On low frequency sound transmission loss of double sidebranches: a comparison between theory and experiment

The sound power transmission losses of various sidebranches installed along a rectangular duct below the first cut-off frequency of the duct are studied experimentally. Special efforts are made to examine how accurately the plane-wave theory predicts the sound-power transmission loss. Four types of sidebranch impedance are established and their effects to the sound power transmission loss discussed. It is found that under the nonresonant conditions the plane-wave theory can give reasonable prediction when the branch separation is large or the original sound transmission loss of the corresponding single side-branch is weak. The theory always overestimates the sound transmission loss at resonant conditions but gives underestimation if the transmission loss is due to the noise breakout in the sidebranches, especially for short branch separation.     

The application of mode coupling theory to the transmission of sound in the sidebranch of a rectangular duct system

Mode coupling is applied to rectangular ducts of the size encountered in modern ventilation systems. Theory is developed to describe the transmission of sound along a straight duct, and along a sidebranch. Acoustic impedance boundary conditions at the duct walls are included. Experimental and predicted results are presented for the frequency spectra, cross-sectional variation and longitudinal variation in the entry and sidebranch ducts caused by a pure tone source. Good agreement is shown between the two sets of results.     

WHISTLES WITH A GENERIC SIDEBRANCH: PRODUCTION AND SUPPRESSION

The present study investigates experimentally the production and suppression of whistle noise resulting from the shear layer instabilities coupled with the acoustic resonances at the interface of two ducts, a main duct and connecting sidebranch. A generic sidebranch adapter is built to allow for mounting downstream of the throttle body in the induction system of a production engine, and the adjustment of sidebranch length. The adapter has also the provision to investigate a number of suppression methods such as: (1) ramps mounted in the main duct right upstream of the sidebranch opening; (2) a spacer to increase the distance between the throttle plate and sidebranch opening; and (3) the rotation of the throttle body from its original position. Experiments with the same hardware are conducted in both a flow laboratory and an engine dynamometer facility. The effectiveness of these suppression techniques is examined experimentally along with the correlation between the two facilities.     

Suppression of flow-acoustic coupling in sidebranch ducts by interface modification

The flow-acoustic coupling of shear layer instabilities with the acoustic resonances at the interface of a closed sidebranch and main duct can produce high-amplitude pure-tone noise, known as “whistle”. This study investigates experimentally the effect of various interface geometry modifications on whistles. The objective of the modifications is to suppress the noise by redirecting the shear layer at the main duct-sidebranch interface. Interchangeable suppressor blocks of varying shapes and sizes mounted upstream and downstream of the sidebranch opening are used to change the geometry. The block shapes include those with square edges, ramps, bevelled edges, and curved (radiused) edges. The experiments are conducted in a flow facility at conditions that include certain ranges of Strouhal numbers known to coincide with significant noise generation. The effectiveness of various suppressors in reducing the noise is assessed by analyzing the measured sound pressure levels.     

On the calculation of the transmission line parameters for long tubes using the method of multiple scales

The present paper deals with the classical problem of linear sound propagation in tubes with isothermal walls. The perturbation technique of the method of multiple scales in combination with matched asymptotic expansions is applied to derive the first-order solutions and, in addition, the second-order solutions representing the correction due to boundary layer attenuation. The propagation length is assumed to be so large that in order to obtain asymptotic solutions which extend over the whole spatial range the first-order corrections to the classical attenuation rates of the different modes come into play as well. Starting with the case of the characteristic wavelength being large compared to the characteristic dimension of the duct, the analysis is then extended to the case where both of these quantities are of the same order of magnitude. Furthermore, the transmission line parameters and the transfer functions relating the sound pressures at the ends of the duct to the axial velocities are calculated.     

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。      

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.     

Propagation and radiation of sound in a finite length duct

An analysis has been carried out to investigate sound transmission in an unflanged circular duct of finite length. A new formulation for calculating the generalized radiation impedance of the opening of a finite length duct with a spinning source inside is presented. The effect of interference between the duct apertures at its two ends is identified in the calculations of radiation impedance, reflection coefficient, and the far field radiation pattern. The results show that the degree of interference between the two ends reduces for high frequency waves and/or for long ducts. Further, the interference effects on the far field for spinning sound are more serious than those for non-spinning sound.     

充水管道声分隔片消声性能研究Ⅰ.理论分析

本文对在充水管道中加入声分隔片的消声结构的消声性能进行了理论研究,用模式匹配法计算了有限和蔗声分隔片的传递损失和功率反射系数。研究结果表明,声分隔片的消声效果优于通常所采用在管壁加吸声衬怪的结构,并且其功率反射系数很小。     

Propagation and radiation of sound from flanged circular ducts with circumferentially varying wall admittances,II: Finite ducts with sources

The radiation of sound from a flanged duct system containing various hard-walled pressure sources and a finite length of non-uniformly lined duct is considered. Reflection coefficients, transmission losses and the directivity of the radiated field are evaluated. Direct comparisons between the results for the non-uniformly lined ducts, a uniformly lined duct and a hard-walled duct are made for fixed values of admittance, liner length and source distributions. Several interesting wave scattering characteristics which relate to the design of aircraft turbofan inlet liners are uncovered.     

三通管路管壁声传递损失测试方法

针对汽车进气系统三通管路的特点,提出了多通管路的管壁传递损失测试方法。并以某车型的双涡轮增压发动机进气三通管道为例,采用该方法评价其用塑料代替铝后的声学性能,主要以声传递损失来评价涡轮增压器噪声通过三通连接管路管壁的辐射和透射特性。测试过程中,三通管道的两个连接涡轮增压器端口分别用声源两次发声,靠近进气歧管端口采用两种不同反射末端,然后在每段管路布置两个压力场扬声器进行测试,并基于平面波分离入射波和反射波,同时在三通管道外用声功率半球面十点分布法自由场扬声器测试,经过3次测量来计算管道管壁的声传递损失。由于声传递损失是管道本身特性决定,所以该测试方法能够准确找出塑料件和金属件在不同频率的声学特性差异。而后,在声传递损失测试结果的基础上,结合近场声全息方法和波束形成原理进行声源识别,可知该三通管路材质改为塑料后主要噪声来自焊缝薄弱处的中高频透射声和管壁结构的低频辐射声。     

A theoretical study of passive control of duct noise using panels of varying compliance.

It is theoretically demonstrated that, in a duct, a substantial amount of sound energy can be transferred to flexural waves on a finite wall panel when the upstream portion of the panel is made to couple strongly with sound. The flexural wave then loses its energy either through radiating reflection sound waves or by internal friction. The effectiveness of the energy transfer and damping is greatly enhanced if the panel has a gradually decreasing in vacuo wave speed, which, in this study, is achieved by using a tapered membrane under tension. A high noise attenuation rate is possible with the usual viscoelastic materials such as rubber. The transmission loss has a broadband spectrum, and it offers an alternative to conventional duct lining where a smooth air passage is desired and nonacoustical considerations, such as chemical contamination or cost of operation maintenance, are important. Another advantage of the tapered panel is that, at very low frequencies, typically 5% of the first cut-on frequency of the duct, sound reflection occurs over the entire panel length. This supplements the inevitable drop in sound absorption coefficient, and a high transmission loss may still be obtained at very low frequencies.     

Sound transmission at low frequencies through the walls of distorted circular ducts

A theoretical treatment of sound transmission through the walls of distorted circular ducts is given, for plane mode transmission within the duct. The transmission mechanism is essentially that of “mode coupling”, whereby higher structural modes in the duct walls are excited, because of the wall distortion, by the internal sound field. The theory is in two parts: an approximate analytical model for the structural response of the walls to the internal sound field, and a structural radiation model. Computed results, based on the theory, are compared to measurements on “long-seam” air conditioning ducts. Where the duct geometry can be reliably specified, reasonably good agreement is obtained between theoretical and experimental data. It is concluded that mode coupling effects serve to account for the discrepancies between ideal and observed behaviour in sound transmission through duct walls.     

Characterizing holes in duct walls using resonance frequencies

The current paper demonstrates that the technique recently used by de Salis and Oldham for the sizing and location of blockages in ducts using resonance and antiresonance value shifts [J. Sound Vib. 221(1), 180-186 (1999)] may be successfully applied to the detection, location, and sizing of small holes in duct walls. It is shown that when the blockage area function reconstruction process is applied using resonance and antiresonance frequencies determined from a single measurement of sound pressure within a duct, the position of a hole in the duct wall is revealed as the beginning of an apparent gradual expansion. This expansion is termed the effective expansion function and emanates from an initial positive dc shift. Once the hole has been located, a simple impedance model of the duct incorporating the hole location and the measured wave number at the first-order duct resonance may be derived which allows the determination of the radius of the side hole from a simple quadratic equation. This hole-sizing technique is also successfully applied to thin slits in duct walls using an equivalent radius approach, and is shown to be highly accurate for slit-shaped holes with cross-sectional length to width aspect ratios of up to 28:1.     

Dual Helmholtz resonator

Acoustic characteristics of a dual Helmholtz resonator which consists of a pair of cylindrical neck and cavity connected in series (neck-cavity-neck-cavity) are established primarily in terms of a lumped-parameter theory. The results are then compared to (a) a two-dimensional analytical approach by applying a pressure/velocity matching technique for a piston-driven model; (b) a three-dimensional boundary element method for a pipe-mounted model; and (c) experiments on an impedance-tube test setup with one fabricated prototype installed in a sidebranch orientation (pipe-mounted model). Closed-form expressions have been developed in the present study for the resonance frequencies and the transmission loss of this two degree-of-freedom system based on the Newton’s second law of motion for a lumped system. The resonance frequencies and the transmission loss from this theory agree well with the analytical results, while showing a reasonable comparison with both the numerical predictions and the measurements.     

Effects of wall admittance changes on duct transmission and radiation of sound

This paper is concerned with the effect of changes in duct wall acoustic properties on the transmission of sound through ducts. Two special problems are considered. The first problem is that of a rectangular infinite-length duct with airflow and a single change in duct wall acoustic admittance. The second problem is that of an axisymmetric field in a finite circular duct without airflow and with an arbitrary number of duct wall acoustic admittance changes. Results for the first problem show the effect of wall admittance change and flow on the acoustic power transmission within the duct. Results for the second problem show the interactive effects of multiple duct liner sections on power radiated from a finite duct.     

管束穿孔板的管腔耦合共振吸声机理研究

为了揭示管束穿孔板共振吸声结构的吸声机理,利用热黏性条件下基于有限元算法的管束穿孔板仿真模型,研究了平面声波正入射条件下,管束穿孔板内部声场分布特征,并利用阻抗管对吸声系数的理论仿真结果进行了试验验证.结果表明,管束穿孔板在低频主要靠腔体共振吸声,在高频主要靠管共振吸声,管束穿孔板整体呈现出较为明显的管腔耦合共振吸声特征。管束穿孔板共振时管中声强和质点法向振速较大,高频次吸声峰频点处管中和腔中均有驻波形成,频率越高驻波数量越多.管束穿孔板的耦合共振受到管长、腔深、穿孔率和管内径等参数变化的影响,管长对高频耦合共振的影响最大,管长增大使高频主吸声峰频点移向低频,并使相邻主吸声峰之间的间距减小.      

切向流对微穿孔共振吸声结构声学性能的影响

切向流对微穿孔共振吸声结构声学性能的影响可以分成三类:(1)对小孔辐射声抗的影响;(2)对结构斜入射吸声性能的影响;(3)对消声通道消声性能的影响。根据声学基本理论,详细讨论这些影响,得到对应的理论分析公式。定性而言,若声波的传播方向与气流的运动方向一致,小孔外侧的辐射声抗、空腔声阻抗函数coth (ξ)的宗量ξ赋值和消声通道的消声系数都会减小;同时呈现多普勒效应,使得结构的吸声系数共振峰频率向低频移动。理论分析得到相应实验研究的支持。      

A displacement-pressure finite element formulation for analyzing the sound transmission in ducted shear flows with finite poroelastic lining

In the present work, the propagation of sound in a lined duct containing sheared mean flow is studied. Walls of the duct are acoustically treated with absorbent poroelastic foams. The propagation of elasto-acoustic waves in the liner is described by Biot's model. In the fluid domain, the propagation of sound in a sheared mean flow is governed by the Galbrun's equation. The problem is solved using a mixed displacement-pressure finite element formulation in both domains. A 3D implementation of the model has been performed and is illustrated on axisymmetric examples. Convergence and accuracy of the numerical model are shown for the particular case of the modal propagation in a infinite duct containing a uniform flow. Practical examples concerning the sound attenuation through dissipative silencers are discussed. In particular, effects of the refraction effects in the shear layer as well as the mounting conditions of the foam on the transmission loss are shown. The presence of a perforate screen at the air-porous interface is also considered and included in the model.