Multiple scattering of an obliquely incident plane acoustic wave from a grating of immersed cylindrical shells

The study of the interaction of acoustic waves with cylindrical structures has numerous applications including the ultrasonic nondestructive testing of materials. In this paper, using a new mathematical model presented for the scattering of obliquely incident plane acoustic waves from a grating of immersed cylindrical shells, a detailed study of the resonant interaction of A-wave resonances originating from the shells is conducted. The nature of A-wave resonances and the effect of center-to-center distance of the shells on these resonances are examined. It is observed that this resonant interaction not only results in the splitting of A-wave resonances, but also causes an increase in resonance amplitudes. This interaction phenomenon is not seen in Rayleigh, whispering gallery and guided wave resonances. It is also shown that increasing the angle of wave incidence to the grating weakens the A-wave resonant interactions. The numerical results obtained from the mathematical model are compared to experimental results available in the literature for gratings composed of two and three aluminum shells. The numerical results are in very good agreement with their experimental counterparts.     

Analysis of coir fiber acoustical characteristics

Coir fiber from coconut husk is an important agricultural waste in Malaysia. Acoustic absorption coefficient of the fiber as a porous material is studied in this paper. Two types of fiber are investigated, fresh from wet market and industrial prepared mixed with binder. Moreover two analytical models, namely; Delany–Bazley and Biot–Allard are used for analysis. Experimental measurements in impedance tube are conducted to validate the analytical outcomes. Results show that fresh coir fiber has an average absorption coefficient of 0.8 at f > 1360 Hz and 20 mm thickness. Increasing the thickness is improved the sound absorption in lower frequencies, having the same average at f > 578 Hz and 45 mm thickness. Delany–Bazley technique can be used for both types of fiber while Biot–Allard method is compensated for the industrial prepared fiber considering the binder additive. This form generally shows poor acoustical absorption in low frequencies. Inevitably, fiber has to be mixed with additives in commercial use to enhance its characteristics such as stiffness, unti-fungus and flammability. Hence other approaches such as adding air gap or perforated plate should be used to improve the acoustical properties of industrial treated coir fiber.     

Space averaging of vibration level differences measured in lightweight building structures

This paper examines space averaging methods applied to structural level differences measured in lightweight building structures (two beam junction and single plate and beam junction). The methods studied include the averaging assuming a constant input force, the averaging based on a constant vibration level across the source element, and the arithmetic averaging of level differences. Tests indicate that a fairly steady input force can be easily produced when using an impulsive source such as a hammer; the constant input force method is then appropriate and can be used to space average level differences (the two beam structure showing a standard deviation of 0.1 dB from the averaging based on levels normalised to the force). On the other hand, acceleration levels can exhibit significant variations with position due to the low mode count and modal overlap of some lightweight elements such as timber beams. Variations in the vibration level across the source element can then be responsible for large inaccuracies in the level difference calculated from the constant level assumption, the two beams’ results showing errors greater than 5 dB at several 1/3 octave band frequencies. The latter averaging method is then inappropriate and should not be used.     

The planar jet-plate oscillator

The aeroacoustic noise generated by a high speed, planar gas jet impinging on a flat plate is investigated experimentally. The jet used in this study is typical of those commonly found in industrial applications such as in various coating control and heat transfer processes. Normal jet impingement on the plate is found to generate strong acoustic tones over a wide range of impingement distances and jet velocities. The characteristics of these tones, as a function of the jet velocity and impingement distance, are quantified. Phase and amplitude measurements of the pressure fluctuations on the impingement plate indicate that the acoustic tones are generated by an antisymmetric instability mode of the jet oscillation. The effect of plate inclination in both the transverse and span-wise directions, with respect to the incident jet, is also studied. The jet-plate tone is found to be much more sensitive to changes in the span-wise plate inclination than to changes in the transverse inclination, but in both cases, a complete suppression of the tone is found to be possible.     

五角芯型保偏光子晶体光纤(Q-PM-PCF)的双折射

提出了一种内层空气孔环含有五个空气孔的新型高双折射光子晶体光纤结构;采用全矢量有限元法进行分析,研究了该光子晶体光纤两正交偏振模的有效折射率和双折射,分别给出了该五角芯型保偏光子晶体光纤双折射随输入光波长和大空气孔半径的变化曲线.分析结果表明:该五角芯型保偏光子晶体光纤的双折射很易达到i0-3量级甚至更高,比传统保偏光纤的双折射至少高出一个数量级,合理设计光纤结构参数,该保偏光纤的双折射在1550 nm处可以达到6.5×10-3以上,甚至更高,适合应用于偏振特性及稳定性要求都较高的实际光纤传感系统,例如光纤陀螺.     

An Exceptional Peroxide Birefringent Material Resulting from d–π Interactions

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb₂VO(O₂)₂F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb₂VO(O₂)₂F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first‐principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O₂^2? anions and V⁵⁺ 3d orbitals, which may be also favorable to the stability in the air for Rb₂VO(O₂)₂F. These findings distinguish peroxides as a brand‐new class of birefringent materials that may possess birefringence superior to the traditional oxides.     

Spectral- and Cepstral-Based Measures During Continuous Speech: Capacity to Distinguish Dysphonia and Consistency Within a Speaker

Spectral- and cepstral-based acoustic measures are preferable to time-based measures for accurately representing dysphonic voices during continuous speech. Although these measures show promising relationships to perceptual voice quality ratings, less is known regarding their ability to differentiate normal from dysphonic voice during continuous speech and the consistency of these measures across multiple utterances by the same speaker. The purpose of this study was to determine whether spectral moments of the long-term average spectrum (LTAS) (spectral mean, standard deviation, skewness, and kurtosis) and cepstral peak prominence measures were significantly different for speakers with and without voice disorders when assessed during continuous speech. The consistency of these measures within a speaker across utterances was also addressed. Continuous speech samples from 27 subjects without voice disorders and 27 subjects with mixed voice disorders were acoustically analyzed. In addition, voice samples were perceptually rated for overall severity. Acoustic analyses were performed on three continuous speech stimuli from a reading passage: two full sentences and one constituent phrase. Significant between-group differences were found for both cepstral measures and three LTAS measures (P < 0.001): spectral mean, skewness, and kurtosis. These five measures also showed moderate to strong correlations to overall voice severity. Furthermore, high degrees of within-speaker consistency (correlation coefficients ≥0.89) across utterances with varying length and phonemic content were evidenced for both subject groups.     

直升机声测设备探测距离估计方法

直升机声测设备用于探测低空飞行的直升机,探测距离是其关键性能。为了精确测定探测距离,可以运用探测结果与机载数据综合分析的方法。此种方法需要较多人员和其它设备,包括机载设备,在设备研制的初期较繁琐。本文提出了一种简便的直升机声测设备探测距离估计方法,此方法不需要使用机载设备,对直升机飞行基本无约束,通过对大量实测数据处理,针对某直升机声测设备该方法的平均误差在7%以内。     

Long distance laser ultrasonic propagation imaging system for damage visualization

Wind turbine blade failure is the most prominent and common type of damage occurring in operating wind turbine systems. Conventional nondestructive testing systems are not available for in situ wind turbine blades. We propose a portable long distance ultrasonic propagation imaging (LUPI) system that uses a laser beam targeting and scanning system to excite, from a long distance, acoustic emission sensors installed in the blade. An examination of the beam collimation effect using geometric parameters of a commercial 2 MW wind turbine provided Lamb wave amplitude increases of 41.5 and 23.1 dB at a distance of 40 m for symmetrical and asymmetrical modes, respectively, in a 2 mm-thick stainless steel plate. With this improvement in signal-to-noise ratio, a feasibility study of damage detection was conducted with a 5 mm-thick composite leading edge specimen. To develop a reliable damage evaluation system, the excitation/sensing technology and the associated damage visualization algorithm are equally important. Hence, our results provide a new platform based on anomalous wave propagation imaging (AWPI) methods with adjacent wave subtraction, reference wave subtraction, reference image subtraction, and the variable time window amplitude mapping method. The advantages and disadvantages of AWPI algorithms are reported in terms of reference data requirements, signal-to-noise ratios, and damage evaluation accuracy. The compactness and portability of the proposed UPI system are also important for in-field applications at wind farms.