Outdoor Sound Propagation Research Articles

Modeling outdoor sound propagation in mountainous areas

Detailed modeling of outdoor sound propagation in mountainous area imposes special requirements. The strong gradients in ground surface create propagation conditions that may lead to noise levels much higher than expected at some distance from the source partly due to a focusing effect. Meteorological conditions are particular: main winds following the valley, strong temperature gradients often including stable inversion, slope winds, etc. Because of these particular conditions, efficient modeling of outdoor sound propagation in mountainous area requires adaptations to be made to general purpose models. In this paper we will present possibilities for making time-domain models terrain following and consider options to decrease the memory requirements. For the higher frequency range and distances one is generally interested in, these adaptations are insufficient. Hence, a Green’s function parabolic equation model is added to the toolbox. To tackle the problem in three dimensions, a number of two-dimensional slices connecting the receiver point to all (segments of) sources considered is made. Propagation results for each slice are added. Numerical simulations are compared to field measurements made close to an Alpine highway.

An exact point-source starting field for the Green’s function parabolic equation in outdoor sound propagation

A method for exactly representing a point source in a Green’s function parabolic equation calculation is presented. The method is no more difficult to use than standard starting-field formulations and has the several advantages: (1) the point-source solution is essentially exact; (2) only the propagating Fourier components are used; and (3) there is no limitation on the source height, which can be taken to be at z=0, if necessary. The method is derived analytically and examples of the numerical implementation are given. [Research supported by U.S. Army Armament, Research Development and Engineering Center (ARDEC).]

Proper orthogonal decomposition for sensitivity analysis and classification of outdoor sound propagation

Simulations of outdoor sound propagation are compromised by many sources of uncertainty and error. A key step in validating computational acoustics models, which may be interpreted as the act of assessing predictive skill, is estimating ensemble sound pressure variability due to uncertainties in the parameters that define the propagation conditions. This paper describes elementary steps toward achieving this goal. The atmospheric surface layer is represented through Monin-Obukhov similarity theory and the acoustic ground properties with a relaxation model. Randomness is assumed across appropriate ranges of the governing parameters. The parameters are modeled herein as independent random variables, but future efforts will extend this formulation to include random field models of the parameters with varying levels of correlation. Sound propagation is predicted with the parabolic equation method. Latin hypercube sampling (LHS) and proper orthogonal decomposition (POD) are employed to develop low-dimension representations, i.e., reduced order models, of the sound pressure random field. Sensitivity of the sound pressure field is studied via the model-induced sensitivity of the POD mode coefficients to the system parameters as well as statistical dependence on the number of LH samples. Attention is also given to connecting the POD coefficients to documented indicators of refraction conditions.

Effect of atmospheric absorption on aircraft noise propagation in various world regions during the year

The attenuation of sound due to atmospheric absorption depends strongly on environmental temperature and humidity. For that reason, it varies in response to changing meteorological conditions on a variety of time scales. To perform long-term prediction of outdoor sound propagation more accurately, it is necessary to examine the fluctuation of the attenuation coefficients for atmospheric absorption during the year. Large variations occur according to the time and place in actual meteorological conditions, on which the attenuation coefficients depend strongly. The attenuation coefficients for atmospheric absorption were calculated from hourly meteorological data obtained at ten international airports throughout the world during a year using the calculation method described in ISO 9613-1. Calculated results show that the variability characteristics of the attenuation coefficients during the year depend strongly upon both the sonic frequency and the place. The effects of atmospheric absorption on aircraft noise propagation were examined using the calculated attenuation coefficients. Results showed that the A-weighted sound pressure levels and sound spectra of aircraft noise vary strongly because of the changes in atmospheric absorption, which depend on actual meteorological conditions of various world regions.

Optimization of multiple edge barriers with genetic algorithms coupled with a Nelder–Mead local search

This research work aims at developing a new multi-criteria optimization method dedicated to complex road noise barriers. Numerical simulations of the acoustical propagation have been achieved using MICADO, a 2D boundary element method (BEM) code developed at CSTB. The optimization part is carried out with the help of a Nelder–Mead algorithm (direct local search method) coupled with an evolutionary strategy in order to globalize the approach. A first application of this combination between an outdoor sound propagation numerical code and an optimization algorithm concerns the optimization of noise barrier caps with the following varying parameters: the cap size, its shape and its surface impedance. The cost function to be minimized is defined through a mean value of the insertion loss due to the added crowning compared to the straight, rigid barrier solution of same overall height, averaged on several receiver points within the barrier shadow zone. Final results show a significant improvement of the efficiency of a multiple edge noise barrier by optimizing values of both size and impedance.

Outdoor sound propagation modeling in realistic environments: Application of coupled parabolic and atmospheric models

Predicting long-range sound propagation over a nonurban site with complex propagation media requires the knowledge of micrometeorological fields in the lower part of the atmospheric boundary layer, and more precisely its characteristics varying in both space and time with respect to local (“small-scale”) and average (“long-term”) conditions, respectively. Thus in this study, a mean-wind wide-angle parabolic equation (MW-WAPE) code is coupled with a dedicated micrometeorological code (SUBMESO) which simulates wind and temperature fields over moderately complex terrain with high resolution. Its output data are used as input data for the MW-WAPE code, which can also deal with different boundary conditions, such as the introduction of impedance jumps, thin screens or complex topography. Both codes are presented in the present paper. Comparisons between numerical predictions, and experimental data are also presented and discussed. Finally, we present an example of such a coupling method (MW-WAPE/SUBMESO) for the estimation of sound pressure levels at almost any site (“local scale”), for mean propagation conditions representative of long-term atmospheric conditions.

Parameter selection in the Green’s function parabolic equation

The accuracy of the Green’s function parabolic equation (GFPE) has already been confirmed for outdoor sound propagation over flat ground with a slowly varying sound speed profile and/or atmospheric turbulence. However, use of parabolic equation methods for prediction is generally limited to experts because of their dependence on numerous algorithm parameters that have significant impact on the accuracy of prediction. The present work offers a set of GFPE parameters, outlined in Table 1, that will provide accurate results for a variety of physical situations involving atmospheric propagation. The guidelines are found by comparing GFPE results to analytical results for a variety of situations and parameter choices and noting which combinations of parameters lead to accurate (within 3 dB of the analytical solution) results and which do not. A significant source of error in GFPE results is inaccuracy in the starting field or inappropriate starting field selection. The selection criteria for starting field and other parameters are discussed here.

Ground‐to‐ground sound propagation in cloud‐topped boundary layers over land

Sound propagation under cloud‐covered skies is discussed. Observations are presented which suggest that under dense cloud cover ground‐to‐ground sound propagation is dominated by a single mode similar to the classic surface mode for outdoor sound propagation. A theory of sound propagation under cloud covered skies has been developed. It is shown that there is a sound duct centered in the cloud layer. Of the modes in this duct a small number (typically one or two for frequencies up to a few hundred Hz) of ground interacting modes, modes whose amplitudes on the ground are significant, are identified. Over land, one of the ground‐interacting modes can be identified with the surface mode.

Acoustic pulse propagation near a right-angle wall

Experimental measurements were conducted around a right-angle wall to investigate the effect of this obstacle on sound propagation outdoors. Using small explosions as the source of the acoustic waves allowed reflected and diffracted arrivals to be discerned and investigated in detail. The measurements confirm that diffraction acts as a low-pass filter on acoustic waveforms in agreement with simple diffraction theory, reducing the peak pressure and broadening the waveform shape received by a sensor in the shadow zone. In addition, sensors mounted directly on the wall registered pressure doubling for nongrazing angles of incidence in line-of-sight conditions. A fast two-dimensional finite difference time domain (FDTD) model was developed and provided additional insight into the propagation around the wall. Calculated waveforms show good agreement with the measured waveforms.

Time-domain calculations of sound interactions with outdoor ground surfaces

A time-domain formulation for sound propagation in rigid-frame porous media, including waveform attenuation and dispersion, is developed. The new formulation is based on inversion of the relaxation functions from a previous model [Wilson DK, Ostashev VE, Collier SL. J Acoust Soc Am 2004;116:1889–92], thereby casting the convolution integrals in a form amenable to numerical implementation. Numerical techniques are developed that accurately implement the relaxational equations and transparently reduce to previous results in low- and high-frequency limits. The techniques are demonstrated on calculations of outdoor sound propagation involving hills, barriers, and ground surfaces with various material properties. We also compare the relaxation formulation to a widely applied phenomenological model developed by Zwikker and Kosten. The two models can be made equivalent if the resistance constant, structure constant, and compression modulus in the ZK model are allowed to be weakly frequency dependent. But if the ZK parameters are taken to be constant, as is typically the case, the relaxation model provides more accurate calculations of attenuation by acoustically soft porous materials such as snow, gravel, and forest litter.

A Wavenumber Independent Boundary Element Method for an Acoustic Scattering Problem

In this paper we consider the impedance boundary value problem for the Helmholtz equation in a half-plane with piecewise constant boundary data, a problem which models, for example, outdoor sound propagation over inhomogeneous flat terrain. To achieve good approximation at high frequencies with a relatively low number of degrees of freedom, we propose a novel Galerkin boundary element method, using a graded mesh with smaller elements adjacent to discontinuities in impedance and a special set of basis functions so that, on each element, the approximation space contains polynomials (of degree $\nu$) multiplied by traces of plane waves on the boundary. We prove stability and convergence and show that the error in computing the total acoustic field is ${\cal O}(N^{-(\nu+1)} \log^{1/2}N)$, where the number of degrees of freedom is proportional to $N\log N$. This error estimate is independent of the wavenumber, and thus the number of degrees of freedom required to achieve a prescribed level of accuracy does not increase as the wavenumber tends to infinity.

Efficient calculation on outdoor sound propagation by FDTD and PE methods

Efficient calculation on outdoor sound propagation by FDTD and PE methods

Coupling Boundary Elements to a Raytracing Procedure using the Singular Indirect Method

AbstractThe coupling of Boundary Elements and a raytracing procedure is presented here. Such a hybrid method is best suited to the study of realistic outdoor sound propagation problems: The noise often acts in a domain where many objects like buildings or sound insulation walls scatter the sound. Thus, diffraction has to be taken into account. BEM is well suited. To study the effects of this noise on a sound receiver far away, raytracing may be preferable for such application, because refraction can be implemented more easily. Hence, a Boundary Element Analysis is performed in noisy nearfield regions, a raytracing procedure at a larger distance from the sound sources. First, the direct Boundary Element algorithm is applied to determine the sound pressure at interface points. Second, a singular indirect Boundary Element formulation is used to find intensities of point sources on the same interface which produce the previously determined sound pressure. Finally, these sound intensities are the input data for the raytracing procedure. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Prediction-step staggered-in-time FDTD: An efficient numerical scheme to solve the linearised equations of fluid dynamics in outdoor sound propagation

The finite-difference time-domain (FDTD) method to solve the linearised equations of fluid dynamics has shown to be very powerful and useful in outdoor sound propagation. Practical applications are however limited due to the large need for computational resources. The numerical discretisation influences computational efficiency to an important degree. In this paper, some possible ways to discretise temporal derivatives are studied. Two obvious ways of time-discretisation namely staggered-in-time (SIT) and a simple collocated-in-time (CIT) scheme are compared to the prediction-step staggered-in-time (PSIT) scheme. The latter is intended to be used for the calculation of sound propagation in the typical low wind speeds encountered in the outdoor environment at low heights above the earth’s surface. It was shown that the PSIT scheme is more stable than the SIT scheme, so practical calculations are possible. Computational efficiency is increased to an important degree compared to the CIT scheme. The numerical accuracy (more precisely the amplitude error) of the PSIT scheme is an important improvement upon SIT. The CIT scheme on the other hand conserves amplitude better. The amplitude error becomes larger with increasing wind speed because of some simplifications during the numerical discretisation. In low wind speeds, the PSIT algorithm can serve as an interesting compromise between numerical accuracy and the required amount of computing power.

Simulation of outdoor sound propagation with a transmission line matrix method

The present paper focusses on the application of the transmission line matrix method (TLM) for outdoor sound propagation simulations. It is shown how the reflection at the ground and vertical sound speed gradients can be taken into account. The comparison with calculations using finite differences in the time domain (FDTD), with calculations using parabolic equation and with analytical solutions showed all very good agreement. Thereby the computational effort of TLM is significantly lower compared to common FDTD calculation schemes.

Outdoor sound propagation effects on aircraft detection through passive phased-array acoustic antennas: 3D numerical simulations

The present paper describes some of the main acoustic issues connected with the SAFE-AIRPORT European Project for the development of an innovative acoustic system for the improvement of air traffic management. The system sensors are two rotating passive phased-array antennas with 512 microphones each. In particular, this study focused on the propagation of sound waves in the atmosphere and its influence on the system detection efficiency. The effects of air temperature and wind gradients on aircraft tracking were analyzed. Algorithms were implemented to correct output data errors on aircraft location due to acoustic ray deviation in 3D environment. Numerical simulations were performed using several temperature and wind profiles according to common and critical meteorological conditions. Aircraft location was predicted through 3D acoustic ray triangulation methods, taking into account variation in speed of sound waves along rays path toward each antenna. The system range was also assessed considering aircraft noise spectral emission. Since the speed of common airplanes is not negligible with respect to sound speed during typical airport operations such as takeoff and approach, the influence of the Doppler effect on range calculation was also considered and most critical scenarios were simulated.

Effect of realistic grounds and atmospheric conditions on single-channel active control of outdoor sound propagation

Engine run-up tests are a part of routine aircraft maintenance at the Vancouver International Airport. A source of noise complaints is the Dash-8 aircraft, which emits low-frequency, tonal noise. Active noise control is a potentially cost-effective alternative to passive noise-control methods, which are ineffective at controlling low-frequency noise. Since the run-up tests are performed outdoors, the effects of outdoor conditions on the performance of an active control system must be considered. In this paper, the results of a preliminary investigation of the effects of realistic meteorological conditions and ground impedance on the performance of a single-channel active-control system are presented. Computer simulations of single-channel active control of a monopole source were performed using the Green’s-function parabolic-equation method. Different realistic atmospheric conditions, and reflective or soft ground, were used in the simulations. The results show that atmospheric refraction causes fluctuations in the noise attenuation achieved by a single-channel control system, and has the overall effect of decreasing its performance, making the system ineffective in some cases.

Book Review

Book Review David A. Bies and Colin H. Hansen: Engineering Noise Control, Theory and Practise, 3rd edition, Spoon Press, London/New York, 2003. Engineering noise control is characterized by the variety of possible applications and the complexity of the problems encountered. The practitioner rarely builds a full mathematical model of the structure of interest, since this would be time consuming and costly, if not impossible. Instead, the problem is analyzed by measurements and simplified calculations and the important sources, transmission paths, and disturbing responses are identified. Idealized models that capture the important aspects of the physics are built and, based on the understanding gained, measures for improvement are suggested. This, by no means simple, procedure requires vast experience and a powerful collection of measurement, analytical and semi-analytical tools. Much of this is provided in “Engineering Noise Control,” which now appears in its third edition. The book broadly divides into three parts: first, the perception and quantification of noise; then, sound sources and sound propagation and, finally, noise reduction. Chapter 1 is entitled “Fundamentals and basic terminology” and this chapter is a must-read, also for those that will use the many solved examples for reference. Among other things, sound level is defined and it is revealed that the book is not for the weak-hearted who fears to evaluate the logarithm of 1Pa2/Hz. Chapter 2 describes the anatomy of the ear, the perception of noise, and hearing damages. Chapter 3 then introduces acoustic instrumentation, such as microphones, tape recorders, sound level meters, dosimeters, and sound intensity analysers. The authors share their experiences in using this equipment and emphasize the importance of calibration. This part of the book is concluded with an extensive chapter on criteria for assessing the severity of noise, which is supplemented with a useful list of acoustic standards at the end of the book. The next part of the book deals with sound generation and sound fields, starting off in Chapter 6 with elementary sources: ranging from monopoles via quadrupoles to incoherent plane radiators. The chapter continues with reflections at interfaces between different media and ground reflections, finishing with a section on outdoor sound propagation, including meteorological effects. Chapter 7 is one, of three, chapters with “sound power” in its title. It starts off with definitions of radiation impedance and near field and far field and continue with procedures for the measurement of sound power from general sources, making valuable references to standards. Chapter 8 is entitled Sound in Enclosed Spaces and provides the basics of room acoustics and some material on sound absorption. The following three chapters are on noise control: one on partitions, enclosures and barriers, one on mufflers, and one on vibration control, including fundamentals of vibration isolation, vibration absorbers and attached damping materials. Chapter 10 also briefly discusses vibration measurements. Chapter 11 provides an extensive list of semi-empirical formulas for the estimation of sound power generated by a great variety of sources. These formulas are mostly motivated by reference to a scientific article, a standard or a technical manual, while not much explanation is provided. Chapter 12, finally, introduces active noise control. The Appendices provides a derivation of the wave equation, an account of the properties of porous materials, and a brief introduction to signal analysis. The book is supplemented with comprehensive lists of references and acoustic standards. Everyone engaged in the art of engineering noise control will benefit from this rich book. It would require hard work to select material for teaching but it seems do-able. However, many students that today aim for a career in noise control will sit by a computer running some extremely sophisticated software. I believe these students would gain more from a book focused on the physics of vibro-acoustic motion and fundamentals of math and mechanics. Nevertheless, these students would also gain by sharing the authors’ experiences in noise control and by the book’s brisk DIY attitude, showing that it is indeed possible to reduce the disturbing noise that surrounds us.Svante FinnvedenRoyal Institute of TechnologyStockholm, Sweden

Application of the Green’s function parabolic equation (GFPE) method to realistic outdoor sound propagation scenarios

The Green’s function parabolic equation (GFPE) is a powerful method for outdoor sound propagation prediction. Given appropriate inputs, GFPE predictions can include the effects of terrain, ground cover, sound speed profile, and turbulence on atmospheric propagation. In many situations, however, access to such input information can be limited. The input data resolution and accuracy can have a significant impact on model output accuracy. In turn, variability of terrain and sound speed profile and statistical aspects of the turbulence impact the model parameter requirements. For example, in the presence of turbulence, lower limits on computational grid height are determined by turbulence correlation scales as well as acoustic wavelength. Input data resolution requirements and some techniques that may be employed to account for insufficient input data will be discussed. Model results for realistic situations will be presented.

Terrain effects on acoustic pulse propagation

Outdoor sound propagation was measured at a variety of locations to determine terrain and environmental effects on acoustic pulses. Small explosive charges were detonated at a height of 1.5 m above the ground, and the resultant acoustic pulses were digitally recorded using pressure sensors located 10 to 100 m from the source. The experiments were conducted in an open area over concrete, grass, or snow; through a forest or thick tropical vegetation; and along a building-lined street in an artificial village. The measurements reveal that waveform changes occur at the shortest measurement ranges and increase as the propagation distance increases. These changes include amplitude attenuation, pulse width, and waveform shape and are caused by the differing ground conditions. The highest peak amplitudes at 100 m were recorded over concrete, and the lowest over snow-covered ground, with peak pressure decay rates of 24- and 32-dB/decade distance in these two situations. In addition, multiple scattering and reflected arrivals were visible in the tropical, forest, and urban areas. [Work funded by U.S. Army.]