空气背衬条件下泡沫材料法向吸声系数混合计算方法研究

把空气背衬看作一层材料,通过理论计算获得空气声传递矩阵,与通过实验测量获得的材料声传递矩阵组合,构建实验与计算相结合的混合模型,从而得到空气背衬复合材料总传递矩阵,计算空气背衬条件下泡沫材料的法向吸声系数。以聚乙烯泡沫材料为例,研究不同厚度空气背衬对泡沫材料法向吸声系数的影响,并与阻抗管中对应的实测结果相比较,两种方法所得结果基本吻合。随着空气背衬厚度的增加,泡沫材料的吸声性能有所提高,并且有效吸声范围向低频区移动。混合计算方法可为空气背衬条件下泡沫材料法向吸声系数的测量研究提供便利有效的手段。     

PVDF弯曲圆盘水听器背衬材料的选择

本文介绍了PVDF弯曲圆盘水听器背衬材料的基本要求。实验表明,聚碳酸酯是一种良好的背衬材料。     

驻波管中测量声学材料的隔声量

在驻波管中用双传声器法测量声学材料的隔声性能。通过测量不同背衬条件（即待测样品的后表面阻抗）下对应的前表面阻抗,可以求解与传递矩阵四个元素相关的中间变量,即可计算声学材料的隔声量。在驻波管中进行了单层多孔均匀材料、双层和三层复合非均匀材料的隔声性能测量,通过与四传声器法直接测量结果的对比,验证了采用双传声器法间接测量声学材料的隔声量是可行的。     

含有空气背衬层的分层多孔材料的吸声性能研究

根据声波在介质中的传播规律,计算了声波垂直入射到含有空气背衬层的分层多孔材料吸声结构的吸声系数。以含有空气背衬层的双层泡沫铝结构为例,研究了各层泡沫铝的设计参数和空气背衬层厚度变化对吸声结构吸声系数的影响规律。研究表明:随着各层孔隙率增加、或厚度增加、或流阻率增加,双层泡沫铝空气背衬层吸声结构的吸声系数逐渐增大;在低频段增加空气背衬层厚度,吸声系数增大,且最高吸声系数表现出向低频迁移的趋势;在中频段,当增加各层孔隙率或流阻率时,没有空气背衬层的双层泡沫铝吸声结构则呈现出更好的吸声性能。合理调整各层材料的设计参数,可在较宽频段上达到满意的吸声效果。     

厚度振动换能器脉冲响应的研究

本文对厚度振动图片换能器的导纳、传递函数和脉冲响应进行了研究。利用换能器等效电路、基尔霍夫定律和傅里叶变换给出了导纳、传递函数和脉冲响应的表达式。通过在ＨＰ—１０００微机上进行大量的计算，得到了ＺＮＬ—２压电陶瓷圆片换能器在不同匹配层和背衬层材料的导纳，传递函数和脉冲响应，得出了匹配材料声阻抗分别为４， ６（ １０６ｋｇ／ｍ２· ｓ）和背衬材料声阻抗分别为 ７．５， １５．５（１０６ｋｇ／ｍ２·时左右比较理想，然后用ＨＰ阻抗分析仪、Ｘ—Ｙ３０３３记录仪和脉冲响应测试仪分别测出几个换能器的导纳和脉冲响应，从实验结果可知：理论和实验结果基本符合。所得结果可供工厂生产时参考。     

高切向流速高声强条件下梯度阻抗吸声超材料研究

在涡扇航空发动机声衬表面，存在高速切向流场与高声强声场耦合问题，吸声机理与调控机制复杂。高切向流速高声强条件下低频宽带噪声的有效吸收极具挑战性。该研究提出考虑高切向流速、高声强以及复杂容抗修正的声阻抗理论方法，设计了梯度阻抗调控吸声超材料。从理论解析计算、有限元数值建模及试验测试三个维度深入分析声波与结构的作用机理和调控规律。结果表明：梯度阻抗吸声超材料可有效提高吸声效率，避免传统声衬吸声频带窄的缺点。在0、30 m/s、60 m/s、98 m/s切向流速和130 dB背景噪声下，提出的梯度阻抗吸声超材料在500～3 000 Hz范围内具备良好吸声效果。该超材料结构简单，在高切向流速高声强复杂边界条件下，仍有深亚波与宽带吸声特性，具有广阔的应用前景。     

圆管超声换能器响应的激光超声检测

文章着重叙述一种用激光超声检测圆管超声换能器响应的方法。为激励和接收轴对称声场，采用压电圆管作为压电换能元件，并分别用环氧、环氧加钨粉为背衬，制作了两种压电圆管换能器。由激光超声方法和自发自收脉冲反射法实验测定了所制作的换能器的频率响应，并比较了频谱分析结果。实验结果不仅在一定程度上说明了两种背衬的效果，同时也表明激光超声检测换能器响应的可行性。     

背衬对计算机配色精度的影响

在具有一定不透明度的纸张上建立配色数据库及预测配方时,选取不同的背衬会得到不同颜色测量结果,影响配色模型的参数,进而得到不同的配色精度。因此,为保证配色的准确性,需规范测色背衬的选取。在计算机配色过程中,使用符合ISO 13655—2017规定的常用标准黑背衬和标准白背衬进行对比研究,分析在黑、白不同背衬下,几种常见印刷介质上的配色精度。研究结果表明,在3种介质上配方与目标色的平均色差,垫黑时为2.575,垫白时为3.553,因此计算机配色在标准黑背衬条件下展现出更加优越的配色精度,建议在配色过程中选用黑色背衬。     

带背衬压电振子的灵敏度频率特性分析

1 前言 G.Kossoff论述了具有背衬的压电陶瓷振子的灵敏度频率特性,本文则对于使用PVDF之类高分子压电材料的场合也作了计算,研究了它和陶瓷振子在特性上的不同。另外,Kossoff设定在背衬上没有反射,而本文则在Mason等效电路中插入衰减,对含有反射时的特性进行了分析。本文分析的是接收及发射灵敏度。2 接收电压灵敏度Mo     

水下覆盖层结构的声学特性分析

根据波动理论应用分层分析的方法得到了不同入射角度、不同背衬情况时声波在水下覆盖层结构中的吸声、反射系数以及透射损失等,并数值计算了不同入射角度下三种背衬条件下的透射损失,分析表明:声波垂直入射时透射损失性能最差;水背衬时水下覆盖层结构的透射损失低频时很小,中高频时急剧增加;单壳背衬低频时壳体本身起主要的吸收作用;中高频时水下覆盖层结构的吸收作用逐渐明显;双层壳背衬时其透射损失频响曲线出现谐振峰,削弱水下覆盖层结构的声学性能,总体来说,敷设非均匀结构要优于敷设均匀结构。     

A novel ultrasonic transducer backing from porous epoxy resin–titanium–silane coupling agent and plasticizer composites

In this paper, a new composite for ultrasonic attenuation backing has been successfully fabricated from porous epoxy resin containing titanium (Ti), silane coupling agent and plasticizer composites. The effect of Ti particles on the network structure and mechanical properties of epoxy resin has been analyzed in detail. The ultrasonic parameters in epoxy composites have been measured by a conventional pulse-echo-overlap technique at a frequency of 1–5 MHz. The effect of Ti content and temperature on the longitudinal sound velocity and attenuation of epoxy resin composites were investigated. Precise in situ observations of the acoustic properties such as attenuation and acoustic impedance of epoxy composites are expected to be useful for ultrasonic transducer systems for new as well as for backing application with high attenuation.     

Acoustic properties of particle/polymer composites for ultrasonic transducer backing applications

The acoustic impedance and attenuation in composites made of particle fillers loaded in polymer matrices for transducer backing applications is investigated. The acoustic impedance of tungsten/vinyl composites was modeled, and an experimental matrix identifying variables that contribute to composite attenuation was established. The variable included the particle type, the particle size and volume fraction of a filler, the physical characteristics of the polymer matrix, and the processing route that determined the composite connectivity. Experimental results showed that with an increase in filler particle size or a decrease in volume fraction of filler, there is an increase in composite attenuation. Overall, the various types of filler, the polymer matrix, and the interface between the two contribute to attenuation in the composite, as confirmed by the acoustic properties and the microstructural analysis.     

Novel multi-layer polymer-metal structures for use in ultrasonic transducer impedance matching and backing absorber applications

This paper presents a novel design principle for designing multilayer polymer-metal structures which are well suited for front surface impedance conversion (matching) and for back surface acoustic absorption. It is shown that a polymer layer with an outer metal layer, when loaded by a low impedance propagation medium, acts as an efficient impedance converter. The resulting impedance seen at the inner polymer surface is increased and the structure provides the same performance as a traditional quarter-wavelength matching layer. Experimental evidence is also shown for a double-matching scheme for a lead zirconate titanate (PZT) transducer using an inner polymer-metal multilayer and an outer polymer quarterwavelength layer, resulting in a 55% bandwidth at 2.6 MHz with air backing. Also, it is theoretically shown that multiple layers of a lossy polymer adhesive-metal structure produce low propagation velocity and high absorption. Experimental proof of this ultrasonic multilayer backing absorber is provided. Design theories based on both a simplified mass and spring model and a rigorous one-dimensional wave model have been developed and show fair agreement.     

Study on the water flooding in the cathode of direct methanol fuel cells

Water flooding phenomena in the cathode of direct methanol fuel cells were analyzed by using electrochemical impedance spectroscopy. Two kinds of commercial gas diffusion layers with different PTFE contents of 5 wt% (GDL A5) and 20 wt% (GDL B20) were used to investigate the water flooding under various operating conditions. Water flooding was divided into two types: catalyst flooding and backing flooding. The cathode impedance spectra of each gas diffusion layer was obtained and compared under the same conditions. The diameter of the capacitive semicircle became larger with increasing current density for both, and this increase was greater for GDL B20 than GDL A5. Catalyst flooding is dominant and backing flooding is negligible when the air flow rate is high and current density is low. An equivalent model was suggested and fitted to the experimental data. Parameters for catalyst flooding and backing flooding were individually obtained. The capacitance of the catalyst layer decreases as the air flow rate decreases when the catalyst flooding is dominant.     

Filled and unfilled temperature-dependent epoxy resin blends for lossy transducer substrates

In the context of our ongoing investigation of low-cost 2-dimensional (2-D) arrays, we studied the temperature- dependent acoustic properties of epoxy blends that could serve as an acoustically lossy backing material in compact 2-D array-based devices. This material should be capable of being machined during array manufacture, while also providing adequate signal attenuation to mitigate backing block reverberation artifacts. The acoustic impedance and attenuation of 5 unfilled epoxy blends and 2 filled epoxy blends - tungsten and fiberglass fillers - were analyzed across a 35degC temperature range in 5degC increments. Unfilled epoxy materials possessed an approximately linear variation of impedance and sigmoidal variation of attenuation properties over the range of temperatures of interest. An intermediate epoxy blend was fitted to a quadratic trend line with R² values of 0.94 and 0.99 for attenuation and impedance, respectively. It was observed that a fiberglass filler induces a strong quadratic trend in the impedance data with temperature, which results in increased error in the characterization of attenuation and impedance. The tungsten-filled epoxy was not susceptible to such problems because a different method of fabrication was required. At body temperature, the tungsten-filled epoxy could provide a 44 dB attenuation of the round-trip backing block echo in our application, in which the center frequency is 5 MHz and the backing material is 1.1 mm thick. This is an 11 dB increase in attenuation compared with the fiberglass-filled epoxy in the context of our application. This work provides motivation for exploring the use of custom-made tungsten-filled epoxy materials as a substitute PCB-based substrate to provide electrical signal interconnect.     

On the drop-weight testing of alumina/aluminum laminated composites

Laminated composites with ceramic front layers and metallic or composite backing layers have gained attractiveness as lightweight armours, as they exhibit the same ballistic performance with lower areal densities as compared to steels. Drop-weight testing (DWT) has potential for evaluating the low velocity impact behaviour of materials. This testing gives significant ideas and information about failure mechanisms and behaviour of materials under low velocity impact. In this study, DWT of alumina/aluminum laminated composites was done in order to investigate the effects of lamination type, density with respect to area and mechanical property of backing material on the low velocity ballistic performance of these composites. The experimental results showed that the laminated composite with ceramic front layer and aged-aluminum alloy as backing layer was the most effective among different investigated specimens against low velocity impact loads.     

Calculation of dissipation resistances in a single-element transducer

In this paper, a new formulation of the electrical input impedance of a single element transducer is presented. The resistive part of the electrical impedance that takes into account acoustic radiation in the front medium and losses in the transducer is split into a radiation resistance on one hand and into dissipation resistances related to each transducer component on the other hand. To confirm these theoretical results, characterization methods based on temperature measurements and pulse-echo response are presented. Measurements have been conducted on 1 MHz transducers, which consist of a piezoelectric ceramic glued on a backing. The results show a good agreement between experience and theory for dissipation resistance and radiation resistance values, which confirms the theoretical approach.     

Low field ac study of PZT/PVDF nano composites

Composites of nanocrystalline Pb_0.96Sr_0.04(Zr_0.53,Ti_0.47)O₃ (PZT) and α-phase PVDF have been developed using solution casting technique. Characterization of the composites has been done using XRD, FEGSEM, DSC and impedance analysis. XRD and FEGSEM determined the size range of PZT as 22–40 nm. XRD shows the successful incorporation of PZT into PVDF matrix and also confirms that no new phase is developed. DSC of the nanocomposites showed decrease in crystallinity with increasing PZT content. Broadband impedance analysis has been carried out to study the effect of the addition of PZT on the low field ac electrical properties of PVDF. Room temperature dielectric permittivity measurement of the PZT-PVDF composites at 1 kHz determined using impedance analyzer gives values of permittivity 2–4 times higher as compared to neat PVDF. It is found that dielectric permittivity values at the lower frequency edge are affected by space charges while the higher frequencies show the influence of relaxation effects in the materials. It is suggested that PZT/PVDF composites are the preferred materials for high temperature and high frequency applications. However, for low frequency use at higher temperatures, these composites do not offer any specific advantage. At room temperature, the composites are again the better choice in the 1 mHz–1 MHz frequency range.     

A 2.5 MHz 2D array with Z-axis electrically conductive backing

The design, fabrication and initial testing of a prototype fully λ/2 sampled, 2500 element 2D phased array is presented. The array utilizes a unique Z-axis electrical conductivity backing layer to provide both acoustic attenuation and electrical interconnect for the signal channels. The electrical interconnect is designed to be in the acoustic shadow of the transducer elements so as to minimize the foot print of the array. A modular, demountable Pad Grid Array interconnect is used to connect to the backing of the array. Results are presented for measurements of the single element properties of electrical impedance, pulse echo waveform and spectrum, directivity, and cross talk