Brief Introduction to Active Noise Control

This paper is a very brief introduction to the field of active noise control. The concept of active noise control based on destructive interference of sounds is presented. The need for high amplitude and phase accuracy of canceling sound for efficient cancellation is shown. Considered is the possibility of noise cancellation in open and closed space. Quiet zone formed in the local noise cancellation related to the noise wavelength is illustrated. Most important approaches used for active noise control such as the feedforward, feedback and hybrid method are laid out. Adaptive feedforward and feedback structures based on adaptive filtering are outlined considering the influence of secondary path transfer function and acoustic feedback. Suitability of particular methods for different sound characteristics like broadband and narrowband noise and availability or absence of the reference signal is considered. Synchronized waveform synthesis is also presented as a solution for active control of periodic noise commonly produced by rotating machinery. General directions for digital implementations such as choice of sampling rate, adaptive filters length, computational cost, circuits for pre-filtering and post-filtering of reference, error, and secondary signal are given. Some typical applications are listed.