area was observed between 1 kHz to10 kHz with the sound pressure reached 48 dB in average. For a 4 by 4 array with 15 mm in diameter circular cells, 1 kHz to 13 kHz, the sound pressure enhanced 34.3 % in the range between. In summary, the concept of using a flattened voids structure to enhance acoustic response of porous electret is verified. In the future, we hope this technology can be applied to flexible speakers with a wider bandwidth and low frequency response.