مجله انجمن مهندسی صوتیات ایران
سال پنجم شماره 2 (پاییز و زمستان 1396)

In this paper, the effects of geometrical parameters in a simple expansion compartment with input and output tubes on its acoustic damping performance are studied by analytical and numerical method. In analytical model, the transfer matrix method (TMM), and in numerical model, the finite element method (FEM) are utilized to study the effective parameters. These effects include parameters such as extending the inlet and outlet tubes into the expansion chamber and increasing their lengths, changing the position of the input and output tubes, adding holes and varing their diameters and their numbers in the chamber. The results show that the performance of the audio damping improves by adding segmentation and increasing length. It can also be found that by adding holes. Into the pit and increasing their numbers, the frequency associated with the peak of acoustic damping is increased. Therefore, they can be used to improve the design of muffler in the desired frequency range.

In this paper, the optimization of sound transmission through finite-length Functionally Graded Materials (FGM) plates by the use of genetic algorithm is studied. The main objective of this paper is to identify and optimize the effective parameters for sound power transmission of these structures. In this regard, at first, the dynamic equations of the plate are obtained using the classic theory of thin plates, then the governing wave propagation equations are extracted according to the Roussos theory. Next, the transmission loss coefficient (TL) of the structure is calculated by applying the boundary conditions. The effect of parameters effective on sound transmission loss is numerically investigated and optimization of sound transmission loss is executed using multi-objective optimization using Non-dominated Sorting in Genetic Algorithm (NSGA). The purpose of optimization is to maximize the sound transmission loss of FGM plate with minimization of its weight simultaneously, and the critical frequency of the plate is also considered as the problem constraint. The results show that the FGM material, plate thicknesses, layers layout, plate dimensions, power law exponent and the angle of incidence play an important role in reducing the sound power transmission through these structures.

Recently, infrasound arrays have been greatly considered by researchers because of their unique ability in long-range monitoring of natural and anthropogenic events. Since different geometric configuration have different performance in terms of detection probability and direction-finding accuracy, it has always been the question of what the optimal arrangement for an infrasound array is. In this paper, three methods for performance evaluation of infrasound microphone arrays are introduced and in each case, associated analytical equations are derived. Firstly, the so-called frequency-wavenumber power spectral density is described as a well-known array-based detection performance evaluation criterion. As the second method, direction-dependency of array-averaged coherence coefficient is proposed for measurement of detection accuracy. Finally, the degree of uncertainty in estimation of parameters of incoming wave over the array is proposed as a performance criterion. Then, by using the proposed methods, performance of different array geometries are studies through several computer simulations. The results of this paper show the required characteristics of an optimum configuration strategy for the deployment of infrasound arrays.

In this work, eigen modes and eigen frequencies of the body (sound board and sound box) of the setar, a persian stringed musical instrument, are studied. Eigen modes of the sound board and the whole body, neglecting the holes, are calculated independently and analyzed comparatively. This scheme of separate modal analyses for different parts of the setar will lead to the understanding of their individual roles in the overall output sound. In categorizing and assessment of sound quality of musical instruments, the study of characteristics of frequency response function (ERF) is important. In case of stringed musical instruments like setar, of the defining parameters are the first peak of FRF and its location. This first peak usually coincides with first air eigen frequency which in the present research occurs at about 377 Hz. Thus, all the other eigen modes like 588 Hz and 740 Hz, construct the rest of the FRF. Also, the effects of such as sound board thickness, wood material properties shape of sound box and other structural features of setar on two value of eigon frequencies can assessed. Furthermore, analysis of the aero-acoustic pressure contours and the eigen modes of the setar structure (sound board and box) and air allows the nature (air, structure or coupled air-structure) of a acoustic mode to be determined.

The sound attenuation in the atmosphere propagation depends on several factors, such as ground material, temperature, and geographical features. In this study, the propagation of sound in the atmosphere above of ground has been studied. For the simulation of the ground surface, the four-parametric model of Attenborough has been employed to compute the specific impedance of the ground surface. At first, the governing equations of sound propagation and Parabolic Equations (PE) method were presented; then, by employing Finite Difference Method (FDM) discretization and Crank-Nicolson approximation, algebraic equations were obtained. The algebraic equations were solved by Three Diagonal Matrix Algorithm (TDMA) in order to evaluate the sound pressure and sound attenuation over the computational domain. After validation of the numerical simulation, the results of the linear and logarithmic sound velocity profiles were presented and compared in frequency range of 10-100 Hz. For the assumed sound velocity profiles, two cases were considered; in the case that the shadow zone could not exist and in another case, where the shadow zone was possible to formed. The sound attenuation over 100 dB was achieved in range of frequency of 40-100 Hz in second studied case. In this situation, with increasing frequency, attenuation was increased but more oscillation was observed in the attenuation. Also for the flat surface ground, sound attenuation was examined for different sound velocity profiles. Finally, the effects of ground surface roughness on the sound attenuation, by considering the logarithmic sound velocity profile, were investigated at different frequencies.

Doppler ultrasound system is of great importance among the diagnostic modalities. To ensure the output of such systems that measure the physiological quantities, performance tests were carried out using AIUM and BSI protocols on 20 pulsed wave Doppler systems in 9 reference centers in Tehran. In these tests, velocity of Doppler ultrasound systems as an acoustic parameter were assessed. To evaluate the performance of pulsed wave Doppler, an in-house string phantom was designed for the first time in the country. This phantom was used to investigate pulsed Doppler ultrasound systems. The results show that the minimum and maximum errors in blood flow velocity measurement on 20 Doppler systems were 23 (cm/s) and 83 (cm/s), respectively. The mean±SD for velocity measurement errors in 6 Doppler systems (7.5 MHz) and 12 systems (3.5 MHz) were 11±16 and 28±9 (cm/s), respectively, the velocities ranged from 10-140 (cm/s). These errors emphasize the necessity for doing performance tests in Doppler ultrasonic systems.

Thermoacoustic speakers are a new generation of speakers that have come to the attention of the scientific community in recent years. These speakers have various applications in the industry due to the lack of use of vibrating parts in sound production and the need for a magnet. Due to the newness of the topic, the need to conduct theoretical studies in this field is felt severely. In response to this need, in this paper, a theoretical approach for investigating the effect of different variables on the performance of a heat sink speaker has been used. The results are very useful for choosing the optimal structure of the acoustic speakers, and determine the amount of different quantities such as thickness, dimensions, electrical input power, substrate type, activity atmosphere and the shape of the sound pressure field for the heat-producing material in the heat sink speaker. Another result of this theoretical approach is to obtain a spectral pressure range at various frequencies from 1 Hz to 1 megahertz for the speaker at various distances.