Study on sound propagation in long enclosures with a vertical or inclined branch of different dimensions

As there may be some branches in long enclosures, such as high-speed railway and freeway tunnels, it will make a difference in the sound propagation in long enclosures if we give more consideration to different designs of the branches at the onset of their construction. However, most researches on the sound propagation in long enclosures available are concerned with straight long enclosures. In this paper, the sound pressure level (SPL) attenuation, early decay time (EDT), and reverberation time (RT30) of long enclosures with a vertical or inclined branch of different dimensions have been studied by comparing experimental results from physical scale models of such enclosures with those of the straight long enclosure. This experimental investigation gives interesting results on the behavior of sound propagation in long enclosures with a vertical or inclined branch of different dimensions. When conducting engineering design for long tunnels of the high-speed railway or freeway, it would be appropriate to consider designing the branch of the tunnels into a widened left inclined or right inclined one to provide a better relative SPL than that of other branches with different dimensions. This study further reveals that with an inclined branch of different dimensions, the EDT of the long enclosure will be different. At both 500 and 1000 Hz 1/3 octave bands, the EDT is the shortest when the long enclosure has a left inclined branch and an extended left inclined branch, while the EDT is the longest when the long enclosure has a widened vertical branch. Generally speaking, in the near field (i.e. the source-receiver distance is shorter than the threshold distance), the EDT measurements are similar, while in the far field (i.e. the source-receiver distance exceeds the threshold distance), the EDT is the longest for the straight long enclosure, second longest for the long enclosure with a widened inclined branch, third longest for the long enclosure with a vertical branch, and the shortest for the long enclosure with an extended inclined branch, which is similar to the trend with RT30.     

The effect of multiple branches on sound propagation in long enclosures

Since the classical room acoustics can not be used for long enclosures due to the inhomogeneous sound field, much work has been carried out recently to investigate the sound propagation in long enclosures, which are helpful to the acoustic design of practical long enclosures, such as the high-speed railway tunnels. However, most of these works focuses on the straight long enclosures without branches or with one branch. In this paper, the effects of the multiple branches on sound propagation of long enclosures are studied. The sound pressure level (SPL) attenuation, early decay time (EDT), and reverberation time (T30) of long enclosures with multiple branches have been investigated by physical scale models based acoustic experiments. Several interesting results have been obtained concerning the sound propagation of long enclosures with multiple branches. It shows that the sound field of long enclosures with multiple branches is more complex and inhomogeneous than that of the long enclosures without branches or with one branch.     

Sound field prediction in long enclosures with branches: A combined method

The propagation of sound in long enclosures with branches has been studied theoretically and experimentally, and an efficient combined method is proposed to predict the sound field in long enclosures with branches. Based on the wave-acoustics theory, the theoretical analysis of the sound field of the long enclosures with branches is performed. This paper also investigated the sound field prediction of long enclosures with branches, by using the acoustic modeling program, ODEON. The results obtained by the theoretical analysis and the numerical simulation ODEON are compared with the experimental measurements, and the characteristics of the two methods for predicting the sound field of long enclosures with branches are analyzed. Compared with the experimental results, it is found that: (1) the results predicted by the theoretical analysis fluctuate relatively large with respect to the source-receiver distance, and the sound pressure level (SPL) attenuation obtained is smaller than that measured; and (2) the results predicted by the numerical simulation is smoother, and the calculated SPL attenuation is larger than that measured. To effectively predict the sound field of long enclosures with branches, a combined numerical method is thus proposed. The effectiveness of the proposed combined method is demonstrated by the scale-model experiments.     

The prediction of speech intelligibility in underground stations of rectangular cross section

Long enclosures are spaces with nondiffuse sound fields, for which the classical theory of acoustics is not appropriate. Thus, the modeling of the sound field in a long enclosure is very different from the prediction of the behavior of sound in a diffuse space. Ray-tracing computer models have been developed for the prediction of the sound field in long enclosures, with particular reference to spaces such as underground stations which are generally long spaces of rectangular or curved cross section. This paper describes the development of a model for use in underground stations of rectangular cross section. The model predicts the sound-pressure level, early decay time, clarity index, and definition at receiver points along the enclosure. The model also calculates the value of the speech transmission index at individual points. Measurements of all parameters have been made in a station of rectangular cross section, and compared with the predicted values. The predictions of all parameters show good agreement with measurements at all frequencies, particularly in the far field of the sound source, and the trends in the behavior of the parameters along the enclosure have been correctly predicted.     

Numerical modeling of the sound fields in urban squares

This paper studies the basic characteristics of sound fields in urban squares surrounded by reflecting building fa?ades and the effectiveness of architectural changes and urban design options. A radiosity model and an image source model are developed, and a parametric study is carried out in hypothetical squares. The results show that the reverberation time (RT) is rather even in a square, whereas the early decay time (EDT) is very low in the near field, and then becomes close to RT after a rapid increase. Compared to diffuse boundaries, with geometrical boundaries the RT and EDT are significantly longer and the sound pressure level (SPL) attenuation with distance is generally smaller unless the height/side ratio is high. With a boundary diffusion coefficient of 0.2, the sound field is already close to that resulting from purely diffusely reflecting boundaries. The SPL in far field is typically 6-9 dB lower if the square side is doubled; 8 dB lower if the height of building fa?ades is decreased from 50 m to 6 m (diffuse boundaries); 5 dB (diffuse boundaries) or 2 dB (geometrical boundaries) lower if the length/width ratio is increased from 1 to 4; and 10-12 dB lower if the boundary absorption coefficient is increased from 0.1 to 0.9.     

On the sound field of a vault with two open ends

The sound field in a vault with two open ends is studied and its sound pressure distribution is compared with that in an enclosure and a long space. The accuracy of estimation on this kind of space by the classic Sabine Formula, the modified Sabine Formula and the ray tracing method are compared through numerical simulations and experiments. It is found that the sound field in this type of non-close space is not uniform, SPL declines faster near the openings, and RT values have a large span when the source is not at the center. Non-exponential decay is also found in this open vault, and the reason and precondition of its occurrence are discussed.     

Insertion loss of an acoustic enclosure

Acoustical enclosures are the common arrangements in reducing airborne noise from shipboard machinery such as engines and generators. In this paper the theoretical models, established based on statistical energy analysis, are presented for predicting the insertion loss of acoustical enclosures in different frequency ranges. In addition to the consideration of resonant modal coupling between internal sound field and enclosure structural vibration, the nonresonant transmission though and the interaction between enclosure walls in the models are also included. It is shown that the insertion loss of enclosures is mainly controlled by the nonresonant modes in the intermediate frequency range. At high frequencies, the insertion loss of enclosures can be improved by increasing the sound absorption at the internal boundaries of enclosures. Experiments were carried out on two enclosures made of different materials. The measured results are compared with the predicted values and the good agreement between them is the initial demonstration of the validity and feasibility of the theoretical models.     

Prediction of reverberation time and speech transmission index in long enclosures

It is known that the sound field in a long space is not diffuse, and that the classic theory of room acoustics is not applicable. A theoretical model is developed for the prediction of reverberation time and speech transmission index in rectangular long enclosures, such as corridors and train stations, where the acoustic quality is important for speech. The model is based on an image-source method, and both acoustically hard and impedance boundaries are investigated. An approximate analytical solution is used to predict the frequency response of the sound field. The reverberation time is determined from the decay curve which is computed by a reverse-time integration of the squared impulse response. The angle-dependence of reflection coefficients of the boundaries and the change of phase upon reflection are incorporated in this model. Due to the relatively long distance of sound propagation, the effect of atmospheric absorption is also considered. Measurements of reverberation time and speech transmission index taken from a real tunnel, a corridor, and a model tunnel are presented. The theoretical predictions are found to agree well with the experimental data. An application of the proposed model has been suggested.     

有孔双层矩形金属机壳屏蔽效能

为了分析带有孔缝双层金属机壳的屏蔽效能,采用广义散射矩阵级联的思想,将多层金属腔体近似等效为若干个能够传输多个模式的波导级联,提出了一种使用模式匹配法和基于矩量法求解混合位积分方程的全波混合算法。该算法考虑了腔体结构、孔缝排列形式、入射波极化方向以及高次模等因素对屏蔽效能的影响。通过将数值仿真结果与经典算法仿真结果进行对比,验证了其具有较高的准确性和有效性。研究结果表明:双层金属腔体的屏蔽效能要优于单层金属腔体的屏蔽效能,适当地增加双层金属腔体上孔缝所在侧壁之间的距离可以提高屏蔽效能,当双层金属腔体侧壁上的孔缝平行排列时,平行极化下的屏蔽效能要优于垂直极化下的屏蔽效能,而当孔缝交叉排列时,平行极化和垂直极化下屏蔽效能的优劣则并不明显。     

A coherent model for predicting noise reduction in long enclosures with impedance discontinuities

A theoretical model has been developed for the prediction of sound propagation in a rectangular long enclosure with impedance discontinuities. Based on the image-source method, the boundaries are assumed to be geometrically reflective. An infinite number of image sources are generated by multiple reflections. The sound pressure of each image is obtained by an approximate analytical solution, known as the Weyl-van der Pol formula. The total sound field is then calculated by summation of the contribution from all images. The phase information of each image and the phase change upon reflection are included in the model. A single change of impedance in a two-dimensional duct is focused on as the fundamental problem of the current study. The diffraction effect at the impedance discontinuity is proved to be insignificant, and it is ignored in the formulation. On the assumption that the diffraction effect is not important, the investigation is moved on to a rectangular long enclosure. Measurements are conducted in two model tunnels to validate the proposed prediction model. The predictions are found to give good approximations of the experimental results. The theoretical model serves as the first attempt to optimize the position and pattern of sound absorption materials in a long enclosure, such as an underground railway station or a building corridor, for the reduction of noise and improvement of sound quality.     

基于电磁拓扑的多腔体屏蔽效能快速算法

提出基于电磁拓扑理论计算开孔多腔体屏蔽效能的快速方法.首先给出双腔体等效电路和电磁拓扑信号流图,并推导孔缝节点处的散射矩阵,给出拓扑网络的散射矩阵方程和传输矩阵方程,获得双腔体的广义Baum-Liu-Tesche(BLT)方程.在此基础上研究了开孔三腔体,包括串型级联三腔体和串并型混合级联三腔体的广义BLT方程.对于串型级联三腔体,其电磁拓扑网络和广义BLT方程在双腔体基础上直接扩展即可获得.而对于串并型混合级联三腔体,通过将位于三腔体公共面上的孔缝等效为三端口网络节点,并根据三端口网络散射参数定义推导获得该节点的散射矩阵,最终得到串并型混合级联三腔体的广义BLT方程.本文方法对双腔体的计算结果与文献结果和实验结果相符合,对3组不同类型和尺寸开孔腔的屏蔽效能的计算结果与时域有限差分法计算结果符合较好.该算法不仅效率高,通过对所有计算结果和实验结果的误差统计分析,表明该算法具有较高的计算准确度.     

Development of a hybrid computer model for simulating the complicated virtual sound field in enclosures

A hybrid computer model, SOFIS, has been developed for the simulation of an enclosed virtual sound field in an arbitrary shaped enclosure. It can be used to calculate the impulse response and acoustical parameters of different positions in a virtual enclosure. This paper describes the way in which SOFIS models the sound source, the receiver and the sound propagation throughout the enclosed space with curved surfaces or barriers. A phase tracing method and the calculation of acoustic indexes are also discussed in this paper.     

声速剖面反演对基阵倾斜失配的敏感性研究

通过推导基阵倾斜失配给声传播时间计算带来的误差,分析了这种参数失配对基于声传播时间的声速剖面反演结果的敏感性。研究结果表明,小角度倾斜时,由基阵倾斜所带来的声速剖面反演误差与倾斜俯仰角近似成正比,与声传播时间近似成反比。当基阵倾斜方位与声源一基阵连线方向一致时带来的误差最大,而垂直时误差最小。理论计算与仿真、实验结果在变化趋势和数量级上具有较好的一致性,为反演误差预报提供了参考。      

Sound field of typical single-bed hospital wards

The aim of this paper is to investigate the basic characteristics of sound fields in the increasingly concerned single-bed hospital wards, the effects of commonly used furniture, and the feasibility and strategic use of acoustic simulation techniques for such spaces. Two typical single-bed wards at a Critical Care Unit were selected as the case study site. Eight different room acoustic conditions were created by gradually moving pieces of furniture out of the ward. It has been shown that in the ward without acoustic ceiling, under empty room condition the RT ranged from 0.8 s, typically at low frequency, to 2.3 s at 630 Hz; the variations in both SPL and RT at different receivers were insignificant except at low frequencies, so that in such single-bed wards a diffuse field could be assumed; and longer RT and higher SPL were found when furniture was gradually moved out of the ward. In the ward with acoustic ceiling, the effect of furniture was less. There was a good agreement between measured and simulated RT and SPL, showing the usefulness of computer simulation for this kind of spaces. In the simulation, the effect of surface diffusion coefficient was generally insignificant, whereas the effect of edge diffusion should be taken into account, especially for the RT prediction. The feasibility of geometric simplification was also demonstrated.     

A study of barriers in enclosures by a ray-tracing computer model

The theoretical study of the behaviour of barriers in enclosures is much more complicated than that of barriers in the free field. As a matter of fact, in the latter case one has to take into account the sound energy reflected at the boundary surface of the enclosure. The barrier-absorption joint effect has to be considered in defining the barrier sound attenuation. From this point of view, the barrier attenuation also depends on the enclosure shape and proportions. In this paper we suggest a numerical method, based on a ray-tracing technique, in which the concepts inherent in Keller's theory of diffraction are included. The results of some simulation examples are reported, together with some qualitative considerations which allow emphasis of the problems connected with the optimization of barrier design in enclosures.     

狭长封闭声场的近似圆锥束跟踪法模拟研究

狭长空间声传播特性研究对于地铁，长廊等场所的音质设计是非常有意义的，本文提出了模拟狭长封闭声场的近似圆锥束跟踪法，利用它研究了矩形和非矩形狭长声场的空间分布特性。仿真结果表明，对于狭长声场，经典扩散场理论并不适用，而利用本文的数值模拟方法能正确地分析这类声场的特性。     

Visualization of three-dimensional flow structures in the wake of an inclined circular cylinder

In the previous measurements of the aerodynamic sound generated from an inclined circular cylinder, it is reported that the sound pressure level (SPL) changes with the aspect ratio and the inclined angle. Therefore, we have investigated the changes in the vortex structure of the wake considered as one of the causes of the SPL variation. Using the low-noise wind tunnel, the velocity fluctuation in the wake is measured to obtain the correlation length. Moreover, the flow visualization is performed with a hydrogen bubble method and a numerical analysis method in order to clarify how the wake structure changes by variations of aspect ratio and inclined angle. As a result of this investigation, it is shown that the spanwise structure of Karman’s vortex is highly influenced by the interference of Karman’s vortex with the bottom endplate, and that the influence on the spanwise structure in the wake becomes greater as the aspect ratio decreases and the inclined angle increases.     

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。     

Modeling and acoustic analysis of irregular sound enclosure by using Chebyshev-variational method

A modeling method for irregular sound enclosures was proposed based on the Chebyshev-variational theory. A rectangular space was first assumed to bound the irregular sound space and the sound pressure in the rectangular space expressed as a triple-Chebyshev series. Next, a coordinate transformation was performed and the Lagrangian functional of the irregular sound space obtained. Finally, the Lagrangian functional was solved under the Ritz method framework, and the enclosure's acoustic characteristic equation deduced and the eigenpairs obtained. The accuracy of the present method was validated according to agreement between the present results and finite element results for an enclosure with a curved surface.Furthermore, the acoustic characteristics of a trapezoidal enclosure and an enclosure with an inner groove were investigated. The results showed that the mode shapes of the trapezoidal sound space changed with increased inclination angle and the natural frequencies, except the first order, of the sound space with a rectangular inner groove decreased with increased groove depth.     

Sound propagation in complicated industrial enclosures

I.IntroductionInordcrtocfficlcntlyprcdictandcontro1thenoiseinindustria1enclosurcs,manypeoplehavestudicdmathmodclsforsoundpropagationinindustria1enclosurcs.Butsofar,thereisnosatisfactoryrcsult.Inthepast,a1mostal1studieswcrercstrictedtoidealemptyindustrialcnc1osurcs,andonlyafewofthcm['-']concerncdtheinfluenceofobstac1cswithinthemodel.Thcscmodc1strcattheobstac1eeffcctsbythemethod-ofimagesandcanonIybeusedtoregu-larhexahcdronroomswhcreobstaclcsareassumedtobedistributesuniformIy.Ondet'sartic1el'],…