Noise cancellation over spatial regions using adaptive wave domain processing

This paper proposes wave-domain adaptive processing for noise cancellation within a large spatial region. We use fundamental solutions of the Helmholtz wave-equation as basis functions to express the noise field over a spatial region and show the wave-domain processing directly on the decomposition coefficients to control the entire region. A feedback control system is implemented, where only a single microphone array is placed at the boundary of the control region to measure the residual signals, and a loudspeaker array is used to generate the anti-noise signals. We develop the adaptive wave-domain filtered-x least mean square algorithm. Simulation results show that using the proposed method the noise over the entire control region can be significantly reduced with fast convergence in both free-field and reverberant environmentsThanks to Australian Research Councils Discovery Projects funding scheme (project no. DP140103412). The work of J. Zhang was sponsored by the China Scholarship Council with the Australian National University

Active control of sound inside a sphere via control of the acoustic pressure at the boundary surface

Here we investigate the practical feasibility of performing soundfield reproduction throughout a three-dimensional area by controlling the acoustic pressure measured at the boundary surface of the volume in question. The main aim is to obtain quantitative data showing what performances a practical implementation of this strategy is likely to yield. In particular, the influence of two main limitations is studied, namely the spatial aliasing and the resonance problems occurring at the eigenfrequencies associated with the internal Dirichlet problem. The strategy studied is first approached by performing numerical simulations, and then in experiments involving active noise cancellation inside a sphere in an anechoic environment. The results show that noise can be efficiently cancelled everywhere inside the sphere in a wide frequency range, in the case of both pure tones and broadband noise, including cases where the wavelength is similar to the diameter of the sphere. Excellent agreement was observed between the results of the simulations and the measurements. This method can be expected to yield similar performances when it is used to reproduce soundfields.Comment: 28 pages de text

Secondary source and error sensing strategies for the active control of sound transmission through a small opening

© 2019 Elsevier Ltd The openings of an enclosure allow natural ventilation and light ingress but also act as a point of entry for noise of the whole structure. In this paper, the active control of the sound transmitted through a small opening in a wall formed by two infinitely-large baffles is investigated up to 4 kHz. Based on an analytical model developed with the modal expansion method, the effects of different secondary source and error sensor strategies are compared numerically for different types of primary sound fields. The upper frequency limit of effective control is found to be determined by the eigen-frequency of the acoustic modes of the opening. Experimental results with an opening of 6 cm by 6 cm on a 31.8 cm thick wall agree well with the numerical results. The upper frequency limit of effective control is found to be 2750 Hz for a single-channel system and 3900 Hz for a 4-channel system with more than 10 dB noise reduction. It is concluded that implementing active control in small openings with an appropriate secondary source and error sensing strategy can extend the frequency range of control significantly, so that the active control systems can be applied to more noise control scenarios which have both noise reduction and ventilation requirements in the middle to high frequency range

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

Theory and Design of Spatial Active Noise Control Systems

The concept of spatial active noise control is to use a number of loudspeakers to generate anti-noise sound waves, which would cancel the undesired acoustic noise over a spatial region. The acoustic noise hazards that exist in a variety of situations provide many potential applications for spatial ANC. However, using existing ANC techniques, it is difficult to achieve satisfying noise reduction for a spatial area, especially using a practical hardware setup. Therefore, this thesis explores various aspects of spatial ANC, and seeks to develop algorithms and techniques to promote the performance and feasibility of spatial ANC in real-life applications. We use the spherical harmonic analysis technique as the basis for our research in this work. This technique provides an accurate representation of the spatial noise field, and enables in-depth analysis of the characteristics of the noise field. Incorporating this technique into the design of spatial ANC systems, we developed a series of algorithms and methods that optimizes the spatial ANC systems, towards both improving noise reduction performance and reducing system complexity. Several contributions of this work are: (i) design of compact planar microphone array structures capable of recording 3D spatial sound fields, so that the noise field can be monitored with minimum physical intrusion to the quiet zone, (ii) derivation of a Direct-to-Reverberant Energy Ratio (DRR) estimation algorithm which can be used for evaluating reverberant characteristics of a noisy environment, (iii) propose a few methods to estimate and optimize spatial noise reduction of an ANC system, including a new metric for measuring spatial noise energy level, and (iv) design of an adaptive spatial ANC algorithm incorporating the spherical harmonic analysis technique. The combination of these contributions enables the design of compact, high performing spatial ANC systems for various applications

Recent Technological Advances in Spatial Active Noise Control Systems

This article provides a broad overview of the recent advances in the field of active noise control techniques to reduce unwanted noise over a certain spatial region of interest. Thanks to commercial and technological advances in local active noise control systems extending the size of the quiet zone seems to be a crucial step to developing the next generation of active control systems for a more personalized and quieter audio product. In this review article, the advances over the past decade the in design and development of spatial active noise control techniques to enlarge the controlled sound zone is reviewed. The focus is specifically on the adaptive control techniques and the methods proposed in the frequency domain to control the sound field. The study has paid specific attention to the most important performance measures in designing a spatial active noise control system such as convergence rate, stability and robustness of the algorithm, the size of the quiet zone and how it can be enlarged by configuring the loudspeaker and microphone array geometries. Finally, the authors will discuss the current and future challenges that should be overcome to improve the effectiveness of the recently proposed methods to expand the silence zone

Acoustics Division recent accomplishments and research plans

The research program currently being implemented by the Acoustics Division of NASA Langley Research Center is described. The scope, focus, and thrusts of the research are discussed and illustrated for each technical area by examples of recent technical accomplishments. Included is a list of publications for the last two calendar years. The organization, staff, and facilities are also briefly described

Spatial dissection of a soundfield using spherical harmonic decomposition

A real-world soundfield is often contributed by multiple desired and undesired sound sources. The performance of many acoustic systems such as automatic speech recognition, audio surveillance, and teleconference relies on its ability to extract the desired sound components in such a mixed environment. The existing solutions to the above problem are constrained by various fundamental limitations and require to enforce different priors depending on the acoustic condition such as reverberation and spatial distribution of sound sources. With the growing emphasis and integration of audio applications in diverse technologies such as smart home and virtual reality appliances, it is imperative to advance the source separation technology in order to overcome the limitations of the traditional approaches. To that end, we exploit the harmonic decomposition model to dissect a mixed soundfield into its underlying desired and undesired components based on source and signal characteristics. By analysing the spatial projection of a soundfield, we achieve multiple outcomes such as (i) soundfield separation with respect to distinct source regions, (ii) source separation in a mixed soundfield using modal coherence model, and (iii) direction of arrival (DOA) estimation of multiple overlapping sound sources through pattern recognition of the modal coherence of a soundfield. We first employ an array of higher order microphones for soundfield separation in order to reduce hardware requirement and implementation complexity. Subsequently, we develop novel mathematical models for modal coherence of noisy and reverberant soundfields that facilitate convenient ways for estimating DOA and power spectral densities leading to robust source separation algorithms. The modal domain approach to the soundfield/source separation allows us to circumvent several practical limitations of the existing techniques and enhance the performance and robustness of the system. The proposed methods are presented with several practical applications and performance evaluations using simulated and real-life dataset