陣列結構駐極體揚聲器系統之設計與研製

Abstract

傳統動圈式揚聲器已經發展超過一世紀，因其重量及體積在應用上而有所限制。近年來，因為可撓式駐極體揚聲器比傳統動圈式揚聲器具有多項優點，例如其簡單的構造可達到超薄及輕量的結構，在4C (電腦、通訊、消費性電子及汽車)產品的應用上具有相當的潛力。本論文所探討的可撓式駐極體揚聲器是由間隔材、背板及駐極體材料薄膜所組成的具有許多電容式致動器的陣列結構。 本論文在模擬方面提出了完整的模擬流程，來分析駐極體單一制動器的共振頻率與模態，以及陣列駐極體揚聲器之聲場特性與指向性。因為駐極體揚聲器的低頻表現主要是受到致動器的第一模態所影響，為了不同的應用而欲調制具寬頻的駐極體揚聲器，則每個致動器的形狀與大小將依不同的需求而有所調整。此外，駐極體揚聲器的指向性對於應用時的效果也有很大的影響，本究所發展的駐極體揚聲器可藉由結構設計的概念而具有不同的指向性，所以，研究過程中利用有限元素分析去評估駐極體揚聲器的振動模態及指向性特性，提出能夠改善駐極體揚聲器效能的設計。 量測系統方面，為了驗證有限元素分析的模擬結果，本研究建置兩套可量測奈米級振動的非接觸式光學檢測系統，其中的雷射都譜勒振動量測儀暨干涉儀(AVIDTM)用來量測駐極體致動器的振動，再使用電子斑點干涉系統(ESPI)去量測駐極體致動器的模態變形。另外一個系統乃是一套可量測大面積駐極體薄膜的表面電壓分布之自動量測系統，此外，為了量測駐極體揚聲器的聲學特性及指向性，亦建置了一套高頻寬、高動態範圍之自動量測系統。模擬結果與實驗有良好的一致性。這些模擬不僅可用來設計可撓式駐極體揚聲器，更可以輔助設計調製駐極體揚聲器之聲學特性。 為了驗證研究過程中所建置的模型與實驗系統，本論文以汽車中揚聲器的應用為平台，研析揚聲器配置於汽車內不同位置時，所可營造出來的聲場分佈，並依照實車大小、材料參數為條件建立了一個模擬模型，並將現有的車用揚聲器與本論文所提出的駐極體揚聲器進行比較與討論。輕薄的駐極體致動器能裁切並組成任意形狀的駐極體揚聲器，而可安裝於汽車內的任意位置，故可重新設計車內聲場，除此之外，駐極體揚聲器還確認具有省能、良好的頻率響應、體積小、輕量及可撓性等優點。這個應用情境中，駐極體揚聲器的成功設計、製造、驗證進一步的確認了本論文所得成果的可應用性和正確性。

Conventional coil speakers have been developed for more than a century. However, expanding coil speakers into more futuristic applications often are limited by their weight and volume. Recently, flexible electret-based loudspeakers triggered many renewed interests due to their advantages over conventional coil speakers such as a simple and compact construction made from ultra-thin lightweight structures. Electret loudspeakers thus have many potential futuristic applications for 4C products (computers, communication, consumer electronics, and cars). The flexible electret-based loudspeaker presented in this dissertation consisted of spacer grids, back plates and electret membranes that were constructed by integrating many small capacitance type actuators to form an array structure. Simulations of electret cell actuator were used to derive the resonance and the corresponding modes. Since the low frequency performance of an electret loudspeaker is greatly affected by the first mode of each electret cell actuator, the shape and size of each cell actuator were tailored according to the specific requirements in order to vary the bandwidth of the electret loudspeaker for different applications. Furthermore, the performance is also mainly determined by the directivity of the electret loudspeaker. It was discovered that the novel electret loudspeaker possesses various directivities from a structural design aspect. Finite element analysis (FEA) was used to evaluate and to design the vibration mode and the directivity characteristics of the electret loudspeaker developed. AVID (advanced vibrometer/interferometer device) system and ESPI (electronic speckle pattern interferometry) system, both of which were non-destructive optical detection tools, were used to measure the vibration and the vibration mode shape of the electret cell actuators, respectively. A large-area surface potential measurement system was designed and constructed. A high bandwidth automatic acoustic measurement system was also developed to investigate the acoustic characteristic and directivity of the electret loudspeaker. Experimental results obtained confirmed the effectiveness of the simulation results. Finally, the sound characteristics of a flexible electret loudspeaker were tailored using the modeling and understanding obtained to improve the performance to the desired specifications. To further test the validity of the simulation and experimental model developed, applying the electret speakers in an automotive condition was attempted. To observe the SPL distribution at the passengers’ ears for loudspeakers installed at various locations, simulation model of the car cabin was first established. The performance of current vehicle installed coil speakers were compared to that of the newly developed electret loudspeaker. It was discovered both by simulation and by experimental verifications that the electret device possesses many advantages including installed locations, power consumption, sensitivity, volume, weight and flexibility when compared to that of the traditional coil speakers. The successful design, construction, and verification of the automotive used electret speakers provided a nice verification to the validity of the research results presented in this dissertation.