Refined multiload method for measuring acoustical source characteristics of an intake or exhaust system

The one-port source characteristics in a duct system, viz., source impedance and strength, can be determined by using the four-load method. In this paper, to avoid the instability problem of the conventional four-load method, a new formulation for the multiload method has been proposed, which employs an error function based on the linear, time-invariant source model. It is shown that the method is less sensitive to input errors compared to the previous methods. For a 10% input error, the proposed method yields a relative error in the source resistance that is about 1/100 times smaller than for the conventional method. The effectiveness of the present method is demonstrated by two test examples, a loudspeaker and a blower, each operating in a duct. It is observed that the conventional and least-squares methods result in large errors, whereas the present method yields far better agreement with the actual source parameters, as measured by the direct method. The present method is then used to obtain the source parameters on the exhaust side of an operating internal combustion engine. The radiated sound spectrum from the exhaust opening is predicted by using the measured source parameters and the calculated result agrees very well with the measured one.     

LED背光源中侧发光导光管长度与出光性能的关系

微细侧发光导光管可以将LED点光源转化成线光源.为了同时发挥LED光源和CCFL光源的优势,本文对导光管模型进行了改进,使不同长度导光管均能满足良好的出光性能.在网点公式基础上引入非线性修正参数α,并找到导光管长度与α的对应关系,得到不同长度导光管的优化网点排布形式.针对较短导光管光能利用率偏低的问题,采用导光管一端入...     

Numerical investigation and electro-acoustic modeling of measurement methods for the in-duct acoustical source parameters

It is known that the direct method yields different results from the indirect (or load) method in measuring the in-duct acoustic source parameters of fluid machines. The load method usually comes up with a negative source resistance, although a fairly accurate prediction of radiated noise can be obtained from any method. This study is focused on the effect of the time-varying nature of fluid machines on the output results of two typical measurement methods. For this purpose, a simplified fluid machine consisting of a reservoir, a valve, and an exhaust pipe is considered as representing a typical periodic, time-varying system and the measurement situations are simulated by using the method of characteristics. The equivalent circuits for such simulations are also analyzed by considering the system as having a linear time-varying source. It is found that the results from the load method are quite sensitive to the change of cylinder pressure or valve profile, in contrast to those from the direct method. In the load method, the source admittance turns out to be predominantly dependent on the valve admittance at the calculation frequency as well as the valve and load admittances at other frequencies. In the direct method, however, the source resistance is always positive and the source admittance depends mainly upon the zeroth order of valve admittance.     

Prediction of intake noise of an automotive engine in run-up condition

It is very important to predict the radiated noise from the engine intake system for the effective noise control and virtual prototyping of in-cavity and outdoor noise of a vehicle. To this end, one should precisely measure the in-duct acoustic source parameters of the intake system, viz., source strength and source impedance. Usually, the noise radiation characteristics need to be expressed as a function of engine speed. In this study, acoustic source parameters of an engine intake system under engine run-up condition were measured by using the direct method. Direct method employed two external loudspeakers, turned on simultaneously, and three microphones for the separation of upstream and downstream wave components. It was noted that the frequency spectra of source impedance hardly changes with the increase of engine speed. Utilizing this fact, source strength under the engine run-up condition was calculated by assuming invariant source impedance. Predicted insertion loss and radiated sound pressure level using the measured source parameters were compared with those of measured data and predicted data using several idealized source models, which have been adopted for the calculations. A reasonably good agreement was observed between measured sound spectra at the intake orifice and predicted one using the measured source data. It was shown that the source data obtained by the present method yielded a far better prediction accuracy than those by the idealized source models.     

Influence of wall vibrations on the sound of brass wind instruments

The results of an experimental and theoretical investigation of the influence of wall vibrations on the sound of brass wind instruments are presented. Measurements of the transmission function and input impedance of a trumpet, with the bell both heavily damped and freely vibrating, are shown to be consistent with a theory that assumes that the internal pressure causes an oscillation of the diameter of the pipe enclosing the air column. These effects are shown to be most significant in sections where there are flaring walls, which explains why damping these vibrations in cylindrical pipes normally produces no measurable effects.     

Acoustic modes induced by flow in a pipe with two closed side-branches

An air flow in a pipe with two closed side-branches can induce high pressure pulsations in a pipe system. This phenomenon has been investigated and the results are reported in this paper. A simple theoretical model based on a wave transmission was used to determine a resonance condition associated with an acoustic coupling between branches. In the model a plane wave approximation and an impedance representation of a branch were applied. The experiment was carried out in a pipe system with a relatively large distance between branches compared to branch lengths. A frequency and a pressure of pulsations were measured in a wide range of length of downstream branch. A support for the theory is provided by a favorable comparison between experimental data and calculated resonant frequencies of the system.     

On whistling of pipes with a corrugated segment: Experiment and theory

Corrugated pipes are commonly used because of their local rigidity combined with global flexibility. The flow through such a pipe can induce strong whistling tones, which is an environmental nuisance and can be a threat to the mechanical integrity of the system. This paper considers the use of a composite pipe: a shorter corrugated pipe segment embedded between smooth pipe segments. Such a pipe retains some flexibility, while the acoustical damping in the smooth pipe reduces whistling tones. Whistling is the result of coherent vortex shedding at the cavities in the wall. This vortex shedding is synchronized by longitudinal acoustic waves traveling along the pipe. The acoustic waves trigger the vortex shedding, which reinforces the acoustic field for a critical range of the Strouhal number values. A linear theory for plane wave propagation and the sound production is proposed, which allows a prediction of the Mach number at the threshold of whistling in such pipes. A semi-empirical approach is chosen to determine the sound source in this model. This source corresponds to a fluctuating force acting on the fluid as a consequence of the vortex shedding. The functional form of the Strouhal number dependency of the dimensionless sound source amplitude is based on numerical simulations. The magnitude of the source and the Strouhal number range in which it can drive whistling are determined by matching the model to results for a specific corrugated pipe segment length. This semi-empirical source model is then applied to composite pipes with different corrugated segment lengths. In addition, the effect of inlet acoustical convective losses due to flow separation is considered. The Mach number at the threshold of whistling is predicted within a factor 2.     

A study of vibroacoustic coupling between a pump and attached water-filled pipes

This paper presents a model for the vibroacoustical behavior of a pump coupled with water-filled pipes. Coupling between (a) the pump and the inlet and outlet pipes, and (b) the pipe wall and the fluid contained in the pipe, is investigated through analytical modeling and numerical simulation. In the model, the pump is represented by a rigid body supported by multiple elastic mounts, and the inlet and outlet pipes by two semi-infinite water-filled pipes. The vibration characteristics of the coupled system under the excitation of mechanical forces and fluid-borne forces at the pump are calculated. The results enhance our understanding about how the input mechanical and fluid excitation energy at the pump is transmitted to the pipes and how to relate the piping vibroacoustical response to the excitations at the pump. This study assists in predicting dynamic stress in pipes for given excitations at the pump, and in developing methods to identify the nature (fluid or mechanical) of the excitation forces at the pump using the vibration and dynamic pressure measurements on the pump/pipe system.     

An electro-acoustics impedance error criterion and its application to active noise control

Characteristics of radiation impedance and its inducing variation of electrical impedance for a controllable source have been investigated. An impedance-based error criterion has been proposed and its application to active noise control is demonstrated through a coil driven loudspeaker. A general formula of radiation impedance is derived for two control strategies, according to the criterion of total acoustic power output. The radiation impedances of some commonly used sound sources are calculated. We discuss in detail the relation between variation of the input electrical impedance and radiation impedance for the two control strategies. An AC-bridge circuit is designed to measure the weak variation of electrical impedance resulted from radiation impedance. The input electrical impedance of a loudspeaker was measured and the experimental result is consistent with that of theoretical analysis. An impedance-based error criterion is proposed since the AC-bridge relative output is unique for a certain control strategy. The implementation of this criterion applied to an active control system is analyzed by simulations. An analogue control system is set up and experiments are carried out in a semi-anechoic chamber to verify the new control approach.     

Inverse estimation of the acoustic impedance of a porous woven hose from measured transmission coefficients

A porous tube, comprised of a resin-coated woven fabric has recently been used as an effective component for use in intake systems of internal combustion engines to reduce the intake noise. For the prediction of the acoustic performance of an engine intake system with a porous woven hose, the acoustic wall impedance of the hose must be known. However, the accurate measurement of the wall impedance of a porous woven hose is not easy because of its peculiar acoustical and structural characteristics. A new measurement technique is proposed herein, that is valid over the low to mid frequency ranges. The acoustics impedance is inversely estimated from an overdetermined set of measured pressure transmission coefficients for specimens of different lengths and the reflection coefficient of end termination. The method involves only one measurement setup, and, as a result, it is very simple. A variation of the proposed method, an inverse estimation method using one of the four-pole parameters is also proposed. An error sensitivity analysis was performed to investigate the effect of measurement error on the accuracy of the final result. The measured TL for samples with arbitrary lengths and arbitrary porous frequency are in reasonably good agreement with values predicted from curve-fitted impedance data.     

Further assessment of forward pressure level for in situ calibration

Quantifying ear-canal sound level in forward pressure has been suggested as a more accurate and practical alternative to sound pressure level (SPL) calibrations used in clinical settings. The mathematical isolation of forward (and reverse) pressure requires defining the The?venin-equivalent impedance and pressure of the sound source and characteristic impedance of the load; however, the extent to which inaccuracies in characterizing the source and/or load impact forward pressure level (FPL) calibrations has not been specifically evaluated. This study examined how commercially available probe tips and estimates of characteristic impedance impact the calculation of forward and reverse pressure in a number of test cavities with dimensions chosen to reflect human ear-canal dimensions. Results demonstrate that FPL calibration, which has already been shown to be more accurate than in situ SPL calibration, can be improved particularly around standing-wave null frequencies by refining estimates of characteristic impedance. Better estimates allow FPL to be accurately calculated at least through 10 kHz using a variety of probe tips in test cavities of different sizes, suggesting that FPL calibration can be performed in ear canals of all sizes. Additionally, FPL calibration appears a reasonable option when quantifying the levels of extended high-frequency (10-18 kHz) stimuli.     

ITER气体注入系统含氚管道的等效漏率评估

氚的泄漏会对公众安全和环境造成危害,有必要对含氚管道的泄漏进行分析。对ITER气体注入系统的含氚管道进行分析,对触发系统氚报警阈值(3×10⁵Bq·m^-3/环境安全值,1×10⁸Bq·m^-3/系统安全值)时,氚送气管、离子源管以及中性化管泄漏的等效空气漏率进行了计算。结果表明,氚送气管泄漏的风险最高。虽然氚送气管的气体压力低于包容管的夹层压力,但由于管道的氚浓度高,漏率大于3.2×10^-8 Pa·m³·s^-1时会触发报警。而在离子源管中虽然氚浓度低,但管道的气体压力远高于包容管的夹层压力,漏率大于6.1×10^-6 Pa·m³·s^-1时会触发氚报警。气体注入系统含氚管道泄漏的氚危害不可忽视,必须进行实时监测和防护。     

Shape identification of underwater objects using backscattered frequency signals

The inverse problems in the area of the acoustic scattering often concern the determination of the size, shape, and orientation of an object using the scattered field data. This paper presents a method to retrieve the shape information of an underwater object using illuminated lengths, which can be obtained from the ramp response signatures of the object. An ellipsoidal object submerged in water is considered. Both the low and high backscattered frequency data have been employed to calculate the illuminated lengths. The calculated results show that the illuminated lengths will be more accurate, if only the high-frequency-range data are employed. For ellipsoidal objects, any three illuminated lengths that are not of a same plane can in theory fully determine the shape of the ellipsoid. As the calculated illuminated lengths contain numerical errors, the calculated results of the three semiaxes of the ellipsoid will deteriorate and become unreliable, especially when the three incident directions of the illuminated lengths become close. The reason is that the condition number of the coefficient matrix becomes big in such situations, which leads to an increase of the relative error upper limit in the calculated results. To avoid such errors in close incident wave cases, it is found that the use of more than three incident waves works very well in the shape identification of an underwater object.     

Accurate experimental two-port analysis of flow generated sound

In the study of in-duct aero-acoustic phenomena two-port analysis based upon measurements has become an important method for the plane wave region. However even at moderate Mach numbers (0.2-0.3) the errors in the results can be hard to suppress. Ways of dealing with this include the use of over-determination methods and methods of obtaining more accurate wavenumbers. Most of the previous published work on this subject deals with the passive (scattering) part of the two-port. In this paper, both the passive and active (sound generating) parts of the two-port are addressed, and different methods for the determination of the source data and the scattering matrix are evaluated. For the source data in the form of a cross spectrum matrix an over-determination method is introduced. Additionally, a method of obtaining the mean Mach number from experimentally determined wavenumbers is described.In order to evaluate the methods measurements are conducted at a mean Mach number of 0.2 for two test cases—an empty duct and a mixer plate inside the duct. The main improvements in the scattering matrix results are achieved by discarding measurements from an abundant set, based upon the measured coherence between excitation and fluctuating pressures. For the source part, it is shown that the error in the magnitude of the source cross spectrum matrix can be significantly suppressed by having additional (>2) reference microphones on each side of the two-port.The mean Mach number obtained from an experimentally determined wavenumber yields more accurate scattering matrix results in both phase and magnitude, than those based upon flow velocity measurements at one point and an assumed flow profile.     

Investigation on Rijke pipe＇s acoustic characteristics by numerical simulation： modeling the pulsing flow field coupled the inner of pipe with its outer space

Based on the results of fluid dynamics, heat transfer and acoustics, a Computational Fluid Dynamics （CFD） method was utilized to study the acoustic characteristics and self-excited pulsation mechanism inside a Rijke pipe. To avoid settling the irrational boundary conditions of the finite-amplitude standing wave in the Rijke thermo-acoustic system, the simulation modeling in the flow field, which coupled the inner of pipe with its outer space, was carried out to replace the traditional way in form of internal flow field numerical investigations. A hypothesis for heat source in energy equation including the relationship on unsteady heat of air around heat source, oscillation pressure and oscillation velocity was presented. To reflect the essence of Rijke pipe, simulation on self-excited oscillation was conducted by means of its own pulsation of pressure, velocity and temperature. This method can make the convergence process steady and effectively avoid divergence. The physical phenomenon of the self-excited Rijke pipe was analyzed. Moreover, the mechanisms on the Rijke pipe＇s self-excited oscillation were explained. Based on this method, comparative researches on the acoustic characteristic of the Rijke pipe with different size and different shape of nozzle were performed. The simulation results agreed with the experimental data satisfactorily. The results show that this numerical simulation can be used to study the sound pressure of nozzle for the engineering application of Rijke pipes.     

Analysis of pipeline networks using two-ports

Majority of vibration problems arise in pipeline networks are attributed to the high-pressure pulsations. Pulsations are generated by fluid machines such as compressors and pumps. These pulsations turn into shaking forces at elements such as pipe bends and pipe reducers, which in turn excite vibrations in the connected piping network. High vibrations beyond the endurance limit of the pipe material may cause damage to pipes, supports, and equipment. In addition, if the source pulsation frequency coincides with one of the natural frequencies of the piping network, resonance will take place and the vibrations will be magnified to a large scale. Obviously, if these vibrations are not well controlled, they might cause damage to the whole system and foundation, and might lead to substantial financial losses. Thus, prediction of pulsations is important for safe and proper operation. In this paper, a pilot plant equipped with a reciprocating compressor, pipes, bends, and terminated by a vessel is built. The network is modeled using the two-port theory that splits the network into several cascaded elements, and predicts the response of the network. The prediction model uses the measured compressor source data as an input, which is determined by the indirect multi-load method that is usually used to characterize internal combustion engines. A pulsation suppression device is designed, modeled, manufactured and inserted into the pilot network. The pressure pulsations are measured with and without the pulsation suppression device, and compared to the predictions using the two-port theory.     

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。     

爆炸载荷作用下焊缝区附近埋地X70钢管的动力响应分析

爆破地震效应对埋地管线的影响已成为工程爆破领域研究的热点。基于有限元软件ANSYS/LS-DYNA,以两种含Y型焊缝(坡口有2 mm余高焊缝和坡口无余高焊缝)的埋地X70钢管为例,数值模拟研究了TNT炸药量相同(4.473 kg)而炸高(60.0、85.0和110.0 cm)不同时,焊缝区附近埋地X70钢管的动力响应规律。研究表明:当炸高为60.0 cm时,焊缝有余高的管道受应力集中的影响较大,且先于焊缝无余高的管道进入屈服阶段;当炸高为60.0和85.0 cm时,焊缝有余高的管道整体抵抗变形的能力明显弱于焊缝无余高的管道。管土间的相互作用对X70管道背爆面有支撑作用,可有效地减小管道背爆面的位移。在相同条件下,焊缝有余高的X70管道抵抗振动的性能弱于焊缝无余高的管道,且与焊缝形式相比,炸高对含焊缝区的X70管道的最大振速起主要影响作用。