主動式噪音控制(Active Noise Control, ANC)的基本概念為利用額外的噪音控制聲源來製造一反噪音(Anti-noise)，使其振幅大小與原噪音相同，而相位卻恰好相反；並利用聲波在空氣中之疊加原理(Principle of Superposition)，使反噪音與原噪音互相產生破壞性干涉，來達成噪音控制的效果。此ㄧ概念最早在1936年即被提出，但由於近年來數位訊號處理器(Digital Signal Processor, DSP)之技術的蓬勃發展才得以實現。主動式噪音控制在諸多交通運輸業、製造業等易產生低頻噪音之載具及機械上皆有多方面的應用性，而其特性優點甚至可應用於助聽器以及耳機等消費商品上。
本文主要利用數位訊號處理器來實現以修正式(Modified)Filtered-x NLMS(MFxNLMS)為控制器主體統之前饋式噪音控制系統。由於利用正規化因子來調變濾波器之參考訊號，其收斂速度將優於修正式Filtered-x LMS(MFxLMS)演算法之控制結果。本文在實驗部份除了達成窄頻、寬頻之噪音消除，更達成了選擇性地消除單一頻率或ㄧ個範圍頻帶之音訊。此一可行性研究將有利往後於日常生活中更廣泛之應用。

Active noise control (ANC) is achieved by introducing a canceling “anti-noise” wave through a control source. The anti-noise of equal amplitude and opposite phase is generated and combined with the undesired noise based on the principle of superposition, thus resulting in cancellation of both noises. Although the idea of ANC was first proposed in 1936, the practical applications were successfully implemented in recent decade due to the available of low cost and efficient digital signal processor (DSP). ANC has been widely applied to transportations, manufacturing, and consumer products.
This thesis will focus on the practical aspects of ANC system in utilizing modified filtered-x NLMS(MFxNLMS) algorithm for real-life applications. The normalized step size can be updated based on the power of reference signal, the convergent speed of this proposed method will faster than traditional modified filtered-x LMS(MFxLMS) scheme. Experiments show that MFxNLMS not only cancels narrow-band and broadband signals quite effectively, but also selectively cancels signals with pre-specified frequency or pre-specified band.

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