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.     

The prediction of airborne sound transmission between two rooms using first-order flanking paths

Airborne sound transmission between adjacent rooms can be predicted using the Standard EN 12354-1 (ISO 15712-1), which is equivalent to a first-order approximation of statistical energy analysis (SEA). This paper analyses airborne sound transmission between adjacent rooms in a masonry building, by comparing results obtained from EN 12354-1 to SEA predictions and measurements. It is shown that the restriction of the Standard to first-order flanking paths can lead to large errors in predictions when compared to measurements and SEA results taking into account all transmission paths. This is observed both for individual flanking paths and overall transmission between rooms, for which the Standard provides results similar to those obtained by the first-order approximation of SEA. The paper also looks at possible reasons why previous studies using the approach in EN 12354 have generally shown good agreement with measurements.     

高真空多层绝热结构中一氧化钯吸氢特性研究

对高真空下的一氧化钯吸附氢气的特性进行了分析和研究,实验测定了一氧化钯吸附氢气的机理,不同条件下一氧化钯对氢气的吸附特性,以及设置一氧化钯对夹层漏放气的影响。实验证明,在夹层中使用恰当的混合吸附剂,可以有效减小夹层的漏放气量,提高低温储罐的寿命。     

均匀电荷密度下的磁绝缘理论研究

在平行板磁绝缘传输线中均匀电荷密度下,分别对压力平衡下的修正层流模型和非压力平衡和非层流模型进行了求解,比较了两种理论模型下的结果。分析表明：相对于非层流模型而言,层流模型因未考虑径向速度,其电荷密度偏低,阳极电场、阳极磁场及阳极电流都偏大。此外,粒子模拟结果显示非层流模型具有更高的精度。最后,对压力平衡条件、均匀电荷密度下的层流以及非层流模型,从平行板理论推广到一般构型。     

Improving the Sound Absorption Properties of Flexible Polyurethane (PU) Foam using Nanofibers and Nanoparticles

Polyurethane foam as the most well-known absorbent materials has a suitable absorption coefficient only within a limited frequency range. The aim of this study was to improve the sound absorption coefficient of flexible polyurethane (PU) foam within the range of various frequencies using clay nanoparticles, polyacrylonitrile nanofibers, and polyvinylidene fluoride nanofibers. The response surface method was used to determine the effect of addition of nanofi- bers of PAN and PVDF, addition of clay nanoparticles, absorbent thickness, and air gap on the sound absorption coefficient of flexible polyurethane foam (PU) across different frequency ranges. The absorption coefficient of the samples was measured using Impedance Tubes device. Nano clay at low thicknesses as well as polyacrylonitrile nanofibers and polyvinyl fluoride nanofibers at higher thicknesses had a greater positive effect on absorption coefficient. The mean sound absorption coefficient in the composite with the highest absorption coeffi- cient at middle and high frequencies was 0.798 and 0.75, respectively. In comparison with pure polyurethane foam with the same thickness and air gap, these values were 2.22 times at the middle frequencies and 1.47 times at high frequencies, respectively. Surface porosity rose with increasing nano clay, but decreased with increasing polyacrylonitrile nanofibers and polyvinyl fluoride nanofibers. The results indicated that the absorption coefficient was elevated with increasing the thickness and air gap. This study suggests that the use of a combination of nanoparticles and nanofibers can enhance the acoustic properties of flexible polyurethane foam.     

Measurement of sound insulation of acoustic louvres by an impulse method

This paper presents results of a study of the sound attenuation of acoustic louvres. At the core of the study is an alternative method of measuring sound insulation, impulse response analysis, which circumvents the limitations imposed by standard and proposed standard methods. Using the impulse method, the sound transmission coefficient is measured at different angles of incidence and the angular dependency of transmission loss obtained. In the low frequency range, the transmission is governed by a mass layer effect. The value of transmission loss is independent of angle of incidence. For the mid and high frequencies, diffraction, interference and absorption determine louvre performance and an angular dependency is observed. The transmission at the angle of incidence, corresponding to a line-of-sight through the louvre blades, is the dominant contribution to the angle average value and a single measurement at the pitch of the louvre approximates the overall transmission loss. For the case considered, the geometry of the blades has little influence on the transmission at low frequencies and the mass of the blades has little influence at higher frequencies. In a companion paper, the impulse data are used to predict the insertion loss provided by the louvre when installed in a plant room.     

Experimental study of the noise emission of personal computer cooling fans

A controlled experimental study of the noise emission of a typical model of computer cooling fan, under different operating conditions, was done. The sound-power levels and directivities of 80-mm-diameter Panaflo computer fans mounted in a test plenum were determined from measurements of sound-pressure level over a hemi-spherical surface above a reflecting plane. The design and testing of the test plenum is discussed. Results of tests performed on one fan unit to gain an understanding of the fan’s noise-radiation characteristics and how they vary with operating conditions are presented. The results show that the noise emission of the Panaflo fan is directly related to the voltage and inversely related to the air-flow rate. It was found that the noise level measured at one particular position corresponds well to the average of those measured at 10 points on the hemisphere. The variability of the noise radiation from these Panaflo fans was determined by testing three other fans; variations of resulting total A-weighted power levels were within 1 dB.     

Analysis of sound absorption by periodic structures using a hybrid-boundary element/mode expansion method

An improved hybrid method is proposed for analyzing sound scattering by a periodic structure. Part of the scattered field formulated with the mode expansion method is combined with other components of the field formulated with the boundary integration method in one period of the structure. Structures treated by this method can have arbitrary periodic forms made of locally reactive boundaries and porous materials. The oblique incident absorption coefficient of the structure is obtained simply from the reflection factor calculated for each elemental wave of the scattered field. The accuracy of the method is demonstrated by the agreement between calculated and measured values of the normal absorption coefficient of some test structures.     

Detection of extraneous abnormal sounds affecting road traffic noise by use of a necessary condition method

When measuring and/or recording road traffic sound levels during a long time interval, extraneous abnormal sounds will inevitably affect the road traffic sound levels of interest. Such sounds include those produced by horns, sirens, animals, construction sites, and the like. The detection and elimination of such extraneous sound requires much time and effort, but are necessary if noise indices such as L_eq, L_max, and L₁₀ are to be properly estimated. This paper proposes a practical detection method of these extraneous interfering sounds by deriving a necessary condition that road traffic sound levels must satisfy. The necessary condition provides an easy method of identifying sound levels not satisfying the condition, and distinguishes them as extraneous abnormal sounds, even in a large volume of observed data. The validity and usefulness of this method are confirmed by application to actually observed data.     

Ocean circumstance effects on the sound scattering of thermocline

I.IntroductionTheformationofoccanthermoc1ineistheresultfromasea-atmosphereintcraction,whichdependsonvariousintensitiesanddepthsofthethermocline,andthedynamictransactionofthermoenergybetweenlayers.Thercfore,thetcmporalandspacialchangcofthethcrmoc1inebecomesanimportantparameterforrcsearchonuppcrboundary1ayerofsea.Thetempera-tUre,salinityandprcssurevarywithdcpth,whichhasformedthcvcrticalprofileofsoundvclooityintheuppcrboundarylaycrsofsca.A1ongthcshallowwatcrareaofChina,thereareobvi-ousnon-unifo…