Outdoor Noise Propagation Research Articles

Comparing outdoor sound propagation simulations based on geometrical and numerical methods

Conventional simulations of outdoor noise propagation often rely on geometrical acoustic methods such as ray tracing or mirror sources. Especially for low frequencies, these methods deliver physically implausible results due to their inherent approach not resolving the properties of the acoustic field. In this contribution, the results of a conventional noise propagation simulation software based on geometrical acoustics are compared to numerical results originating from solving the Helmholtz equation with the Finite Element Method (FEM). A 2.6 km long cross-section through an alpine valley with a railway line is considered as an exemple. In comparison with the FEM results, the shortcomings of geometrical acoustic methods are demonstrated, which underline the need for powerful and computationally efficient numerical approaches for outdoor noise propagation.

Enhancing sustainable urban air transportation: Low-noise UAS flight planning using noise assessment simulator

Low-altitude flights of unmanned aircraft systems (UASs), such as drones, can cause substantial noise pollution in urban areas. This study aims to tackle the crucial issue of noise in UAS flight planning and analyze the feasibility of implementing low-noise urban flights at a micro level. We employ a virtual flight simulator to accurately model the UAS noise impact in realistic flights and optimize the flight path via a heuristic algorithm accordingly. The flight noise assessment incorporates a sound source modeling technique for practical UAS models and an efficient Gaussian beam tracing method for outdoor noise propagation in realistic environments. Case studies for flight simulations are conducted in a representative resident community and metropolitan area to evaluate the noise impact resulting from different flight paths. By comparing our approach with benchmark shortest-distance paths, we validate its effectiveness in significantly reducing en-route noise exposure, encompassing both instantaneous and continuous noise levels. This method holds great potential for contributing to the planning and management of urban air transport systems. It effectively mitigates noise impact, promoting quieter and more environmentally friendly UAS operations and sustainable urban transportation.

Application of the finite element method to atmospheric sound propagation over impedance discontinuities

Outdoor noise propagation typically involves the propagation of sound over mixed ground conditions, including variations in the surface impedance. Diffraction occurs at the interface between different ground conditions, and this can significantly affect local sound attenuation. Popular approaches to modelling impedance discontinuities include the boundary integral method, parabolic equations, and semi-empirical methods. This paper presents an alternative approach that takes advantage of the generality of the finite element method. It was shown recently that the semi analytic finite element method can be used to solve the exact governing wave equation for atmospheric sound propagation in a complex vertically stratified medium. This allows for the inclusion of arbitrary temperature and wind profiles provided the problem is range independent. It is shown here that it is relatively straightforward to extend this approach to include multiple surface impedance discontinuities, provided each discrete surface is uniform in the axial direction. This is achieved using point collocation to enforce the axial boundary conditions over each discontinuity. Predictions are generated for single and multiple discontinuities, and comparisons are made against predictions obtained using other computational methods.

Prediction of Outdoor Noise Propagation Induced By Single-Phase Power Transformers

Prediction of Outdoor Noise Propagation Induced By Single-Phase Power Transformers

Extending standard urban outdoor noise propagation models to complex geometries

A hybrid method that combines a noise engineering method and the 2.5D boundary element method approximates outdoor sound propagation in large domains with complex objects more accurately than noise engineering methods alone and more efficiently than reference methods alone. Noise engineering methods (e.g., ISO 9613-2 or CNOSSOS-EU) efficiently approximate sound levels from roads, railways, and industrial sources in cities for simple, box-shaped geometries by first finding the propagation paths between the source and receiver, then applying attenuations (e.g., geometrical divergence and atmospheric absorption) to each path, and finally incoherently summing all of the path contributions. Standard engineering methods cannot model more complicated geometries but introducing an additional attenuation term quantifies the influence of complex objects. Calculating this extra attenuation term requires reference calculations but performing reference computations for each path is too computationally expensive. Thus, the extra attenuation term is linearly interpolated from a data table containing the corrections for many source/receiver positions and frequencies. The 2.5D boundary element method produces the levels for the real and simplified geometries and subtracting them yields a table of corrections. For a T-shaped barrier with two buildings, this approach reduces the mean error by approximately 2 dBA compared to a standard engineering method.

Enabling noise engineering methods to model complex geometries

Many governments use standard engineering methods (e.g., CNOSSOS or Nord2000) to model outdoor noise propagation from cars, trains, and trams. These methods provide efficient approximations of long-term averaged noise levels based on the geometrical divergence, atmospheric absorption, ground effect, refraction, reflection, and diffraction. However, these methods are limited to diffraction around simple, box-shaped geometries; for example, engineering methods cannot model a T-barrier. In addition, reference methods like boundary elements or finite differences are too computationally expensive for many realistic cases. Thus, a hybrid method has been developed to improve the accuracy of the engineering methods for complex geometries and surfaces. This approach interpolates a table of 2.5D boundary element results to estimate the influence of the complex objects. The hybrid method produces more accurate results than the standard engineering methods compared to a fast-multipole boundary element method for a small test case using a T-barrier up to almost 2 kHz. However, significant differences between the hybrid and boundary element methods remain because the boundary element method sums the pressures coherently and the hybrid method sums them incoherently like the engineering methods do for simplicity and efficiency.

A Transmission Line Matrix model for sound propagation in arrays of cylinders normal to an impedance plane

The present paper focuses on two of the acoustic phenomena involved in sound propagation through forested areas, namely multiple scattering caused by tree trunks at mid-frequencies and ground effect at low frequencies. The use of time domain methods can be of interest for the simulation of transient phenomena such as scattering. The study aims at evaluating the ability of an alternative time-domain approach, the Transmission Line Matrix (TLM) method, to model sound scattering by cylindrical scatterers. The TLM method is applied to the study of both single and multiple scattering coupled to ground effects, in two- and three-dimensional domains. Keeping in mind the initial purpose of this study, the size and the location of the scatterers (tree trunks), as well as the noise frequency range, are related to outdoor noise propagation in realistic forests. In order to validate the TLM method, numerical simulations are compared to analytical solutions as well as measurements on 1:10 scale-models. The most complete cases of cylinders arrays placed normal to impedance floors are in agreement with the measurement results.

The reduction of gunshot noise and auditory risk through the use of firearm suppressors

Law enforcement, security, and military personnel train with small-caliber firearms that present a significant risk of noise induced hearing loss for the operator and range instructors. Measurements of three rifles and one pistol equipped with suppressors were conducted at an outdoor firing range using subsonic and supersonic ammunition. Suppressed and unsuppressed recordings were analyzed. Microphones were located to the left of the muzzle, to the right and left of the shooter’s head, and one meter behind the shooter’s head at the nominal instructor’s position. Recordings were collected with a National Instruments PXI 1082 chassis with an NI 4499 data acquisition board at a 200 kHz sampling rate. Analysis of the peak sound pressure levels (dB SPL) and 8-h equivalent A-weighted energy (LAeq8) were conducted. The suppressors reduced the peak between 15 and 25 dB SPL and the LAeq8 between 8 and 28 dB. Reduced noise levels at the source will reduce auditory risk but do not necessarily eliminate the need for hearing protection. An outdoor noise propagation model based on ISO 17201-1 will be used to investigate the effect of suppressors to reduce noise in surrounding communities.

Time-Domain Simulations of Outdoor Sound Propagation with Suitable Impedance Boundary Conditions

Finite difference time-domain methods are attractive for the study of broadband outdoor noise propagation, because they can accurately take into account both atmospheric and ground effects. Moreover, these methods allow moving sound sources to be modeled, which can be interesting in the context of transportation noise. A recently proposed method to obtain an impedance boundary condition is implemented in a linearized Euler equations solver. A long-range propagation configuration in a two-dimensional geometry is studied in homogeneous conditions and in downward-refracting conditions with an impedance ground over a distance of 500 m. Two impedance models corresponding to a grassy ground and to a snow-covered ground are considered. Numerical results are compared in the time domain to an analytical solution in homogeneous conditions and to results from a ray-tracing code in downward-refracting conditions. Near the ground, surface waves are detected in the two cases and are the dominant arrivals in the homogeneous case.

The Effects of Wind on Outdoor Noise Propagation

The Effects of Wind on Outdoor Noise Propagation

Prediction of Outdoor Noise Propagation

Prediction of Outdoor Noise Propagation

Orthogonal coding sequences for multiple‐source study of outdoor noise propagation.

This paper describes recent developments in the design of a coded acoustic signal for the study of outdoor sound propagation. Low signal-to-noise ratio is a common limitation in outdoor propagation investigations, and time variance of the air limits the amount of averaging. Recent work by the authors presented a coded acoustic signal approach consisting of a frequency carrier biphase-modulated by a specially designed pseudo-random code sequence to overcome these limitations. The Inner and Outer code sequence was specially designed for environmental sound propagation investigations, combining simultaneous fine time resolution and large range ambiguity, together with an ability to average and probe the propagation path. This approach is extended to multiple acoustic sources for the study of outdoor sound propagation, with the design of signals using orthogonal correlation sequences. The results illustrate that combinations of orthogonal coding and multiple carrier frequencies can permit continuous sound speed measurements in numerous vertical and horizontal directions, and might therefore be useful for acoustic tomographic applications.

Investigation of traffic noise from additional tolling lanes at Portage toll plaza

Electronic toll collecting systems quickly and efficiently move traffic through toll facilities by automatically collecting tolls from individual vehicle transponders. An ongoing study investigates the addition of three electronic toll lanes to the existing twelve lanes at the Portage toll plaza on the Indiana Toll Road (Portage, Indiana, USA), scheduled to be completed by December 2008. The study aims to predict noise levels of the plaza after the addition of the lanes, the hypothesis being that the additional lanes will have little or no effect on the toll plaza noise levels. Noise measurements are being made at locations in the vicinity of the existing plaza with consideration for a nearby residential area. An outdoor noise propagation software (SoundPlan) is used to create a comprehensive traffic noise model for the area under investigation. Test data from the existing site will be compared to the model and used to predict future noise levels.

A spread spectrum technique for the study of outdoor noise propagation

This paper describes field measurements to assess innovative correlation techniques for the study of meteorological and topographical effects on sound propagation. To take advantage of the properties of coded signals in a time-varying system, the correlation signal is produced by the modulation of a code sequence onto an acoustic carrier. An established method of increasing signal-to-noise ratio is to use correlation techniques with maximum length sequences. However, this standard method is restricted in its use outdoors because of the time-variant nature of the atmosphere. On the other hand, the correlation properties of a directly carrier-modulated code sequence modulation signal may be exploited in a time-varying environment. An experiment is described in which the correlation properties of the spread spectrum signal are demonstrated and are used to calculate accurate times of flight that compare well with sonic anemometer measurements of speed of sound. The results illustrate that an acoustical spread spectrum system can provide significantly improved ways of measuring sound propagation outdoors.

Spread spectrum sequence design for the study of outdoor noise propagation

The design of carrier-modulated code sequence signals that can be used for the study of outdoor propagation, are presented. In particular, the construction of these signals such that they are robust to the time-variance of the outdoor acoustic path is discussed. The design of these signals so that they can also occupy known bandwidths is also described. The result is an audible frequency signal that is biphase-modulated by two pseudorandom code sequences in a way that is specially designed for simultaneous fine time resolution and large range ambiguity. A particular advantage of this signal is that an estimate of the time-varying Doppler can be derived by recovering the suppressed carrier from the received signal, and various means of extracting the Doppler are explained. Extensions of these signals to allow multiple simultaneous measurements are also described. [Work supported by EPSRC UK]

An acoustic spread spectrum technique for the study of outdoor noise propagation

An investigation of the properties of a directly carrier-modulated code sequence modulation signal is presented for the study of outdoor propagation. The audible frequency carrier is biphase-modulated using a pseudorandom code sequence specially designed for simultaneous fine-time resolution and large-range ambiguity. An experiment is described in which the correlation properties of the spread spectrum signal are demonstrated, and the amplitude of the transfer functions between the receivers, together with accurate times-of-flight, are calculated from the cross-correlation of the measured acoustic spread spectrum signals. The results illustrate that an acoustic spread spectrum technique can provide significantly improved ways of measuring sound propagation outdoors. In particular, the fine-time resolution for time-of-flight suggests an application in acoustic tomography, for example in the investigation of nocturnal boundary layers, horizontal gradients, and of turbulence structures in the atmospheric surface layer. [Work supported by EPSRC UK.]

Phase-error analysis of high-order finite difference time domain scheme and its influence on calculation results of impulse response in closed sound field

To obtain accurate impulse responses in a sound field by numerical analysis, a high-order finite difference time domain (FDTD) method was formulated on the basis of a simple Taylor expansion. The accuracy of the FDTD scheme was evaluated by Von Neumann and Richtmyer dispersion analysis. It was found that the unavoidable phase error obtained from the conventional FDTD algorithm is reduced using the high-order scheme with 8 or more reference points. When using higher orders in the scheme used in this study, the time resolution should be small to reduce the phase error. To validate the FDTD scheme for calculating the impulse response in a closed sound field, the numerical result was compared with an analytical solution obtained by the separation of variables method. It was found that the phase error for a low-order FDTD appeared as fluctuations of the arriving pulses and that the fluctuations can be reduced using the high-order scheme with a small time resolution. The accuracy of the FDTD calculation for a large-scale sound field or a long-time calculation, which we should consider when making a sound field analysis of room acoustics or outdoor noise propagation, is strongly influenced by the phase error and therefore the error should be considered when performing such numerical analyses.

A predictive noise model for the resurfacing of Nebraska Highway 2

A detailed investigation of the noise soundscape resulting from traffic on a 3-mile segment of Nebraska Highway 2 prior to resurfacing work was undertaken by the students enrolled in the Acoustics Program at Columbia College, Chicago. Noise data were acquired at six different locations using established protocols and included daytime and nighttime conditions. The test data were compared to the predictions from an outdoor noise propagation computer model and the results were found to be within ±2 dB for all locations under consideration. The computer model was then used to assess expected noise conditions under various concrete and asphalt scenarios for resurfacing. Data show that reductions of up to 6 dB in the overall noise levels may be achievable depending on the choice for the surface of the roadway.

A comprehensive noise study for the city of Lincoln, Nebraska

A comprehensive noise study to define the soundscape of the city of Lincoln, NE, has been undertaken by the students enrolled in the Acoustics Program at Columbia College, Chicago. The study includes (1) a global review of best practices in the field of environmental noise control using a perspective emphasizing close attention to effective and proven solutions that can be implemented via policies and effective legislation, (2) the creation of a large-scale outdoor noise propagation computer model that incorporates all primary traffic arterial and readily identifiable noise sources, (3) a comprehensive large-scale testing of noise levels using specially modified noise dosimeters allowing for determination of Leq, and Ldn over 24-h intervals, and (4) a large scale attitudinal survey of the population using a web-based survey engine that allows for the geographical location of the respondents. The results of the survey are used to refine the density of the test locations and the scale resolution of the software model, as well as to provide city and county officials with a targeted course of action pertaining to regulations and noise abatement policies. The project is expected to last 3 years with up to 1200 test locations being analyzed.

Cooling tower noise control

Cooling towers are typically provided with multiple large-diameter propeller fans mounted on top of the unit and discharging upward. These fans generate a significant level of low-frequency noise. The induced-draft intakes for these fans are typically located on the sides of the unit, so fan noise is also radiated horizontally from these openings. Traditional methods for noise control include sound-barrier walls, silencers, or complete noise enclosures. Barrier walls usually have no adverse effect on the performance of the cooling towers. However, silencers and close-fitting noise enclosures must be carefully designed to minimize resistance to the airflow that is required to maintain proper operation of the cooling tower. This paper discusses the current techniques used to reduce outdoor noise propagation from cooling towers. Some case studies are presented to illustrate successful implementation of these noise-control methods.