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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Blind signal separation

Chan, D. C. B. January 1997 (has links)
The separation of independent sources from mixed observed data is a fundamental and challenging signal processing problem. In many practical situations, one or more desired signals need to be recovered from the mixtures only. A typical example is speech recordings made in an acoustic environment in the presence of background noise and/or competing speakers. Other examples include EEG signals, passive sonar applications and cross-talk in data communications. The audio signal separation problem is sometimes referred to as <I>The Cocktail Party Problem</I>. When several people in the same room are conversing at the same time, it is remarkable that a person is able to choose to concentrate on one of the speakers and listen to his or her speech flow unimpeded. This ability, usually referred to as the <I>binaural cocktail party effect</I>, results in part from binaural (two-eared) hearing. In contrast, a person with a severe hearing loss in one ear finds it is difficult to focus on a particular speaker under the same circumstances. A signal separation pre-process would be desirable in such circumstances. Signal separation techniques can also be applied in many other areas such as noise reduction, speech recognition and multi-media applications. The term 'Blind Signal Separation' refers to the lack of any propagation model: only statistical independence of the sources is assumed. The lack of other prior information underlines the difficulty of the problem. Observations may be modelled as linear mixtures of a number of source signals, i.e. a linear multi-input multi-output system. In this dissertation, the general n-source n-sensor (<I>n x n</I>) linear time invariant wide-band system is studied, in which, <I>n</I> random signals are received at <I>n</I> sensors and these signals originated from <I>n</I> sources. The problem is to recover the sources from observed signals only. Various block-based iterative algorithms are proposed which use output decorrelation as a signal separation criterion. These algorithms search for a linear transformation that minimises the statistical correlation between the components. Some existing solutions are reviewed and compared.
42

Weak compression waves in relaxing gases

Scott, E. M. January 1972 (has links)
Studies have been made of the structure of weak compression (shock) waves in relaxing gases. These studies have been primarily concerned with the development of separate theoretical and experimental techniques. These techniques are not inter-related in general except in the overall context of Vibrational relaxation. The theoretical studies have been concerned with the influence on the structure of weak normal shock waves of translational non-equilibrium. bimodal relaxation and second order unimodal relaxation. The affect of translational nonequilibrium on the relaxation process has been studied by forming an asymptotic expansion in the ratio of the viscous length to the relaxation length. The perturbation scheme was singular and required the application at the method of matched asymptotic expansions. Bimodal relaxation has been studied by forming an asymptotic expansion in the ratio of the energy of the secondary vibrational mode to the total Vibrational energy. The addition of a second order term to the rate equation describing the behaviour of a single vibrational mode has also been studied by forming an asymptotic expansion. In this case. the perturbation parameter was the ratio of the two relaxation times concerned. The experimental studies have been concerned with the production and study or weak normal shock waves in the Cranfield Institute or Technology 2&quot; shock tube. A time resolved quantitative schlieren system has been used for the study of the weak normal shock waves. This particular system had been developed previously for this purpose. and further developments and refinements have been made to it. Experimental studies have been made with the schlieren system of the structure or strong incident shock waves in carbon dioxide. The vibrational relaxation time of carbon dioxide determined in this way for translational temperatures from 300 o K to 1200 o K has been found to be in reasonable agreement with measurements made elsewhere. A technique has been developed for the production of weak incident shock waves in t.i.1e shock tube, which involved the positioning of a perforated plate in the channel of the shock tube. The vibrational relaxation time of carbon dioxide determined in this way for translational o temperatures of approximately 300 K has been found to be in good agreement with measurements made elsewhere. Good agreement has also been obtained between the experimentally measured density gradient profiles and theoretical profiles. The curvature of the shock waves obscured the transition from a fully dispersed to a partly dispersed shook wave. Further improvements and refinements have been made to a technique which had been developed previously for the propagation of weak normal shock waves through the reflected shock region of the shock tube flow. This technique was used to study the behaviour of fully dispersed shock waves at high temperatures. The relaxation time of carbon dioxide determined in this way for temperatures from 300 o K to 600o K has been found to be in good agreement with measurements made elsewhere.
43

Vibration of thin cylindrical shells of variable thickness

Falkiewicz, A. January 1952 (has links)
No description available.
44

Subjective judgements in concert halls in relation to current room acoustic criteria

Sotiropoulou, Alexandra January 1982 (has links)
No description available.
45

Angular deflection of light beams using ultrasonic techniques

Holt, D. January 1969 (has links)
No description available.
46

The acoustics and psychoacoustics of the distributed-mode loudspeaker (DML)

Harris, Neil John January 2001 (has links)
No description available.
47

The use of high frequency ultrasound to study dimensions and properties of skin

Edwards, C. January 1984 (has links)
No description available.
48

A hemispherical acoustic resonator for the measurement of the speed of sound in gases

Angerstein, Jeanette Louise January 2000 (has links)
A hemispherical acoustic resonator is described which was designed and constructed for the measurement of the speed of sound in gases, at pressures up to 40 MPa and at temperatures in the range from 300 K to 400 K. The hemispherical geometry retains many of the advantages characteristic of the spherical geometry but affords a major advantage at high pressures because one of the transducers may be placed at a position of maximum acoustic density for the radial modes and so loss of signal strength is minimised. A detailed description is given of the resonator and pressure vessel, the thermostat and the various measurement techniques employed. Characterisation of the resonator was achieved using a prototype equatorial plate for which the sound source could be moved over the radius of the cavity. Using the prototype plate, measurements performed in air at room temperature and pressure allowed the transducer configuration to be optimised. Calibration of the resonator was possible by comparison of the values of ula(pj) obtained isothermally in nitrogen with data obtained previously using a spherical resonator. These measurements allowed the resonator's geometry to be characteriseda nd the dependenceo f the radius on temperaturea nd pressuret o be modelled. The semi-empirical model developed using the results of the calibration was tested using measurements obtained in argon; results were obtained simultaneously from the hemispherical resonator and a well-characterised spherical resonator. Measurements on propene together with the results from nitrogen allowed the halfwidths to be modelled and enabled useful information about the loss mechanisms occurring to be extracted from the measured halfwidths. Tetrafluoromethane was subsequently studied and the acoustic virial coefficients and vibrational relaxation times were measured and compared with literature values.
49

The acoustics of small rooms at low frequencies

Gilford, Christopher Leonard Sidney January 1994 (has links)
No description available.
50

Application of laser doppler velocimetry to rotor vibration measurement

Bell, John R. January 2001 (has links)
Vibration measurement is of fundamental importance in many machinery applications including for the development and monitoring of rotating machinery. In such applications, measurement of the vibration transmitted from the rotor into a non-rotating part of the structure is the most common arrangement but this cannot always be relied upon because vibration transmission may be low. In such cases, the use of a non-contacting vibration transducer capable of measuring vibration directly from the rotor itself is desirable. Laser Doppler Velocimetry (LDV) is a non-contacting vibration technique capable of such measurements but vibration measurements on rotating structures using LDV have been shown to be ambiguous. The sensitivity of the measured velocity to other rotor vibration components can be significant enough to mask the intended vibration measurement entirely. This thesis examines the use of LDV for vibration measurements on rotating structures more comprehensively than in any previous study. A new and completely general theory is developed to allow the velocity sensitivity of LDV measurements taken from rotating structures to be described for laser beam incidence in an arbitrary direction on a target element requiring 6 degrees of freedom to define its vibratory motion fully. Extension of the' theory to optical configurations incorporating multiple laser beams is also included with a number of useful instrument configurations established. The theory enables some fundamental questions regarding the use of LDV on rotating structures to be answered. Of particular importance is the confirmation that direct measurement of radial or pitch and yaw vibration is not possible because the measurements will always be unavoidably cross-sensitive to other motion components. Resolution of these components is possible, however and a new method of resolving steady state, non-synchronous radial, pitch and yaw vibrations is presented enabling a range of measurements to be made for the first time using LDV. Several of these measurements were made on a running IC engine and of special note are the angular vibration measurements made using a novel instrument incorporating 3 beams, the laser angular vibrometer, designed specifically for the task. Errors within the resolution technique are considered in detail and, looking forward, a number of promising means by which to reduce error magnitudes are introduced and recommended for further investigation. LDV has great potential for rotating machinery diagnostics and such developments are key to achieving this potential.

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