Sound propagation in long enclosures with a vertical or inclined branch

It is of interest to investigate the sound propagation in long enclosures with a vertical/inclined branch, since this may be relevant to long enclosures in practical applications, such as in high-speed railway tunnels. However, much research on sound propagation in long enclosures available is 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 vertical/inclined branches have been studied by comparing experimental results from physical scale models, of such enclosures with those for a straight long enclosure. It has been found that, compared with the straight long enclosure, the inclined branch makes more-difference to the SPL, EDT and RT30 of the long enclosure than the vertical branch, even though both kinds of branches have the equivalent volume.     

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.     

Noise reduction in tunnels by hard rough surfaces

This paper examines the feasibility of using two-dimensional hard rough surfaces to reduce noise levels in traffic tunnels with perfectly reflecting boundaries. First, the Twersky boss model is used to estimate the acoustic impedance of a hard rough surface. Second, an image source model is then used to compute the propagation of sound in a long rectangular enclosure with finite impedance. The total sound fields are calculated by summing the contributions from all image sources coherently. Two model tunnels are built to validate the proposed model experimentally. Finally, a case study for a realistic geometrical configuration is presented to explore the use of hard rough surfaces for reducing traffic noise in a tunnel which is constructed with hard boundaries.     

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.     

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.     

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。     

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.     

Effect of bone cortical thickness on velocity measurements using ultrasonic axial transmission: a 2D simulation study

In recent years, quantitative ultrasound (QUS) has played an increasing role in the assessment of bone status. The axial transmission technique allows to investigate skeletal sites such as the cortical layer of long bones (radius, tibia), inadequate to through-transmission techniques. Nevertheless, the type of propagation involved along bone specimens has not been clearly elucidated. Axial transmission is investigated here by means of two-dimensional simulations at 1 MHz. We focus our interest on the apparent speed of sound (SOS) of the first arriving signal (FAS). Its dependence on the thickness of the plate is discussed and compared to previous work. Different time criteria are used to derive the apparent SOS of the FAS as a function of source-receiver distance. Frequency-wave number analysis is performed in order to understand the type of propagation involved. For thick plates (thickness>lambdabone, longitudinal wavelength in bone), and for a limited range of source-receiver distances, the FAS corresponds to the lateral wave. Its velocity equals the longitudinal bulk velocity of the bone. For plate thickness less than lambdabone, some plate modes contribute to the FAS, and the apparent SOS decreases with the thickness in a way that depends on both the time criterion and on the source-receiver distance. The FAS corresponds neither to the lateral wave nor to a single plate mode. For very thin plates (thickness< lambdabone/4), the apparent SOS tends towards the velocity of the lowest order symmetrical vibration mode (S0 Lamb mode).     

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.     

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.     

Sound propagation and energy relations in churches

The results of an acoustic survey carried out in a group of Italian churches differing in style, typology, and location were used in order to study how the acoustic energy varies inside this kind of space. The effect of different architectural elements on sound propagation was investigated by means of three-dimensional impulse responses measured using a B-format microphone with sweep signals. Side chapels, columns, and trussed roofs appeared to scatter the reflections, so that the purely diffuse exponential sound decay begins after a time interval which grows with the source-receiver distance and with the complexity of the church. The results of the measurements were then compared with predictions given by existing theoretical models to check their accuracy. In particular a model previously proposed by the authors for a specific type of Romanesque churches was further refined taking into account the new findings and making some simplifications. Its application to the wider sample of churches under analysis showed that strength, clarity, and center time can be predicted with reasonable accuracy.     

Three-dimensional linearised Euler model simulations of sound propagation in idealised urban situations with wind effects

A three-dimensional numerical time-domain model based on the linearised Euler equation is applied to idealised urban situations with elongated, isolated buildings beside a straight street with sound emissions. The paper aims at the investigation of principle relationships between the source-receiver geometry (street and building facades) and sound propagation under the consideration of ground and wind. By applying cyclic lateral boundary conditions for either one or both horizontal co-ordinates, two different idealised urban environments were considered: a single street and parallel streets. Numerical experiments were performed to elaborate the effects of different roof types, ground properties, wind flow, and turbulence in both urban environments with the focus on the back facades (‘quiet’ sides) of the buildings. As a result it was found that the back facades of flat-roof buildings are quieter than those of hip roof buildings despite equal cross-cut areas. The wind effect (resulting in quieter upwind and louder downwind facades) is more pronounced for hip-roof buildings. In the case of parallel streets upwind facades are slightly louder than downwind facades because they are simultaneously exposed to downwind propagating sound from the next parallel street.     

Research on Propagation Characteristics of Millimeter Wave in Tunnels

This paper analyzes the propagation characteristics of the millimeter wave in various circular section tunnels. The received power, path loss and delay spread are discussed in detail using the verified software of Wireless InSite. The investigated tunnel models with circular section include the long straight tunnel, the tunnel with train inside and the curved tunnels with different curvature. The simulated results are verified by the experimental value in the actual tunnel. Several valuable conclusions are obtained, which would offer theoretical and practical references for the millimeter wave communication in limited spaces.     

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.     

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.     

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.     

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.     

Sound propagation in complicated industrial enclosures

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