Numerical modeling of urban sound fields by a diffusion process

In this paper, we present the numerical implementation of a sound field model used in urban acoustics. The mathematical model being based on a classic diffusion equation for the sound energy, a simple finite difference scheme is applied. We give also some finite difference equations for simple boundary conditions, like absorption by a wall and at building edges. The two-dimensional numerical scheme is then compared to analytical solutions of the sound field propagation in a rectangular street with a good agreement, both in the steady state and in the time varying state. Finally it is suggested that the adjustment of usual softwares for heat transfer could be an interesting and low cost way to develop powerful acoustic softwares for the prediction of noise in urban areas.     

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。     

On generation of sound by an internal bore

Internal bores are nonlinear internal waves that move toward the coast and have a characteristic steplike shape to their fronts. The bore is a frequent phenomenon for shelf zones of seas and oceans. The passage of the bore leads to a substantial change in the conditions of sound propagation in the sea medium and, generally, to the formation of a near-bottom sound channel. In this work, we theoretically consider another effect of the aforementioned phenomenon on underwater acoustics, namely, the generation of sound by the internal bore propagating over the bottom.     

A ray model for hard parallel noise barriers in high-rise cities

A ray model is developed and validated for prediction of the insertion loss of hard parallel noise barriers placed in an urban environment either in front of a row of tall buildings or in a street canyon. The model is based on the theory of geometrical acoustics for sound diffraction at the edge of a barrier and multiple reflections by the ground, barrier and fa?ade surfaces. It is crucial to include the diffraction and multiple reflection effects in the ray model as they play important roles in determining the overall sound pressure levels for receivers located between the fa?ade and the near-side barrier. Comparisons of the ray model with a wave-based boundary element formulation show reasonably good agreement over a broad frequency range. Results of scale model experimental studies are also presented. It is demonstrated that the ray model agrees tolerably well with the scale model experimental data.     

Modelling traffic noise mathematically

In this paper we describe a mathematical model for the prediction of traffic noise levels in an urban or suburban situation. The model is ultimately intended to provide an alternative to existing methods of prediction. At the present time, only noise levels produced by stationary sound sources have been considered.Any point in a chosen area is described by its grid co-ordinates. A detailed plan of the buildings or other structures in the area and the position(s) of the sound source(s) are needed as input to the model. Noise levels at all grid positions in the area are then calculated on the basis of the attenuation of sound due to direct propagation, diffraction and reflection.The results obtained so far are given and since the model is in an early stage of development, and has yet to be proved against measurements in real situations, possible refinements and future developments are discussed in some detail.     

超流Hell中第一声、第二声和第四声声速的测量

本文介绍了在超流Hell中传播的第一声、第二声和第四声的特性以及利用声共振器测量它们的传播速度的实验装置。我们初步测量所得的结果与Rudnick教授等人的结果基本一致。     

Interfering modes and an axial wave in an underwater sound channel

Experiments on long-range propagation of low-frequency sound that were conducted starting from the mid-1980s indicate a complex character of propagation in an underwater sound channel, in which a source and a receiver are located close to the channel axis. A burst of energy propagating along the axis follows early arrivals, which are well described by the formulas of geometrical acoustics, in plots of acoustic intensity as a function of propagation time and hydrophone depth. This energy burst cannot be described using geometrical acoustics because of caustics with caustic beaks located near the channel axis. Very complex interference processes occur near these caustics. As the distance from the source grows, the dimensions of the interference vicinity increase and start to overlap producing a peculiar “axial wave.” For an arbitrary two-dimensional underwater sound channel, the axial wave can be represented as a sum of the first normal modes and a residue. This conclusion is based on the use of two representations for an acoustic field. The first of them includes the sum of ray components and an axial wave. The second representation consists of ray addends, the sum of the first normal modes, and a residue. Numerical results are obtained for a canonical profile of sound velocity at the frequency of 200 Hz for the distances of 1600–1650 km.     

Sound and light propagation in a weakly inhomogeneous atmosphere

Small-scale inhomogeneities caused by atmospheric turbulence have a considerable effect on sound and light propagation, producing the fluctuations of these wave fields. V A Krasilnikov [1, 2] performed experiments on phase and amplitude variations of a sound wave propagating through the atmosphere. Fluctuations of light-wave parameters occur, for example, in the well known phenomenon of star scintillation, apparently strongly connected with turbulent irregularities of the atmospheric temperature field [3, 4]‡.

Some calculations of phase (arrival angle) and amplitude fluctuations for a wave propagating through a turbulent medium are described by Krasilnikov [1, 3, 8]. All these and similar calculations are based on the geometrical optics (acoustics) approximation, which may be the reason for disagreement between calculation and experimental data in some cases. Thus, for example, amplitude fluctuations in the geometrical approximation turn out to be proportional to the distance of propagation through a turbulent medium to the power of 3/2. However, observations usually show much slower fluctuation growth.

This paper represents an attempt to consider the problem of amplitude and phase variations for a scalar wave field in terms of more general equations including some diffraction effects. Incidentally, the range of validity of geometrical optics theory becomes clear.     

A low order flow/acoustics interaction method for the prediction of sound propagation using 3D adaptive hybrid grids

A low-order flow/acoustics interaction method for the prediction of sound propagation and diffraction in unsteady subsonic compressible flow using adaptive 3-D hybrid grids is investigated. The total field is decomposed into the flow field described by the Euler equations, and the acoustics part described by the Nonlinear Perturbation Equations. The method is shown capable of predicting monopole sound propagation, while employment of acoustics-guided adapted grid refinement improves the accuracy of capturing the acoustic field. Interaction of sound with solid boundaries is also examined in terms of reflection, and diffraction. Sound propagation through an unsteady flow field is examined using static and dynamic flow/acoustics coupling demonstrating the importance of the latter.     

Subjective response to noise in rural villages,particularly from road traffic

Subjective response to noise, particularly from road traffic, arising from 10 villages in rural Hampshire and Wiltshire is compared with response from a previous survey carried out on a representative sample of adults resident in England. This English sample consisted of individuals occupying, for the most part, urban and suburban areas. The background (ambient) levels of noise experienced were, on the whole, lower in the rural study; especially at sites exposed to most traffic. There was no evidence of a difference in dose/response relationships between the two studies with respect to road traffic noise.     

Computer models in room acoustics: The ray tracing method and the auralization algorithms

Computer algorithms are described for constructing virtual acoustic models of various rooms that should satisfy some specific sound quality criteria. The algorithms are based on the ray tracing method, which, in the general case, allows calculation of the amplitude of an acoustic ray that survived multiple reflections from arbitrary curved surfaces. As a result, calculations of room acoustics are reduced to tracing the trajectories of all the acoustic rays in the course of their propagation with multiple reflections from reflecting surfaces to the point of their complete decay. For this approach to be used, the following physical properties of a room should be known: the geometry of the reflecting surfaces, the absorption and diffusion coefficients on each of these surfaces, and the decay law for rays propagating in air. The proposed models allow for the solution of the important problem of architectural acoustics called the auralization problem, i.e., to predict how any given audio segment will sound in any given hall on the basis of computer simulation alone, without any full-scale testing in specific halls.     

格子Boltzmann方法应用于气动声学研究

应用格子Boltzmann方法(LBM)对不同类型的气动声学问题进行数值研究.通过模拟一维平面声波和二维点源声波的传播,得到沿传播方向的声压脉动,其振荡幅值和衰减趋势与理论值相吻合.其次进行声波衍射和干涉现象的数值模拟.最后,模拟处于流场中运动声源辐射声场的多普勒效应.模拟结果说明LBM方法能较好地模拟低马赫数下的声学问题,包括声压脉动的传播,声波的波动特性以及流动与声波间的相互作用.     

The predicted barrier effects in the proximity of tall buildings

A ray model is developed and validated for the prediction of the insertion loss of barriers that are placed in front of a tall building in high-rise cities. The model is based on the theory of geometrical acoustics for sound diffraction at the edge of a barrier and multiple reflections by the barrier and fa?ade surfaces. It is crucial to include the diffraction and multiple reflection effects in the ray model, as they play important roles in determining the overall sound pressure levels for receivers located between the fa?ade and barrier. Comparisons of the ray model with indoor experimental data and wave-based boundary element formulation show reasonably good agreement over a broad frequency range. Case studies are also presented that highlight the significance of positioning the barrier relative to the noise-sensitive receivers in order to achieve improved shielding efficiency of the barrier.     

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.     

Point-to-point correlation of sound pressures in reverberation chambers

The reverberation chamber is such a fundamental tool in general acoustics and, in particular, architectural acoustics, that most of its properties have been under intensive study for some years. However, the point-to-point correlation of the sound pressures has received little attention except for an elegant paper by Cook et al. published in 1955.     

The physics exhibition, 1971

There has been a considerable upsurge of interest in musical acoustics over the last 10 to 15 years and this article is an attempt to summarize the present position in this multidisciplinary field. It first looks at the essential physics involved, reduced to its simplest terms, goes on to consider the relationships between real musical instruments and the resonance tubes and sonometers of the old text-books on ‘sound’ and outlines the history of the subject up to the Second World War. The main section then discusses areas of current study and the relevant measurement techniques in relation to three of the main groups of musical instruments. The article concludes with a discussion of some aspects of the difficult, but all-important, subject of psycho-physics and a brief attempt to predict the future prospects for research in musical acoustics     

The prediction of noise from urban traffic under interrupted flow conditions

A computer model is described for predicting noise levels generated by urban road traffic under interrupted flow conditions. The model is composed of two subsections. The first predicts the propagation characteristics of sound in typical street configurations and the second simulates the flow of road traffic in urban areas. The two subsections are combined to yield a model capable of predicting traffic noise levels in urban conditions.Predictions obtained from application of this model are compared with those given from application of predictive models based upon field measurements. The agreement between the predictions is good. It is shown that the model described in this paper can predict noise levels for situations which existing field-based models cannot handle.     

Sightlines and soundlines—The design of an audience seating area

Many new criteria for good auditorium acoustics have been proposed in recent years. These criteria take into account the details of human perception of sound, as well as the physics of sound propagation—for example, the temporal and directional distribution of reflected sounds received binaurally is thought to be very important. However, many seats in well-known auditoria have inadequate sightlines and thus people sitting in them lack direct sound components. Since it is the direct sound that gives information regarding the temporal sequence and location of sound sources, and, to some extent, also determines the ability to perceive subsequent reflected sound components, it is essential that all seats in an auditorium should have good sightlines and thus direct soundlines.     

A craft-project loudspeaker to serve as an educational demonstration

Moving-coil loudspeakers typify the interdisciplinary nature of acoustics: in order to reproduce sound, these devices employ principles of electricity, magnetism, mechanics, and acoustics. The widespread use of loudspeakers has made them a familiar and valuable opportunity to introduce students to acoustics. A low-cost loudspeaker project/demonstration is presented here that is built from scratch using common household items and craft supplies. A variety of educational topics may be illustrated with this device, making it appropriate for a wide range of academic levels.     

High-frequency modes in a two-dimensional rectangular room with windows

We examine a two-dimensional model problem of architectural acoustics on sound propagation in a rectangular room with windows. It is supposed that the walls are ideally flat and hard; the windows absorb all energy that falls upon them. We search for the modes of such a room having minimal attenuation indices, which have the expressed structure of billiard trajectories. The main attenuation mechanism for such modes is diffraction at the edges of the windows. We construct estimates for the attenuation indices of the given modes based on the solution to the Weinstein problem. We formulate diffraction problems similar to the statement of the Weinstein problem that describe the attenuation of billiard modes in complex situations.