Helicopters can be a source of loud noise. The high production noise of helicopters makes them unsuitable for densely populated areas, especially in low-flying conditions. This study examines methods and offers solutions to reduce or eliminate the noise produced by helicopters. Several methods have been considered to reduce the noise produced. Solutions such as muffler for motors, use of modulated blade spacing, X-force control, reduced tip speed, blade tip modification, airfoil tailoring, Increasing the number of main rotor blades, active blade control and variable diameter rotor in The rotor and the use of the strut in the gearbox were introduced. According to the studies conducted in the present study, combined methods haveas the best noise reduction. although Independent methods reduce noise, depending on the amount of reduction required in the targeting, combining the methods can be achieved the appropriate answer.

In this study we describe an experiment, which considers the output beam power of fibers with different materials at a fixed length by using the LED, white lamp and a laser as the illumination light source. Laser is a coherent light source and other two light sources are incoherent light sources. In another experiment the length effect is studied and for this purpose two similar fibers with the length of 4 cm and 52/5 cm are tested and the output beam powers of these fibers for three illumination sources are measured. The obtained results are compared and our results show that among tested light sources laser beam shows the maximum output power for the short length fiber and therefore laser light source is the most efficient light source for the fiber optic transmission lines.

The isoseismal maps for 22 earthquakes in different regions of Iran were analyzed to study the attenuation of seismic intensity with the distance from the epicentre under a certain surface wave magnitude (Ms). The attenuation relationships were derived by using an iterative least squares fit procedure. These equations were derived from more basic concepts assuming that the intensity is proportional to the logarithm of seismic energy density at any location empirically. The isoseismal maps are elongated in the direction of local structural trend of causative faults. Therefore, attenuation relationships for the main direction of fault, transverse to it and average attenuation were derived. Ia = 11. 564 + 0. 943 M − 2. 508 Ln(Ra + 33) ,  ,  ,  ,  ,σ = 0. 79 ,  ,  ,  ,  ,  ,Ra <, 200 km along the main direction of fault Ib = 9. 469 + 0. 717 M − 2. 121 Ln(Rb +13) ,  ,  ,  ,  ,  ,σ = 0. 49 ,  ,  ,  ,  ,Rb<, 140 km transverse to the main direction of fault I =11. 926 + 0. 831 M − 2. 7 Ln(R + 22)  ,  ,  ,  ,  ,  ,  ,  ,σ = 0. 49 ,  ,  ,  ,  ,  ,  ,  ,R<, 167 km average attenuation Where M is the surface wave magnitude and I is the intensity at a distance R (km) from the epicenter. The attenuation of seismic intensity in Iran is faster than the East and West of China and Sicily and Calabria in Italy, apart from different tectonic characteristics of these regions.

Structural dynamic analyses need different earthquake parameters such as design base acceleration, design spectra and earthquake records. In order to obtain base acceleration and design spectra of a site, prevailing method is based on seismic hazard analyses. In this method, base acceleration and design spectra are obtained from seismicity of the region, site condition based on probable statistical methods. One of the obligations in this method is having attenuation relations for different parameter such as PGA. In this research we update and make improvements of the PGA attenuation relationship, presented by Ghodrati et al. (2007) for site rock and soil bed conditions of the seismo-active states of Zagroz and, Alborz and central Iran. A total of 858 strong-motion accelerograms recorded for Alborz and central Iran and Zagros are used to obtain attenuation relation, which 477 records due to Alborz and central Iran. Regression analysis has been done for maximum of horizontal components and PGA attenuation relation coefficients are obtained.

Introduction: The advent of dual-modality PET/CT scanners has revolutionized clinical oncology by improving lesion localization and facilitating treatment planning for radiotherapy. In addition, the use of CT images for CT-based attenuation correction (CTAC) decreases the overall scanning time and creates a noise-free attenuation map (mmap). CTAC methods include scaling, segmentation, hybrid scaling/segmentation, bilinear and dual energy methods. All CTAC methods require the transformation of CT Hounsfield units (HU) to linear attenuation coefficients (LAC) at 511 keV. The aim of this study is to compare the results of implementing different methods of energy mapping in PET/CT scanners.Materials and Methods: This study was conducted in 2 phases, the first phase in a phantom and the second one on patient data. To perform the first phase, a cylindrical phantom with different concentrations of K2HPO4 inserts was CT scanned and energy mapping methods were implemented on it. For performing the second phase, different energy mapping methods were implemented on several clinical studies and compared to the transmission (TX) image derived using Ga-68 radionuclide source acquired on the GE Discovery LS PET/CT scanner.Results: An ROI analysis was performed on different positions of the resultant µmaps and the average mvalue of each ROI was compared to the reference value. The results of the µmaps obtained for 511 keV compared to the theoretical values showed that in the phantom for low concentrations of K2HPO4 all these methods produce 511 keV attenuation maps with small relative difference compared to gold standard. The relative difference for scaling, segmentation, hybrid, bilinear and dual energy methods was 4.92, 3.21, 4.43, 2.24 and 2.29%, respectively. Although for high concentration of K2HPO4 the three methods; hybrid scaling/segmentation, bilinear and dual energy produced the lowest relative difference of 10.91, 10.88 and 5%, respectively. For patients it was found that for soft tissues all the mentioned energy mapping methods produce acceptable attenuation map at 511 keV. The relative difference of scaling, segmentation, hybrid, and bilinear methods compared to TX method was 6.95, 4.51, 7, and 6.45% respectively. For bony tissues, the quantitative analysis showed that scaling and segmentation method produce high relative difference of 26 and 23.2%, respectively and the relative difference of hybrid and bilinear in comparison to TX method was 10.7 and 20%, respectively.Discussion and Conclusion: Based on the result obtained from these two studies it can be concluded that for soft tissues all energy mapping methods yield acceptable results while for bony tissues all the mentioned methods except the scaling and segmentation yield acceptable results.

Unmanned aerial vehicles (UAVs) like drones (quadcopters, hexacopters, octocopters, etc.) can be a source of significant acoustic noise. High noise makes them less suitable for use in densely populated urban areas, particularly during take-off, landing, and low-level flight, due to the noise annoyance. This paper reviews the methods and proposes some concepts for the attenuation of UAV noise. Both passive and active solutions are considered. Solutions with piston engine silencer, Q-tip propeller, more propeller blades, absorptive and reflective barrier, ducted propeller, sound absorbing ducts, synchrophaser, multichannel active noise cancellation (ANC) system with secondary sources circularly arranged around the propeller and the system with magnetically excited propeller blades are mentioneds

The main objective of this study is to present new method on the basis of genetic algorithms for attenuation relationship determination of horizontal peak ground acceleration and spectral acceleration. The proposed method employs the optimization capabilities of genetic algorithm to determine the coefficients of attenuation relationships of peak ground and spectral accelerations. This method has been applied to 361 Iranian earthquake records with magnitudes between 4.5 and 7.4 obtained from two seismic zones, namely Zagros and Alborz-Central Iran. The obtained results indicated that the proposed method can be characterized as a powerful tool for prediction horizontal peak ground and spectral accelerations.

As seismic energy propagates through the earth medium, its energy (amplitude) decays due to geometrical spreading, intrinsic attenuation and scattering. Owing to anelastic absorption, intrinsic attenuation converts the seismic energy to heat while scattering redistributes the energy at random heterogeneities. Knowledge of the relative contributions of scattering and intrinsic attenuation is important for appropriate subsurface material identification, tectonic interpretations and quantification of the ground motion. Besides, investigating seismic wave attenuation inside lithosphere allows for a more thorough knowledge as to Earth’ s deep structures. The attenuation of short-period S waves, expressed as the inverse of the quality factor (Q− 1), helps fathom the physical laws related to the propagation of the elastic energy of an earthquake through the lithosphere. Coda wave attenuation is considered as the combination of scattering and anelastic attenuation. In this study, the quality factor of coda wave was estimated in NW Iran making use of single back scattering method of Aki and Chouet (1975). For this purpose, we analyzed 3720 waveforms recorded by 8 short-period stations of Tabriz network from 1996 to 2013. So as to calculate the frequency relationships for Qc, nine frequency bands with central frequencies of 1. 5, 2, 3, 4, 6, 8, 12, 16 and 20 Hz were considered and the lateral and depth variations of Q0 (Qc in 1 Hz) were investigated in the research area. In order to study the lateral variations, we chose coda waves recorded in epicentral distances less than 80 km, in a lapse time window of 30 s. The reason for the selection of such short distance (< 80 km) and narrow lapse time (30 s) was to avoid coda waves reflected from deep scatterers, which ultimately helps compare and contrast the attenuation of shallower structures in the study area. Investigation of lateral attenuation variation demonstrated that in the northwest of Sahand volcano (in station AZR), in the northwest of Sabalan volcano (in station SRB) and around Marand (station MRD), the attenuation underwent a faint increase relative to other areas. Because of the shortage of significant lateral variations in the study area, we presented an average frequency relationship for coda quality factor in a lapse time window of 30 s as Qc = 68± 1 f 0. 84± 0. 01. The low amount of the quality factor (= 68) in the mentioned lapse time window reveal the thermal effects of the study area on the estimation of the quality factor. In order to investigate the depth variation of Qc, seventeen lapse time windows from 30 s to 100 s (time interval of 10 s) were extracted for two different datasets, one including an epicentral distance <= 80 km, the other comprised of a distance range of 80-150 km. The Qc factor was calculated for each lapse time in both datasets. The obtained quality factor indicated that Q0increased with the augment in the lapse time due to the effects of wave propagation inside the deeper parts. Frequency relationship parameter presented unexpected variations; it increased with the increase in the lapse time which is the opposite of typically-observed trends. Anomalous variations in frequency relation parameter versus the lapse time show heterogeneous uppermost mantel beneath the study area. The average frequency parameter obtained in this research was ~ 1. 0, a value indicating that the frequency dependency of lithospheric attenuation is negligible in NW Iran.