Air-ground interaction in long range propagation of low frequency sound and vibration—field tests and model verification

An extensive program of intermediate and long range impulsive sound propagation field tests have been conducted. The test program and the performed measurements are presented. Particular focus is given on the air-ground interaction and its effect on low frequency sound and vibration propagation. It is found that the pressure wave interaction with the viscoelastic Rayleigh wave in the ground may have a significant effect on the ground impedance and the sound and vibration propagation. This introduces an important mechanism not covered in commonly used ground impedance models. Numerical simulation models have been developed and verified against the test data. The ground impedance does not only effect the sound pressure propagation. If either acoustically induced ground vibration, or ground to building transmitted vibration, is to be considered, the acousto-seismic impedance has a dramatic effect on the level of ground vibration induced by a given sound pressure. For a site where Rayleigh wave interaction appears at the dominant frequencies of the sound pressure, the ground vibration may be greater than a factor 100 (40 dB) than at a site with ground conditions not making the interaction happen.     

Absorption of a rigid frame porous layer with periodic circular inclusions backed by a periodic grating

The acoustic properties of a periodic rigid frame porous layer with multiple irregularities in the rigid backing and embedded rigid circular inclusions are investigated theoretically and numerically. The theoretical representation of the sound field in the structure is obtained using a combination of multipole method that accounts for the periodic inclusions and multi-modal method that accounts for the multiple irregularities of the rigid backing. The theoretical model is validated against a finite element method. The predictions show that the acoustic response of this structure exhibits quasi-total, high absorption peaks at low frequencies which are below the frequency of the quarter-wavelength resonance typical for a flat homogeneous porous layer backed by a rigid plate. This result is explained by excitation of additional modes in the porous layer and by a complex interaction between various acoustic modes. These modes relate to the resonances associated with the presence of a profiled rigid backing and rigid inclusions in the porous layer.     

A practical acoustical absorption analysis of coir fiber based on rigid frame modeling

An analytical study based on rigid frame model is demonstrated to evaluate the acoustic absorption of coir fiber. Effects of different conditions such as combination of air gap and perforated plate (PP) are studied in this work. Materials used here are treated as rigid rather than elastic, since the flow resistivity of coir fiber is very low. The well-known rigid frame Johnson-Allard equivalent-fluid model is applied to obtain the acoustic impedance of single layer coir fiber. Atalla and Sgard model is employed to estimate the surface impedance of PP. Acoustic transmission approach (ATA) is utilized for adding various consecutive layers in multilayer structure. Models are examined in different conditions such as single layer coir fiber, coir fiber backed with air gap, single layer PP in combination with coir fiber and air gap. Experiments are conducted in impedance tube on normal incidence sound absorption to validate the results. Results from the measurement are found to be in well agreement with the theoretical absorption coefficients. The performance of the rigid frame modeling method is checked more specifically in all conditions, by the mean prediction error rate of normal incidence sound absorption coefficients. Comparison between the measured absorption coefficients and predicted by rigid frame method shows discrepancy lower than 20 and 15% for most of the conditions in the frequency range of 0.2?C1.5 and 1.5?C5 kHz, respectively. Moreover, acoustic absorption of various single and multilayer structures is compared with the simpler empirical methods such as Delany-Bazley and Miki model; and complicated method such as Biot-Allard Model and Allard Transfer Function (TF) method. Comparisons show that the presented method offers a better accuracy of the results than the empirical models. Subsequently, it can provide almost same absorption plot with Biot-Allard model (single layer combination) and TF method (multilayer combination) proving it to be a comprehensively easy and general analytical tool. Therefore, the rigid frame model can be implemented relatively easier than other similar models to analyze the acoustic absorption of coir fiber in most of the conditions.     

高声压级时多孔金属板的吸声特性研究

针对高声压级下有限厚度多孔金属板在线性阻抗背衬条件下(背衬表面声压与声质点速度为线性关系)的吸声问题,提出了一个描述不同声压级下材料层法向吸声性能的一维模型,并给出求解材料层内部声质点速度的线化与差分方法,以预测多孔金属板在高声压级下的非线性吸声特性。在阻抗管中对两块多孔金属板进行了声学测试,得到了材料层法向表面阻抗和吸声系数随入射声压级变化的实验结果。研究表明:实验与理论预测符合良好,验证了模型与数值方法的正确性。本文所提原理和方法,可用于一般硬质多孔材料。      

混凝土箱梁相似模型结构噪声对比分析*

为探讨混凝土箱梁噪声的时变特性,以京沪高铁32 m混凝土简支箱梁为原型,制作了1:10的缩尺模型,通过模态试验的方法验证箱梁模型与原型的相似关系,通过声学试验验证箱梁声学计算模型的正确性。然后,建立了箱梁缩尺模型与原型两种计算模型,利用有限元和边界元法求出两种模型的瞬态结构噪声。研究发现,箱梁缩尺模型与箱梁原型的材料参数满足一定关系,模型试验的方法能够验证箱梁模型与箱梁原型之间符合相似关系,箱梁模型的振动噪声测试结果能真实反映原型振动噪声水平。两种模型的结构噪声在时域内声压级及对应场点的声压存在一定相似关系。该研究可为箱梁缩尺模型结构噪声反演至箱梁原型提供依据,所采用的方法和得到的结果对桥梁结构振动与声辐射实验研究具有参考作用。     

Fast asymptotic solutions for sound fields above and below a rigid porous ground

The current study simultaneously addresses the problem of reflection and refraction of sound from a rigid porous ground surface. A more rigorous approach is used to derive more accurate asymptotic solutions that can be cast in a convenient form for ease of numerical implementations. The solutions provide means for rapid computations of the sound fields above and below the rigid porous ground. The improved asymptotic formulas for both situations agree well with numerical results obtained by other numerical schemes, which are more accurate but computationally more intensive. More importantly, the asymptotic solutions can be written in the well-known form of the Weyl-van der Pol formula, which provides a direct correlation between the reflected wave term for the sound field above the porous ground and the transmitted (refracted) wave term for the sound field below.     

Low frequency in situ metrology of absorption and dispersion of sound absorbing porous materials based on high power ultrasonic non-linearly demodulated waves

The present work is related to acoustic in situ free-field measurements of sound absorption in porous materials, such as cellular plastic foams, glass-wool or recycled felt materials. The emphasis is given towards fine metrology of absorption in view of potential industrial applications. A powerful ultrasonic array working at 40 kHz is used. It enables to measure absorption acoustical data down to 100 Hz due to the exploitation of the non-linear ultrasonic demodulation phenomenon in air. Fine measurements of acoustic absorption are compared to numerical predictions based on the “equivalent-fluid” model (when the squeleton frame is motionless), and to some measurements performed on a Brüel and Kjaer impedance tube. Dispersion curves are also measured and compared to the numerical predictions for some automotive felt materials which are compressed at various ratios. Data obtained with a dedicated portable instrument are also discussed for the same type of materials and configurations.     

含三聚氰胺多孔材料分层复合介质吸声特性*

三聚氰胺泡沫材料是一种具有高开孔率的多孔材料,具备优良的吸音、防火隔热及环保性能,可以作为吸声材料与弹性板、空腔介质形成复合结构,在建筑、航空、交通工具等工程领域有广泛的应用。该文基于Biot理论和分层介质在交界面处的不同边界条件,建立非均匀复合介质背衬刚性壁面结构的理论声学模型,详细分析了多孔材料布局对复合结构吸声特性的影响。该文理论模型计算的结果与阻抗实验得到的垂直入射吸声系数基本一致,验证了理论模型的正确性。结果表明:在多孔材料前面增加空气层可以改善高频吸声特性;在多孔材料后面增加空气层可以改善复合结构低频吸声特性。通过合理配置多孔材料,可以在应用需求频段上达到满意的吸声效果。     

On the sensitivity analysis of porous material models

Porous materials are used in many vibroacoustic applications. Different available models describe their behaviors according to materials' intrinsic characteristics. For instance, in the case of porous material with rigid frame, and according to the Champoux–Allard model, five parameters are employed. In this paper, an investigation about this model sensitivity to parameters according to frequency is conducted. Sobol and FAST algorithms are used for sensitivity analysis. A strong parametric frequency dependent hierarchy is shown. Sensitivity investigations confirm that resistivity is the most influent parameter when acoustic absorption and surface impedance of porous materials with rigid frame are considered. The analysis is first performed on a wide category of porous materials, and then restricted to a polyurethane foam analysis in order to illustrate the impact of the reduction of the design space. In a second part, a sensitivity analysis is performed using the Biot–Allard model with nine parameters including mechanical effects of the frame and conclusions are drawn through numerical simulations.     

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.     

Acoustic properties of unconsolidated granular mixes

Simple empirical relations have been proposed to relate a limited number of directly measurable non-acoustic properties of an unconsolidated granular mix to its characteristic acoustic impedance and propagation constant. These properties are: characteristic particle dimension, porosity, tortuosity and the density of the grain base. It is believed that the model accounts heuristically for the mechanical friction between the elements of the rigid frame, the absorption in the frame micro-pores, and the degree of compaction. These effects can be important and are linked to the value of material density. This work presents practical applications of the proposed model for the prediction of the acoustic characteristics of hard-backed layers of loose granular mixes which can be used for acoustic absorption and insulation. It is shown that the predictions are in excellent agreement with the measured data for a representative range of loose granular mixes. A comparison of the results of the 4-parameter Attenborough model for the acoustic properties of porous media and the experimental data is made also. This model is used extensively to predict the acoustic properties of porous ground and granular media. The accuracy of this model with respect to loose granular materials is discussed.     

Experimental measurements of acoustical properties of snow and inverse characterization of its geometrical parameters

Snow is a sound absorbing porous sintered material composed of solid matrix of ice skeleton with air (+water vapour) saturated pores. Investigation of snow acoustic properties is useful to understand the interaction between snow structure and sound waves, which can be further used to devise non-destructive way for exploring physical (non-acoustic) properties of snow. The present paper discusses the experimental measurements of various acoustical properties of snow such as acoustic absorption coefficient, surface impedance and transmission losses across different snow samples, followed by inverse characterization of different geometrical parameters of snow. The snow samples were extracted from a natural snowpack and transported to a nearby controlled environmental facility at Patsio, located in the Great Himalayan range of India. An impedance tube system (ITS), working in the frequency range 63–6300 Hz, was used for acoustic measurements of these snow samples. The acoustic behaviour of snow was observed strongly dependent upon the incident acoustic frequency; for frequencies smaller than 1 kHz, the average acoustic absorption coefficient was found below than 0.4, however, for the frequencies more than 1 kHz it was found to be 0.85. The average acoustic transmission loss was observed from 1.45 dB cm⁻¹ to 3.77 dB cm⁻¹ for the entire frequency range. The real and imaginary components of normalized surface impedance of snow samples varied from 0.02 to 7.77 and −6.05 to 5.69, respectively. Further, the measured acoustic properties of snow were used for inverse characterization of non-acoustic geometrical parameters such as porosity, flow resistivity, tortuosity, viscous and thermal characteristic lengths using the equivalent fluid model proposed by Johnson, Champoux and Allard (JCA). Acoustically derived porosity and flow resistivity were also compared with experimentally measured values and good agreement was observed between them.     

Acoustical impedance models for outdoor ground surfaces

A theory which predicts the acoustical characteristics of rigid porous materials in terms of four physical parameters is used to provide impedance versus frequency models for various types of ground surface. It is found possible to obtain tolerable agreement with measurements of surface normal impedance for grass-covered ground, bare ground and layered forest floor when measured values of flow resistivity and porosity and estimated values of the other two parameters are used. The agreement with measured acoustical data is shown to be superior to that obtainable with the single-parameter, semi-empirical model that is widely used to predict ground effect even when the single parameter is adjusted for best-fit. A low frequency/high flow resistivity approximation is derived in a form that requires the adjustment of only a single parameter when fitting impedance versus frequency data. This model predicts that the real part of the surface normal impedance, or resistance, and imaginary part, or reactance, are equal and decrease as the inverse square root of frequency. The low frequency/high flow resistivity approximation makes possible the derivation of a two-parameter impedance versus frequency model for a medium with a porosity that decreases exponentially with depth. If the two parameters are considered to be adjustable for best-fit, then some measurements of surface impedance are found to be explicable by means of such a model. The low frequency/high flow resistivity approximation is used, together with the assumption of layer thickness small compared with internal wavelength, to derive another simple two-parameter model for grounds that behave acoustically as thin hard-backed porous layers. Examples of the validity of this model are provided. Both of the two-parameter models predict values of reactance which are greater than those of resistance over the audio-frequency range. Methods of adjusting the two parameters of each model in fitting short-range excess-attenuation data are suggested as alternatives to the single-parameter method, and as the basis for predicting ground effect at long range and low frequencies.     

Sound reduction by barriers on the ground

An existing theory on the propagation of spherical sound waves over ground with a finite acoustic impedance is to a large extent verified by field measurements over different types of ground surfaces. It is then shown that it is reasonable to combine this theory with ordinary diffraction theory to get a solution of the mixed problem with a barrier on the ground. Full-scale measurements have been carried out with a 3 m high wall and the results show good agreement with the predicted values. It is also shown that it is generally of minor importance whether a thin screen is sound absorbing or not.     

Acoustic absorption modeling of porous concrete considering the gradation and shape of aggregates and void ratio

The results of acoustic absorption modeling of porous concrete considering the gradation and shape of aggregates and void ratio are presented. To model the void texture of porous concrete, the multi-layered micro-perforated rigid panel model considering air gaps [1] and [2] is adopted. The parameters used in this acoustic absorption modeling are determined by a geometrical and experimental approach considering the gradation and shape of aggregates and void ratio. The predicted acoustic absorption spectra are compared with experimental results to verify the proposed acoustic absorption modeling approach. Finally, a parametric study is conducted to investigate the influence of design factors on the acoustic absorption properties of porous concrete.     

The application of source methods in estimation of an interior sound field

In earlier studies, one has successfully developed three different source methods (SSM, similar source method; IPSM, internal parallel source method; ISM, internal source method) to estimate radiation and scattering sound fields. All these methods are estimations of exterior sound fields. In the present study, the similar source method is modified to estimate an interior sound field. The modification is to move the imaginary sources outside the boundary surface. In addition, general boundary conditions in terms of acoustic admittance or impedance are considered by introducing the suitable least square error functions. Finally, a two-dimensional interior sound field with alternative boundary conditions is evaluated to simulate sound fields inside a car. The results are in agreement with those of boundary integral method.     

The diffraction of sound by an impedance sphere in the vicinity of a ground surface

The problem of sound diffraction by an absorbing sphere due to a monopole point source was investigated. The theoretical models were extended to consider the case of sound diffraction by an absorbing sphere with a locally reacting boundary or an extended reaction boundary placed above an outdoor ground surface of finite impedance. The separation of variables techniques and appropriate wave field expansions were used to derive the analytical solutions. By adopting an image method, the solutions could be formulated to account for the multiple scattering of sound between the sphere and its image near a flat acoustically hard or an impedance ground. The effect of ground on the reflected sound fields was incorporated in the theoretical model by employing an approximate analytical solution known as the Weyl-van der Pol formula. An approximation solution was suggested to determine the scattering coefficients from a set of linearly coupled complex equations for an absorbing sphere not too close to the ground. The approximate method substantially reduced the computational time for calculating the sound field. Preliminary measurements were conducted to characterize the acoustical properties of an absorbing sphere made of open cell polyurethane foam. Subsequent experiments were carried out to demonstrate the validity of the proposed theoretical models for various source/receiver configurations around the sphere above an acoustically hard ground and an impedance ground. Satisfactory comparative results were obtained between the theoretical predictions and experimental data. It was found that the theoretical predictions derived from the approximate solution agreed well with the results obtained by using the exact solutions.     

Model and estimation method for predicting the sound radiated by a horn loudspeaker - With application to a car horn

This paper deals with a new car horn device made of a sound synthesizer and an electrodynamic horn loudspeaker. It presents an one-dimensional model allowing to predict the loudspeaker efficiency and a specific method to estimate experimentally the model parameters. First, this model aims at reducing the time spent in the design process. Second it aims at correcting the sound emitted by the sound synthesizer in order that the listener hears the sound designed for creating the warning message. The study gives a survey of the vast loudspeaker literature. It is based on the conventional electroacoustic approach used for electrodynamic loudspeakers and on wave propagation models used for characterizing acoustic horns. The estimation of the model parameter values is performed using measurements of the electrical impedance of the loudspeaker and of the acoustic impedance of the horn. The model is assessed by comparing the calculated and measured electrical impedances and horn efficiencies. Results show that the model predicts well the horn efficiency up to 2500 Hz, the limitation being due to the horn radiation impedance modelization.     

Apparent mass of seated man—First determination with a soft seat and dynamic seat pressure distributions

Data of the impedance and/or the apparent mass of the sitting human body during the exposure to whole-body vibration in z-direction using rigid seats were standardized in the ISO 5982. These data are available as target functions for model developments. Models developed on this data basis should also apply to driver seats with a soft seat and backrest cushion, although the qualitative different contact conditions were neglected. Due to insufficient technical prerequisites, the determination of forces at the interface between subject and soft seat was impossible until very recently. Results of studies during static conditions showed clear differences in the pressure distributions between the rigid and the soft contact areas. In this experimental study pressure distributions on a seat cushion were measured during whole-body vibration in z-direction (random signal in the frequency range between 0.3 and 20 Hz, vibration magnitudes 0.25, 0.8, and 1.6 m s⁻² unweighted root mean square measured at the seat base) with a sampling rate of 32 m s. The apparent masses were calculated by the forces derived from the pressure distributions and accelerations measured at one point of the seat cushion near the buttocks. The moduli of the apparent masses derived for the soft seat were clearly lower than those determined for a rigid seat. These apparent masses showed a similar dependence on the vibration magnitude as the apparent mass functions derived in the usual way for rigid seats. Factors that could explain differences between the apparent mass functions derived for the soft and rigid seat were discussed and evaluated. The data of this study indicate the possibility and necessity to consider the contact conditions at the interface when deriving target functions for the model development. Recommendations for technical improvements and further experimental studies with driver seats were derived.     

Analysis of structural - acoustic coupling of elastic rectangular enclosure with arbitrary boundary conditions

The structural acoustic coupling characteristics of a rectangular enclosure con- sisting of two elastic supported flexible plates and four rigid plates are analyzed.A general formulation considering the full coupling between the plates and cavity is developed by using Hamiltonian function and Rayleigh-Ritz method.By means of continuous distributions of ar- tificial springs along boundary of flexible plates,a wide variety of boundary conditions and structure joint conditions are considered.To demonstrate the validity of the analytical model, the responses of sound pressure in the cavity and plate velocity are worked out.The analytical results coincides well with Kim's experimental results.The result is satisfactory.Finally,an- alytical results on the structure vibration and the sound field inside the cavity are presented. These results indicate that the coupling of the combined structure is relatively weak,so the internal cavity sound is controlled by plate directly excited,and the translational stiffness affects the sound more than the rotational stiffness does.