مجله انجمن مهندسی صوتیات ایران
سال هفتم شماره 2 (پاییز و زمستان 1398)

Around the word, traffic growth is causing growing air and noise pollution. Noise levels in a given area are affected by traffic on the streets as well as effective factors, including existing infrastructure and industrial centers, and so on. The purpose of this research is to model and estimate the amount of acoustic emission in the streets of Tehran's third district, using the 3D spatial information system. In this research, effective parameters on noise emission such as air absorption, distance, absorption by earth and the like are modeled with the help of the spatial information system. A three-dimensional map is presented to illustrate the distribution of noise in space, taking into account the above parameters. A hybrid model has been used to account for the noise reduction behavior of objects in reducing noise. Modeling and mapping of noise reduction, due to objects, in 3D space are calculated with higher accuracy, and are shown. The three dimensional maps presenting the noise levels, have proxided better understanding and helpful information of existing noise distribution status. Evaluations indicate the validity of the proposed hybrid model.

Noise pollution is considered as one of the most annoying factors in educational and office environments. In these environments where learning processes and intellectual activities are ongoing, an effective acoustic design is necessary. The aim of this paper is to assess the reverberation time as one of the important indicators in describing the acoustic characteristics of rooms. In this study, the reverberation time was measured in accordance with ISO 3382-2: 2008. This indicator was calculated based on experimental models for number of classrooms and offices. The calculated values were compared to that of measured corresponding ones. The mean values of reverberation times measured in classroom and office rooms were 0.99 and 0.58 seconds, respectively. In 36% of the classrooms, the amount of reverberation time was higher than the recommended amount. But, the amount of reverberation time measured in all office rooms was less than the recommended amount. The mean of all the reverberation times calculated by the models were greater than the amounts of measured reverberation time. The results also showed that the theoretical  models for predicting reverberation time were not valid for the understudy places with the mean absorption coefficients of less than 0.1.

In this paper, the acoustic wave propagation in multilayer Thue-Morse structures has theoretically been studied. The composing layers were assumed do be Cu, Al, Mgo and Pb materials. Two, three or four different materials have been used in a typical structure. By using a perpendicular incident acoustic wave, the transmission coefficient was calculated and then by its old, the frequency gaps wasinvestigated. These structures can have different applications such as acoustic filters, acoustic insulations, acoustic diodes, etc. it is shown that the quantization of the transmission coefficient with respect to the number of layers in an assumed structure, can be utilized to use minimize the materials used in an experiment to fabricate a structure with the desired frequency gap. Now, the frequency gaps and their positions on the frequency axis are tunable by using the Thue-Morse structures. Finally, the effects of the number of layers, total system length, material type and the arrangement of the layers on the frequency filtering have been studied. It is also shown that the 4-material systems can filter a wider range of frequencies than the 3-material ones and 3-material systems can filter a wider range of frequencies than the 2-material ones.

Medical ultrasound imaging due to close behavior of cancer tumors to body tissues has a low contrast. This problem with synthetic aperture imaging method has been addressed. Although the synthetic aperture imaging technique solved the low-contrast problem of ultrasound images, to an acceptable limit, but the performance of these methods is not even acceptable when the signal to noise ratio (SNR) is low or the length of data is short. Bayesian method is a time-spatial image reconstruction algorithm proposed to compensate for the poor performance of adaptive Beamforming methods in low signal to noise ratio. The main advantage of the Bayesian methods as compared to synthetic aperture imaging once is due to the use of a pre-defined distribution function wich aids image reconstruction. In this study, the minimum mean squared error (MMSE) and maximum a posteriori (MAP) criteria have been used for image reconstruction.

In this paper, three types of two-dimensional phononic crystals comprising of steel rods (with circular, square, and triangular cross-sections) in water-ground matter were investigated. The frequency band structure for the x-y mode was calculated using Finite Element Method. What the results show complete and incomplete gaps for phononic crystals with a circular and triangular cross-sections and for the square cross-section an incomplete gap appeared. The results show the maximum intensity for phononic crystal with circle, square and triangle cross section, 1.15 MHz, 1.11 MHz and 1.27 MHz, respectively. The effect of change of source distance to sheet, effect of sheet thickness change on focal point intensity and focal point distance to source were investigated. In general, the obtained results are in good agreement with other existing results.

Studies on jet noise are usually conducted on "transient" and "steady" jets. Transient jet refers to the staring processes on jet formation (e.g. initial shock wave and starting vortex ring). The quasi-steady jet forms after these transient phenomena. In the present study, the common facilities for studing the noise sources in the transient and steady jet are reviewed. In the next step design of a reflected shock tube is presented. This facility makes it possible to study the noise sources in both transient and steady jets. The results of experimental aeroacoustic studies on a hot supersonic jet with the facility are presented an analyzed for the next step. The Mach number of the generated jet is 1.4 with a total temperature of 950 K. Comparison of the results of the designed reflected shock tube with those of common facilities shows the advantages of reflected shock tube for experimental aeroacoustic studies on transient and steady hot jets.

Nowadays, the issue of noise and annoying sounds is one of the major challenges in design of mufflers, that is used to reduce the noise level generated by output current of vehicles' engines. Therefore, the noise generated by the exhaust system can be controlled through predicting and calculating the transmission loss level, which is considerable and cannot be neglected. The investigated muffler has been reported to have an elliptical cross section, three straight porous tubes, four expansion chamber and three divisive plates. Generally, the main purpose of this study is to analyze the acoustic attenuation performance by using the Boundary Element Method. Also, in order to approach maximum transmission loss level, the effective geometrical parameters are optimized by using Response Surface and Taguchi methods. The results of this two methods are compared with each other. Finally, due to non-dependence of the Sound Transmission Loss level at the resonant frequencies on dimentions variation of geometrical parameters, by adding sound absorbing materials to the muffler’s shell, the acoustic attenuation performance is increased more than double and the effect of different absorptive materials in this numerical simulation is investigated.