基于声场重建的材料吸声系数测量方法

针对现有方法对材料吸声系数进行现场测量时存在低频测量误差大的问题,本文提出了一种利用扬声器线阵列对材料吸声系数进行现场测量的新方法。该方法使用基于能量比值约束的最小二乘法在待测材料表面进行平面波声场重建并结合双传声器传递函数法对材料的吸声系数进行测量。数值仿真表明在100～1600 Hz频率范围内,新方法在未加约束时能够对材料的吸声系数进行准确测量。在半消声室中利用新方法测量了三聚氰胺泡沫的吸声系数,分析了能量比值约束值对测量结果的影响,并和阻抗管以及其它两种现场测量方法的测量结果进行了对比。结果表明该方法能够对吸声材料在160～1600 Hz频段内的吸声系数进行准确测量,并且相较于现存的现场测量方法,新方法具有更低的测量频率下限。     

基于声压-质点速度声强探头的材料吸声系数的测量

通过由一个声压换能器和一个质点速度换能器所构成的传感器(p-u声强探头)同时测量材料表面附近的声压和质点振动速度,可直接得到其声学阻抗,进而得到材料的反射因子、吸声系数。本文利用一个p-u探头声强测量系统,在半消声室内测量了三聚氰胺泡沫的吸声系数,分析了声源高度和入射角度、材料样本尺寸和厚度对吸声系数测量的影响,并和阻抗管中测量得到的法向吸声系数进行了对比。最后分析了声阻抗率的幅值和相位误差对吸声系数的影响,推导了它们的误差传递公式。     

一种现场测量材料吸声特性的新方法

现有的材料吸声系数测量方法主要有混响室法和驻波管法,都属于实验室测量方法,不适合现场测量。使用普通扬声器的反射法可以对材料的吸声特性进行现场测量,但是对材料尺寸和测试环境有较高的要求。本文利用参量阵非线性自解调可听声的高指向性和在阵长距离内的平面波特性,结合传递函数法,测量材料的吸声系数,并与传统驻波管测量结果进行了对比。结果表明在普通房间条件下,不需要驻波管,混响室等实验环境,即可对小尺寸的材料进行吸声系数的现场快速测量,具有较大的实用性。      

金属橡胶材料吸声特性研究

理论及实验研究了金属橡胶材料吸声性能参数.由金属橡胶材料声学特性参数理论计算式,推导出金属橡胶材料吸声性能参数──声阻抗率和吸声系数的理论计算公式.实验研究了金属橡胶材料结构常数和压缩模量,并建立了结构常数、压缩模量与材料结构参数的关系式,结果表明:相同平均孔隙直径金属橡胶材料具有相同的结构常数值;在一般频率范围内,相同结构参数金属橡胶材料压缩模量近似为常量.通过金属橡胶材料吸声系数的实验测量和理论计算,理论结果与实验结果符合较好,验证了理论和实验研究方法的正确性.     

声学二维平面周期结构的吸声性能建模

声学周期结构兼具优化室内声场环境,节约建筑吸声材料的特性而在现代建筑中广泛使用。针对二维平面周期结构的声场优化特性,建立了一种吸声系数计算模型。首先,根据边界条件理论推导了吸声系数的线性方程组,继而通过数值分析方法进行求解,最后在驻波管和混响室中分别进行实验验证。实验结果表明,测量的吸声系数和理论计算曲线吻合良好,该模型可以准确测算二维平面周期结构的吸声系数。同时分析表明:在平面周期结构中,相同吸声材料面积情况下,吸声材料占比越大,吸声效果越好;在相同吸声材料面积和占比情况下,材料边缘长度越长,高频段吸声效果越好;随着材料边缘长度的减少,边缘效应影响减弱。      

窄脉冲声用于大样品的吸声测量

本文利用逆滤波器原理,在空间产生了波形可控、长度在毫秒量级的窄脉冲声信号,分别采用脉冲分离法和脉冲叠加法,对一种毛毡材料和三种不同厚度的海绵材料进行了吸声系数的测量。实验证明,基于窄脉冲声信号的吸声测量结果与ISO13472-1:2002中的MLS脉冲法及阻抗管的测量结果基本吻合。采用窄脉冲进行吸声测量,可以减少样品边缘和周围环境对测量信号的干扰,提高现场测量的准确性。     

一体化有源吸声终端实现及其在行波管测量中的应用

提出了一种利用矢量水听器在声管中实现一体化有源吸声终端的方法。该吸声终端采用矢量水听器作为传感元件,以实现入射波和反射波的分离,克服了传统双水听器声波分离方法中传感器间距及测量频率的限制,显著拓宽了一体化有源吸声终端的低频吸声频段。由声管中水声材料的测试原理出发,重点分析了吸声终端中传感元件灵敏度误差对吸声终端性能的影响,并给出了反射、透射系数的修正方法。实验结果表明:该有源吸声终端在100~2000 Hz频段内吸声系数可以达到0.98以上,测量得到材料的声压反射系数、声压透射系数及理论计算基本一致。      

空腔结构吸声器的吸声系数计算方法的研究

探讨计算空腔结构吸声器吸声系数的方法。以粘弹体能量守恒方程为基础导出吸声器各层媒质的表达式,用声波在分层吸收媒质中传播模型计算能量吸收系数。用所导出的等效密度、弹性模量表达式和文献1[声学学报,1965;2(4):192-197]、文献2[同济大学学报, 1979(1):96-104]所给的等效参数,分别计算了尖劈吸声器和空腔结构的尖劈吸声器的吸声系数,比较表明对于前者三种方法都有效,对于后者本文的结果与实测吸声系数曲线较符合,另两文与实验相差较大;又用本方法分别计算了平板吸声器含空腔及空腔结构变化时的吸声系数,所得结果合理地反映了空腔结构对吸声性能的影响;对原空腔结构吸声器的吸声系数模拟结果作了实验验证,表明计算值和测量结果基本相符。结果表明用本文所给等效参量表达式和声波在分层吸收媒质中传播模型,近似计算空腔结构吸声器的吸声系数是可行的。     

基于黄金分割搜索算法的行波管测量方法

为了提供声学材料的测试环境,该文通过有源吸声技术在行波管中建立了一套测量系统,声管中实现了模拟无限水域的行波场环境,从而对样品的声学参数进行测量。提出了一种利用黄金分割搜索算法对最佳时延与幅值进行搜索从而完成行波场建立的方法。在上述行波场建立的基础上,利用双水听器法分别对水柱与钢板样品的声学参数进行了声学测量,透射系数测量值与理论值符合较好。该方法为声学参数的测量方法提供了有意义的参考。     

释压法混合吸声系统中多孔材料厚度优化研究

研究释压法主被动混合吸声结构吸声性能与多孔吸声材料参数的关系,利用释压法对材料厚度敏感的特性,选择合适的多孔材料厚度提高系统的低频吸声性能。提出一种释压法主被动混合双层吸声结构,将误差传声器置入两层吸声结构之间的空气层中,分别优化各层的厚度,在较宽频带内获得较好吸声。对流阻系数为15000NSm^-4的一类典型的玻璃棉数值仿真,发现选择优化厚度分别为2.8 cm和6 cm,中间空腔长为2 cm,可使有效吸声频带向下大幅扩展。实验结果验证了所提宽带吸声结构。     

General regression neural network for prediction of sound absorption coefficients of sandwich structure nonwoven absorbers

In this paper, we propose a more general forecasting method to predict the sound absorption coefficients at six central frequencies and the average sound absorption coefficient of a sandwich structure nonwoven absorber. The kernel assumption of the proposed method is that the acoustics property of sandwich structure nonwoven absorber is determined by some easily measured structural parameters, such as thickness, area density, porosity, and pore size of each layer, if the type of the fiber used in nonwoven is given. By holding this assumption in mind, we will use general regression neural network (GRNN) as a prediction model to bridge the gap between the measured structural parameters of each absorber and its sound absorption coefficient. In experiment section, one hundred sandwich structure nonwoven absorbers are particularly designed with ten different types of meltblown polypropylene nonwoven materials and four types of hydroentangled E-glass fiber nonwoven materials firstly. Secondly, four structural parameters, i.e., thickness, area density, porosity, and pore size of each layer are instrumentally measured, which will be used as the inputs of GRNN. Thirdly, the sound absorption coefficients of each absorber are measured with SW477 impedance tube. The sound absorption coefficient at 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and their average value are used as the outputs of GRNN. Finally, the prediction framework will be carried out after the desired training set selection and spread parameter optimization of GRNN. The prediction results of 20 test samples show the prediction method proposed in this paper is reliable and efficient.     

Experimental studies of sound absorption by church pews

The influence of pews on the acoustical characteristics of churches may be significant because they occupy large areas in rooms lacking absorbing surfaces. However, the information about this particular category of seating is sparse and, sometimes, contradictory. Different types of pews, differing in materials and construction, were analysed in a reverberant chamber by means of Bradley’s method, measuring the absorption coefficients of blocks with different perimeter-to-area ratios. A substantial dependence on the latter parameter was found, allowing prediction of absorption coefficients as a function of actual block dimensions. The presence of upholstered kneelers showed improved absorption properties which were also replicated on other pew types by adding strips of polyester fibre. On-site measurements in three churches were used to validate the method by means of direct application of Sabine’s formula and by means of virtual acoustic modelling. The first only provided satisfactory results in rooms complying with Sabine’s assumptions, while the second also gave good results in the other church with markedly non-uniform absorption. Finally, the application to computer models of absorption coefficients measured in the chamber was discussed.     

Predicting the sound absorption of natural materials: Best-fit inverse laws for the acoustic impedance and the propagation constant

Natural materials are becoming a valid option for sound absorption treatments. In particular, among them, natural fibers have received increasing attention given their good thermal insulation properties, lack of harmful effects on health, and availability in large quantities. This paper discusses an inverse method to predict the acoustical properties of nine natural fibers. Six vegetative fibers: kenaf, wood, hemp, coconut, straw, and cane; one animal fiber, sheep wool; recycled cardboard; and granular cork are investigated. The absorption coefficient and the flow resistance for samples of different thickness have been measured. Moving from the Delany-Bazley model, this study compares the impedance tube results with the theoretically predicted ones. Then, using a least-square fit procedure based on the Nelder-Mead method, the coefficients that best predict both the acoustic impedance and the propagation constant laws are calculated. The inverse approach used in this paper allows to determine different physical parameters and to obtain formulas to include the investigated natural fibers in software modelling for room acoustics applications.     

薄纤维层对穿孔板的声学影响研究

为了提高附着薄纤维层穿孔板的声学预测精度,通过实验及理论分析方法研究薄纤维层对穿孔板的声学影响。采用声学阻抗管系统测试穿孔饰面板及背腔的吸声系数。测试结果显示,薄纤维层对吸声层的吸声性能具有明显的提高作用。以穿孔板声阻抗模型为基础构建附着薄纤维层的穿孔饰面板等效声阻抗模型,以附加修正参数等效薄纤维层的声学影响。依据吸声系数测试结果计算薄纤维层的等效声学修正参数,结果显示薄纤维层造成的附加阻性修正比较显著。     

A simple empirical model of polyester fibre materials for acoustical applications

A new empirical model has been developed by the authors to predict the flow resistivity, acoustic impedance and sound absorption coefficient of polyester fibre materials. The parameters of the model have been adjusted to best fit the values of airflow resistivity and sound absorption coefficient measured over a set of 38 samples. Calculated results are compared with normal incidence measurements carried out using two different techniques: the transfer-function method in an impedance tube (ISO 10534-2) and the free-field impulse response method (ISO 13472-1). Measurements performed on polyester fibre materials with different density and thickness values, and diameter ranging from 18 to 48 μm, are in good agreement with the predictions of the new model. It is concluded that the new model can predict the basic acoustic properties of common polyester fibre materials with any practical combination of thickness and density².     

A criterion for the use of linear regression analysis in the determination of low values of absorption coefficient using the impedance tube method

This article describes a quick and convenient method for estimating the percentage error associated with any measurement of absorption coefficient using the impedance tube method. A parameter Z is calculated, using constants appropriate to the tube and the decibel difference between a pressure maximum and minimum. The value of this parameter can be used as a criterion as to whether a more accurate determination, using linear regression, should be employed. The technique is valuable as an aid when low values of absorption coefficient, particularly at low frequencies, are to be measured.     

Low-frequency sound propagation in porous media: Glass spheres and pea gravel

The sound propagation properties of two air-filled granular materials: large sifted pea gravel and 10 mm diameter glass spheres have been measured in an impedance tube. The experimental method was essentially the same as reported earlier [Swenson et al. Low-frequency sound wave parameter measurement in gravels. Appl Acoust 2010; 71: 45–51] for two other kinds of gravel: crushed limestone and undifferentiated pea gravel. Additional sampling and processing steps were applied to the microphone signals such that instead of tones, band-limited random noise was used as the input signal, and spectral domain complex pressures are now offered as input to the estimation algorithm. The estimation process extracts the best-fit attenuation coefficient, phase velocity, and characteristic impedance for the material over the signal frequencies, all with better precision than we previously obtained. Quadratic approximations for the acoustical parameters are given over the frequency range 25–160 Hz. The media are both slightly attenuating and dispersive, having attenuation coefficients within 0.13–0.34 Np/m, phase velocities smaller than those in air (180–240 m/s), and characteristic impedance approximately 3–5 times that for air. Pea gravel was more attenuating, and had slightly higher characteristic impedance, but lower phase velocities than the glass spheres.     

A model comparison of the absorption coefficient of a Microperforated Insertion Unit in the frequency and time domains

The absorption coefficient of acoustic materials can be measured either in the frequency or the time domain. At normal incidence, a sample of the material is fitted within an impedance tube and the absorption coefficient is calculated in the frequency domain from the measurement of the transfer function between two microphones [ISO 10534-2. Acoustics - determination of sound absorption coefficient and impedance in impedance tubes - Part 2: transfer function method. ISO, Geneva, Switzerland; 1996]. When the acoustic material must be characterized at oblique incidence or in situ (noise barriers, for instance) the absorption coefficient is calculated from measurements of the loudspeaker-microphone impulse response in the time domain, both in free field and in front of the sample [CEN/TS 1793-5. Road traffic noise reduction devices - test method for determining the acoustic performance - Part 5: intrinsic characteristics - in situ values of sound reflection and airborne sound insulation. CEN, Brussels, Belgium; 2003, ISO 13472-1. Acoustic measurement of sound absorption properties of road surfaces in situ - Part I: extended surface method. ISO, Geneva, Switzerland; 2002]. Since the absorption is an intrinsic property of the acoustic material, its measurement in either domain must provide the same result. However, this has not been formally demonstrated yet. The aim of this paper is to carry out a comparison between the absorption coefficient predicted by the impedance model of a Microperforated Insertion Unit and the absorption coefficient predicted from a simulated reflection trace taken into account the finite length of the time window.     

In situ measurements of surface impedance and absorption coefficients of porous materials using two microphones and ambient noise

An in situ measurement method is proposed for obtaining the normal surface impedance and absorption coefficient of porous materials using two microphones located close to the material without a specific sound source such as a loudspeaker. Ambient environmental noise that does not excite distinct modes in the sound field is employed as the sound source. Measurements of the normal surface impedance of glass wool and rockwool have been made using this method in various sound fields. The repeatability and wide applicability of the method are demonstrated by comparing results of measurements in one room with different noise conditions and in three other environments (corridor, cafeteria and terrace). The assumed diffuse nature of the sound field on the material is validated by using absorption characteristics obtained experimentally at oblique incidence. This method allows simple and efficient in situ measurements of absorption characteristics of materials in a diffuse field.     

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.