Study of acoustic and aerodynamic performance of reactive silencer with different configurations: Theoretical, modeling and experimental.

Periodic sound reduction at some frequencies and high pressure loss could be considered as the disadvantages of simple expansion silencers. Generally, reactive elements are applied in silencers to optimize their acoustic and aerodynamic performance. In this paper, we were going to investigate the combined and independent effects of reactive elements, baffle and extended tubes at the inlet and outlet of simple expansion silencer, on the acoustic performance and pressure loss of simple expansion silencer. Firstly, simple expansion silencer dimensions were determined based on theoretical calculations to attenuate the noise by 10 dB. Secondly, the simulation was performed using COMSOL acoustical module software based on finite element method to predict silencer sound transmission loss and pressure loss, respectively, before and after adding reactive elements. Then, according to ISO7235, ISO3744 and ISO3746 standards noise insertion loss of various silencers under study was measured in a free field and in the following that silencers pressure loss evaluated. Finally, the predicted results with the software were compared with the experimental ones. The addition of the extended tubes increased the transmission loss and insertion loss at low and medium frequencies and mitigated the pressure loss of the silencer compared to the simple expansion silencer. The baffle increased the transmission loss curve in the medium frequencies compared to the simple expansion silencer. The combination of baffle and extended tube elements caused an extraordinary increase in insertion loss and transmission loss at the medium frequencies and a decrease in pressure loss. It could be concluded from the present study that using extended tube might be probably the best choice to reduce pressure loss and increase the acoustic performance of simple expansion silencer at low and medium frequencies and what's more is that the best acoustic performance in medium frequencies can be achieved by using a combination of baffle and extended tube.

Investigating the Effect of Welding Fume in Pulmonary Function of Welders in an Automobile Industry.

Context: Respiratory exposure to welding fumes directly or indirectly, in the long run, can lead to systematic effects among welders. Aims: This study aimed to investigate respiratory symptoms and pulmonary function parameters among welders working in the automotive industry. Material and Methods: This cross-sectional study was performed among 2304 workers from two groups in the manufacturing and administrative staff (as exposure and control groups). Pulmonary function parameters and respiratory symptoms were collected through periodic spirometry examinations and a standard respiratory symptoms questionnaire. Exposure to welding fumes was obtained from the annual measurement data based on the NIOSH7300 method. Data were analyzed using SPSS software version 22 and linear and multiple regression statistical tests. Results: The mean age of the subjects in the case and control groups were 37.5 and 38.5 years, respectively. All the apparent symptoms of respiratory diseases (cough, sputum) in the welder's group were more than in the control group. Also, there was a significant difference between the respiratory symptoms in the two groups (P < 0.001). The mean value of all spirometric parameters (FVC, FEV1, FEV1/FVC, FEF25-75, PEF) in the case group (welders) was significantly lower than the control group. Conclusion: The results of this study revealed that the variables of age and work experience effectively reduce all spirometric parameters of welders. Also, regarding the effect of metal vapors, a significant relationship has been seen between Cu in welding fume and FEF25-75 and FEV1 spirometric parameters.

The Effect of Mobile Radiation on the Oxidative Stress Biomarkers in Pregnant Mice.

Objective: Due to the growing use of communication instruments such as cell phones and wireless devices, there is growing public concern about possible harmful effects, especially in sensitive groups such as pregnant women. This study aimed to investigate the oxidative stress induced by exposure to 900 MHz mobile phone radiation and the effect of vitamin C intake on reducing possible changes in pregnant mice. Materials and methods: Twenty-one pregnant mice were divided into three groups (control, mobile radiation-exposed, and mobile radiation plus with vitamin C intake co-exposed (200 mg /kg)). The mice in exposure groups were exposed to 900 MHz, 2 watts, and a power density of 0.045 µw /cm2 mobile radiation for eight hours/day for ten consecutive days. After five days of rest, MDA (Malondialdehyde), 8-OHdG (8-hydroxy-2' -deoxyguanosine), and TAC (Total Antioxidant Capacity) levels were measured in the blood of animals. The results were analyzed by SPSS.22.0 software. Results: The results showed that exposure to mobile radiation increased MDA (P=0.002), and 8-OHdG (P=0.001) significantly and decreased Total Antioxidant Capacity in the exposed groups (P=0.001). Taking vitamin C inhibited the significant increase in MDA and 8-OHdG levels in exposed groups. Conclusion: Although exposure to mobile radiation can cause oxidative stress in the blood of pregnant mice, vitamin C as an antioxidant can prevent it.

Effect of Occupational Exposure to Radar Radiation on Cancer Risk: A Systematic Review and Meta-Analysis.

OBJECTIVE: Microwave radiation is one of the most growing environmental workplace factors that exposes too many workers in the various workplaces. Regard to concerns about cancer incidence in these workers and lack of systematic or meta-analytic studies about this object, so, we conducted a meta-analysis to acquire an understanding of the association between cancer risk and occupational exposure to radar radiation. METHODS: A systematic search was carried out on case-control, cohort and clinical control trial studies that published in the Cochrane Library, PubMed, ISI Web of Science, Scopus and Google scholar databases that accomplished from March 2017 to March 2018 and updated on 30 September, 2018 in English and Persian articles without time limit in publication date. Keywords were selected based on PICO principle and collected from MeSH database. After removal of duplicated studied, taking into inclusion and exclusion criteria, the process of screening was carried out and data were extracted after preparation of the full text of included articles. Article collection was completed by manually searching for a reference list of eligible studies. For quality assessment of included studies, Newcastle-Ottawa scale was used. RESULTS: a total of 533 studies was found in the first step of literature search, only 6 were included with 53,008 sample size according to inclusion and exclusion criteria. Estimated pooled random effects size analysis showed no significant increasing effect of occupational exposure to radar radiation on mortality rate (MR=0.81, 95%CI: 0.78, 0.83) and relative risk (RR=0.87, 95%CI: 0.75, 0.99, P <0.0001) of cancer with a significant heterogeneity between the selected studies. CONCLUSIONS: In conclusion, the results of this meta-analysis study have shown no significant increase in overall mortality ratio and cancer risk ratio from occupational exposure to the radar frequency of workers. But, these results are not conclusive. As regards to some limitation such as fewer numbers of included studies, lack of data about exposure characterizations and demographic characterizations in this meta-analysis, this result is not certain and conclusive. It is recommended to conduct future studies.