Acoustic instabilities in a combustion chamber arise due to the coupling of acoustic pressure with in-phase heat-release, and are characterized by large amplitude oscillations of one or more natural modes of combustor. Even though an array of studies, both theoretical and experimental, has been conducted by a number of authors in this field to extract the flame response, most of these are based on kinematic flame models. In this dissertation, an experimental study of a subsonic flame's intrinsic response to acoustic pressure perturbations is performed for the case of a tube closed at one end and the other end opened to the atmospheric conditions. Pressure fluctuations inside the tube are measured for hot and cold side flows, and their varying trend is explained. The frequencies obtained from Fourier transform analysis exhibit a strong dependence with the distance between the stabilized flame position and open end of the tube. For different values of flame position (xf ), the values of growth constant 's' are calculated from the pressure versus time data readings procured from acoustic pressure transducer and dominant frequencies are analyzed from windowed FFT of the same. The expression for obtaining response function from the measured pressure fluctuations has been derived from the 1-D linearized conservation equations. The undamped response function plot is obtained by adding the decay rates at different frequencies inside the tube to the corresponding growth rates. Finally, the effect of blockage of pre-mixed flow on the growth rates inside the tube and consequently, the flame response values, is studied by repeating the experiment with different types of flame holders. A large number of theoretical flame-response models have been developed in modern literature, and some of these models are compared with the experimentally obtained response. Suggestions are also cited in this study so as to account for the observed deviations in trends. This includes a revisit of the intrinsic flame model by incorporating the effect of flame-area perturbations, with the aid of analyzed steady flame images.

Acoustic pressure in environment protection programs of developed countries is the one of most important issue after air and water pollution. Acoustic pressure is all around us, such like machines, traffics and some other things. It is not possible to completely stop the noise, but there are some certain ways were used to reduce it up to acceptable limits. The aim of this work is to measure the acoustic noise and find solution to reduce it. The first measurements were done indoor (workplace studio) focused on room acoustic. We measured the acoustic pressure for three heights in different places,but finally the most important results were selected and compared with respect to each others. The next measurements were performed outdoor to collect the results about spread of the noise, which were analyzed with certain limits. The main goal was to find how to safe the human health and maintain peaceful and healthy environmental.

No description available.

The conceptions in acoustics. The description of the methods measurement of the noise mechanical equipment. The express the uncertainty of measurement. The analyse and determination basic emission values of acoustic pressure and power. The summary of results and programming of the calculation basic emission values acoustic pressure and power for fine implementation measurement.

Master thesis deals with creating of the numerical model of the human vocal folds. Calculation algorithm is designed to include vocal chordsinteraction with the air flow. Analysis of the results achieved by the numerical simulations and calculations are focused on the pressure and velocity conditions in the areas under vocal folds, between vocal folds and above vocal folds. Movement and stress analysis of individual layers of vocal folds has been made. This analysis is limited only for physiological health vocal folds without pathology and disease. Modal analysis of structural and acoustic environment, backround research of vocal folds function and summary of some published overviews of numerical models is part of this work.

Master thesis deals with creating numerical model of the human vocal folds. Calculation algorithm includes interaction between vocal chords and the air flow. Modal analysis of structural and acoustic environment, backround research of vocal folds function and summary of some published overviews of numerical models are parts of this work. Analysis of the results achieved by the numerical simulations and calculations are focused on the pressure and velocity conditions in the areas under vocal folds, between vocal folds and above vocal folds. Movement and stress analysis of individual layers of vocal folds has been made. Impact of tissue thickness on resulting behaviour has been assessed.

This diploma thesis solves creation of a computing model of sound propagation for determination of acoustic pressure level around multiple point sound source with consideration of sound directivity of each source (compresor and fan). Thesis contains neccesary theoretical knowledge for creation of computing model and basics of sound measuring, theoretical computing model, measurement data of real unit and model configuration for real measured values. Computing code of the model in Visual Basic for MS Excel is a part of the thesis.

The acoustic emissions from supersonic jets represent an area of significant research needs; not only in the field of aero-acoustics, but in industry as well where high pressure let down processes have been known to cause acoustically induced vibrations. A common method to reduce the acoustic emissions of such processes involves dividing the single larger supersonic flow into several smaller ones. Though this is common practice, there is not yet a current model which describes the reduction of acoustic emissions from an array of smaller supersonic jets. Current research which studies supersonic jet arrays are mainly focused on the effects of screech. Though screech is important, due to its high amplitude acoustic pressure, this research focuses on the overall acoustic emissions radiated from supersonic jet arrays which can cause severe acoustic loadings. This research investigated the acoustic emissions and shock formations from several eight by eight arrays of axisymmetric jet experimentally. The array nozzle diameters investigated ranged from 1/8 inch to 1/4 inch and the spacing over diameter ratio ranged from 1.44 to 3. The net pressure ratios investigated ranged from 2 to 24. Results revealed a strong correlation between the acoustic emissions and the shock formations of the flow. Up until a critical net pressure ratio, the overall sound pressure levels were comparable to that of a single jet within an array. At net pressure ratios beyond the critical the overall sound pressure levels transitioned to higher decibel levels; equivalent to a single jet with an equivalent exit area of an entire array. Also, the characteristic acoustic frequency emitted from a nozzle array remained ultrasonic (above 20 kHz) at lower net pressure ratios and then shifted to audible levels (between 20 Hz to 20 kHz) at net pressure ratios beyond the critical. Also, before the critical net pressure ratio the shock cells from the jets within the array remained unmerged, but at net pressure ratios beyond the critical the shock cells merged and formed lattices of weak oblique shocks at first and then strong oblique shocks as the net pressure ratio continued to increase. The critical net pressure ratio was investigated by non-dimensional analysis. The non-dimensional analysis revealed that the critical net pressure ratio was a strong linear function of the spacing over diameter ratio. A linear model was derived which is able to predict the critical net pressure ratio, and in turn, predict a critical shift in the acoustic emissions of a nozzle array.

Although several studies have proven the accuracy of using a non-contact, air-coupled receiver in nonlinear ultrasonic (NLU) Rayleigh wave measurements, inconsistent results have been observed when working with narrow specimens. The objectives of this research are first, to develop a 3D numerical finite element (FE) model which predicts nonlinear ultrasonic measurements and second, to apply the validated model on the narrow waveguide to determine causes of the previously observed experimental issues. The commercial FE-solver ABAQUS is used to perform these simulations. Constitutive law and excitation source properties are adjusted to match experiments conducted, considering inherent effects of the non-contact detection, such as frequency dependent pressure wave attenuation and signal averaging. Comparison of “infinite” and narrow width simulations outlines various influences which impair the nonlinear Rayleigh wave measurements. When the wave expansion is restricted, amplitudes of the fundamental and second harmonic components decrease more significantly and the Rayleigh wavefronts show an oscillating interaction with the boundary. Because of the air-coupled receiver’s finite width, it is sensitive to these edge effects which alter the observed signal. Thus, the narrow specimen adversely affects key factors needed for consistent measurement of material nonlinearity with an air-coupled, non-contact receiver.

The diploma thesis is focused on analysis of noise level of mobile power vehicles in relation to motor operation mode and its evaluation. The thesis core lies within measurement of acoustic pressure levels at four vehicles in different motor operation modes by two audio-noise meters. One meter was places within the cabin, the other outside. The results of the work showed a development in reducing noise at modern power vehicles.