以聲阻抗和電阻抗法測量振膜等價彈簧係數之研究

Abstract

動圈式耳機為電—機—聲整合系統，其性能的展現，由系統間的相互配合決定。機械系統將電能轉換成聲能，由振膜與振膜框所組成的振膜系統為此中的一員。機械系統於低頻作動時，可以等效質量—彈簧—阻尼系統來模擬，前述三個參數決定了此頻率範圍下系統的性能，然而振膜系統之等價彈簧係數於耳機成品製成前，幾乎難以事前得知，因此本研究嘗試利用現有的聲學阻抗管與電聲學類比電路的理論，分別構思出量測方法。聲學阻抗管法測量振膜組合—腔體之聲學比阻抗，再計算機械阻抗與曲線擬合，得出等價彈簧係數；電聲學類比電路法測量揚聲器—腔體—振膜組合之共振頻率，再透過理論模擬與迭代，得出等效彈簧係數。從實驗結果發現，聲阻抗法與電阻抗法所得的等價彈簧係數值接近。利用此兩種方法來量測振膜系統等價彈簧係數具可行性。

Moving-coil earphone is an integration of electrical, mechanical and acoustical system. Its performance is determined by the collaboration of these systems. Mechanical system converts electrical energy to acoustical energy, and diaphragm system, which composed of diaphragm and diaphragm rim is part of it. At low frequency, mechanical system can be simulated by an equivalent mass-spring-damper system. The equivalent mass, spring constant, and damping coefficient determine the performance of mechanical system at this frequency range. However, the equivalent spring constant of diaphragm system cannot be known before the earphone is assembled. Therefore, in this research, acoustic and electrical impedance method are adopted to determine the equivalent spring constant of the diaphragm system. In acoustic impedance method, the specific acoustic impedance of diaphragm system with an air-backed cavity was measured, and through curve fitting, the equivalent spring constant can be obtained. In electrical impedance method, the resonant frequency of system comprising of loudspeaker, cavity, and diaphragm system was measured, and by iteration, the equivalent stiffness value can be acquired. It can be shown from the result that the equivalent spring constant measured by the respective method are close in the order of magnitude and these two methods are feasible.