EUROPEAN LABOUR PRODUCTIVITY AND CORPORATE E-LEARNING ACTIVITIES: AN EMPIRICAL ANALYSIS

The purpose of this analysis is to test the hypothesis which growth in workers’ competency level is affected by educational, training and workplace features. We focused above all on the corporate e-learning activities and labour productivity, in order to identify differences between European countries. Our findings showed some statistical significances related to six variables concerning a macro view of knowledge and innovation in the workplace, whereby we highlighted the comparison of mutual positions of European countries on the basis of a potential component of investments in human capital which is e-learning. According to statistical significativity we specifically noted that most Northern European countries have a comparative advantage in terms of labour productivity and direct investments than those in the south.corporate e-learning, European labour productivity, principal component analysis

Broadening sound absorption coefficient with Hybrid Resonances

In the last years, a great research effort has been focused on the noise mitigation at low frequencies. Membrane-type acoustic metamaterials (AMM) are one of the most promising solutions to meet the growing demand for low frequency sound absorbers. Typically, acoustic membrane absorbers require large back cavities to achieve low frequency sound absorption, which is usually categorised by a single narrow absorption peak. This paper presents an acoustic resonator unit cell, comprising of a thin elastic silicone plate with an air gap cavity with broadband absorption in a frequency range between 250 and 400 Hz. The broadband and multiple peak sound absorption showed by the proposed resonator is due to hybrid resonances which occur in the frequency range due to coupling of the structural dynamic response of the plate with the acoustic response of the air cavity. A numerical model based on acoustic-structural interaction, validated for experimental data, has been used to explain how the broadening gain in the sound absorption level is strictly related to the hybrid resonances of the unit cell resonator. We demonstrated that hybrid resonances are a function of the geometrical parameters and the ratio between the Young's modulus and the density of the material plate, thus the proposed resonators absorption frequency range is tuneable at low frequencies allowing a wider broadband not achievable with acoustic membrane absorbers.</p

Broadening sound absorption coefficient with Hybrid Resonances

In the last years, a great research effort has been focused on the noise mitigation at low frequencies. Membrane-type acoustic metamaterials (AMM) are one of the most promising solutions to meet the growing demand for low frequency sound absorbers. Typically, acoustic membrane absorbers require large back cavities to achieve low frequency sound absorption, which is usually categorised by a single narrow absorption peak. This paper presents an acoustic resonator unit cell, comprising of a thin elastic silicone plate with an air gap cavity with broadband absorption in a frequency range between 250 and 400 Hz. The broadband and multiple peak sound absorption showed by the proposed resonator is due to hybrid resonances which occur in the frequency range due to coupling of the structural dynamic response of the plate with the acoustic response of the air cavity. A numerical model based on acoustic-structural interaction, validated for experimental data, has been used to explain how the broadening gain in the sound absorption level is strictly related to the hybrid resonances of the unit cell resonator. We demonstrated that hybrid resonances are a function of the geometrical parameters and the ratio between the Young's modulus and the density of the material plate, thus the proposed resonators absorption frequency range is tuneable at low frequencies allowing a wider broadband not achievable with acoustic membrane absorbers.</p

Microperforated Panel and deep subwavelength Archimedean-inspired spiral cavities for multi-tonal and broadband sound absorption

In recent years, metamaterial-structures and Microperforated panel (MPP) absorbers have been proposed as a valid alternative to porous materials for sound absorption in the low frequency range. However, high and broadband absorption cannot be achieved in the low frequency range as required in some engineering applications where large thicknesses of the absorbers are unsuitable. In this work, a deep subwavelength hybrid parallel-arranged MPP and Archimedean-inspired spiral (AIS) absorber is proposed to obtain broadband sound absorption at low frequency (i.e. 400–2000 Hz). The absorption properties are investigated using an equivalent electro-acoustic model and a parametric analysis is performed to optimise the geometric parameters of the device for the desired frequency range. Two parallel arranged MPPs and AIS structures were designed to achieve sound absorption in frequency ranges between 550–1650 Hz and 380–1250 Hz, with a total thickness less than 1/28 wavelength (24.3 mm). In addition, a parallel arrangement of an AIS and a double layered MPP were also considered to achieve absorption in a wider frequency range (i.e. 480–2800 Hz). The prototypes were then fabricated and tested with an impedance tube to evaluate the normal absorption coefficient via the Transfer Function method (TFM). Experimental results show a good correlation with the analytical models, with a absorption coefficient above 60% over the respective frequency ranges. Moreover, absorption peaks occur at the resonance frequencies and higher harmonics of the structures, with measured values above 95%. The low frequency broadband absorption shown by the proposed subwavelength hybrid structures makes the device suitable for many acoustic engineering applications.</p

Microperforated Panel and deep subwavelength Archimedean-inspired spiral cavities for multi-tonal and broadband sound absorption

In recent years, metamaterial-structures and Microperforated panel (MPP) absorbers have been proposed as a valid alternative to porous materials for sound absorption in the low frequency range. However, high and broadband absorption cannot be achieved in the low frequency range as required in some engineering applications where large thicknesses of the absorbers are unsuitable. In this work, a deep subwavelength hybrid parallel-arranged MPP and Archimedean-inspired spiral (AIS) absorber is proposed to obtain broadband sound absorption at low frequency (i.e. 400–2000 Hz). The absorption properties are investigated using an equivalent electro-acoustic model and a parametric analysis is performed to optimise the geometric parameters of the device for the desired frequency range. Two parallel arranged MPPs and AIS structures were designed to achieve sound absorption in frequency ranges between 550–1650 Hz and 380–1250 Hz, with a total thickness less than 1/28 wavelength (24.3 mm). In addition, a parallel arrangement of an AIS and a double layered MPP were also considered to achieve absorption in a wider frequency range (i.e. 480–2800 Hz). The prototypes were then fabricated and tested with an impedance tube to evaluate the normal absorption coefficient via the Transfer Function method (TFM). Experimental results show a good correlation with the analytical models, with a absorption coefficient above 60% over the respective frequency ranges. Moreover, absorption peaks occur at the resonance frequencies and higher harmonics of the structures, with measured values above 95%. The low frequency broadband absorption shown by the proposed subwavelength hybrid structures makes the device suitable for many acoustic engineering applications.</p

Graphite-oxide hybrid multi-degree of freedom resonator metamaterial for broadband sound absorption

Low frequency broadband sound absorption for thin structures is still a great challenge. A new concept of a stackable hybrid resonator metamaterial is proposed which exhibits super broadband low-frequency sound absorption. The proposed metamaterial is based on micrometric scale thickness Graphene Oxide (GO) embedded in a stacked structure or used as external skin in a designed honeycomb (HC) structure. The stackable nature of the proposed structure allows the GO-HC cores to be embedded within micro-perforated panels (MPP) providing enhanced stiffness/strength to the structure and high absorption characteristics. We demonstrate how the exploitation of the GO elastic and mass properties result in multiple hybrid structural–acoustic resonances. These resonances are tailored to occur in a frequency range of interest by the theoretical calculation of the sound absorption coefficient. The theoretical model combines the mutual interaction between the structural dynamic of the GO foil and acoustic higher modes of the HC core cell as well as stacked MPP-HC/GO-HC cores. The result is a multi-degree of freedom hybrid resonator which provides subwavelength scale broadband sound absorption in low frequency range between 300 and 2500 Hz

Genetic algorithm for space debris and space objects attitude motion reconstruction through optical measurements

Space debris has recently become a major problem in the planning and execution of space missions. Due to the recent widespread placement of satellite mega- constellations in Low Earth Orbits (LEO), where most of the catalogued debris is located, the need to monitor such uncontrolled objects and maintain an up-to-date catalogue has increased. Moreover, estimating the attitude motion of a space object is fundamental to improving methods for orbit determination and supporting eventual Active Debris Removal (ADR) missions. The Sapienza Space System and Space Surveillance Laboratory (S5Lab), whose researchers have years of experience in space debris detection, operates an extensive observation network that can exploit different observation strategies. This paper illustrates the reconstruction of an object’s attitude motion from its light curve, which can be extracted using scientific Complementary Metal-Oxide Semiconductor (sCMOS) sensors installed on high-slew rate telescopes. The method is based on a comparison between the object's actual light curve and a synthetic curve created by changing the initial conditions for the attitude motion, considering the observer's motion, the Sun’s position, the object’s position and its 3D model. A genetic algorithm is used to create multiple synthetic light curves by varying the initial conditions for the attitude motion until one of them matches the observed one. In addition to extracting the light curves and reconstructing the attitude, observational strategies for acquiring light curves are discussed. Finally, the results of the investigation of potentially hazardous debris are presented

A highly-detailed anatomical study of left atrial auricle as revealed by in-vivo computed tomography

The left atrial auricle (LAA) is the main source of intracardiac thrombi, which contribute significantly to the total number of stroke cases. It is also considered a major site of origin for atrial fibrillation in patients undergoing ablation procedures. The LAA is known to have a high degree of morphological variability, with shape and structure identified as important contributors to thrombus formation. A detailed understanding of LAA form, dimension, and function is crucial for radiologists, cardiologists, and cardiac surgeons.This review describes the normal anatomy of the LAA as visualized through multiple imaging techniques such as computed tomography (CT), magnetic resonance imaging (MRI), and echocardi-ography. Special emphasis is devoted to a discussion on how the morphological characteristics of the LAA are closely related to the likelihood of developing LAA thrombi, including insights into LAA embryology