Influence of vehicle noise emission directivity on sound level distribution in a canyon street. Part I: Simulation program test

Simulation programs may be useful tools for controlling an environmental noise. The computer simulation program PROP11 that enables predictions of the time-average sound level within an urban system is used here. A roadway as a noise source is represented by a sum of the sound exposures due to individual vehicle pass-bys. Different representations of equivalent point sources for various classes of vehicles are allowed including directivity characteristics other than omnidirectional. Propagation throughout an urban system contains multi-reflections from the walls and single and double diffraction at their edges. In this paper, the PROP11 program is used to predict the sound level between opposing façades of buildings in a canyon street. A multi-lane road is assumed to contain two classes of vehicles (light and heavy) ones. The equivalent source representing vehicles is defined by the source power spectrum, its position above the ground and directivity characteristics. The consequences of introducing equivalent source directivity are analyzed.     

Noise abatement schemes for shielded canyons

Access to quiet areas in cities is important to avoid adverse health effects due to road traffic noise. Most urban areas which are or can become quiet (L_A,eq < 45 dB) are shielded from direct road traffic noise. By transfer paths over roof level, many road traffic noise sources contribute to the level in these shielded areas and noise abatement schemes may be necessary to make these areas quiet. Two real life shielded courtyards in Göteborg have been selected as reference cases for a numerical investigation of noise abatement schemes. The selected areas are modelled as canyons with a road traffic noise source modelled outside the canyon by a finite incoherent line source, which is more realistic than both a coherent and an incoherent line source of infinite length. The equivalent sources method has been used for the calculations. For all studied noise abatement schemes in the shielded canyon, the reductions are largest for the lower canyon observer positions. Façade absorption is the most effective when placed in the upper part of the canyon and can typically yield a reduction of 4 dB(A). Constructing 1 m wide walkways with ceiling absorption reduces the level typically by 3 dB(A). These effects are most effective for narrower canyons. For treatments at the canyon roof, reductions are independent of the canyon observer position and amount to 4 dB(A) for a 1 m tall screen and 2 dB(A) for a grass covering of a saddle roof. Downward refracting conditions increase the levels for the lower canyon observer positions and higher frequencies. For sources located in canyons, abatement schemes therein are more effective for noise reduction in the shielded canyon than similar abatement schemes in the shielded canyon itself, given that all contributing source canyons are treated.     

Quasi-wavelet calculations of sound scattering behind barriers

Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions.     

Excess attenuation for sound propagation over an urban canyon

Because quiet areas in dense urban environments are important to well-being, the prediction of sound propagation to shielded urban areas is an ongoing research focus. Sound levels in shielded areas, such as canyons between rows of buildings, are strongly influenced by distant sources. Therefore, propagation factors such as metrology, screening, and intermediate canyons—as occur between a source canyon and a receiver canyon—must be addressed in an engineering propagation model. Though current models address many important propagation factors, engineering treatment of a closed urban canyon, subject to multiple internal reflections, remains difficult.A numerical investigation of sound propagation across the open tops of intermediate urban canyons has been performed, using the parabolic equation and equivalent sources methods. Results have been collected for various canyon geometries, and the influences of multiple canyons, canyon/rooftop absorption, variable rooftop height, wind gradient, and correlated versus uncorrelated source models have been investigated. Resulting wideband excess attenuation values ranged from −1 dB to −4 dB per canyon, and were fairly constant with frequency in many useful cases. By characterizing the excess attenuation of canyons intermediate to the source and receiver, the influence of these intermediate canyons could be addressed simply, without the overhead of a detailed numerical calculation.     

Parameter study of sound propagation between city canyons with a coupled FDTD-PE model

A parameter study is performed for the case of two-dimensional sound propagation from a (source) city canyon to a nearby, identical (receiver) city canyon. Focus was on sound pressure levels, relative to the free field, in the shielded canyon. An accurate and efficient coupled FDTD-PE model was applied, exploiting symmetry of the source and receiver canyon. With the proposed calculation method, simulations were necessary in only half the sound propagation domain. The shielding in the receiver canyon in case of a coherent line source was compared to the shielding by an incoherent line source, by means of sound propagation calculations in a number of 2D cross-sections through source and receiver. It was found that the shielding is rather insensitive to the width-height ratio of the canyons. The presence of diffusely reflecting façades and balconies lead to an important increase in shielding compared to flat façades. Rigid façades yield significantly lower shielding compared to partly reflecting façades. Effects of a moving atmosphere were modeled in detail. Shielding decreases significantly in case of downwind sound propagation when comparing to sound propagation in a non-moving atmosphere. Refraction is the most important effect in the latter. In case of upwind sound propagation, turbulent scattering plays an important role and the shielding is similar to the one of a non-moving atmosphere for the parameters used in this paper. The combination of effects, as is shown by some examples, is in general not a simple addition of the separate effects.     

The influence of turbulence on noise propagation from a point source above a flat ground surface

The presence of turbulence in the atmosphere affects the interaction between an acoustic wave and the ground surface. The noise attenuation by the ground in the presence of atmospheric turbulence is smaller than in non-turbulent atmosphere.A simple engineering model of noise propagation above a flat ground surface, for stationary and moving point sources, has been proposed. The model takes into account the air absorption and ground effect in the presence of turbulence.As well as parameters for type of ground and air absorption, the model introduces two adjustable parameters which must be deduced from in situ measurements at two ranges or two heights. The model’s free parameters have been obtained as a function of the resultant sound speed gradient on the basis of the field measurements performed for a stationary noise source. Also, using field data for a vehicle moving at steady speeds up to 100 km/h, the model has been verified for a moving point source.     

Focused sound from three-dimensional sound propagation effects over a submarine canyon

Ship noise data reveal an intensification of the near-surface sound field over a submarine canyon. Numerical modeling of sound propagation is used to study the effect. The noise data were collected during an ocean acoustic and physical oceanography experiment northeast of Taiwan in 2009. In situ measurements of water sound-speed profiles and a database of high-resolution bathymetry are used in the modeling study. The model results suggest that the intensification is caused by three-dimensional sound focusing by the concave canyon seafloor. Uncertainties in the model results from unsampled aspects of the environment are discussed.     

Nature of inlet turbulence and strut flow disturbances and their effect on turbomachinery rotor noise

Results of an investigation in which turbomachinery rotor sound spectra were correlated with aerodynamic measurements of the inlet turbulence, strut wake, and vortex flow strengths are reported. Aerodynamic measurements included mean velocity profiles, turbulence intensity, and axial length scales. Inlet turbulence data indicate that the major effect of flow contraction appears to be the elongation of turbulent eddies. Eddies of this size dominate the blade passing frequency (BPF) tones. Decreasing eddy size by use of a grid revealed vortex flow strength to be the second major sound source. A doubling of vortex flow strength produced a 6 dB increase in the SPL of the first BPF. The sound pressure level showed less than a 2 dB change with doubling of strut wake turbulence intensity or velocity defect. A discussion of the relative importance of various sources of noise due to flow non-uniformities at the inlet is given.     

The convergence of sound field in a South China Sea canyon

The sound propagation characteristics in a canyon environment are analyzed using the data from the northern South China Sea. The experimental results show that the energy begins to bend to a deeper place with the depth change of the canyon and the sound transmission losses(TLs) reach the maximum near the center of the canyon due to the combined effect of the negative gradient sound speed profile and the seabed topography. As the sea become shallow gradually, the energy is concentrated due to the seabed reflection, thus the TLs reduce by over 20 d B compared with the TLs results got from the simple slope to the deep water environment. Moreover, when the convergence effects are less than the loss caused by diffusion,seabed reflection, etc. the TLs are minimized and then increase with an increase in distance.In the last segment, the gradual convergence of the energy is highlighted again, and the TLs decrease 10 d B when the water depth suddenly decreases from 850 m to 311 m within 8 km.Finally, the convergence of sound field, which is caused by the topographic change in the canyon environment, is explained by the ray theory and the parabolic equation method.     

Double-leaf microperforated panel space absorbers: A revised theory and detailed analysis

A double-leaf microperforated panel space absorber (DLMPP) is composed of two microperforated panels (MPPs) placed in parallel with an air-cavity in-between, without a back wall or any backing structure. This was proposed as a space sound absorber, which can be used for a sound absorbing screen or partition. A conventional MPP absorber with a rigid back wall is effective only around its resonance frequency, which is usually at middle frequencies, and not effective at low frequencies. However, a DLMPP can be effective also at low frequencies, because an additional sound absorption is produced by its acoustic flow resistance. In the authors’ previous work, theoretical analyses on the acoustic properties of a DLMPP were carried out using a simplified electro-acoustical equivalent circuit model. However, the equivalent circuit model includes an approximation, and more sophisticated theory is required for a better prediction and detailed discussion. In this paper, a revised theory for a DLMPP is presented: A Helmholtz integral formulation is employed to obtain a rigorous solution for more precise prediction of the absorptivity of a DLMPP. The result of the present revised theory is compared with that of the equivalent circuit model, and the difference between them is discussed. A parametric survey is made through numerical examples by the present revised theory to discuss its acoustic properties.     

CAA broadband noise prediction for aeroacoustic design

The current status of a computational aeroacoustics (CAA) approach to simulate broadband noise is reviewed. The method rests on the use of steady Reynolds averaged Navier-Stokes (RANS) simulation to describe the time-averaged motion of turbulent flow. By means of synthetic turbulence the steady one-point statistics (e.g. turbulence kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations having statistics that very accurately reproduce the initial RANS target-setting. The synthetic fluctuations are used to prescribe sound sources which drive linear perturbation equations. The whole approach represents a methodology to solve statistical noise theory with state-of-the-art CAA tools in the time-domain. A brief overview of the synthetic turbulence model and its numerical discretization in terms of the random particle-mesh (RPM) and fast random particle-mesh (FRPM) method is given. Results are presented for trailing-edge noise, slat noise, and jet noise. Some problems related to the formulation of vortex sound sources are discussed.     

Measurement of noise associated with model transformer cores

The performance of a transformer core may be considered in terms of power loss and by the noise generated by the core, both of which should be minimised. This paper discusses the setting up of a suitable system for evaluation of noise in a large model transformer core (500 kV A) and issues associated with noise measurement. The equivalent continuous sound pressure level (LAeq) was used as a measure of the A-weighted sound level and measurements were made in the range 16 Hz–25 kHz for various step lap core configurations. The selection of optimum sound insulation materials between core and ground support and for enclosing the transformer is essential for minimisation of background noise. Core clamping pressure must be optimised in order to minimise noise. The use of two laminations per layer instead of one leads to an increase in noise arising from the core. Provided care is taken in building the core, good reproducibility of results can be obtained for analysis.     

Experimental Estimation of the Emission Directivity of a Moving Surface Vessel in Shallow Water

The possibility of constructing an acoustic model of a surface ship’s noise emission in the far field using monopole-type emitters uniformly distributed along the hull is investigated. Experimental data obtained in shallow water are used to calculate the characteristics of equivalent monopole emission sources that form a total sound field similar to the sound field from a moving surface ship. The powers of each monopole and the cross-correlations between them are calculated. For the selected discrete components and linear model of an extended source, the directivity patterns are constructed, reduced to the free space. In the experiments and calculations, technical tools and algorithms were used that ensure high-precision positioning of the vessel with respect to the receiving elements of the array. An equivalent model of the waveguide transfer function in the operations area was preliminarily obtained by acoustic waveguide calibration using specially developed equipment, experimental techniques, and processing algorithms. This made it possible to use adequate seafloor models and the waveguide transfer function when calculating the equivalent sound field and directivity pattern. Good agreement is shown between the calculated and experimental data, both of the directivity pattern and field distribution along the transit characteristics. Practical recommendations are given for developing methods to measure the noise fields of surface vessels.

     

Sound attenuation by multiple barriers

A method (tentatively called ‘the method of equivalent sound source’) of predicting the sound attenuation achieved by multiple barriers with knife edges and/or right-angled wedges is proposed. This paper shows that the sound pressure level in the shadow region behind the multiple barriers can be obtained by successively setting imaginary sound sources for respective edges and/or wedges. The locations of the imaginary sound sources are determined by means of the traditional solution for sound attenuation by a single edge. The calculated results for several kinds of small-scale model, obtained by ‘the method of equivalent sound source’, are compared with experimental results.     

Validation of a complex urban noise model close to a road

The adequacy of a model for the sound level close to a road is investigated by comparing resulting predictions for the sound level over a building façade with measurements. The road model involves the road geometry (the number and positions of traffic lanes), the traffic structure (vehicle flow rates and their average speeds in each lane) and equivalent omnidirectional point sources representing the vehicles. It is found that the assumed road traffic noise source model is adequate only for predicting levels over the higher part of the façade. However the investigation has allowed definition of what improvements are needed in the road source modelling to enable adequate predictions over the whole of the building façade.     

Sound generation in a flow near a compliant wall

The generation of sound near an infinite compliant wall is studied, with account taken of a uniform mean flow. Stable and unstable configurations are looked at. It is shown that a possible influence of the wall on the sound generation occurs only via a modification of the turbulence if hydrodynamic non-linearities are responsible for the levelling-off of the instabilities. Then no fundamentally more efficient sound sources are found. An increase of the radiated sound may be possible because of the mirror sources and because of their possibly reduced compactness.     

大空间人群声学模型

经典扩散声场理论难以适用于大空间建筑。为了探究大空间建筑中人群分布对声场的影响,本文用模拟与实测相结合的方法,提出了一种适用于大空间的人群声学模型,得到了确定等效声源和人群声源的简化方法,以及人群密度与声压级之间、人群总数与发声人数之间的关系。用实测数据对该人群声学模型进行了验证。     

A hybrid approach to reconstruct transient sound field based on the free-field time reversal method and interpolated time-domain equivalent source method

A hybrid approach is presented in the current work, which reconstructs the transient sound field radiated from the two-dimensional sources with unknown locations and sizes, by combining the free-field time reversal method and the interpolated time-domain equivalent source method (TDESM). In the first step of the proposed method, the time reversal focusing algorithm is performed to estimate the source locations on the source plane. And then, the interpolated TDESM is applied to reconstruct the transient sound field on the reconstruction plane by assuming that the equivalent sources are located near the estimated source locations found in the previous step. The proposed technique, in principle, requires fewer microphones in the measurement since the equivalent sources are only placed in the vicinity of the ‘real’ sound sources. Reconstruction of the transient sound field radiated from the dual-planar-piston model is studied by numerical simulation for feasibility demonstration. A measurement of the sound fields radiated from two baffled loudspeakers is performed in the anechoic chamber, which shows that a better reconstruction result can be achieved by using the proposed hybrid scheme than the original interpolated TDESM with relatively the same number of sampling channels.     

Efficient calculation of two-dimensional periodic and waveguide acoustic Green's functions

New representations and efficient calculation methods are derived for the problem of propagation from an infinite regularly spaced array of coherent line sources above a homogeneous impedance plane, and for the Green's function for sound propagation in the canyon formed by two infinitely high, parallel rigid or sound soft walls and an impedance ground surface. The infinite sum of source contributions is replaced by a finite sum and the remainder is expressed as a Laplace-type integral. A pole subtraction technique is used to remove poles in the integrand which lie near the path of integration, obtaining a smooth integrand, more suitable for numerical integration, and a specific numerical integration method is proposed. Numerical experiments show highly accurate results across the frequency spectrum for a range of ground surface types. It is expected that the methods proposed will prove useful in boundary element modeling of noise propagation in canyon streets and in ducts, and for problems of scattering by periodic surfaces.     

南海海域跨海沟环境的声场会聚特性

利用南海北部海域的声传播实验数据分析了跨海沟条件下的声传播特性。结果表明:由于负梯度声速剖面和海底地形的共同作用,导致声能量在开始随着海沟深度变化向更深层位置上弯曲传播,传播损失在海沟中心位置附近达到最大。在海深逐渐变浅的距离上,由于海底反射使得声能量逐渐会聚,声传播损失比单纯陆坡变到深海环境下要减小20 dB以上。当会聚效果小于扩散和海底反射等引起的损失时,声传播损失达到最小,之后随着距离增大而增大;在海沟最后一段,当海深在8 km范围内从850 m突然变浅到311 m,声场能量逐渐会聚效果再次凸显,使得声传播损失减小10 dB。利用射线理论和抛物方程近似数值分析,解释了海沟环境下的由于地形变化引起的声场会聚传播特性。