金属烧结丝网在内场消声中的应用

利用试验手段研究暂冲式风洞稳定段内安装不同规格烧结金属丝网对风洞上游控制阀后气流噪声和湍流度抑制作用。试验结果表明:多层金属烧结丝网可在全频段内大幅度降低上游气流的噪声,最大可达21 dB;消声量与金属烧结丝网无量纲的压力损失系数成正比,压力降与金属烧结丝网层数呈现出非线性叠加的结果。另外发现烧结金属烧结丝网对气流速度脉动亦具有突出的抑制效果。例如,试验段马赫数Ma=1:5时,120目26层+160目26层组合烧结金属丝网出口气流速压脉动幅值减小为入口来流的18%,湍流度由11.7%降至3%。因此金属烧结丝网适合于暂冲式风洞的内场降噪。     

A mathematical model for a single screen barrier in open-plan offices

In open-plan offices, single screen barriers are widely used to separate individual workplaces as a means of improving acoustical privacy. In this paper, a general model for calculating the insertion loss of a single screen barrier in the presence of a floor and a ceiling is developed using the image source technique. In addition to the acoustical properties of the floor and ceiling, this model also takes the sound absorption of the screen, the sound transmission through the screen and the interference between the sound waves into account. This model is able to separate the contribution of reflected sound and diffracted sound from the total sound pressure level at the receiving point, which can help indicate how best to improve the acoustical design of an open office. The mean differences between the predicted 1/3 octave band insertion loss values behind the screen and the corresponding measured results are within 2 dB.     

Numerical evaluation of tree canopy shape near noise barriers to improve downwind shielding

The screen-induced refraction of sound by wind results in a reduced noise shielding for downwind receivers. Placing a row of trees behind a highway noise barrier modifies the wind field, and this was proven to be an important curing measure in previous studies. In this paper, the wind field modification by the canopy of trees near noise barriers is numerically predicted by using common quantitative tree properties. A realistic range of pressure resistance coefficients are modeled, for two wind speed profiles. As canopy shape influences vertical gradients in the horizontal component of the wind velocity, three typical shapes are simulated. A triangular crown shape, where the pressure resistance coefficient is at maximum at the bottom of the canopy and decreases linearly toward the top, is the most interesting configuration. A canopy with uniform aerodynamic properties with height behaves similarly at low wind speeds. The third crown shape that was modeled is the ellipse form, which has a worse performance than the first two types, but still gives a significant improvement compared to barriers without trees. With increasing wind speed, the optimum pressure resistance coefficient increases. Coniferous trees are more suited than deciduous trees to increase the downwind noise barrier efficiency.     

汽车涡轮增压器同步谐波噪声仿真与优化*

针对增压发动机急加速急减速时产生的增压器同步谐波噪声问题,该文通过噪声仿真技术进行分析与优化。首先,分析该噪声的特征与传播路径;其次,建立流场仿真模型。利用剪切应力输运湍流模型与分离涡流模拟湍流模型对增压器进行稳态与非稳态瞬态流场分析,提取非稳态流场的叶轮与压气机流道表面的偶极子声源;最后,建立噪声传播模型,计算该增压器压气机的进气口声场分布。通过理论分析与试验相结合的方法,优化叶轮轮缘与压气机壳体的配合型线,将该增压器噪声的阶次峰值最大降低约15.3 dB(A),消除了同步谐波噪声,且对发动机性能几乎无影响。该噪声的解决方法可以为压气机气动噪声优化提供有价值的参考。     

Surface tension effects in breaking wave noise

The role of surface active materials in the sea surface microlayer on the production of underwater noise by breaking waves is considered. Wave noise is assumed to be generated by bubbles formed within actively breaking whitecaps, driven into breathing mode oscillation at the moment of their formation by non-equilibrium, surface tension forces. Two significant effects associated with surface tension are identified-a reduction in low frequency noise (<1000 Hz) due to the re-fragmentation of actively radiating bubbles by fluid turbulence within the whitecap and a reduction in overall noise level due to a decrease in the excitation amplitude of bubbles associated with reduced surface tension. The impact of the latter effect on the accuracy of Weather Observations Through Ambient Noise estimates of wind speed is assessed and generally found to be less than ±1 m?s(-1) for wind speeds less than 10?m s(-1) and typical values of surfactant film pressure within sea slicks.     

Verbal communication and noise in eating establishments

A new simple prediction model has been derived for the average A-weighted noise level due to many people speaking in a room with assumed diffuse sound field. Due to the feed-back influence of noise on the speech level (the Lombard effect), the speech level increases in noisy environments, and the suggested prediction model gives a 6 dB reduction of the noise level by doubling the equivalent absorption area of the room. This is in contrast to the lowering by 3 dB by doubling of the absorption area for a constant power sound source. The prediction model is verified by experimental data found in the literature. In order to achieve acceptable conditions for speech communication within a small group of people, a guide for the recommended minimum absorption area per person in eating establishments is provided.     

The extended Fourier pseudospectral time-domain method for atmospheric sound propagation

An extended Fourier pseudospectral time-domain (PSTD) method is presented to model atmospheric sound propagation by solving the linearized Euler equations. In this method, evaluation of spatial derivatives is based on an eigenfunction expansion. Evaluation on a spatial grid requires only two spatial points per wavelength. Time iteration is done using a low-storage optimized six-stage Runge-Kutta method. This method is applied to two-dimensional non-moving media models, one with screens and one for an urban canyon, with generally high accuracy in both amplitude and phase. For a moving atmosphere, accurate results have been obtained in models with both a uniform and a logarithmic wind velocity profile over a rigid ground surface and in the presence of a screen. The method has also been validated for three-dimensional sound propagation over a screen. For that application, the developed method is in the order of 100 times faster than the second-order-accurate FDTD solution to the linearized Euler equations. The method is found to be well suited for atmospheric sound propagation simulations where effects of complex meteorology and straight rigid boundary surfaces are to be investigated.     

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.     

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.     

Coherence function and mean field of plane and spherical sound waves propagating through inhomogeneous anisotropic turbulence

Inhomogeneity and anisotropy are intrinsic characteristics of daytime and nighttime atmospheric turbulence. For example, turbulent eddies are often stretched in the direction of the mean wind, and the turbulence statistics depends on the height above the ground. Recent studies have shown that the log-amplitude and phase fluctuations of plane and spherical sound waves are significantly affected by turbulence inhomogeneity and anisotropy. The present paper is devoted to studies of the mean sound field and the coherence functions of plane and spherical sound waves propagating through inhomogeneous anisotropic turbulence with temperature and velocity fluctuations. These statistical moments of a sound field are important in many practical applications, e.g., for source detection, ranging, and recognition. Formulas are derived for the mean sound field and coherence function of initially arbitrary waveform. Using the latter formula, we also obtained formulas for the coherence functions of plane and spherical sound waves. All these formulas coincide with those known in the literature for two limiting cases: homogeneous isotropic turbulence with temperature and wind velocity fluctuations, and inhomogeneous anisotropic turbulence with temperature fluctuations only. Using the formulas obtained, we have numerically shown that turbulence inhomogeneity significantly affects the coherence functions of plane and spherical sound waves.     

尖楔模型结构对脉动压力测量影响实验研究

高超声速边界层转捩与湍流研究是当前空气动力学研究的一个热门领域,脉动压力测量技术在高超声速风洞背景噪声测量和边界层内扰动波发展研究实验中得到了广泛应用.脉动压力传感器由于灵敏度高、测量频率范围宽,其测量结果受多方面因素影响.文章以尖楔模型为研究对象,在常规高超声速风洞中开展了模型背面凸起对表面脉动压力测量结果影响的实验研究,获得了3种背面结构（方型凸起、斜坡型凸起和无凸起）模型表面脉动压力信息,对比发现模型背面方型凸起和斜坡型凸起使得模型表面声压级明显增加,其中方型凸起的影响比斜坡型凸起的影响更大,模型背面凸起结构对表面脉动压力测量的影响沿着展向逐渐增加.分析认为模型背面凸起结构影响表面脉动压力测量有三种可能途径:①模型背面大尺度凸起改变模型流场波系,从而影响模型表面流动;②模型背面非定常流动产生的噪声通过固壁传播至脉动压力传感器,影响测量结果;③模型周围非定常非对称流动给模型施加一个非定常的作用力,使得模型振荡,从而影响表面脉动压力测量.      

Effect of room absorption on human vocal output in multitalker situations

People increase their vocal output in noisy environments. This is known as the Lombard effect. The aim of the present study was to measure the effect as a function of the absorption coefficient. The noise source was generated by using other talkers in the room. A-weighted sound levels were measured in a 108 m(3) test room. The number of talkers varied from one to four and the absorption coefficients from 0.12 to 0.64. A model was introduced based on the logarithmic sum of the level found in an anechoic room plus the increasing portion of noise levels up to 80 dB. Results show that the model fits the measurements when a maximum slope of 0.5 dB per 1.0 dB increase in background level is used. Hence Lombard slopes vary from 0.2 dBdB at 50 dB background level to 0.5 dBdB at 80 dB. In addition, both measurements and the model predict a decrease of 5.5 dB per doubling of absorbing area in a room when the number of talkers is constant. Sound pressure levels increase for a doubling of talkers from 3 dB for low densities to 6 dB for dense crowds. Finally, there was correspondence between the model estimation and previous measurements reported in the literature.     

Air-borne sound generated by sea waves

This paper describes a semi-empiric model and measurements of air-borne sound generated by breaking sea waves. Measurements have been performed at the Baltic Sea. Shores with different slopes and sediment types have been investigated. Results showed that the sound pressure level increased from 60 dB at 0.4 m wave height to 78 dB at 2.0 m wave height. The 1/3 octave spectrum was dependent on the surf type. A scaling model based on the dissipated wave power and a surf similarity parameter is proposed and compared to measurements. The predictions show satisfactory agreement to the measurements.     

A linearized Eulerian sound propagation model for studies of complex meteorological effects

Outdoor sound propagation is significantly affected by the topography (including ground characteristics) and the state of the atmosphere. The atmosphere on its part is also influenced by the topography. A sound propagation model and a flow model based on a numerical integration of the linearized Euler equations have been developed to take these interactions into account. The output of the flow model enables the calculation of the sound propagation in a three-dimensionally inhomogeneous atmosphere. Rigid, partly reflective, or fully absorptive ground can be considered. The linearized Eulerian (LE) sound propagation model has been validated by means of four different scenarios. Calculations of sound fields above rigid and grass-covered ground including a homogeneous atmosphere deviate from analytic solutions by < or = 1 dB in most parts of the computed domain. Calculations of sound propagation including wind and temperature gradients above rigid ground agree well with measured scale model data. Calculations of sound propagation over a screen including ground of finite impedance show little deviations to measured scale model data which are probably caused by an insufficient representation of the complex ground impedance. Further calculations included the effect of wind on shading by a screen. The results agree well with the measured scale model data.     

Traffic noise reduction by means of surface wave exclusion above parallel grooves in the roadside

A new method to reduce traffic noise by means of an ‘invisible’ wall has been investigated both theoretically and experimentally. A formula was derived for the frequency dependent impedance of an infinite structure of parallel ribs on an impedance boundary. From the definition of surface waves it followed that these waves can only exist for certain combinations of frequencies, heights of ribs and phases of the complex reflection coefficient of the underlying surface. Upon making this surface softer, more low frequency sound is absorbed. Outdoor experiments above an array of 16 or 21 low brick walls showed a considerable absorption of sound. Attenuations occurred up to 20 dB in the one-third octave bands from 125 to 400 Hz and amplifications up to 12 dB in the range of 400–1000 Hz. It was possible to explain these measurements qualitatively by the theory of surface waves. The wall structure caused an insertion loss of approximately 4 dB(A) in the total sound pressure level of the A-weighted one-third octave bands from 100 to 12,500 Hz.     

Influence of turbulence on train noise

The subject of this paper is the long distance propagation of train noise. The sound exposure level of train noise L_AE was measured. To describe the results of measurements, a semi-analytical model was used. It takes into account the wave-front divergence, air absorption, ground effect, and the turbulence destroying the coherent nature of the ground effect. The model contains three adjustable parameters that must be estimated at the site. To verify the model, we performed measurements of L_AE at the distance D = 450 m from the train track center. The difference between the calculated and measured mean values of L_AE equals 1.3 dB.     

Wind dependence of ambient noise in shallow water of Bay of Bengal

This work reports on investigations into the wind dependence of ambient noise in the Bay of Bengal. Ambient noise measurements were made in the shallow water of Bay of Bengal using a portable broadband, high frequency data acquisition system together with a sensitive hydrophone suspended from the measuring platform at a depth of 5 m from the surface where the ocean depth was 25 m. Periodic measurements were carried out for one year corresponding to a wind speed range between 2 m/s and 9 m/s during summer, monsoon and winter seasons. The proportionality of the noise level with wind speed for frequencies ranging from 500 Hz to 6 kHz for each season was studied. The analysis reveals that the correlation between the wind speed and the ambient noise spectrum level was higher at lower frequencies. The results of empirical fitting based on analysis were used for noise level prediction and the model predictions compare well with the measured noise level. Further it was observed that the wind generated noise level measured during summer was approximately 8 dB less than that in other seasons. On the other hand the proportionality between the noise level and the wind speed was less during winter.     

海面波浪谱对深海低频环境噪声的影响

提出了一种适用于深海低频环境噪声的波浪谱,通过声压谱和波浪谱的理论关系,分析了深海低频噪声在百赫兹以下的谱特征,解释了不同频段噪声谱的主要产生机理。将深海传播条件下海面波浪谱与海面风速相结合,利用波浪发声理论得到一种低频海洋环境噪声理论表示方法。仿真结果表明,波浪谱决定着辐射噪声谱的强度和斜率,本模型得到的理论噪声谱可以对低频海洋环境噪声进行预报。2016年的深海实验观测数据分析显示,统计的环境噪声谱级在1 Hz至100 Hz频段范围内大于70 dB,并且噪声谱在低频段呈倒“N”型,在34 Hz处为噪声谱的谷值,噪声级为70 dB,在50 Hz处为噪声谱的峰值,噪声级为92 dB,通过理论计算和实验对比,相关系数为0.95,理论结果和实验测量对比结果符合较好。      

Testing meteorological classifications for the prediction of long-term average sound levels

The paper describes the results of a preparatory study for the planned introduction of a harmonised method to assess long-term sound levels in Europe. The purpose of this study is to quantify the error in the prediction of long-term sound levels when propagation calculations refer to a limited number of between 1 and 121 representative meteorological situations. This error is determined with respect to the average sound level resulting from parabolic-equation simulations for 1452 meteorological situations of consecutive 6-h intervals during one year. The meteorological situations were taken from meteorological tower measurements at Garching near Munich, Germany. The various meteorological conditions enter the model through two-parameter logarithmic-linear vertical profiles of the effective speed of sound which were fitted to the tower measurements. While source power and ground impedance were held constant, the model results elucidate the sound level variations due to the meteorological variability. A spread of 18 dB was found in the calculated instantaneous levels at 200 m range for rigid ground. The spread was 42 dB at 1000 m. Because of the asymmetric frequency distribution of wind directions, a spread of still up to 5.5 dB was found in the calculated annual average levels at 1000 m range for various directions of propagation. As a consequence of the specific meteorological conditions the annual average night-time level exceeds the annual average day-time level by up to 5 dB. Only 25 logarithmic-linear profile classes are necessary to determine the long-term average sound level at 1000 m range with an accuracy better than 2 dB. The use of purely linear profile classes slightly impairs the results, whereas the use of purely logarithmic profile classes cannot be recommended.     

Noise control for rapid transit cars on elevated structures

Noise control treatments for the propulsion motor noise of rapid transit cars on concrete elevated structures and the noise reduction from barrier walls were investigated by using acoustical scale models and supplemented by field measurements of noise from trains operated by the Port Authority Transportation Corporation (PATCO) in New Jersey. The results show that vehicle skirts and undercar sound absorption can provide substantial cost-effective reductions in propulsion noise at the wayside of transit systems with concrete elevated guideways. The acoustical scale model noise reductions applied to PATCO vehicles on concrete elevated structures show reductions in the A-weighted noise levels of 5 dB for undercar sound absorption, 5 dB for vehicle skirts, and 10 dB for combined undercar absorption and vehicle skirts. Acoustical scale model results for sound barrier walls lined with absorptive treatment showed reductions from 7 dB to 12 dB of noise from vehicles in the far track, depending on the height of the wall, and reductions from 12 dB to 20 dB of noise from vehicles on the near track. Transit vehicles at high speeds where propulsion system noise dominates are 7 dB(A) noisier at 50 ft on concrete elevated structures than on at-grade on tie and ballast. Of this amount, 3 dB is due to loss of ground effect, and 4 dB is due to the absence of undercar absorption provided by ballast.