Reverberation time measurements in non-diffuse acoustic field by the modal reverberation time

The increasing presence of low frequency sources and the lack of acoustic standard measurement procedures make the extension of reverberation time measurements to frequencies below 100 Hz necessary. In typical ordinary rooms with volumes between 30 m³ and 200 m³ the sound field is non-diffuse at such low frequencies, entailing inhomogeneities in space and frequency domains. Presence of standing waves is also the main cause of bad quality of listening in terms of clarity and rumble effects. Since standard measurements according to ISO 3382 fail to achieve accurate and precise values in third octave bands due to non-linear decays caused by room modes, a new approach based on reverberation time measurements of single resonant frequencies (the modal reverberation time) has been introduced. From background theory, due to the intrinsic relation between modal decays and half bandwidth of resonant frequencies, two measurement methods have been proposed together with proper measurement procedures: a direct method based on interrupted source signal method, and an indirect method based on half bandwidth measurements. With microphones placed at corners of rectangular rooms in order to detect all modes and maximize SNRs, different source signals were tested. Anti-resonant sine waves and sweep signal turned out to be the most suitable for direct and indirect measurement methods respectively. From spatial measurements in an empty rectangular test room, comparison between direct and indirect methods showed good and significant agreements. This is the first experimental validation of the relation between resonant half bandwidth and modal reverberation time. Furthermore, comparisons between means and standard deviations of modal reverberation times and standard reverberation times in third octave bands confirm the inadequacy of standard procedure to get accurate and precise values at low frequencies with respect to the modal approach. Modal reverberation time measurements applied to furnished ordinary rooms confirm previous results in the limit of modal sound field: for highly damped modes due to furniture or acoustic treatment, the indirect method is not applicable due to strong suppression of modes and the consequent deviation of the acoustic field from a non-diffuse condition to a damped modal condition, while standard reverberation times align with direct method values. In the future, further investigations will be necessary in different rooms to improve uncertainty evaluation.     

Quantitative temperature field measurements on a non-gray multi-materials scene by thermoreflectometry

This article addresses the problem of measuring an accurate temperature field on a multi-materials scene composed of two dielectric and one metallic materials. The measurements of thermo-radiative properties demonstrate that the scene exhibit very different emissivity spectra and thermal conductivities inducing high thermal gradients. From these radiative properties, the calculation of the theoretical temperature error of conventional passive methods highlights that a method may be suitable for measuring only one material but no method provides satisfactory measurements of the sets of materials. The proposed method, called thermoreflectometry, performs a simultaneous measurement on all materials thanks to an on-line indirect determination of emissivity based on a bidirectional reflectivity measurement. Its temperature error is compared to that of the selected passive method for each material through an experimental validation on the multi-materials scene. These results highlight the accuracy of thermoreflectometry and shows opening prospects for the on-line temperature control of dynamical processes.     

目标表面发射率对红外热像仪测温精度的影响

介绍了红外热像仪测温原理,分析了影响红外热像仪测温精度的因素,计算了不同表面发射率下红外热像仪的测温误差曲线。理论分析表明,目标表面发射率越高,红外热像仪测温精度越高。实验改变表面发射率的设置,计算了不同表面发射率对应的总辐射亮度,得到TP8型长波红外热像仪能够精确测温时,目标表面发射率必须大于0.5的结果。最后,对表面发射率分别为0.96、0.93和0.3的3种材料进行实际测温,结果表明,材料表面发射率较高时,红外热像仪具有较好的测温精度。     

目标表面发射率对红外热像仪测温精度的影响

介绍了红外热像仪测温原理,分析了影响红外热像仪测温精度的因素,计算了不同表面发射率下红外热像仪的测温误差曲线。理论分析表明,目标表面发射率越高,红外热像仪测温精度越高。实验改变表面发射率的设置,计算了不同表面发射率对应的总辐射亮度,得到TP8型长波红外热像仪能够精确测温时,目标表面发射率必须大于0.5的结果。最后,对表面发射率分别为0.96、0.93和0.3的3种材料进行实际测温,结果表明,材料表面发射率较高时,红外热像仪具有较好的测温精度。     

AN ANALYTICAL INVESTIGATION OF THE DIRECT MEASUREMENT METHOD OF ESTIMATING THE ACOUSTIC IMPEDANCE OF A TIME-VARYING SOURCE

This paper presents an analytical investigation of the direct method of measurement of the source impedance of a linear time-variant source. The direct method yields a frequency-dependent effective source impedance which is routinely used in a time-invariant analysis to determine the insertion loss of two different acoustic loads applied to the same source. In such an analysis the strength of the source is assumed to be invariant with load. It is shown here that there is generally no precise correspondence between the effective source impedance as given by the direct method and the characteristics of the actual source. Furthermore, it is shown that the effective source impedance values given by the direct method are functions of the acoustic load and the location of the injected signal as used in the measurement. However, the effective source resistance is always found to be positive, in accordance with experimental measurements. In this regard the direct method is an improvement on the indirect method, where physically implausible negative resistance values are often found. Finally, it is shown that the effective impedance values as given by the direct method when used with a time-invariant analysis give rise to very accurate predictions of insertion loss, even when the strength of the actual time-variant source is allowed to vary with the acoustic load.     

Une nouvelle méthode calorimétrique périodique de mesure de l'émissivité des matériaux opaques à température ambiante

This paper presents a new calorimetric periodic technique for measuring emissivity of opaque materials at room temperature, without surface temperature measurement. The sample is in a vacuum chamber, so the thermal losses are only radiative. The presented technique requires thermal modulation of one side of the sample (the front side). The measured signals are the sample's front side temperature and the infrared flux of the other side (back side). Experimental data and a heat conduction model are compared, yielding the Biot number which characterizes the thermal losses. Using the identified value of the Biot number, it is possible to access the total hemispherical emissivity. Measurements have been carried out on a PVC sample (5 mm thick) coated with black paint and on a PVC sample coated with aluminium paint. The results are concordant with the expected ones ; the repeatability error is about 3 %.     

基于激光旋转加热的非导电材料高温光谱发射率测试方法与装置

光谱发射率是表征材料热物理性能的重要参数。对于非导电材料的高温光谱发射率测试,一般采用高温加热炉加热或辐射加热的方式来进行发射率测试,存在的问题是采用高温石墨炉加热时,样品可能会与高温石墨发生化学反应,从而破坏材料原有物性;采用辐射加热,一般是单向静止加热,会存在样品温场梯度非均匀分布的问题。基于激光旋转加热和样品/黑体整体一体化设计,提出了一种“样品动中测”的非导电材料高温光谱发射率测试新方法,建立了相应的测量模型,突破了传统的 “样品静中测”的局限,样品与参考黑体共形一体化设计,采用微区域光谱辐射成像方法,同时测量参考黑体和样品的光谱辐射能量与温度。建立了激光旋转加热状态下的热传导方程,对典型样品材料的温度分布进行了仿真计算,结果表明旋转样品温场分布较为均匀,分析了温场分布与红外光谱发射率测量误差间的关系,给出了适用于本测试方法的材料的热导率下限值。基于该方法,搭建了相应的测量装置,对典型材料碳化硅在1 000 K时的光谱发射率进行了测试,在4 μm处对各个典型高温温度点的光谱发射率进行了测试,得到了碳化硅材料在红外波段的光谱发射率波长变化和温度变化规律特性。与国外的测量结果进行了比对,结果较为一致,验证了激光旋转加热光谱发射率测试方法的可行性。采用此方法,不破坏样品本身的理化特性,样品加热升温速度快,测量温度范围上限高,有效减小了激光静止单向加热带来的温度不均匀性,可同时测量出样品和参考黑体的光谱辐射亮度及温度,无需另外再设计标准高温黑体,解决了现有非导电材料高温光谱发射率测试中非均匀加热和辐射能量同步比对测量的问题,可应用于多种非导电材料高温光谱发射率的测试。     

Multiwavelength infrared pyrometry: optimization and computer simulations

We present here a preliminary study concerning a multi-wavelength pyrometry method. This technique may be useful for temperature measurements on materials of unknown, selective, rapidly varying emissivity. From a (simultaneous) measurement of the thermal emission at N wavelengths, a fitting procedure leads to the simultaneous identification of the temperature and the spectral emissivity. We present first a comprehensive discussion of the various parameters influencing the accuracy of the method. Then a number of simulated experiments are calculated in order to test the efficiency of various emissivity models and to estimate the robustness of the method. It appears that measurements in the (thermal) infrared spectral range may yield satisfying temperature measurements down to T = 600 K.     

Non-contact charge temperature measurement on industrial continuous furnaces and steel charge emissivity analysis

Continuous furnaces are commonly used for steel billet reheating before a rolling operation. It is necessary to perform a number of measurements to set-up and operate the optimization system of the furnaces correctly. A charge temperature measurement using infrared detectors can be one of the usable measurement techniques. This non-contact measurement method is based on the detection of infrared radiation emitted from a measured surface. The radiation intensity depends on the surface temperature and emissivity, which is one of the most important parameters for infrared measurements. Advantages of the non-contact temperature measurement, as well as some problems regarding the surface emissivity, are presented. The direct steel billet temperature measurement procedure using infra-red detectors, emissivity determination procedures, and example results are introduced. It is shown that steel emissivity can vary from approx. 0.17 to 0.8, depending on the surface state, scale formation, and wavelength interval. These problems are critical for the charge temperature measurement using the infra-red detectors, and are discussed in this paper.     

发射率限定辐射测温原理

辐射测温技术随着辐射测量传感器技术的进步而不断进步,已经由单波长测温发展到多波长和多波段测温,由点温测量发展到二维甚至三维温度场测量。但是在辐射测温更精确反演方面,却很难克服因发射率未知性而引起的模型构建误差。发射率行为难以确定并极大地影响了测温精度,急需发展一种具有通用性,不受发射率具体行为限制,具有较高稳定性的辐射测温方法。双波长测温适用于发射率具有灰体行为的物体温度测量,一系列的发射率补偿算法和波长选择方法均未能很好地实现通用性测量,往往直接单色测量可能误差比比色法更小。多波长测温得到广泛应用,但并不是波长越多越好,发射率模型仍然具有较大局限性。提出了发射率直接限定算法和发射率松驰限定算法来反演温度。在发射率限定条件相同时,这两种方法是等价的。发射率松驰限定算法基于最小二乘算法和松驰因子进行真温求解。推导了松驰限定法的误差传递公式,发现在保证测量信号强度的前提下,λT越小温度误差越小;发射率行为对温度相对误差具有重要影响,在相同的λT条件下,发射率随波长变化越大,在限定区间上覆盖越均匀,测量误差越小。但从直接限定算法可以看出所测波长数越多,测量误差越小。两种方法均可以看出,减少限定区间长度也可以显著地提高测量精度。     

Thermal conductivity: A review of some important developments

Some recent developments are presented which relate to the thermal conductivity of metals and its measurement. These include the influence of sample size on the electronic thermal conductivity of very pure metals, the thermal conductivity minimum of aluminium and a few other metals at sub-normal temperatures, the high-temperature thermal conductivity and increasing Lorenz function of platinum, with particular reference to the experiments of Flynn and O'Hagan (1967), the thermal conductivity of molten metals and the recently reported Lorenz functions decreasing with increase in temperature to well below the theoretical value, and the encouraging results of an investigation in progress at the Thermophysical Properties Research Center (TPRC) on direct electrical heating methods for the measurement of metallic thermal conductivities to high temperatures. Modern computer techniques avoid the restrictions and approximations introduced in the many existing methods and allow thermal conductivity to be accurately evaluated from the observed temperature profile. This method has the additional advantage that many other properties can be obtained to high temperatures for the same sample and experimental conditions. The account shows that despite the amount of effort already devoted to thermal conductivity determinations, this still remains a most rewarding field requiring further accurate measurements.     

Infrared technique for simultaneous determination of temperature and emissivity

This paper presents, in the context of materials dynamic behaviour study, a method for simultaneous measurement of the temperature and emissivity of a solid’s surface, by the use of infrared radiation. In contrast to existing methods, this method has no need for a pre-measurement of the surface emissivity. The emissivity and the temperature are measured simultaneously, by detecting the variations of emitted radiation and infrared radiation reflecting on the surface, at two different spectral zones. In this way, the accuracy of the measured temperature is greatly improved in cases were the surface optical properties vary during the measurement. Several experiments were carried out in order to complete the theoretical foundation of the method and to outline its accuracy and some of its limitations. There are various industrial applications of this method, for example the control of the temperature of the mechanical parts during work machining. One of them may be the measurement of the temperature of a sample during mechanical testing. An application of the method is proposed, that is easy to employ with non-sophisticated infrared and optical components. The results confirm the accuracy of the proposed method with an order of 3% of precision for temperature determination.     

基于发射率模型约束的多光谱辐射真温反演法

温度测量是工业生产或科学实验中保证产品质量、降低生产成本和确保实验安全的重要因素之一。目前非接触的测温方法以辐射测温法为主,二次测量法是多光谱辐射测温中一种常用的方法。但是,二次测量法不适用于实时数据处理。针对此问题,基于多光谱亮度温度数学模型引入了发射率模型约束条件,提出了一种多光谱辐射真温快速反演法。对于非黑体,根据不同波长下的亮度温度的关系,得出当亮度温度在一个区间内是增函数或者常数函数时,发射率在该区间内是增函数;当亮度温度在一个区间内是减函数时,则发射率在该区间内满足一个关于发射率和波长的不等式。该发射率模型约束条件根据亮度温度的信息,将发射率假设值的构建由多类减少到一类,避免了不必要的发射率的构建。实验分别采用实际发射率随波长单调下降、单调上升、先下降再上升、先上升再下降和随机变化的具有代表性的五个被测目标,针对两个被测温度点进行了仿真对比分析。结果表明,与二次测量法相比,对于同一个被测目标,在相同的温度初值和相同的发射率搜索范围下,新算法在保证精度的情况下,不仅所得结果相同,而且处理速度提升了19.16%～43.45%。     

Emissivity measurements of 3D photonic crystals at high temperatures

An accurate methodology is presented to measure photonic crystal emissivity using a direct method. This method addresses the issue of how to separate the emissions from the photonic crystal and the substrate. The method requires measuring two quantities: the total emissivity of the photonic crystal–substrate system, and the emissivity of the substrate alone. Our measurements have an uncertainty of 4% and represent the most accurate measure of a photonic crystal's emissivity. The measured results are compared to, and agree very well with, the independent emitter model.     

IRS400型材料发射率测试装置的研制

为了准确评估红外材料和涂层的隐身性能,研制了一套IRS400型材料发射率测试装置,主要用于温度范围（50～400）℃,光谱范围（3～5）m和（8～12）m的固体不透明材料和涂层定向发射率测量。给出IRS400型材料发射率测试装置的技术指标,阐述其工作原理,IRS400的光学系统采用全反射式设计,探测器选用钽酸锂热释电探测器,采用50 ℃～1 000 ℃黑体辐射源标准装置对黑体发射率B（1,2）进行标定。通过解决标定和环境温度补偿等关键技术,确保发射率测量不确定度小于2%(k=2)。     

Determining the temperature of an opaque object by its thermal radiation spectrum: forms of initial data presentation and methods

Spectral distributions of intensities, relative emissivity, and inverse radiance temperatures of an opaque, free-radiating object in a condensed state are used as the initial data. The methods of determining the thermodynamic (true) temperature corresponding to these distributions, when object emissivity is previously unknown, are considered. The advantages and disadvantages of each method and corresponding form of an initial data presentation are discussed. It is shown that the spectral distribution of inverse radiance temperatures gives the greatest information about the true temperature and emissivity of the measured object. The estimates of the temperature range to which the true temperature belongs are given based on the known experimental data for tungsten. The methods for additional verification of reliability of the obtained results are presented.     

Infrared Reflectance and Emissivity Spectra of Nanodiamonds

Nanosized diamonds obtained by detonation of explosives are the subject of active studies in several scientific and technological fields. Besides their interesting peculiarities for solid-state physics studies, their presence in various circumstellar environments has been inferred from infrared (IR) spectra. Nanodiamonds are unique materials, allowing investigation of size-dependent properties of matter. We report measurements of IR properties of detonation nanodiamonds in reflectance at room temperature and in emissivity in a dedicated setup at temperatures up to 873 K. The spectra are characterized by the presence of sharp bands due to surface-bound functional groups. Thermal desorption of oxygen-containing groups leads to corresponding spectral changes. The maximal emissivity of nanodiamond powder reaches 0.985.     

Measurement of thermophysical properties by a pulse-heating technique

A technique is described for the dynamic measurement of selected thermophysical properties of electrically conducting solids in the temperature range from 1100 K to the melting point. Based on rapid resistive self-heating of the specimen from room temperature to any desired high temperature in several seconds by the passage of an electical current pulse through it, this technique measures the pertinent quantities such as current, voltage, randiance temperature, with sub-millisecond time resolution. The pulse-heating technique is applied to strip specimens. The radiance temperature is measured by high-speed pyrometry,normal spectral emissivity of the strips is measured by integrating sphere reflectometry. The normal spectral emissivity results are used to compute the true temperature of the specimens. The heat capacity,electrical resistivity, total hemispherical emissivity are evaluated in the temperature range from 1100 K to the melting point.     

A method to retrieve blackbody temperature errors in the two points radiometric calibration

Measurements of the atmospheric thermal emission using an infrared Fourier transform spectrometer need to be radiometrically calibrated. As for several existing instruments (IASI-MetOp, IASI-balloon, NAST-I, AERI, etc.), the two points radiometric calibration is generally performed using two well-characterized blackbodies at two different temperatures (or one blackbody and the cold space), which allow to determine the response and the self-emission of the instrument. To perform an accurate radiometric calibration, the emissivity and the temperature of each blackbody should be known accurately. The blackbody temperatures are chosen to fit the characteristics of the instrument. The measurement of these temperatures is essential. This paper proposes a method to perform an accurate radiometric calibration even when one of the two blackbodies temperatures is not perfectly measured, by numerically retrieving this erroneous temperature.     

Investigation of the Spectral Characteristics of Heat Radiation of a Heated Gaseous Medium for Remote Determination of Its Temperature

A method for processing spectral emissivity data of carbon monoxide and dioxide obtained by pyrometry methods is suggested to estimate the kinetic temperature of a gas volume. The radiative temperatures obtained with allowance for the spectral dependence of the emissivity of a mixture of the above-mentioned gases calculated by the direct line-by-line method are taken as the input data. The procedure is based on detecting spectral intervals with minimum and maximum temperature dependences of the emissivity, so that the relative difference between the corresponding radiative temperatures is unambiguously related to the kinetic temperature.