Theory and Design of Feasible Active Noise Control Systems for 3D Regions

This thesis advances Active Noise Control (ANC) over three-dimensional (3D) space using feasible loudspeaker and microphone array systems. By definition, ANC reduces unwanted acoustic noise by generating an anti-noise signal(s) from secondary loudspeakers. The concept of spatial ANC aims to reduce unwanted acoustic noise over a continuous 3D region, by utilizing multiple microphones and multiple secondary loudspeakers to create a large-sized quiet zone for listeners in three-dimensional space. However, existing spatial ANC techniques are usually impractical and difficult to implement due to their strict hardware requirements and high computation complexity. Therefore, this thesis explores various aspects of spatial ANC, seeking algorithms and techniques to promote the reliability and feasibility of ANC over space in real-life applications. The spherical harmonic analysis technique is introduced as the basis of conventional spatial ANC systems. This technique provides an accurate representation of a given spatial sound field using higher-order microphone (spherical microphone array) recordings. Hence, the residual noise field in a spatial ANC system can be effectively captured spatially by applying the spherical harmonic technique. Incorporating conventional spatial ANC methods, we developed a series of algorithms and methods that optimize conventional methods regarding array geometries and ANC algorithms, towards improving the feasibility of a conventional spatial ANC system involving the spherical harmonic analysis. Overall, motivated by feasible and realistic designs for spatial ANC systems, work included in this thesis mainly solves the three problems of: (i) the impracticality of realizing spherical microphone and loudspeaker arrays, (ii) achieving secondary channel estimation with microphones remote from their desired locations, and (iii) unreasonable delays inherent to frequency domain spatial ANC methods. Based on our work, we have stepped towards achieving a spatial ANC system in a real-world environment for people to enjoy silence in the control region with the reliable usage of resources and algorithms. Several contributions of this work are: (i) designing a 3D spatial ANC system using multiple circular microphone and loudspeaker arrays instead of spherical arrays, (ii) proposing a 3D spatial ANC method with remote microphone technique such that noise reduction over a region is achieved with microphones remote from the region, (iii) proposing a secondary channel estimation method using a moving higher-order microphone such that usage of an error microphone array is not necessary, (iv) deriving a time domain spherical harmonic analysis method for open spherical microphone array recording with less delay than in the frequency domain, and (v) designing a feed-forward adaptive spatial ANC algorithm incorporating the time domain spherical harmonic analysis technique to better minimize the noise in the region of interest

Active control of outgoing noise fields in rooms

Active noise control is a strategy to suppress a noise by superimposing it with a carefully designed secondary noise. The strategy has been under research over the past half century with active noise control aided devices surging on the market over the last decade. However, up to now, the most successful applications of active noise control are still limited to the single channel systems, where noises propagate in ducts or in the human ear canals. Many researchers attempted to extend the application of active noise control to spatial noise fields, such as controlling the tire rolling noise in cars, the ventilation noise in workplaces, or the pump engine noise outdoors, which account for the majority of noises we encounter in our everyday lives. They developed spatial active noise control systems based on room modes, spherical modes, or the Helmholtz integral equation. The attempts have found limited success in the real world because of two problems. The first is that a spatial noise field is the complicated interaction of a number of noise sources with the environment, both of which can be non-stationary and time-varying. This problem makes it extremely difficult to obtain clean reference signals for spatial active noise control systems. The second is that due to the lack of a time-domain spatial sound field control theory, the existing spatial active noise control systems process the acoustic quantities in the time-frequency domain. The time-frequency domain processing introduces the frame delay and thus probably makes the systems violate the causal control constraint. This thesis proposes an outgoing noise field control system based on the frequency-domain sound field separation method. The method decouples the outgoing field (due to the noise sources) from the incoming field (due to the environment) on a sphere surrounding the noise sources. By canceling the outgoing field only, the proposed system reduces the noise entirely in a room without estimating the secondary paths in real-time and with negligible influence on the desired sound field in the room. This thesis further derives a time-domain sound field separation method, based on which a low latency outgoing field control system with random noise field cancellation capacity is developed. Multiple circular arrays of vector sensors for three-dimensional sound field analysis are developed based on the time-domain method. The designed arrays have a compact geometry, and thus can be integrated with small sized wearable devices and provide them with real-time sound field analysis capacity