A compound secondary source for active noise radiation control

Based on the analysis of active noise control system with multi-channel monopole secondary sources, a kind of compound secondary source is proposed in this paper. The proposed compound secondary source consists of two closely located monopole sources with their distance much smaller than a wavelength. The characteristics of the compound source are analyzed, and the performances of the active noise control system with the compound secondary sources on the monopole primary sound field and sound radiated by a plate are investigated both numerically and experimentally. It has been found that the proposed compound secondary sources control system can provide higher noise reduction for free field noise radiation control with the same number of control channels. It is shown that the better performance in noise reduction of the compound secondary sources control system is mainly due to the directivity of the secondary sources where the energy radiation pattern of the compound sources is similar to that of the primary sources.     

次级源近场和管壁弹性对充液管路有源消声性能的影响EI北大核心CSCD

建立了含次级源结构的充液直管有源消声系统数值模型,重点分析了声激励下次级源近场和管壁弹性对有源消声性能的影响。结果表明:次级源近场为非均匀声场,误差点位于该区域时部分频点控制效果较差甚至放大,而处于声场均匀区域时可使降噪量提高10 dB以上,增加误差点数量可使绝大多数频点的降噪量提高5 dB以上;管壁弹性使次级源与管壁间的耦合较强,非对称分布的次级源容易激起管壁振动,导致降噪谷值的出现,采用对称分布的次级源可显著提升控制效果;增加次级源数量能够提高系统的有源无源复合控制效果,但使得管内声场变得复杂,多次级源模型的有源消声效果随频率升高而有所降低。     

A study of sound intensity control for active noise barriers

Active noise control systems have been applied to increase the insertion loss of noise barriers where the squared sound pressure or the total acoustic energy density is used as the cost function in previous works. The absolute value of the mean active sound intensity is chosen as the cost function to obtain extra sound insertion loss in the dark area of a hybrid active noise barrier system in this note. The strategy of minimizing the near-field sound intensity at discrete locations along the edge of the passive barrier is shown to be able to provide better far-field noise reduction than that of minimizing the squared sound pressure control. Both numerical simulations and off-line experiments are carried out with a three-channel demonstration system, where the locations of the secondary sources and the error sensors are optimized and comparisons are made between the extra sound pressure attenuation of the sound intensity control and that of the squared sound pressure control.     

复杂声学环境中人耳附近空间有源降噪研究综述

复杂声学环境中人耳附近空间降噪是有源噪声控制研究的重要课题,目前采用的主要方法为有源降噪头靠(AHR)和虚拟声屏障(VSB).本文简述AHR与VSB的发展历史和研究现状,介绍其物理原理和设计方法,评述其在实际应用中的优缺点,讨论了目前存在的问题与未来相关的研究方向.已有理论、数值仿真和实验研究验证了相关技术在人耳附近空间产生静区的可行性. AHR系统需要较少控制源,系统相对简单易实现,但静区范围较小,结合虚拟传声器技术和人头跟踪技术后可实现随人头移动的静区,降噪频率可达中高频; VSB产生的静区范围较大,但控制源个数较多,系统复杂和成本高,可通过代价函数和控制源优化,以及主被动混合控制技术来提高有效降噪频率范围和减少控制源个数.     

壁面分布次级声源对管道中高阶模式声波的有源控制

针对管道中高阶模式声波的有源控制问题,探究了管道壁面布放次级声源的控制机制。首先根据管道声场模式理论和有源控制准则,推导了无限长管道内次级声源激励声场实现控制的必要条件,然后论证了次级声源沿管道壁面分布的可控性,并提出将空间分布离散化后的控制策略;最后通过数值仿真验证了沿壁面分布的有效性。结果表明,当次级声源在管道壁面的周向分布满足空间采样定律且沿轴向分布具有足够长度时,可以取得显著的有源控制效果.      

应用于有源声辐射控制的一种组合次级声源

在分析多通道单极子次级源控制系统的基础上,提出一种幅度调节型组合次级声源,这种组合源由两个单极子声源构成,单极子声源的间距远小于声波的波长。文中就组合次级源在自由场中对不同初级声源辐射噪声的控制进行理论分析和实验验证,并与同等条件下的单极子次级源控制系统的降噪性能进行了比较分析。数值计算和实验结果均表明,对于不同的初级声场,在次级通道数相同的情况下,组合次级源控制系统可以得到比单极子次级源控制系统高的降噪量。     

Error sensor location optimization for active soft edge noise barrier

Active control is used to improve the performance of noise barrier at low-frequency range and the excess insertion loss due to the active control system is influenced by the positions of the error sensors. The positional optimization for the error sensors in an active soft edge noise barrier was investigated. Both numerical simulations and experiments show that there is an optimum in the distance between the secondary sources and the error sensors, and that the error sensors should be placed above the secondary sources.     

基于平面声源进行结构声辐射有源控制的实验研究

采用分布式平面声源作为次级声源,对振动钢板的声辐射进行了抵消实验,验证了以往研究中的一系列关键理论。实验研究结果表明:一个平面声源可以控制钢板奇-奇模态的声辐射,两个平面源可以控制结构偶-奇或奇-偶模态的声辐射,同时也可以控制结构奇-奇模态的声辐射;平面声源的面积和布放位置对降噪效果有重要影响,采用单个平面声源控制时,平面声源面积越大,控制效果越好;基于近场声压的误差传感策略是有效可行的,实际中,将近场测量面的声功率作为有源控制的目标函数与总声功率作为目标函数是一致的;控制后远场声压和声强都得到有效降低,部分区域的声能向声源流动,近场声压及声强分布也发生显著变化。     

Performance of an independent planar virtual sound barrier at the opening of a rectangular enclosure

Planar virtual sound barrier systems have been used successfully to reduce noise radiation through an opening without affecting natural ventilation and lighting. However, the complexity of a fully coupled control system grows at the rate proportional to the square of the number of channels and this make the system implementation become impractical for enclosures with large openings. To reduce the system complexity, this paper proposes an independent planar virtual sound barrier, which is a multi-channel system consisting of many independent single channel active noise control systems. Each single channel system is “independent” in the sense that the control source output of the system is updated only with the signal from its own error sensor. Based on the analytical model of sound radiation through the opening of a rectangular enclosure, the transfer functions from both primary and control sources are calculated first. Then the noise reduction performance, the stability, and the convergence behavior of both fully coupled and independent planar virtual sound barrier systems are investigated. It is found that the independent system with no control output constraint becomes inherently unstable at some frequencies; however its stability can be improved by applying some control output constraint. Reducing the number of channels and the distance between secondary loudspeakers and error microphones can also increase system stability but at the cost of smaller noise reduction. When the system is inherently stable and there is no constraint on control output, the independent system can provide the same noise reduction as the fully coupled one but with faster convergence speed.     

Investigation of spherical loudspeaker arrays for local active control of sound

Active control of sound can be employed globally to reduce noise levels in an entire enclosure, or locally around a listener's head. Recently, spherical loudspeaker arrays have been studied as multiple-channel sources for local active control of sound, presenting the fundamental theory and several active control configurations. In this paper, important aspects of using a spherical loudspeaker array for local active control of sound are further investigated. First, the feasibility of creating sphere-shaped quiet zones away from the source is studied both theoretically and numerically, showing that these quiet zones are associated with sound amplification and poor system robustness. To mitigate the latter, the design of shell-shaped quiet zones around the source is investigated. A combination of two spherical sources is then studied with the aim of enlarging the quiet zone. The two sources are employed to generate quiet zones that surround a rigid sphere, investigating the application of active control around a listener's head. A significant improvement in performance is demonstrated in this case over a conventional headrest-type system that uses two monopole secondary sources. Finally, several simulations are presented to support the theoretical work and to demonstrate the performance and limitations of the system.     

An experimental study of swinbanks' method of active attenuation of sound in ducts

The concept of attenuating sound propagating down a duct by means of an antiphase copy of the sound was first put forward forty years ago, but to date no practical system has been produced. The principal problem is to introduce the antiphase signal in such a way that it propagates only in the direction of propagation of the original sound and to ensure that this property can be maintained over a useful frequency range.A suitable arrangement of secondary sound sources has recently been proposed by Swinbanks and this paper describes experiments carried out in an attempt to realise a practical system.A unidirectional array of secondary sources has been successfully constructed around a rectangular duct, using loudspeaker drive units and electronic delays. Sound propagating in the direction of these sources was sampled and a control signal applied to the sources which in turn acted to significantly reduce the amplitude of the sound. Pure tones at frequencies around 150 Hz have been attenuated by more than 50 dB but results with band-limited noise have been less successful. Further work is suggested which should result in a device having significant advantages over conventional splitter silencers, at low frequencies.     

Diffraction of sound from a dipole source near to a barrier or an impedance discontinuity

Pierce's formulation for the diffraction of spherical waves by a hard wedge has been extended to the case of the sound field due to a dipole source. The same approach is also used to extend a semiempirical model for sound propagation above an impedance discontinuity due to a dipole source. The resulting formulas have been validated by comparing their numerical solutions with that computed by summing the sound fields due to two closely spaced monopole sources of equal magnitude but opposite in phase. These new formulations are then used to develop a simple model for calculating the dipole sound field diffracted by a barrier above an impedance ground. Applications of these models relate to transportation noise prediction, particularly railway noise abatement, for which dipole sources are commonly used. The numerical predictions have been found to compare reasonably well with indoor measurements using piezoceramic transducers as dipole sources.     

Active noise control with linear control source and sensor arrays for a noise barrier

A study is conducted on minimizing the sum of the squared acoustic pressures with a linear array of control sources and a perpendicular linear array of error sensors, placed above the top of a noise barrier. Particular angular orientations, with respect to the center of the barrier top, and spacings of the linear arrays of control sources and error sensors result in moderate to significant additional reduction of the acoustic pressure in the shadow zone. Visual inspection of the sound pressure field, with and without active noise control, found that uniform and significant additional insertion loss can be generated near the barrier. Numerical simulations were conducted to test the proposed method. For separations between control sources and error sensors much less than a quarter wavelength of the primary noise disturbance, results show that the angular orientation, of the combined linear control source and sensor arrays, is a weak factor for acoustic pressure reduction in the shadow zone. Weak angle dependence serves as an advantage to the proposed method, which yields uniform performance for any angular orientation. An angular orientation involving the alignment of the furthest error sensor with the first diffracting edge of the barrier and the primary source was observed to perform well for a variety of frequencies, since the spacing between error sensors and between control sources is of the order of a quarter-wavelength. Improved noise control in the shadow zone of a barrier is achieved by the use of two control sources and angular orientation as mentioned above. Further spatial extension of the area of reduced acoustic pressure is possible by utilizing an increased number of control sources.     

Active noise attenuation in ventilation windows

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.     

平板扬声器用于结构声有源噪声控制实验研究

对NXT技术的分布式平板扬声器和集中参数扬声器的频响和指向性等进行了对比测量，通过对组合平板扬声器的声场研究及有源消声实验，证明了平板扬声器用于有源噪声控制的可行性。综合平板扬声器和集中参数扬声器的物理特点和声场特性，根据基于平面声源有源噪声控制的已有的理论研究结果和本文的实验结果，可以认为，NXT平板扬声器适合作有源噪声控制的次级源，有利于有源噪声控制的工程推广应用。     

Secondary actuation and error sensing for active acoustic structure

A typical approach to active control of sound radiation or transmission from vibrating structures involves active structural acoustic control (ASAC) and active noise control (ANC), which introduce respectively force input and compacted sound source to apply on or be close to the vibrating structure. However, for the ASAC approach, arrangement for secondary force and error sensor is heavily dependent upon the properties of the primary structure and acoustical space; for the ANC approach, a large number of compacted secondary sources are required. Hence, in this paper, based on distributed secondary sound source and near-field error sensor, active acoustic structure is proposed to construct adaptive or smart structure as a versatile module or element for controlling sound radiation or transmission at low frequencies. First, a theoretical model based on a minimization of the total sound radiation from the primary and secondary panel is established, after which, taking into consideration the relationship between the vibration modes pattern and sound radiation characteristics for secondary panels, optimal arrangement for the secondary panels is examined in detail. Finally, a near-field pressure-based error sensing approach is presented, based on two kinds of object function, and active control of sound radiation is performed.     

Multi-domain active sound control and noise shielding

This paper describes an active sound control methodology based on difference potentials. The main feature of this methodology is its ability to automatically preserve "wanted" sound within a domain while cancelling "unwanted" noise from outside the domain. This method of preservation of the wanted sounds by active shielding control is demonstrated with various broadband and realistic sound sources such as human voice and music in multiple domains in a one-dimensional enclosure. Unlike many other conventional active control methods, the proposed approach does not require the explicit characterization of the wanted sound to be preserved. The controls are designed based on the measurements of the total field on the boundaries of the shielded domain only, which is allowed to be multiply connected. The method is tested in a variety of experimental cases. The typical attenuation of the unwanted noise is found to be about 20 dB over a large area of the shielded domain and the original wanted sound field is preserved with errors of around 1 dB and below through a broad frequency range up to 1 kHz.     

Acoustic comfort in large dining spaces

This study carried out a questionnaire field investigation in two typical large dining spaces. The results suggest that the acoustic comfort of diners has an influence on the comfort evaluation of the overall dining environment, and background noise is an important factor affecting the acoustic comfort evaluation of diners. The role of various individual sound sources in background noise has been investigated, considering general background music, speech sound, activity sound, and mechanical noise, and it has been revealed that background music, other diners’ speech sound and tableware’s impact sound has a dominant impact on the acoustic comfort evaluation of diners. Compared with the existence of background music in background noise, diners’ acoustic comfort evaluation is higher than that without background music. The loudness, articulation, noise level and preference degree of various individual sound sources are factors which affect diners’ acoustic comfort evaluation on sound sources. In terms of demographic and social factors, gender and the frequency of dining out have a significant impact on diners’ acoustic comfort evaluation.     

Binaural active noise control using parametric array loudspeakers

This paper reports the binaural active noise control (ANC) system developed to deal with factory noise. The control points are located in the vicinity of the left and right ears of a worker sitting along the production line. Due to the complicated safety requirements in the factory, secondary sources and error microphones are not allowed to be placed near the worker. Therefore, the proposed ANC system employs the feedforward structure and adopts the parametric array loudspeakers (PALs) as the secondary sources. The PAL is a type of directional loudspeaker that generates a much narrower sound field as compared to the conventional loudspeaker. Once the proposed ANC system has been trained offline, the error microphones can be removed. The performance of the binaural ANC system is successfully demonstrated based on a digital signal processor (DSP) implementation.     

双层加筋板低频声的隔离与有源控制

为探讨加筋对双层结构低频隔声及有源控制的影响,分析了筋条数目及布放位置对双层加筋结构低频隔声性能、有源控制策略选取及有源隔声性能的影响。首先利用模态叠加与声-振耦合理论对双层加筋结构建模,然后采用数值算例对上述问题展开探讨。研究发现,筋条数目增多或筋条靠近基板的中间位置布放,将有利于双层加筋结构低频隔声性能的提高。对于有源控制措施,声控制策略与力控制策略相比,前者的控制效率较高且降噪效果较好。由于筋复杂的耦合影响,添加多条筋或筋条靠基板中间布置时有源控制效果减弱,需施加多个点源才能获得较好的降噪效果。