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The movement and rotation of the human head can significantly degrade the control performance of active headrests with virtual microphones. This paper investigates the performance robustness of an active headrest with virtual microphones against the head rotation in a pure tone diffracted diffuse sound field. A physical model of a headrest system with a diffractive sphere is firstly developed, based on which the influence of acoustic transfer responses on the performance robustness against head rotation is analyzed. Then, the multi-objective optimization with constraints is developed to improve the performance robustness of the system, by designing optimized plant responses. Simulation results show that the method is effective in improving the robustness of single and dual channel systems. For a dual-channel headrest system with the proposed arrangements of secondary sources and microphones, the promotion of the minimum noise reduction is more than 7 dB after optimization when the head rotates from −90 degrees to 90 degrees at 125 Hz, 250 Hz and 500 Hz. In addition, the influence of different parameters on the optimization method is discussed for this dual-channel system. Finally, experiments carried out in an ordinary room validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Remote acoustic sensing can be used to estimate the error signals in human ears without placing any physical microphones there. In this paper, the coherence between the signals picked up by physical microphones over a sphere surface and the signal obtained at the sphere center is investigated. Based on the multiple channel coherence formulas in the time domain and frequency domain, the relationship between the coherence and the placement of physical microphones is analyzed by numerical simulations first, then the experimental results obtained in a reverberation chamber and a car cabin are presented to verify the simulation results. Finally, a placement of physical microphones for active control of road noise in car cabins is discussed. Both the numerical and experimental results show that an upper limit frequency exists for accurate sound pressure estimation at the center of a sphere with the sound pressure on the sphere surface. For a sufficiently complex sound field such as that in a reverberation room or in a car, half the wavelength of the upper limit frequency is about the average distance among the physical microphones. [ABSTRACT FROM AUTHOR]

The phase-shifting based active noise control (ANC) algorithm without secondary path modeling is appealing due to its simple implementation and high computational efficiency. However, the algorithm suffers from the influence of an implicit circular convolution caused by directly applying the most appropriate phase shift to the reference signal in the frequency domain. The implicit circular convolution results in a poor linear relationship between the phase-shifted reference signal and the error signal, which considerably degrades the convergence performance of the ANC system. In this paper, two methods are proposed to deal with this problem. In the first method, the circular wraparound artifacts are removed by the time-domain constraining operation. In the second method, a noncausal phase-shifting filter is used to regularize the phase of the reference signal. Compared with the standard frequency-domain implementation, the two proposed algorithms both eliminate the influence caused by the wraparound artifacts of the circular convolution and improve the linear relationship between the phase-shifted reference signal and the error signal, leading to a higher level of noise attenuation with a tolerable increase in computational complexity. Simulation results validate the better noise attenuation ability of the proposed algorithms. [ABSTRACT FROM AUTHOR]

Single screen barriers are widely used in open-plan offices as a means of improving acoustical privacy. However, the insertion loss of an indoor barrier is limited in low frequency range. Active control systems have been shown to be able to enhance the insertion loss of outdoors noise barrier in low frequency range, so its feasibility for improving the performance of indoor barriers is investigated in this paper. The simulation results show that about 10 dB extra insertion loss below 500 Hz can be obtained with the active control system when the squared sound pressure inside the entire observation area is minimized. The performance of the active noise barrier deteriorated significantly due to the reflecting ceiling, and the performance is hardly improved by increasing the ceiling height or the ceiling absorption coefficient. Finally, the effects of the sensing strategy on the control performance and the impacts of the active control system on the source side are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Since a practical virtual sound barrier (VSB) system has to be implemented surrounding a human’s head, the effects of such a diffracting sphere on the performance of the system are considered in this paper. It is found that the performance of the VSB system with a diffracting sphere might be improved or decreased depending on the size of the zone surrounded by the error sensors and the noise frequency, and the noise reduction distribution around the sphere tends to be uniform due to the diffraction of the sphere. Numerical simulations show that the tendency of the control performance with respect to the configuration of the system with a diffracting sphere is similar to that without the sphere. The diffracting sphere might move in the quiet zone of the VSB system, and the performance decreases with the increase of the distance between the sphere and the center of the system. However, the noise reduction level can remain more than 10dB even when the sphere is near the border of the quiet zone. Experiments with a 16-channel cylindrical VSB system in a normal room validate the above conclusions. [Copyright &y& Elsevier]

• Factors affecting the performance of virtual feedback control system are analyzed. • Reducing delay or increasing physical microphones improves noise reduction. • Experiments in an electric car running at 50 km/h are conducted. • A binaural noise reduction of 3.4 dBA is obtained inside an electric car cabin. This paper analyzes the factors that affect the noise reduction performance of feedback control based on remote microphone technique (RMT) and how these factors can be exploited to improve the control performance. First, simulations were conducted to compare the noise reduction performance of employing feedback control directly at the virtual microphone (located at the ear position) and on the physical microphone (located on the headrest), as well as the RMT-based feedback control system. Then the impact of the delay of the virtual secondary path and the coherence between physical and virtual signals on the noise reduction performance of the RMT-based feedback control system was analyzed. It is found that the noise reduction performance can be improved by reducing the delay of the virtual secondary path or increasing the number of physical microphones. Finally, experiments of a dual-channel control system conducted inside an electric car cabin demonstrate that the feedback control strategy based on RMT achieves a binaural noise reduction of 3.4 dBA when employing 4 physical microphones to estimate sound pressure at the ear positions. This approach achieves similar performance but is more cost-effective than a feedforward system using multiple reference sensors. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this case study, the upper limit frequency for global noise control in car cabins is investigated with numerical simulations, and experimental results of global control are presented for the first time. To simulate the complicated noise field in a running car, a 1.90 m × 1.50 m × 1.10 m rectangular enclosure, which is approximately the size of a typical car is used with point monopoles evenly distributed near the boundaries generating the primary sound field. Besides verifying the fact that the upper limit frequency of effective control increases with the number of secondary sources, a formula is proposed in this paper to estimate the number of secondary sources required to achieve global control up to a certain frequency. The simulations are carried out in the time domain based on the transfer functions measured between the secondary loudspeakers and 40 evaluation points evenly distributed in a typical car, which show that the upper limit frequency of 3 dB global control is about 200 Hz while using 4 secondary sources and it can be improved to 315 Hz with 7 secondary sources. An active noise control system is realized with 16 accelerometers picking up vibration reference signals and 4 or 7 loudspeakers as secondary sources, and the real time test results in the car running at 50 km/h on a typical asphalt road demonstrated the validity of the findings in numerical simulations. [ABSTRACT FROM AUTHOR]