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Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2009 / In acoustics, pulse echo methods are well known as a means of measuring time of Hight. Traditional techniques for generating acoustic waves in solid ferromagnetic waveguides include piezoelectric, capacitive and magnetostriction. Piezoelectric and capacitive techniques are preferred due to the inefficiency of magnetostriction caused by electro-mechanical coupling losses and the fact that most ferromagnetic materials show low levels of magnetostriction. The aim of this study was to optimise the magnetostrictive effects for sensing applications based on a ferromagnetic waveguide using the pulse echo method. The results obtained were implemented in the design of an acoustic thermometer. Two configurations for signal generation and recovery were examined, the use of a single wound copper coil acting as a transceiver coil, and the use of separate transmit and receive coils. Results obtained using the latter configuration indicated better signal to noise ratio's and provided the flexibility to manipulate the point of signal recovery. The pulse echo method was implemented and optimised. An acoustic thermometer based on an existing design was developed by inducing a partial reflection from a set position in the waveguide, defining a sensing probe. Awareness of the elastic properties of the waveguide material enabled the guaging of its temperature by measuring the acoustic pulse velocity in the probe. The accuracy of the instrument was increased through signal conditioning, examined together with cross correlation and an increased sampling frequency. Systematic errors were resolved through calibration, giving the instrument an overall accuracy of ±O.56"C for the range of temperatures between 2O"C and 400"C.

Most musical instruments have undergone evolution over several hundred years. The pursuit of excellence in instrument building helps instruments in meticulously translating musicians's movement and gesture into sound. Musicians spend much of their time perfecting their movements and gestures to achieve the desired sound and expression. Digital technologies brought new perspective and possibilities to this performer-instrument relationship by detaching the tight relationship between gesture used and sound produced. This relationship can then be re-created with almost no restrictions, and composers have freedom to design their own controller or electronic instrument. The intention of this thesis is to further the development of gestural controller usage by understanding the evolution that has lead to the current trends in their use; how this evolution may inform newer development, and creating two new interactive systems for informed by the above. It also consists of a discussion of problems and challenges in interface building in terms of art and design. To demonstrate proficiency in traditional compositional genres, the first half of this dissertation consists of a composition portfolio. The second part of this dissertation is an investigation into the use of gestures and controllers in interactive electroacoustic music.

<p> The study described in this thesis aims to assess the effects of wind-generated noise on potential infrasound measurements on future Mars missions. Infrasonic sensing on Mars is being considered as a means to probe the long-scale atmospheric dynamics, thermal balance, and also to infer bolide impact statistics. In this study, a preliminary framework for predicting the principal wind noise mechanisms to the signal detected by a sensor placed inside a hemispherical porous dome on the Martian surface is developed. The method involves calculating the pressure power density spectra in the infrasonic range generated by turbulent interactions and filtered by dome shaped filters of varying porosities. Knowing the overall noise power spectrum will allow it to be subtracted from raw signals of interest and aid in the development of infrasound sensors for the Martian environment. In order to make these power spectral predictions, the study utilizes the Martian Climate Database (MCD) global circulation model, developed by Laboratoire de Meteorologie Dynamique in Paris, France. Velocity profiles are generated and used in semi empirical functions generated by von K&aacute;rm&aacute;n along with equations for describing the physical turbulent interactions. With these, turbulent interactions in the free atmosphere above the Martian surface are described. For interactions of turbulence with the porous filter, semi-empirical formulations are adapted to the Martian parameters generated by the MCD and plotted alongside contributions in the free atmosphere outside and inside the dome to obtain the total wind noise contribution from turbulence. In conclusion, the plots of power spectral densities versus frequency are analyzed to determine what porosity filter would provide the best wind-noise suppression when measured at the center the dome. The study shows that 55% (0.02 to 5 Hz) and 80% (6 to 20 Hz) porosities prove to be the better of the five porosities tested. </p><p>

<p> This dissertation is primarily concerned with the development of an effective methodology for reducing structure-borne sound radiation from an arbitrarily shaped vibrating structure. There are three major aspects that separate the present methodology from all the previous ones. Firstly, it is a non-contact and non-invasive approach, which is applicable to a class of vibrating structures encountered in engineering applications. Secondly, the input data consists of a combined normal surface velocity distribution on a portion of a vibrating surface and the radiated acoustic pressure at a few field points. The normal surface velocities are measured by using a laser vibrometer over a portion of the structural surface accessible to a laser beam, while the field acoustic pressures are measured by a small array of microphones. The normal surface velocities over the rest surface of the vibrating structure are reconstructed by using the Helmholtz Equation Least Squares (HELS) method. Finally, the acoustic pressures are correlated to structural vibration by decomposing the normal surface velocity into the forced-vibro-acoustic components (F-VAC). These F-VACs are mutually orthogonal basis functions that can uniquely describe the normal surface velocity. The weightings of these F-VACs represent the relative contributions of structural vibrations into the sound radiation. This makes it possible to suppress structure-borne acoustic radiation in the most cost-effective manner simply by controlling the key F-VACs of a vibrating structure. The effectiveness of the proposed methodology for reducing structure-borne acoustic radiation is examined numerically and experimentally, and compared with those via traditional experimental modal analyses. Results have demonstrated that the proposed methodology enables one to reduce much more acoustic radiation at any selected target frequencies than the traditional approach.</p><p>

Low-frequency noise is a health concern for workers in industrial workshops; rooms of highly varying size and dimensions, usually containing obstacles (the ‘fittings’). Low-frequency noise can be generated from sources such as reciprocating or rotating machinery, or ventilation systems. As the exposure time to the noise lengthens, workers are increasingly at risk to harmful effects such as hearing loss, communication difficulty, personal discomfort, and even nausea from induced body vibrations. Passive methods of noise control, such as absorption or barriers, generally perform better at high frequencies, but are inadequate at low frequencies. A proposed solution is active noise control, which relies on destructive interference of sound waves to reduce noise levels. However, this depends on phase, and how it is affected when sound waves encounter diffracting obstacles. In addition, the geometrical configuration of the active-control system must be optimized, which can be done using a prediction model. Sound-prediction models can also estimate the decibel level of sound within a given room configuration created by a source and the attenuation provided by the control system. Therefore, it is of interest to develop a model that predicts sound propagation in fitted rooms with phase. In this thesis, sound-pressure fields were investigated in rooms containing parallelepiped obstacles at low frequencies for which the wavelength is comparable to the obstacle dimensions. The geometric theory of diffraction (GTD) was used to model edge diffraction from an obstacle and, thus, the pressure field in shadow regions. A ray-tracing prediction model was improved to consider both the amplitude and phase of sound fields, and also the effects of edge diffraction. To validate the prediction model, experiments were performed in an anechoic chamber where a source and diffracting objects were located. In collaboration with Dr Valeau at the Université de Poitiers in France, a second model based on the finite element method (FEM) was used to compare prediction results. It was found that the phase depends mostly on the direct unblocked source-to-receiver distance. The FEM and experimental results showed that occluding objects cause phase shifts. The implementation of first-order diffraction into the ray-tracing program was successful in predicting shadow zones, thus producing a better prediction of realistic sound fields in rooms with obstacles. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

This thesis presents the results of analysis of the spectral shape and variability properties of the radio to X-ray continua of a sample of BL Lac objects and OVV quasars ("blazars"). These results have important implications for the unification of BL Lac objects, OVV quasars and low polarisation quasars into a single class of objects showing a smoothly continuous distribution of properties. The spectral shape of the violently variable millimetre to ultraviolet emission indicates that in this wavelength region the blazar emission is dominated by a very compact, single synchrotron component which becomes self-absorbed at wavelengths longer than 3 mm. The centimetre emission can be attributed to a separate, more slowly varying component, which can be identified with an underlying 'quiescent" component, similar to that observed in the low polarisation quasar 3C273. The OVV quasars also exhibit evidence of an optical excess, which we identify with the optical excesses which are observed in low polarisation quasars. The variability behaviour of the BL Lac objects is consistent with repeated injections or reaccelerations of electrons within in a small "flaring" synchrotron component, with subsequent radiative energy loss. In the case of the OVV quasars, however, the variability behaviour at near-infrared wavelengths is not consistent with emission from a single synchrotron component. It is shown that the behaviour of the Off quasars provides evidence for the presence of an additional non-variable, near-infrared spectral component, reminiscent of that observed in 3C273. The sizes of the regions responsible for the flaring and quiescent emission are deduced to be 10-2 and 10 parsecs, respectively. It is suggested that the flaring regions may represent regions of enhanced emission within a relativistic jet, possibly due to shock-waves within the jet.

Sound propagation is a complex subject, especially in an enclosure. The study of room acoustics involves not only a research into how sound is propagated in a room, but also a search into how to measure sound under different condition and how to control sound in the case of various wall materials. For an acoustical environment, there are three separated parts: sound sources, room acoustics, and the listens. These three items form a source-medium-receiver chain, which is typical for most of communication models. In this thesis, the image method is applied to predict the acoustical quality of a real room, and the experiment for room acoustic measurement is set up. The simulation model using image method proved the design of the measurement system is efficient for room acoustics.

The purposes of this research have been to determine a) the basic physical causes of acoustic scattering from zooplanktonic organisms and b) necessary criteria for future acoustic studies involving these animals. In situ measurements at 102 kHz have been made of the scattering of sound from a volumetric distribution of a zooplanktonic organism, the euphausiid. Quantitative information was recorded on analog magnetic tape and subsequently converted to digital form for analysis. Based on simultaneous measurements of side and back scattering from euphausiids and on a mathematical model, four-fifths of the scattered sound is considered to be caused by the compressibility contrast between the animals and the sea water. The remaining one-fifth is attributed to density contrast. Acoustic energy distribution curves are characteristic of the number and size of the animals contained in the small volume of the ocean that was studied. Acoustic counting of the animals gave results that compared favourably with simultaneous controlled net sampling. The back scattering cross-section of a typical euphausiid at 102 kHz has been found to be 1.4 x 10⁻⁴ cm² . Based on this value it is possible to predict the optimum frequency and intensity of incident sound for future acoustic studies involving these animals. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The acoustic signature of a particular speech sound varies according to its surrounding phonetic context. How listeners overcome these context dependent acoustic differences is not well understood. Researchers have attempted to find a one-to-one relationship between speech sounds and their acoustic realizations for over 50 years. More recently, relative acoustic representations have been proposed as a means of addressing the lack of acoustic invariance problem. Unlike absolute acoustic cues that are dependent on the context of a given speech signal, relative acoustic representations consist of abstract acoustic patterns that are context independent. The primary goal of this dissertation was to evaluate the perceptual significance of a relative acoustic representation of speech called a relative formant deflection pattern and formally test the relative formant deflection pattern hypothesis. A series of experiments were used to 1) establish how the relative formant deflection pattern hypothesis is different from other relative acoustic hypotheses of place of articulation perception and 2) explicitly test the ability of the relative formant deflection pattern hypothesis to predict listeners' place of articulation perceptions. Chapter 2 established that the relative formant deflection pattern hypothesis significantly predicted listeners' identifications more often than a locus-equation-inspired hypothesis in three out of four vowel contexts. Chapter 3 validated a novel acoustic measurement capable of extracting relative formant deflection patterns from recorded speech and then used the developed tool to evaluate listeners' perceptions of relative formant deflection patterns in natural, reduced speech. The experiment found that the relative formant deflection pattern hypothesis correctly predicted the dominant relative formant deflection pattern that solicited listeners' %/b/ identifications and %/g/ identifications, but not listeners' %/d/ identifications. Chapter 4 established that in a more controlled experimental setting the relative formant deflection pattern hypothesis accounted for listeners’ phonetic identifications across vowel contexts and speech manipulation conditions (i.e. simulated versus sine-wave speech). Additionally, the final experiment in Chapter 4 established that relative deflection in all three formant frequencies was necessary to fully account for the predictive power of the relative formant deflection pattern hypothesis. Taken together, the research provides insight into the perceptual significance of different relative acoustic representations of speech and proposes that a relative acoustic representation called a relative formant deflection pattern may be a perceptually significant solution to the lack of acoustic invariance problem.