Effect of Mean Flow on Acoustic Wave Propagation in a Duct with a Periodic Array of Helmholtz Resonators

Sound propagation properties of a duct system with Helmholtz resonators(HRs) are affected by mean flow.Previous studies have tended to focus on the effects of mean flows on acoustic response of a duct system with a finite number of HRs. Employing an empirical impedance model, we present a modified transfer matrix method for studying the effect of mean flow on the complex band structure of an air duct system with an infinite periodic array of HRs. The efficiency of the modified transfer matrix is demonstrated by comparison between an example of transmission response calculation for a finite single HR loaded duct and the finite element simulation result calculated using the COMSOL software. Numerical results are presented to analyze the effect of mean flow on the band structure and transmission loss of the sound wave in the duct system. It is hoped that this study will provide theoretical guidance for acoustic wave propagation of HR silencer in the presence of mean flow.     

Wave propagation in a duct with a periodic Helmholtz resonators array

Helmholtz resonator is often used to reduce noise in a narrow frequency range. To obtain a broader noise attenuation band, combing several resonators is a possible way. This paper presents a theoretical study of sound propagation in a one-dimensional duct with identical side-branch resonators mounted periodically. The analysis of each resonator was based on a distributed-parameter model that considered multi-dimensional wave propagation in its neck-cavity interface. This model provided a more accurate prediction of the resonant frequency of the resonator than traditional lumped-parameter model. Bloch wave theory and the transfer matrix method were used to investigate wave propagation in these spatially periodic resonators. The results predicted by the theory fit well with the computer simulation using a three-dimensional finite element method and the experimental results. This study indicates that the wave coupling in this periodic system results in the dispersion of the frequency band into the stop and the pass bands. The long-term significance is that periodic resonators may more effectively control noise in ducts by broadening the bandwidth they attenuate and increasing the magnitude of sound attenuation.     

Active resonators for sound control in narrow pipes

A method of active sound control by a local device, namely, an active resonator that provides a resonance absorption or scattering of sound in a broad frequency band, is proposed. The active resonator is modeled as a sound source with an acoustoelectric feedback, which allows the correction of its motion depending on the sound field measured in the vicinity of the resonator. The performance of the method is demonstrated by two examples.     

Acoustic impedance at the interface between a plain and a perforated pipe

This paper considers the effective impedance that pertains as low frequency sound in a plain pipe radiates into a general perforated pipe of equal diameter. A previous theory that considered only the reactance is extended to also include resistance. Experimental measurements are made of the response of a Helmholtz resonator to an external sound field, where the neck of the Helmholtz resonator has both plain and perforated pipe sections. A complete theoretical model of this resonator allows for comparison between measured and predicted results of transfer functions from the external to internal sound fields of the resonator. The Nyquist plot of the admittance transfer function is extremely sensitive to the small resistance values, whereas the pressure transfer function gives more accurate results for resonant frequency and hence reactance than the usual method. In particular the results for resistance are so sensitive that it becomes possible to infer which of the current models for aperture resistance within the perforate is the most appropriate.     

A theoretical and experimental investigation of sound intensity distribution within a splitter silencer

Results are presented of an investigation into the sound intensity distribution in a ventilation duct splitter silencer subject to plane waves incident in the axial direction, under conditions of zero flow. A theory is developed which accounts for the generation of nonplane fields at the cross-sectional discontinuities of the duct-silencer-duct system. A local reaction model of the splitter material is employed, and analytical solutions and numerical results are presented for the sound intensity distribution within the silencer. Experiments were conducted to measure the distribution of sound intensity in a silencer subject to incident plane waves. The conventional cross-spectral method of measuring sound intensity could not be used because of the practical difficulties in manoeuvring the two-microphone assembly through the narrow air passage and obtaining measurements at positions close to the wall of the splitters. Instead, a modified cross-spectral method was devised which employs only one microphone for measurement. In this method, the transfer functions between the loudspeaker input signal and the microphone output signal were measured at an array of microphone positions within the air passages of the silencer. The divergence equation for sound intensity was employed to infer transverse intensity distributions from the computed axial distribution. Reasonable qualitative agreement is obtained between theoretical and observed sound intensity distribution, but the local reaction model of the splitter is found not to be satisfactory at the lower frequencies.     

Sound absorption by a Helmholtz resonator

Absorption characteristics of a Helmholtz resonator positioned at the end wall of a circular duct are considered. The absorption coefficient of the resonator is experimentally investigated as a function of the diameter and length of the resonator neck and the depth of the resonator cavity. Based on experimental data, the linear analytic model of a Helmholtz resonator is verified, and the results of verification are used to determine the dissipative attached length of the resonator neck so as to provide the agreement between experimental and calculated data. Dependences of sound absorption by a Helmholtz resonator on its geometric parameters are obtained.     

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。     

Sound absorption by an active resonator array near an impedance surface

Absorption of sound waves incident on a plane surface with arbitrary impedance by a planar active resonator array consisting of monopole or dipole resonators and positioned near the surface is considered. Appropriate tuning of active resonators ensures complete absorption of sound waves incident at a fixed angle in a broad frequency band. The effect of tuning errors on the efficiency of sound absorption by the systems under study is investigated. It is shown that, for rigid surfaces, a monopole resonator array yields a higher absorption efficiency, whereas for soft surfaces, a dipole resonator array is the more efficient one.     

内置谐振器的轻薄宽频吸声器结构设计与分析*

微穿孔板吸声器的吸声频带相较于亥姆霍兹谐振器更宽,但其低频吸声的实现需要较大的空气背腔,这对结构尺寸有限制的场合存在一定局限性。本文设计了一种轻薄吸声降噪结构(内置亥姆霍兹谐振器的微穿孔板吸声器,简称MPPHR),将微穿孔板吸声器与亥姆霍兹谐振器进行了结合,提升吸声器的低频吸声性能的同时兼具了微穿孔板宽带吸声的优点。首先基于微穿孔板和亥姆霍兹谐振器理论建立了等效电路模型并计算了结构的声阻抗。然后通过有限元对MPPHR的吸声特性进行了参数研究。最后验证了MPPHR的声阻抗模型和有限元仿真的准确性。研究结果表明：MPPHR结构拥有更宽吸声频带,厚度仅为30mm的MPPHR的半吸收频带可达1294Hz,相较于同等厚度下的微穿孔板吸声器宽近500Hz。此外,MPPHR拥有更好的低频吸声效率。     

Duct effects on the heave stability of plenum air cushions

Linear heave stability boundaries for a fun-duct-plenum air cushion suspension system are presented. These were obtained by using a quasi-static pressure-flow relationship for the fan, a finite element discretization for one dimensional unsteady duct flow, and a lumped capacitance model for the plenum. They indicate that even relatively short ducts can have a major effect at practical flow rates on the stability of cushion systems typically used for amphibious and overland operations. Some results obtained from a non-linear analysis, in which the method of characteristics was used for the duct flow, suggest that the linear approach should be adequate for practical stability calculations. Comparisons with lumped parameter models indicate that here the duct effect is associated primarily with the inertance of the air in the duct. It is also shown that for some operating conditions the duct-plenum system behaves as an Helmholtz resonator. Good agreement is obtained with an earlier transmission line analysis based on the wave equation.     

On the Helmholtz resonator

This paper develops the theory of the excitation of a Helmholtz resonator by external disturbances located arbitrarily close to the mouth of the resonator. The classical approach of Rayleigh is thereby extended to situations in which the disturbance at the mouth is not necessarily equivalent to a uniform, time dependent pressure perturbation. The analysis involves the derivation of the Green function of the resonator in a manner similar to that described in an earlier paper. The use of the Green function is illustrated by two examples in which the resonator is excited by a low Mach number stream of air. In the first case the air stream has a periodic large scale structure such as may be caused by a Kelvin-Helmholtz type of instability. The second example models the case of excitation by a shear layer possessing a continuous spectrum of turbulent eddies. In both of these applications the orders of magnitude of the sound pressure levels involved are illustrated for a typical resonator.     

Estimation of the far-field directivity of broadband aeroengine fan noise using an in-duct axial microphone array

This paper presents a measurement technique for estimating the far-field directivity of the sound radiated from a duct using measurements of acoustic pressure made inside the duct. The technique is restricted to broadband, multi-mode sound fields whose directivity patterns are axi-symmetric, and whose modes are mutually uncorrelated. The technique uses a transfer function to relate the output from an in-duct axial beamformer to measurements of the far-field polar directivity. A transfer function for a hollow cylindrical duct with no flow is derived, and investigated in detail. Transfer functions for practical cases concerning aeroengine exhausts are also presented. The transfer function is shown to be insensitive to the mode-amplitude distribution inside the duct, and hence can be used to predict the directivity in practice where the noise source distribution is unknown. The technique is then validated using a no-flow facility, and is shown to be able to predict variations in the far-field directivity pattern and also estimate the far-field sound pressure levels to within 2 dB. It is suggested that the proposed technique will be especially useful for fan rig experiments, where direct measurement of directivity, for example by use of an anechoic chamber, is impossible.     

水下材料声学性能宽频段测量方法

本文根据脉冲法和传递函数法的测试原理,建立水下声学材料吸声性能宽频段测量方法,在同一水声声管,相同状态下,对橡胶材料吸声性能进行测试分析。分析表明:常压及加压环境下,采用传递函数法和脉冲法测量材料的吸声性能,在重合频段(4 kHz～5.5 kHz)测试结果基本一致,测试偏差小于5%,从而验证了不同测量原理,测试材料水下声学性能的通用性。因此,采用传递函数法和脉冲法,可以实现水下材料声学性能宽频带测试。     

集中送风式陈列柜的CFD整体模拟

将集中送风式陈列柜的五大部件送风管道、送风结构、陈列柜风幕、回风结构和回风管道结合成为一个整体,采用双流体改进模型进行CFD模拟,分析了CFD模拟的陈列柜温度场分布特性,并与测试结果进行了对比分析。对比分析的结果表明,CFD模拟温度与实测温度十分吻合,说明所采用的CFD整体模拟是成功的。     

Aeroacoustic interaction in a corrugated duct

The sound generation by an air flow in a corrugated tube is studied experimentally for different values of the corrugation pitch and different tube lengths. The Strouhal numbers of sound generated in different tubes with different flow velocities lie within 0.4–0.6. As the flow velocity increases, the Strouhal number decreases. The effect of sound absorption by an air flow in a corrugated duct is described: in a corrugated tube with a flow, at frequencies below the generation frequency, the absorption of sound produced by an external source is observed. A semiempirical model of aeroacoustic interaction in a corrugated tube is proposed. The model provides a qualitative agreement with the experiment.     

基于圆柱定程干涉法测量气体黏度的探索

黏度是流体的重要输运性质, 实验测量是获取黏度数据的基本手段. 圆柱定程干涉法是目前测量气相声速最精确的方法之一, 其测量参数为工质的声学共振频率和共振峰半宽. 共鸣腔中气相工质的黏性会导致共振频率的偏移和共振峰半宽的增加, 是声速测量中的重要非理想影响因素. 但通过对共振频率和共振峰半宽的精确测定, 并结合热边界层、进气导管、声学传感器及壳体振动等其他非理想因素的修正, 可以精确反推获得黏度. 本文从理论上探讨了应用圆柱定程干涉法测量共振频率或者共振峰半宽来得到黏度的新方法, 以氩 (Ar) 为例进行了实验验证, 测量结果与文献值具有较好一致性, 证实了方法的可行性. 关键词： 黏度 圆柱共鸣腔 共振频率 共振峰半宽     

Acoustic performance prediction of Helmholtz resonator with conical neck

There is no accurate analytical approach for the acoustic performance prediction of Helmholtz resonator with conical neck,which has broad band acoustic attenuation performance in the low frequency range.To predict the acoustic performance of the resonator accurately,a general theory model based on the one-dimensional analysis approach with acoustic length corrections is developed.The segmentation method is used to calculate the acoustic parameters for sound propagation in conical tubes.And then,an approximate formula is deduced to give accurate correction lengths for conical tubes with difierent geometries.The deviations of the resonance frequency between the transmission loss results obtained by the general theory with acoustic lengths correction and the results from the finite element method and experiments are less than 2 Hz,which is much better than the results from one-dimensional approach without corrections.The results show that the method of acoustic length correction for the conical neck greatly improved the accuracy of the one-dimensional analysis approach,and it will be quick and accurate to predict the sound attenuation property of Helmholtz resonator with conical neck.     

Disorder in a periodic Helmholtz resonators array

This paper considers the disorder in a periodic duct–resonator system. The transfer matrix method is used to investigate wave propagation in the duct. Two cases are investigated: the disorder in periodic distance and the disorder in the geometries of Helmholtz resonators. Different from the original attenuation characteristic brought about by pure periodic system, it is found that the disorder in the geometries of resonators with the periodic distance being kept unchanged provides a useful way for the design of such a system to achieve a relatively wide noise attenuation band and to track some narrow noise peaks within it.     

锥形颈部赫姆霍兹共振器声学性能预测

锥形颈部赫姆霍兹共振器具有更好的低频消声能力,而其声学性能尚无准确解析预测方法。为了研究其声学性能,在声学长度修正的基础上,利用一维解析方法建立了用于计算传递损失的一维修正模型。运用分割法计算锥形管内部声传播的声学长度修正,并给出了声学修正长度计算公式。采用得到的锥形管声学修正长度和一维修正模型,计算出的锥形颈部赫姆霍兹共振器频率与有限元及实验测试结果偏差在2 Hz以内,明显优于不修正的计算结果。表明锥形管声学长度修正法提高了一维解析方法的精度,从而可以快捷准确的预测锥形颈部赫姆霍兹共振器的消声性能。      

封闭腔体噪声控制中亥姆霍兹共振器的优化设计方法

提出用亥姆霍兹共振器控制声腔内噪声时计算共振器最优阻尼比和最优工作带宽的理论公式,并进行实验验证。首先,建立共振器与待控腔体的声学耦合方程,以最小化腔体内目标声压幅值为参考,对共振器的阻尼比和工作带宽进行理论分析,求出最优阻尼比和最优工作带宽的计算公式。接着,提出在声腔噪声控制中使用最优亥姆霍兹共振器的实施步骤。最后,以一维声学腔体内的噪声为控制对象,通过对比控制前后的理论结果与实测数据,验证最优阻尼比和最优工作带宽的理论公式。结果表明,本文开发的亥姆霍兹共振器优化设计方法能准确地预报共振器的最优阻尼比与最优工作带宽,在声腔中低频噪声控制中有广泛的应用前景。