126 research outputs found

    Study of sound attenuation in elliptic ducts

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    The acoustic emission resulting from the reciprocating motion of machinery parts, such as the movement of pistons and the intake/exhaust of gases in an automotive engine, or the pressure pulsation generated in compressors cause noise pollution. Passive noise--attenuating devices such as mufflers are usually employed in the intake or the exhaust flow path of such systems. A typical muffler element consists of an expansion chamber with inlet and outlet ports for the passage of the exhaust fluid. The difference in the size of the ports from the chamber dimensions presents an impedance mismatch to the propagating acoustic wave, resulting in partial or complete reflection of the sound energy. Mufflers can broadly be classified as reactive type or dissipative type. Reactive mufflers employ the principle of impedance mismatch by incorporating area changes in the transmission path. Dissipative mufflers usually have some kind of absorptive lining that converts sound energy into heat. In comparison to reactive mufflers, dissipative mufflers usually have a sound attenuation over a wider frequency range. However, pressure drop and particle clogging limit their application. The sound attenuation characteristics of a muffler is dependent on various factors like shape and size of the muffler, orientations of inlet/ outlet ports, and number of inlet/outlet ports. Since the parameter space involved is potentially very large, only a few muffler geometries have been extensively studied in the past, viz. circular or rectangular chamber mufflers. More recently, mufflers having elliptical cross-sections have received some attention. These types of mufflers require smaller space across one of the dimensions, and are suitable for applications where space constraints are an important design consideration. The existing literature on elliptic reactive mufflers is not extensive; therefore, further attention to this geometry is required. The present work aims to study the sound attenuation in elliptic reactive mufflers using a perturbation-based approach. Although exact solutions for such geometries are available in terms of the elliptic eigenfunctions, the perturbation based approach may be implemented for more general asymmetric distortions (within moderate eccentricity). Using this approach, the effects of the ellipticity and port-extensions on the sound attenuation are analyzed in this work. The perturbation method is then extended to study the breakout noise from distorted ducts. It has been long established that distortions in ducts significantly alter the sound radiation when subjected to an internal acoustic field. However, most of the reported work had been confined to very small distortions, and plane-wave (acoustic) propagation within the duct. The present work aims to improve on these limitations and provide a solution method for studying the sound radiation from moderately distorted ducts when excited by a point acoustic source. Using an analytical approach suggested earlier, the radiated sound field of an elliptic duct (eccentricity e=0.6) is observed to be much higher (as much as 30 dB, Lw) than a cylindrical shell of comparable dimensions. The final part of this dissertation aims to determine the effect of a liquid phase on the sound attenuation in a muffler geometry. The liquid phase is expected to act like a compliant surface, thereby affecting the sound transmission. However, analytical study of a simple rectangular duct and experimental measurement of transmission loss indicate that the wall compliance effects are negligible and may be neglected

    Fifteen-year incidence rate and risk factors of pterygium in the Southern Indian state of Andhra Pradesh.

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    PURPOSE: To report 15-year incidence rate and associated risk factors of pterygium among people aged 30 years and above at baseline in the rural clusters of longitudinal Andhra Pradesh Eye Disease Study (APEDS III). METHODS: The baseline APEDS I included 7771 participants of which 6447 (83%) were traced and 5395 (83.7%) were re-examined in APEDS III. To estimate the incidence of pterygium, we selected participants who were 30 years and above at baseline (4188), of which 2976 were traced and 2627 (88.3%) were examined, and based on inclusion criteria, 2290 participants were included in the study. The incidence rate of pterygium was defined as the proportion of people free of pterygium at baseline who had developed the condition at 15-year follow-up (range 13-17 years). Univariate and multivariable analyses for risk factors were undertaken. RESULTS: The sex-adjusted incidence rate of pterygium was 25.2 per 100 person-years (95% CI 24.8 to 25.7) which was significantly higher for men than women (26.3 per 100 person-years (95% CI 25.6 to 27.0) and 24.7 (95% CI 24.1 to 25.3) respectively). At the multivariable analysis, male gender (RR: 1.35, 95% CI 1.0 to 1.83), no formal education (RR: 2.46, 95% CI 1.22 to 4.93), outdoor occupation (RR: 1.47, 95% CI 1.14 to 1.9) and lower body mass index (BMI) (<18.5) (RR: 1.25, 95% CI 1.02 to 1.55) were associated with increased risk of pterygium. CONCLUSIONS: The overall incidence rate of pterygium was high in this rural population, especially in men and those engaged in outdoor activities, lack of formal education and with lower BMI. It is likely that greater exposure to ultraviolet light is a major contributing factor, thus warranting preventive strategies

    Incidence, Incident Causes, and Risk Factors of Visual Impairment and Blindness in a Rural Population in India: 15-Year Follow-up of the Andhra Pradesh Eye Disease Study.

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    PURPOSE: To report 15-year incidence rate of visual loss (blindness and visual impairment [VI]), causes, and risk factors for participants in Andhra Pradesh Eye Disease Study III (APEDS III). DESIGN: Population-based cohort study. METHODS: From 2012 to 2016, all rural participants were interviewed and underwent a comprehensive eye examination, including dilated fundus examination and imaging. Presenting visual acuity (PVA) and best-corrected visual acuity (BCVA) were measured using a standard logarithm of Minimum Angle of Resolution chart at 3 meters. World Health Organization (WHO) and United States of America (USA) categories of VI and blindness were used. Incident visual loss was defined as the development of or worsening of visual loss of one or more categories. RESULTS: In APEDS I, 7,771 rural participants were examined using stratified, random-cluster systematic sampling; in APEDS III, 5,395 participants (69.4% of rural or 52.4% of total participants) were re-examined. Using WHO categories, the crude incidence rate of any visual loss based on PVA and BCVA were 14.6 (95% confidence interval [CI]:13.6-15.7) and 6.3 (95% CI: 6.1-6.4) per 100 person-years, respectively. Using USA criteria, the values were 22.6 (95% CI: 22.3-23.0) and 10.6 (95% CI: 10.3-10.8) per 100 person-years, respectively. More than 90% of visual loss was attributable to cataract and uncorrected refractive error. Using WHO categories, significant independent risk factors for the incident visual loss were increasing age, female gender, illiteracy, past or current smoker, and current use of alcohol. Using the USA definition, an additional risk factor was lower level of education. CONCLUSIONS: The high incidence likely reflects poor access to eye care in this population, which needs to be taken into account when planning eye care programs

    Fifteen-Year Incidence Rate of Primary Angle Closure Disease in the Andhra Pradesh Eye Disease Study.

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    PURPOSE: To report on the 15-year incidence of primary angle closure disease (PACD) among participants aged ≥40 years in rural southern India DESIGN: Population-based longitudinal incidence rate study METHODS: Setting: 3 rural study centres. STUDY POPULATION: Phakic participants aged ≥40 years who participated in both examination time points. OBSERVATION PROCEDURES: All participants at the baseline and at the mean 15-year follow-up visit underwent a detailed interview, anthropometry, blood pressure measurement, and comprehensive eye examination. Automated perimetry was attempted based on predefined criteria. Main outcome measures included development of any form of PACD, as defined by the International Society for Geographical and Epidemiological Ophthalmology (ISGEO), during the follow-up period in phakic participants, who did not have the disease at baseline. RESULTS: We analyzed data obtained from 1,197 (81.4% out of available 1,470) participants to calculate the incidence of the disease. The mean age (standard deviation) of the study participants at the baseline was 50.2 (8.1) years, with 670 male (45.5%) and 800 female (54.4%) participants. The incidence rate per 100 person-years (95% confidence interval) for primary angle closure suspect, primary angle closure, and primary angle closure glaucoma was 8.8 (8.4, 9.2), 6.2 (5.9, 6.6), and 1.6 (1.4, 1.8), respectively. Thus, the incidence of all forms of PACD was 16.4 (15.9, 17) per 100 person-years. On logistic regression analysis, female gender was a significant risk factor whereas presence of myopia was protective. CONCLUSIONS: This study reports long-term incidence of PACD from rural India. It has implications for eye health care policies, strategies, and planning
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