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Dual-band bandstop filters using open-circuited stub-loaded resonators are presented. The first and second resonant frequencies of the opencircuited stub-loaded resonator can be flexibly controlled by tuning the length and position of the open-circuited stub. To illustrate the concept, a dual-band bandstop filter has been designed, fabricated and measured. The prototype of the dual-band bandstop filter achieved attenuation of 42 and 44 dB at 2.4 and 5.2 GHz. [PUBLICATION ABSTRACT]

The SC EXCELL (Stratiform/Convective EXponential CELL) model, originally devised for the prediction of the complementary cumulative distribution function of rain attenuation (henceforth P(A)) on Earth-space radio links, is tested here in its ability to predict also the P(A) on terrestrial links. Thanks to its physical soundness in representing the rainfall environment, the proposed method does not require any calibration procedure on existing data, as is typically the case of most of the semi-empirical models proposed so far. When tested against the global DBSG3 database made available by the International Telecommunication Union - Radiocommunication Sector (ITU-R), the proposed model shows an improvement in the prediction accuracy with respect to the method currently recommended by the ITU-R. [PUBLICATION ABSTRACT]

This paper proposes a type of substrate integrated waveguide (SIW) 'corner cavity'. For each corner cavity, two transmission zeros (TZ) can be introduced and the location of either TZ can easily be controlled by adjusting suitable geometrical parameters. Using the corner cavities, a novel SIW filter with wide stopband characteristic is implemented. The central frequency of the filter is 20 GHz, and the first spurious appears at about 40 GHz, which is two times the central frequency. In addition, the measured stopband attenuation of the filter is better than 50 dB from 21.24 to 37.71 GHz.

This study addresses the issue of robust H^sub ∞^ control for a class of uncertain Takagi-Sugeno fuzzy systems with distributed time delay and non-linear perturbations. By employing a new Lyapunov-Krasovskii functional together with linear matrix inequality (LMI) technique, a new set of delay-dependent conditions are derived to achieve the robust stabilisation of uncertain fuzzy systems for a prescribed disturbance attenuation level γ > 0. The state feedback H^sub ∞^ control law can be obtained by solving the corresponding LMIs. Further, some numerical examples are provided to demonstrate the effectiveness of the obtained results. The results reveal that the proposed theory significantly improve the allowable upper bounds of the delays over some existing results. [PUBLICATION ABSTRACT]

The traditional acoustic attenuation coefficient is derived from an analogy of the attenuation of an electromagnetic wave propagating inside a non-ideal medium, featuring only the attenuation of wave propagation. Nonetheless, the particles inside viscous solids have mass, vibrating energy, viscosity, and inertia of motion, and they go through transient and damping attenuation processes. Based on the long-wavelength approximation, in this paper, we use the energy conservation law to analyze the effect of the viscosity of the medium on acoustic attenuation. We derive the acoustic attenuation coefficient by combinations of the dynamical equation of a solid in an acoustic field with conventional longitudinal wave propagation under a spring oscillator model. Considering the attenuation of propagating waves and the damping attenuation of particle vibration, we develop a frequency dispersion relation of phase velocity for the longitudinal wave propagating inside viscous solid media. We find that the acoustic impulse response and vibrational system function depends on the physical properties of the viscous solid media and their internal structure. Combined with system function, the impulse response can be an excellent tool to invert the physical properties of solids and their internal structures. We select a well-known rock sample for analysis, calculate the impulse response and vibrational system function, and reveal new physical insight into creating acoustic attenuation and frequency dispersion of phase velocity. The results showed that the newly developed acoustic attenuation coefficients enjoy a substantial improvement over the conventional acoustic attenuation coefficients reported in the literature, which is essential for industrial applications; so are the dispersion characteristics.

This paper presents an experimental study on the mechanism of interaction between a cavitation bubble and an air bubble. The cavitation bubble was generated by means of the low-voltage discharge method, and the combination of high-speed photography and a pressure measurement system allowed for simultaneous observation and measurement of the evolution of the shock wave and the change in shock wave strength with the presence of the air bubble in the vicinity. The high-speed imaging revealed the predominant roles of the relative distance φ and relative size ε between the cavitation and air bubbles in the determination of the stratification effect that the air bubble exerted on the shock wave produced from the first collapse of the cavitation bubble. The pressure measurement indicated that, when the air bubble did not merge with the cavitation bubble, the aforementioned factors, together with the angle $\\alpha $ formed by the air bubble, cavitation bubble and the measuring point, would jointly affect the attenuation of the pressure peak and energy of the shock wave. Quantitatively, the attenuation magnitude was proportional to $a{(\\alpha \\varphi /\\varepsilon )^b}$, where the values of the coefficients a and b depended on whether the shock wave was stratified or not. When the cavitation bubble and the air bubble merged, the energy and the pressure peak of the shock wave decreased to less than 40 % of the values in the absence of the air bubble. With the new insight into bubble–bubble interaction mechanisms, the findings will facilitate a better understanding and development of cavitation utilization and prevention technology in water--air two phase systems.

For over 50 years, pure or doped silica glass optical fibres have been an unrivalled platform for the transmission of laser light and optical data at wavelengths from the visible to the near infra-red. Rayleigh scattering, arising from frozen-in density fluctuations in the glass, fundamentally limits the minimum attenuation of these fibres and hence restricts their application, especially at shorter wavelengths. Guiding light in hollow (air) core fibres offers a potential way to overcome this insurmountable attenuation limit set by the glass’s scattering, but requires reduction of all the other loss-inducing mechanisms. Here we report hollow core fibres, of nested antiresonant design, with losses comparable or lower than achievable in solid glass fibres around technologically relevant wavelengths of 660, 850, and 1060 nm. Their lower than Rayleigh scattering loss in an air-guiding structure offers the potential for advances in quantum communications, data transmission, and laser power delivery. Hollow core fibers have low light attenuation because the light travels through air rather than glass, but other sources of loss have limited the performance so far. Here the authors design and demonstrate a Nested Antiresonant Nodeless hollow core fiber that has losses competitive with standard solid-core fiber at several important wavelengths.

The identification of small non-penetrating defects in polyethylene (PE) pipes, utilizing ultrasonic-guided waves, serves as the cornerstone for ensuring the safe operation of these pipes. However, owing to the PE pipe material characteristics, the guided wave has high attenuation in PE pipe, which seriously limits the detection range and accuracy of the guided wave. To address this problem, the dispersion and dissipation characteristics of ultrasonic-guided waves in PE pipes were derived, and the results indicated that the excitation frequency was the important parameter affecting the propagation distance. Then, an experimental platform for PE pipe testing was built using macro fiber composites. The acoustic attenuation coefficient and dispersion were calculated. After considering the effects of dissipation and dispersion on the guided wave, the optimal excitation frequency was selected to extend the guided wave detection distance to 4 m. Finally, an experimental study on ultrasonic-guided wave detection of defects in PE pipes was conducted. The experimental results showed that non-penetrating small defects with a section loss rate of 8% could be effectively identified and located using ultrasonic-guided waves.

An experimental investigation of wave propagation coefficients determination of the granite was presented in the present study. Firstly, a series of pendulum impact tests were performed to investigate the stress wave properties of the granite. High-speed digital image correlation (HDIC) was utilized to capture the displacement and velocity at the free end of the impacted granite bar. Subsequently, the HDIC-based non-contact method was introduced for the determination of wave propagation coefficients of the granite. Finally, experimental studies based on the traditional contact method using strain gauges were performed to validate the present HDIC-based non-contact method. The results show that both the attenuation coefficient and wave number increase as frequency increases. Moreover, the propagation coefficients (attenuation coefficient and wave number) determined by the present HDIC-based non-contact method agree well with that determined by the traditional contact method using strain gauges. The present HDIC-based non-contact method can be used to predict the stress wave propagation through the granite effectively.HighlightsA non-contact method by high-speed digital image correlation (HDIC) was proposed.The HDIC-based non-contact method was validated by traditional strain gauges measurement.The propagation coefficients can be determined by the present method efficiently.The HDIC-based non-contact method can predict the stress wave propagation.

Attenuating low-frequency sound remains a challenge, despite many advances in this field. Recently-developed acoustic metamaterials are characterized by unusual wave manipulation abilities that make them ideal candidates for efficient subwavelength sound control. In particular, labyrinthine acoustic metamaterials exhibit extremely high wave reflectivity, conical dispersion, and multiple artificial resonant modes originating from the specifically-designed topological architectures. These features enable broadband sound attenuation, negative refraction, acoustic cloaking and other peculiar effects. However, hybrid and/or tunable metamaterial performance implying enhanced wave reflection and simultaneous presence of conical dispersion at desired frequencies has not been reported so far. In this paper, we propose a new type of labyrinthine acoustic metamaterials (LAMMs) with hybrid dispersion characteristics by exploiting spider web-structured configurations. The developed design approach consists in adding a square surrounding frame to sectorial circular-shaped labyrinthine channels described in previous publications (e.g. (11)). Despite its simplicity, this approach provides tunability in the metamaterial functionality, such as the activation/elimination of subwavelength band gaps and negative group-velocity modes by increasing/decreasing the edge cavity dimensions. Since these cavities can be treated as extensions of variable-width internal channels, it becomes possible to exploit geometrical features, such as channel width, to shift the band gap position and size to desired frequencies. Time transient simulations demonstrate the effectiveness of the proposed metastructures for wave manipulation in terms of transmission or reflection coefficients, amplitude attenuation and time delay at subwavelength frequencies. The obtained results can be important for practical applications of LAMMs such as lightweight acoustic barriers with enhanced broadband wave-reflecting performances.