No abstract

No abstract

A systematic analysis is proposed to predict the performance of a typical feedforward single channel ANC headset in terms of the delay, especially the non-causal delay caused by different noise coming directions. First, the performance of a non-causal feedforward system for a band-limited noise is analyzed by using a simplified pure delay model, where it is found that the noise reduction bandwidth is narrowed and the maximum noise reduction is decreased with the increase of the non-causal delay. Second, a systematic method is developed, which can be used to predict the system performance with measured primary and secondary path transfer functions in most practical sound fields and to study the effects of the control filter length and the path delay on the performance. Then, the causality of a typical feedforward active noise control headset with the primary source at 0° and 90° positions in an anechoic chamber is analyzed, and the performance for the two locations predicted by the systematic analysis is shown in good agreements with the experiment results. Finally, an experiment of a typical feedforward active noise control headset in a reverberation chamber is carried out, which shows the validity of the proposed systematic analysis for other more practical sound fields.Active noise control (ANC) headsets apply active control technique to reduce low frequency noise in headsets [1], where many methods have been used, such as analogue feedback controller [2], digital feedforward controller [3], and the hybrid controller combining the digital feedforward technique with feedback technique [4]. This paper is focused on a typical feedforward active headset composed by an external reference microphone, an internal error microphone and a secondary source in the earmuff [3], where the reference microphone senses the reference signal, the filtered-x least mean square (FXLMS) algorithm is used to adapt the digital filter driving the secondary source to minimize the mean square of the noise at the error microphone.The FXLMS algorithm is a popular ANC algorithm due to its robust performance, low computational complexity and ease of implementation. However, the performance depends on many factors, such as the degree of the coherence between the reference signal and the noise signal [5-7], acoustic feedback [7-8], the time-varying property of the primary noise [4,9], the estimation error between the modeling and practical secondary path [10], and the causality caused by the system delay [5]. For feedforward ANC systems, the primary path contains the acoustic delay from the reference microphone to the error microphone, and the secondary path contains the same kind of acoustic delay from the secondary source to the error microphone and the total electrical system delay from the antialiasing filter, analog to digital (AD) converter, digital to analog (DA) converter, reconstruction filter, and one sampling period for processing etc.. The primary path delay should be larger than the total secondary path delay to guarantee a feedfor...

No abstract

No abstract

The feasibility of applying active noise control techniques to attenuate low frequency noise transmission through a natural ventilation window into a room is investigated analytically and experimentally. The window system is constructed by staggering the opening sashes of a spaced double glazing window to allow ventilation and natural light. An analytical model based on the modal expansion method is developed to calculate the low frequency sound field inside the window and the room and to be used in the active noise control simulations. The effectiveness of the proposed analytical model is validated by using the finite element method. The performance of the active control system for a window with different source and receiver configurations are compared, and it is found that the numerical and experimental results are in good agreement and the best result is achieved when the secondary sources are placed in the center at the bottom of the staggered window. The extra attenuation at the observation points in the optimized window system is almost equivalent to the noise reduction at the error sensor and the frequency range of effective control is up to 390 Hz in the case of a single channel active noise control system.

No abstract

Accelerating beams have attracted considerable research interest due to their peculiar properties and various applications. Although there have been numerous research on the generation and application of accelerating light beams, few results have been published on the generation of accelerating acoustic beams. Here we report on the experimental observation of accelerating acoustic beams along arbitrary convex trajectories. The desired trajectory is projected to the spatial phase profile on the boundary which is discretized and sampled spatially. The sound field distribution is formulated with the Green function and the integral equation method. Both the paraxial and the non-paraxial regimes are examined and observed in the experiments. The effect of obstacle scattering in the sound field is also investigated and the results demonstrate that the approach is robust against obstacle scattering. The realization of accelerating acoustic beams will have an impact on various applications where acoustic information and energy are required to be delivered along an arbitrary convex trajectory.