Research On Active Noise Control System For Open Sound Field

Due to the improvement of industrialization,there are more and more industrial equipment around the living environment,such as engines,air conditioners,factory manufacturing and construction,etc.,which have induced serious noise pollution.Continuous noise pollution will have a great impact on people’s physical and mental health.Noise cancellation is mainly through passive cancellation and active cancellation.Passive elimination mainly uses sound-absorbing materials,such as earplugs,to hinder the propagation of sound waves and reduce the energy of sound waves.Passive noise reduction can better suppress the noise in the high frequency band,but passive noise reduction for low frequency noise signals faces the problem of increased equipment size and high cost,so the low frequency noise is mainly eliminated by means of active noise control.Active noise control(ANC)is the use of digital signal processing technology to generate a signal with the same amplitude and opposite phase as the noise signal for realtime superposition and cancellation.Due to its strong adaptability,simple equipment and low cost,active noise control technology is widely used in noise cancellation.In this thesis,the technical points of noise reduction system structure,least mean square(LMS),and secondary path modeling analysis involved in active noise control are analyzed in detail.In an open sound field environment,in order to use the equipment flexibly,the secondary path and the primary path face sudden changes.In order to improve the stability of the system,the convergence conditions of each filter in the online secondary path modeling in the sense of mean value and mean square value are analyzed,especially the convergence conditions when the secondary path error is large.Secondly,analyze the steady-state error of the system and reveal the influence of the auxiliary white noise signal on the overall system.Simulation analysis shows that the proposed stability conditions improve the robustness of the system in various extreme environments.In order to improve the convergence speed and steady-state performance of the system,this thesis proposes the optimal step factors of the ANC filter and the secondary path modeling filter according to the idea of the normalized adaptive algorithm.When performing online modeling on the secondary path,the injected white noise signal power is too large to affect the steady-state performance of the system and reduce the noise control effect.Too small white noise power will affect the speed of secondary path modeling,which will affect the convergence performance of the ANC filter.In order to balance the slightly contradictory needs,this thesis proposes variable power white noise.The power of white noise is adjusted according to the convergence performance of the ANC filter.When the error of the ANC filter is large,the power of white noise is also large.When the system becomes stable,the power of white noise is small and hardly affects the power level of residual noise.After analyzing the structure of ANC system,including feedback type,feed-forward type,single-channel and multi-channel analysis,this thesis designs a single-channel feedforward ANC system.It is verified by experiments that the system has a good noise reduction ability for narrowband noise of single frequency,multiple harmonic frequencies and broadband noise of 100-400 Hz and 400-800 Hz.