We describe here a class of acoustic metamaterials with fractal Hilbert space-filling and coiled geometry with equal tortuosity for noise mitigation. Experiments are performed using a four-microphone impedance tube and benchmarked against non-viscous and viscothermal finite element models related to configurations spanning up to five fractal/geometry orders. We show that the acoustic absorption can be predicted by the resonance of the cavities associated with the tortuous paths. For a given fractal/geometry order, the acoustic absorption at specific frequencies is also enhanced by maximizing the difference between the minimum and maximum fluid particle velocity of the air inside the patterns. These principles can be used to design high-performance acoustic metamaterials for sound absorption over broad frequency ranges.
Experiments and numerical methods were used to investigate the key parameters which affect the transmission loss behaviour of Hilbert fractal acoustic metamaterials. The tests were conducted using a four-microphone impedance tube, and the numerical simulation was performed using COMSOL Multiphysics software. Fractal order and cavity slot widths on Hilbert fractal metamaterials were investigated. 3D printing manufacturing techniques were used to make polylactic acid specimens. The COMSOL model developed utilised thermo-viscous and lossless domains with boundary layer mesh in the fractal cavities. The tests and simulations' frequency range were 0.2 kHz to 3.0 kHz, with parametric gap widths ranging from 0.5 mm to 3.0 mm. There is a reasonable agreement between the numerical models and the experimental results; the second-order Hilbert fractal had the most significant effect on transmission loss, with an experimental peak of nearly 50 dB around 1600 Hz. Moreover, multiple transmission loss peaks were observed as a function of the gap width in the five Hilbert fractal orders studied. The gap width is one of the critical parameters for optimising the performance of the Hilbert fractal as an acoustic metamaterial.
Introduction: The aim of the study was to evaluate the possible association of early vascular abnormalities and the metabolic syndrome (MS) in normotensive patients at risk of developing diabetes mellitus Methods: The presence (MS+) or absence (MS-) of MS, according to the ATP III criteria, was assessed in 77 subjects (mean age: 50 years), with blood pressure (BP) <140/90 mmHg and family history of diabetes, obesity or impaired fasting glucose. Fifty age-matched healthy control subjects were also recruited. Carotid intima-media thickness (IMT), brachial artery endothelium-dependent flow-mediated dilation (FMD) and endothelium-independent dilation by glyceryl trinitrate (GTN, 25 μg sublingual) were evaluated by high resolution ultrasound and automatic computerised edge detection system. Carotid-femoral pulse wave velocity (PWV) and radial augmentation index (AI) were assessed by applanation tonometry (Sphygmocor ® ). Results: Age and gender distribution did not differ among MS+ (n=27), MS-(n=50) patients and controls. No significant differences were found among MS+, MS-patients and controls for carotid IMT (0.70±0.21, 0.71±0.25 and 0.67±0.19 mm), response to GTN (9.0±3.0, 8.7±3.8 and 8.1±3.8%%) and radial AI (20.8±11.1, 18.3±11.2 and 17.1±9.8 units). FMD was significantly (pMS+ (6.1±3.7%) and MS-(5.8±2.7%) patients as compared to controls (6.9±2.5%). PWV was significantly (pMS+ (9.0±1.9 units) as compared to both MS-patients (7.7±1.2 m/s) and controls (7.2±1.5 m/s). Conclusions: MS is associated with a selective alteration of central PWV in normotensive patients at risk of developing diabetes. Among the components of the MS, only blood pressure values negatively influence arterial stiffness and also endothelial function.
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