无次级通道建模有源噪声控制算法的实验研究

以管道中有源噪声控制为实验平台,基于ADSP21161N数字信号处理评估板实现了本文作者早先提出的不需要次级通道建模的基于LMS方法的四方向搜索型有源控制算法,并给出了该算法在实际应用中收敛系数的选取原则。针对单频和多频噪声的实验结果表明,前人两方向搜索型有源控制算法在大部分频率上都能收敛,但在次级通道相位为±90°的频率不能收敛,而本文作者所提出的四方向变收敛系数搜索型算法,在所有频率都能快速收敛。     

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。     

声屏障和有源噪声控制声屏障

概述了声屏障降低噪声的原理和常用方法，提出了存在的不足，介绍了有源声屏障的降噪性能与发展动态，提出了进一步研究的设想。     

空间有源消声实验研究

本文分别在半声室和普通房间进行有源消声实验，证明了作者以前提出的算法是收敛的，稳定的，以此算法构成的多通道有源消声系统对直达声和混响声都能进行了有效的控制。     

基于声场复现的有源噪声控制支撑技术*

有源噪声控制是一种有广泛用途的低频噪声控制技术,目前已在部分场合取得商业化应用,然而该技术的大规模应用仍然面临诸多困难,声场复现技术为解决其中的关键问题提供了有效手段。该文论述了声场复现的两种实现方式(声场重构和声场再现)的基本原理、国内外现状及其在有源控制技术开发中的应用,尤其是基于声压匹配法的初级声场重构、基于球阵的声场再现以及电声器件布放优化设计。     

调节水床效应的双梯度有源噪声控制自适应算法*

为了使自适应反馈有源噪声控制系统能够兼顾水床效应,通过约束次级信号的能量以调节噪声放大,设计了一种双梯度算法。当次级信号满足约束,算法沿着最小化误差信号能量的梯度方向迭代,反之,则沿着最小化次级信号能量的方向迭代。在有源降噪耳机实例中的对比结果表明,该算法能调节噪声放大并保持较大的降噪带宽,且运算量没有显著增加。     

气流管道有源降噪研究

如何消除气流的影响是管道有源降噪的难点之一，本文分析了湍流对有源降噪系统的影响，并开发了一种抗湍流传声器探管，最后在某种通风系统的进气管段进行了降噪试验，气流速度为２０ｍ／ｓ时，在６０－６３０Ｈｚ频带取得了１５ｄＢ（Ａ）的降噪效果。     

一种能量比调控的有源噪声控制算法

在前馈有源噪声控制系统中,建模信号与控制信号相互影响,建模信号的引入会导致系统降噪性能变差。为了减小建模信号的影响,提出一种基于能量比调控的次级通道在线建模有源噪声控制算法。利用控制过程与建模过程的误差能量比构造步长调控函数,分别调节控制过程与建模过程的步长值,从而减弱两者的相互影响。在次级通道建模过程中,对建模步长值采取分段调控的方法,并通过建模步长值的变化来调节建模信号,从而提升系统降噪性能。仿真结果表明,对于低频噪声信号的有源噪声控制,相比已有算法,提出的算法能获得较快的建模收敛速度和较高的降噪量。     

管道有源噪声控制中壁面分布次级声源的空间分布优化

为了改进管道中高阶模式声波的有源控制效果,研究了壁面分布次级声源的空间分布优化问题。首先提出管道中次级声源独立控制高阶模式声波的理论模型。然后推导次级声源在管道中各方向上空间分布对控制高阶模式声波的贡献,得到了次级声源空间分布的优化准则。通过将空间分布离散化,采用最小化管道中声能流的控制策略得到次级声源的最优驱动强度。最后通过数值仿真对比最优驱动强度时各种次级声源空间分布对控制性能的影响,验证了通过优化次级声源空间分布能显著提高控制效果。仿真结果表明,当次级声源分布于管道所有壁面且沿管道轴向分布范围较大时,高阶模式的控制效率最高。     

A maze structure for sound attenuation

Sonic crystals are periodic arrangement of scatterers made of material with low acoustic impedance or sound hard materials [1]. Sonic crystals have numerous applications such as green belts and sound barriers. Here we showed that a typical maze structure at children playground can attenuate noise effectively for frequencies ranging from 12.5 Hz to 20,000 Hz. The original designer for the maze structure probably does not have that in mind. The maze structure can be viewed as a sonic crystal structure with sound attenuation characteristics. We found that the maze was able to attenuate noise up to 17.9 dBA for frequency range below 1000 Hz and 23 dBA for higher frequency range up to 20,000 Hz. The maze structure was able to mitigate noise at a wide range of frequencies in addition to the center frequency (f_c$f_c$) of 478 Hz which was estimated based on the Bragg’s Law. The periodic effects of the maze was also proven by numerical studies. Our results demonstrated that the maze structure commonly found in children playgrounds was able to attenuate noise covering the whole human hearing range.     

基于声线跟踪法对闭空间声场预测的接收球半径计算

接收球半径是采用声线跟踪法进行声场预测的一个重要参数。以前所用到的接收球半径一般是通过经验估计得到的,缺乏详细的理论分析。本文从声波传播机理角度分析了根据声线密度来确定接收球半径的原因,并推导了矩形闭空间中声线密度和接收球半径的计算方法。声线密度可以通过原始声线数目,声场空间体积与形状,边界吸声系数来确定。在一给定闭空间里,声线密度可看作是均匀分布的,所以接收球半径与空间位置无关,可看作是一个常数。但对不同的声场空间来说,由于空间体积、形状和吸声系数的变化,声线密度是不同的,因此接收球半径也会不同。声线密度越大,接收球半径越小;声线密度越小,接收球半径越大。实验表明,所提出的接收球模型能用来较准确地预测闭空间里的声压级和混响时间等声学参数。     

应用匹配滤波法估测水下介质声阻抗的初步研究

对一种新的反演方法──匹配滤波法应用于水下地声探测中的声阻抗定量估测问题作了研究,论述了这种方法的原理和算法,并且给出了对其进行改进的方法。模拟声呐记录的试算结果表明,匹配滤波法是一种原理简单,计算量小,容噪性好的水下介质声阻抗反演方法.     

A further study of a mathematical model for a screen in open-plan offices

In order to obtain a good acoustical environment in open-plan offices, the acoustic performance of a single screen needs to be well estimated. Based on the sound pressure or the sound energy summation, the models proposed by previous investigators provided the predictions of the speech intelligibility index (SII) at the receivers behind a single screen in the open-plan offices. In the development of these models, the expression describing the diffracted field of a screen was an empirical formula and the sound diffraction is assumed to be appeared in the shadow region of the screen. In the present study, the MacDonald analytical formula is applied to describe the diffracted field of a screen, and both the diffractions in the visible region and in the shadow region are considered in the proposed model though it seems that the diffraction in the visible region is small compared with that in the shadow region. The comparison of the predicted and experimental results of the Wang and Bradley and the predicted results of the improved model shows that the improved model can provide a more accurate prediction, and further calculations indicate that the improvement is mostly due to the application of the MacDonald analytical diffraction formula.     

虚拟声屏障的数值及实验分析

虚拟声屏障(Virtual Sound Barrier,VSB)由若干控制声源和误差传感器构成,使用有源噪声控制方法在噪声环境中产生局部安静区域,“阻隔”声音但不“阻隔”空气和光,像一个无形的屏障对声音起作用。本文建立立体结构的VSB系统模型,从数值模拟和实验两方面说明在噪声来自于多个方向的普通房间中,通过该系统产生人头大小的静区是可行的。数值模拟表明即使在中频VSB系统也有良好的降噪效果。实验给出一种实用的圆柱状分布的16通道的VSB系统,在中低频条件下产生人头大小的静区,降噪可达10 dB以上。     

封闭腔内纳米流体强化自然对流换热的数值模拟

采用F luen软件对封闭腔内Cu-H2O纳米流体强化自然对流换热进行了数值模拟,重点分析Cu纳米粒子添加量和Gr数对换热性能的影响,并解释其换热机理。研究结果表明:在水基液中加入Cu纳米粒子可以显著提高基液的自然对流换热特性。对于一给定的Gr数,随着纳米粒子质量分数的增加,纳米流体的速度组成部分增加,纳米流体质量分数越大,x方向和y方向的速度峰越大,因此加速了流体中能量传输。另一方面,随着Gr数的增加,流线图中旋涡逐渐变大,流线间强度增加,说明换热效果逐渐增强。当Gr数较小时,传热主要是由热壁和冷壁之间的热传导引起的,随着Gr数的增大,换热逐渐变为由对流换热占主导地位。     

Study on sound transmission characteristics of a cylindrical shell using analytical and experimental models

A circular cylindrical cavity enclosed by a thin elastic shell is found in many practical devices such as expansion volume mufflers, hermetic compressors and aircraft cabins. Analytical and experimental studies are conducted in this work to understand the characteristics of sound transmission through the cylindrical wall of such a system. Using an infinitely long circular cylindrical shell subjected to a plane incident wave, an exact solution is obtained by solving the classical shell vibration equations and the acoustic wave equations simultaneously. Transmission losses obtained from the solution are compared to the transmission losses that are measured for a cylindrical shell of finite length and the same cross-sectional dimensions. The comparison suggests that the theoretical model can be used as an effective design tool despite considerable simplifications involved.     

Enhancement of noise reduction efficiency based on compensation in the ANC headset using fixed-point DSP

In this paper, we have proposed a new algorithm considering commutation error and feedback effect to enhance the convergence rate and noise reduction efficiency of ANC controller. In order to improve noise reduction performance of the ANC headset with fixed-point DSP, we have proposed a new FxLMS algorithm, FxLMS CF, which considers the commutation error and feedback. Also, using a non-real-time simulation, we have decided the phase and amplitude compensation factors of anti-noise signal considering round-off and quantization error, nonlinear distortion and delay of analog device. We estimated the phase and amplitude compensation factors by simulation without using any special measuring devices or analysis devices, and reduced the broadband noise by 24 dB.     

A comprehensive study of sound pressure in a finite-length fluid-filled multi-walled carbon nanotube

The aim of this paper is to analyze vibrational behavior and the sound wave propagation in the finite-length fluid-filled multi-walled carbon nanotubes (MWCNTs) and to determine the exact sound pressure load effect on it, and compare it to what has been used by the other researchers. For this purpose, the solution of the modified complex Helmholtz equation is derived by considering the non-rigidity of the CNT and the wave reflections at the open ends of the MWCNT. These investigations are very important for potential application of CNT-filled polymeric foam that is used as sound absorber. In this paper, in formulating the sound pressure load exerted on the innermost tube of the finite-length fluid-filled MWCNT, the following points have been studied for the first time: (i) the energy loss in the fluid, which cannot be ignored in the high frequency analysis; (ii) the non-rigidity of the MWCNT through considering finite acoustical impedance for its walls; (iii) the wave reflections at the open ends of the finite-length MWCNT to calculate the sound pressure load term which is coupled with the dynamic equations of motion for the finite-length fluid-filled MWCNT. The results show that ignoring the mentioned points would cause errors in the prediction of the sound pressure load exerted on the finite-length fluid-filled MWCNT.     

A note on the sound field due to a point source inside an absorbent-lined enclosure

It is shown that the field distribution due to a point source of sound within an absorbent-lined enclosure can be simply calculated using a geometrical ray treatment and the appropriate plane-wave pressure-reflection coefficient. A practical application of this result is in predicting the performance of free-field test chambers of the type commonly used for industrial measurement of the sound power of noise sources.