Статьи журнала - Техническая акустика
Все статьи: 276
Simplified instrumentation for ultrasonic measurements
Статья научная
Simplified instrumentation for ultrasonic measurements to generate and detect ultrasonic pulses in liquids and solids is described. High frequency pulse generator is assembled using integrated circuits (74LS00, 74LS90, 74LS93, 4093, 74121 and 7407), which generates variable frequencies (0.625, 1, 1.25, 2.5, 5 and 10 MHz), having pulse width 2 microseconds to 60 microseconds. The wideband receiver is developed using radio frequency amplifier (IC CA3028), zero-cross detector (LM393), and buffer amplifier (AD 826). The gain and bandwidth of the receiver are 50 dB and 15 MHz respectively. Transit time measurement has been taken on personal computer using analog to digital converter card. The system is found suitable, accurate and versatile for ultrasonic velocity and attenuation measurements.
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Simulation and visualization of loudspeaker'S sound fields
Статья научная
The visualization of sound fields is useful for understanding the directional features of sound radiation by an electro-acoustic transducer. The aim of this work is to present a numerical technique named here the Convolution Method (CM) for the simulation of sound pressure field radiated by a loudspeaker which is represented by the simple concept of vibrating rigid piston located in a baffle. Computational and visualization program is developed for sound fields emitted by such transducer. The numeric results of simulation for normalized contour surface map of acoustic pressure field are shown. For comparing simulation results, the transverse acoustical beam section of sound pressure field produced by an electrodynamical loudspeaker is measured using intensimetry techniques.
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Статья научная
2D-Array transducers are used in real-time 3D ultrasound imaging. Appropriate design of the received and emitted beam pattern from 2D-Array transducers is the aim of simulation of the beam pattern. In most methods of the simulation of acoustic fields, the far-field approximation was applied in order to increase the speed of the calculation. In this paper a new method that without applying any approximation converts the two dimensional Rayleigh integral over the transducer surface to one dimensional integral expression is used. It has shown proper speed and good accuracy both in near-field and in far-field. So it will be useful in intravascular sonography or low depth imaging, where the desired tissue is very near to the transducer. Also it can be used for optimizing array parameters in designing stage and also for education and research purposes. Simulation results are shown for different parameter settings to evaluate the effects of every parameter on the field of 2-D arrays.
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Статья научная
It is very important to explore the compound effect of heterogeneous environmental factors for improving the quality of living environment with popularization of electronic and/or information equipments. First, for an objective evaluation, three kinds of environmental factors: sound, illuminance and electromagnetic field of video (or visual) display terminal (VDT) are focused, especially from a stochastic viewpoint. From this experiment, some mutual correlation between sound and illuminance in peripheral environment of VDT was found. And also, another mutual correlation between electromagnetic field and illuminance environment of VDT was obtained. Secondly, for a subjective evaluation, three kinds of environmental factors: sound, temperature and illuminance are especially focused. If an optimum relationship among these environmental factors can be found, it seems that some improvements of our living environment can be discussed from psychological and/or stochastic viewpoint. From these studies, subjective and objective evaluation methods for room environmental factors will be considered.
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Spoken Tamil character recognition
Статья научная
Speech is one of the most complex signals and the powerful tool for communication. It has been a long desire of the scientists that the machine should recognize the speech of the human beings either for the machine to function on voice commands or for giving a text output of the speech. Automatic recognition of speech by machine has been a goal of research for more than four decades. Now speech recognition tool has become a necessity for busy executives and industrial applications. Since beginning the research in this direction has been concentrating on English Speech recognition. Only from the last few years works are being carried out for recognizing speech in other languages. The Indian languages are structurally and syntactically different from Latin. This paper presents an approach for the recognition of spoken characters in Indian languages particularly Tamil using acoustic features of individual letters. A three layered back propagation neural network approach used for solving the problem is presented. The efficiency of the method presented is highlighted by applying the same to Tamil characters recognition.
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Static versus low frequency dynamic elastic modulus measurement of thin films
Статья научная
A new experimental technique for evaluating Young's (or elastic) modulus of a vibrating thin film from a dynamic measurement is presented. The technique utilizes bending resonance from a remote acoustic excitation to determine Young's modulus. Equations relating the natural frequencies to the mechanical properties are obtained, and Young's modulus is subsequently determined. Young's modulus values from dynamic test are compared with those (static) obtained by a standard tensile test, and consistent results are obtained. The proposed technique is relatively simple and could be used to determine Young's modulus of a wide variety of sheet materials initially having no bending stiffness. It can also be used for determining other mechanical properties, such as compliance methods in connections with fracture mechanical testing, fatigue and damage measurements. This work emphasizes the feasibility of a damage assessment of components in-service by evaluating changes in the material characteristics.
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Strain sensing of interior structural radiation modes on a simply supported panel
Статья научная
Radiation modes have shown great promise as a suitable measure for estimating the power radiated from structures into both the free field and enclosures. These modes may be sensed with either discrete out-of-plane transducers or continuous strain transducers. Here appropriate sensor shape equations are derived to accurately quantify the interior radiation modes from a simply supported panel radiating into a cavity using piezo-electric strain transducers.
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Статья научная
The purpose of this work is an implementation the newly developed measurement technique for structural energy estimation in-situ. The technique based on reusable PVDF strain gauges in measurement of structural energy flows transferred by longitudinal and shear forces, bending and torsional moments in water filled pipes are presented and discussed. Developed technique was implemented for determination of the efficiency of noise control means in the pipeline with the test rig "Flow". The technique also applied in order to determine the components of vibrational energy flows generated by pump incorporated into the rig "Impedance". Measurements were performed when real pumps have been run. The relationships between four structural components in pipe walls and water ones transferred with plane waves in the pipe channel were determined. Degree of uncertainty in proposed technique was revealed using error analyses taking into account the real data about coherence function and measured phase angle between force/moment and linear/angular velocity. The reliability of preformed measurements at tonal components of structural energy spectra proofed employing an error analyses and energy balance calculation in pipe cross section. The possibilities and limitations of the technique were addressed.
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The importance of spectral magnitude in signal through a noisy channels in data transmission
Статья научная
In this paper we present a novel technique that can be applied in data transmission noisy channels. The method is based on the reconstruction of one or two dimensional signal with finite support from only its Fourier transform magnitude computed from the converted original signal. This method is the Saxton and Gershberg iterative algorithm for reconstructing a non minimum phase sequence from magnitude, using this method we have proved that spectral magnitude is less sensitive to additive white Gaussian noise than spectral phase and signal. The algorithm is operated on converted non minimum to minimum phase signal by concentrating the higher energy on the left hand end sequence than the right hand end sequence, thus dealing with magnitude we have achieved a good performance at all SNR levels with limited extra computational complexity.
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The influence of the temperature on the power structure behaviour
Статья научная
The functional limits of a thyristor are defined by an essential parameter: the allowable maximum junction temperature. It can be of 125°C. The technical specification of each type of thyristor gives the limit values of the functional temperature, which will be for example of 55°C to 125°C. The electrical energy dissipated in the thyristor at any direction of the current, appears under the form of a thermal energy across the junctions, where the total power lost in the crystal causes an increase of the junction temperature. In fact, this value represents only a very little part of the power available in the circuit. But it shouldn't be forgotten that the ambient temperature represents an energetic level from which, the increase of the junction temperature is done. However, this latter can reach a value relatively increased, especially if we know that the industrial ambient temperature is very often approximately equal to 50°C or 60°C.
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The performance analysis of an HVDC link
Статья научная
This paper presents the results of a simulation study on a 12-pulse HVDC (High Voltage Direct Current) using a system in Simulink, specialized Toolbox in Matlab for simulating control systems. The object of the study was to investigate the steady state and dynamic performance of the system. First, response of current regulator after change in current reference is examined in order to see the behaviour of the controllers in controlling the desired current. Next, the digital simulation of a test system is presented and the response to a DC fault in the line and the AC fault at inverter side is shown. The results are evaluated to enhance the recovery of the system from the disturbances and voltage dependent current limits (VDCOL) for a full range of typical system disturbances. The presented approach benefits from Simulink's advantages in modeling and simulating dynamical systems. The simulation results are compared with other studies using EMTP (Electromagnetic Transient Program) and EMTDC (Electromagnetic Transient Program DC Analysis).
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The utilization of the PWM inverter feeding in the asynchronous motor command
Статья научная
In this paper, the acquisition of new control tools and the improvement of the PWM inverter feeding in the asynchronous motor command are considered. In the asynchronous machine, the directions flow and the electromagnetic torque are closely tied. The scalar command law allows to maintaining the flow and consequently the electromagnetic torque constant for high frequencies. However, this law is very insufficient at low frequencies, what leads us to proceed with the field-oriented asynchronous pulse-width modulation, for high-performance asynchronous machine drives operating at low frequency. The performances of this command are evaluated by using results of simulation realized by the MATLAB software under the SIMULINK environment. The obtained simulation results prove the efficiency of the PWM signal in the command and give excellent electrical magnitude performances of the system. With this easy model, the simulated system, constituted by the asynchronous machine and the two levels voltage inverter, is achieved in a very short time interval and the obtained results are interesting.
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Two dimensional signal reconstruction from its spectral phase
Статья научная
In this paper, a new algorithm of image reconstruction from the phase of its Fourier transform is presented. It uses the eigenfilter method to approximate a given phase response in the least square sense. This method has proved its efficiency as the iterative methods. An example will be given to show the importance of this algorithm and the high image reconstruction quality.
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Статья научная
The article presents a study of guided ultrasonic waves in a tri-layer structure formed of two aluminium plates assembled with a layer of epoxy resin. The resolution of the dispersion equation was used to plot the dispersion curves and to determine the energy distribution of a given mode propagating within the structure. On the other hand, the evaluation of the interaction of guided modes at low frequency with hidden defect placed either in one of the aluminium plates at the interface between aluminium and the adhesive layer was presented. The analyse of the reflected and transmitted waves in the dual space (wave number, frequency) permitted the separation and the identification of the propagating guided modes and showed the conversion phenomenon of the incident mode into other guided modes after the reflection and the transmission. Finally a quantification of the energy of the reflected and transmitted modes is presented.
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Uncertainty in structural intensity measurement (review)
Статья научная
The purpose of the paper is to point out the main sources of errors for widespread experimental methods used in estimation of vibrational energy flows. This paper comprises the list of the main errors and ways of errors estimation using real data obtained during measurements. The errors analysis was carried out to show a methods sensitivity to various sources of errors. The main sources of uncertainty for the most popular and state-of-art techniques namely Direct Method, Strain Gauge Method, Complex Transmissibility Technique, Mobility Energy Flow and Finite Difference Technique are emphasized and reviewed. The practical examples of errors estimation with knowledge about structure materials, real experimental data and channels features are shown in the paper. The analysis can help researcher in determining the confidence level of measured data. The practical formulae and measurement set-ups are presented.
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Using stationary wavelet transform in bch image coding
Статья научная
This paper propose a novel technique that use efficient image compression based on stationary wavelet transform (SWT) decomposition and Bose-Chauhuri-Hochquenghem (BCH) coding for image transmission over additive white Gaussian noise (AWGN) channel. We know that, discrete wavelet transform (DWT) decomposition suffers a drawback which is the not time-invariant transform, leading to a non-redundant decomposition. This means, even with periodic signal extension, the DWT of a translated version of a signal X is not, in general, the translated version of the DWT of X. In such case, the use of an adequate wavelet transform that restore the translation invariance, appear efficient to give high-quality image reconstruction. Also, BCH coding is a linear block procedure that permits multiple-error bit correction over noisy channel. The idea is to combine efficient image compression gived by SWT decomposition with an exam of the selection of the Bose-Chauhuri-Hochquenghem (BCH) error correction codes applied to the low pass version at different decomposition level. By using BCH(63,45,3) code against the channel degradation, a significal improvement is obtained.
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V/ U-ритм речи при чтении как индикатор состояния функции речевого дыхания у заикающихся
Статья научная
В статье показано, что чередование длительностей в речевом акустическом сигнале, когда голосовые складки колеблются с частотой основного тона, и длительностей, когда таких колебаний нет, образует Voice/Unvoice-ритм (V/U ритм). Ранее исследованиями автора было обнаружено, что такая структура ритма хорошо описывается экспериментально квадратичным логистическим отображением и соответствующим сценарием Фейгенбаума для перехода системы к хаосу. Начальная часть нижнего крыла сценария отвечает U- сегментам, а начальная часть верхнего крыла соответствует V-сегментам. За критической точкой эти два крыла смешиваются, образуя V/U-ритм нормальной речи. Докритическая область сценария соответствует регулярному чередованию голосовых (V) и неголосовых (U) сегментов, свойственному запинкам типа повторов, причем в этой области выполняются регулярные соотношения между суммарными длительностями голосовых и неголосовых сегментов (отношение их суммарных длительностей примерно равно 3/2). В зоне хаоса эта регулярность утрачивается. Предлагается использовать утрату этой регулярности как способ и индикатор исправления заикания. Манипулируя режимом дыхания, можно добиться, чтобы указанная регулярность утрачивалась. В силу закона функциональной композиции утрата регулярности будет свидетельствовать о том, что система находится в закритической области. В этом случае увеличение возбуждения фонации в системе речеобразования обеспечивает появление речевого V/U-ритма без заикания. В работе приведены статистически достоверные данные, подтверждающие этот тезис.
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Viscoelastic damping and piezo-electric control of structures subjected to aerodynamic noise
Статья научная
Analytical and numerical simulations are carried out in order to identify physical parameters affecting acoustic and motion control by viscoelastic piezo-electric and material damping. Numerical simulations in terms of critical parameters, such as relaxation functions of the structure and piezo-electric devices, aerodynamic coefficients, Mach number, are carried out to evaluate their sensitivity to system responses, sensing and structural control. Computational simulations indicate that light weight piezo-electric viscoelastic devices can be effectively used to actively or passively control flight vehicle structural response to aerodynamic noise.
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Статья научная
In this paper voltage stability is evaluated by both dynamic PV curve and time domain simulations with a developed and detailed model of STATCOM. Also effect of load with different power factors in voltage stability assessment is studied. The results presented in this paper show that STATCOM can greatly improve the voltage stability and maximum loading capacity of power systems, especially in lower power factors. It is also shown that the margin of stability can be easily and effectively evaluated by PV curves because they cover a wide range of system operating conditions. Also it is illustrated that reactive power has more effects on voltage stability rather than real power.
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Vortex-acoustic instability in chambers with mean flow and heat release
Статья научная
Acoustic instability appearing in chambers with isothermal or reacting mean flow is an important engineering problem. The subject of this work is the instability that is coupled with vortex shedding and impingement, which can also be accompanied by heat release. A reduced-order theory is formulated that includes the chamber acoustics, vortex-structure interaction, and unsteady heat addition. Assuming that acoustic sources are localized in space and time, the kicked oscillator concept is applied. Model results are compared with experimental data. Possible applications for flow control are discussed.
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