Journal of Theoretical and Applied Vibration and Acoustics
Volume:7 Issue: 2, Summer & Autumn 2021

To improve teaching and learning conditions, room acoustics tries to adjust the educational environment. This research addresses the room acoustics of a small classroom in the Department of Mechanical Engineering at Isfahan University of Technology. It has been studied by modeling and simulating the classroom in the COMSOL software using the ray tracing method, and the reverberation time has been calculated. The model is validated by field measurements and using the impulse response function according to ISO 3382-2 in terms of the reverberation time. Then, it is compared by the Eyring's and Sabin's empirical relationships, as well. The acoustics modes of the classroom have been obtained using the modal analysis in the COMSOL software. ANSI/ASA S12.60 is employed to design the suitable acoustical environment for the classroom and absorption panels and bass traps are utilized to reach the desired condition. After applying the adjustments, the reverberation time has reached the recommended standard value, which increases the sound quality. Other acoustical parameters such as Definition (D_50), Clarity (C_50) and Sound Transmission Index (STI) have been calculated before and after the acoustic correction. Results revealed that speech intelligibility have been fallen in the acceptable range.

In this paper, the Bouc-Wen friction model parameters of two elastic rough interfaces are determined analytically based on multi-asperity contact theory. To this, firstly, the three main contact characteristics parameters of the rough interface, including slippage friction force, slippage displacement limit, and tangential interface stiffness are determined. According to multi-asperity contact theory, these three main contact characteristics are physically meaningful parameters that are related to measurable rough surface parameters such as standard deviations of asperity heights, mean radius of asperity summits, and areal density of asperities. Then the new frictionally contact model of the rough interface is produced as a first-order differential equation form. This model is obtained using exponential function curve-fitting on the interface stiffness behavior of the rough interface. Finally, the constant parameters of the Bouc-Wen friction model are related to three main contact characteristic parameters using a first-order differential equation. In other words, this paper proposed an analytical way to establish the Bouc-Wen model by measurable topography parameters of contacted rough surfaces. The capability and application of the proposed model in engineering problems are investigated and validated experimentally.

The complexity of tire/road noise generation and amplification mechanisms has made it challenging for tire builders to reduce emitted sound. Statistical methods help to model complex problems. This paper predicts tire noise level by a superior regression method in machine learning, relevance vector machine, with a total noise prediction error of 0.62 dB(A). The tire’s noise sensitivity to its parameters is analyzed by applying a small central composite design to the developed model. The effect of grooves’ shapes on tire noise is preserved in the results, unlike the previous publications. For a case study, grooves’ depth has been recognized as critical in controlling tire noise. Based on the variance analysis results, the interaction of this parameter with the number, length, and width of transverse grooves has also been identified as significant. According to the parametric study’s striking tips, two sets of tread pattern specifications are proposed for noise reduction, utilizing the response surface method. They reduce the noise level by 1.72 and 1.54 dB(A) for a tire with a measured noise of 75.88 dB(A)

An Euler–Bernoulli nanobeam is stabilized using a robust adaptive sliding mode control. Using nonlocal strain gradient theory and Hamilton’s principle, a nonlinear partial differential equation is derived to demonstrate the vibration behavior of the considered nanobeam. Moreover, the obtained partial differential equation is converted to an ordinary differential equation using the Galerkin technique. To suppress the nonlinear vibration of the nanobeam and overcome the uncertainties, robust adaptive vibration control is designed using an extended Kalman filter and sliding mode control. Finally, simulation results show the performance of the designed robust adaptive controller. Furthermore, the traditional control schemes are used to illustrate the superiority of the proposed controller over them.

Metro stations as public places are very important in terms of speech clarity, safety, and security. However, due to the size and physical-special characteristics of these places, the use of non-acoustic materials, and providing acoustical comfort is practically not possible, and in emergencies, hearing voice messages is not possible for people with different mental and physical conditions and workers are prone to hearing damage. The purpose of the study is to assess the acoustic conditions of metro stations to provide auditory satisfaction. Two crucial and distinct stations of Tabriz city were measured using B&K2260 sound level meter. SPL and RT are two of the most significant parameters in users' auditory satisfaction, which are used in the assessment of sound level and speech perception by humans. The measurements and evaluations show that (Lt) in Saat and Khayyam Stations are 106.4 and 104.2 dB, and the minimum is 85.6 and 82.4 dB, respectively. The measured maximum reverberation time (RT) is 7.21 and 5.17 seconds, respectively, at frequencies of 500 and 630 Hz with Gain=-20. According to the values of international standards, both parameters are in the unacceptable range, and in addition to causing irreparable damage to human hearing, in the long run, it covers all sounds, and people are not able to hear the sounds with lower levels than the level of the environmental noise. Therefore, by increasing the surfaces and reducing the volume via architectural elements, it is possible to help improve the acoustical conditions in metro stations.

In this paper, the vibration analysis methods and shock pulse method (SPM) are compared in order to detect the unhealthy condition as well as fault type in the early stages of rolling element bearing (REB) degradation. To analyze vibration signals, three weak signature detection methods based on continuous wavelet transform (CWT), empirical mode decomposition (EMD) and envelope technique are employed. A set of accelerated life tests on REBs was designed and performed in CM lab of Sharif university of technology. Seven tests were conducted and vibration signals, as well as shock pulse signals, were recorded regularly. The trend of vibration level and shock pulse level are compared for early detection of the unhealthy condition in REBs. In addition, the extracted spectrums from SPM, CWT, EMD, and envelope techniques are studied to detect bearing characteristics frequencies (BCFs) to diagnostics. Results show that SPM has better performance on early fault detection of REBs rather than vibration analysis techniques.