Thesis (M.Sc.(Engineering))--University of the Witwatersrand, Faculty of Engineering, 1981. / The majority of acoustic measurement criteria are derived from the integral of the squared sound pressure. This work shows the development of an instrument capable of measuring any of the Integral Square (IS) criteria l.n acoustic scale models of rooms using an impulse sound source. The variety of measurements is made possible through the use of a computer-based instrument. The microphone signal is filtered and then digitised. All further signal processing is done digitally in the computer. The results are either displayed numerically or plotted directly on an X-Y plotter. The instrument uses logarithmic quantisation of the signal and an algorithm developed by Hanrahan (103-105) which greatly simplifies the IS calculations to a process of table look-up and summation. The decay curves are calculated using Schroeders Method (27). This study shows the design and testing of the logarithmic converter having 54dB dynamic range and 3dB quantising intervals and interfaced to a NOVA 1200 minicomputer. The software to calculate RT, EDT and IRT was developed and tested on a 1/8th scale model of a reverberation room with known characteristics.

The effectiveness of partitions erected to, among other things, provide acoustic insulation between adjacent cavities in buildings, ships and aero-space structures is often limited by the presence of acoustical flanking paths. These paths, which can be provided by air gaps, ventilation ducts, etc., or by continuous walls or floors, allow sound and vibrational energy to pass between adjacent cavities without suffering the attenuation of the primary acoustic barrier (the partition). Techniques for the measurement of flanking transmission in existing structures and for its theoretical prediction have to date been awkward and imprecise, (the measurement technique requiring the erection of a second barrier to cover the barrier under test) and limited in range of application, (the theory only applying at frequencies for which the panel responses are mass controlled). The work described here was directed at removing these limitations on the present ability to measure and predict the effects of flanking transmission as they act to reduce the noise insulation attainable between two adjacent acoustic cavities. A cross correlation - Fourier transform technique was employed to measure the contributions of individual airborne flanking paths to the total sound field in a receiving cavity. It was discovered that cross correlation between two microphones, one in each of the source and receiving rooms, could not yield useful information about individual flanking paths because of the strong correlation of natural modes common to both cavities. This problem was overcome by replacing the source room microphone signal with the input signal to the noise source. The common room modes could not then correlate since only the receiving cavity microphone signal contained the mode components. This alteration, however, meant that only the relative magnitudes of the various flanking path transmission spectra could be obtained since the source room microphone signal was no longer available to provide a reference spectrum. However, for purposes of determining which flanking path contributes most to the receiving room sound field, relative magnitudes of their transmission spectra are all that is required. The altered technique allowed measurement of the transmission spectrum of an induced airborne path. Agreement with the measured difference in sound pressure levels with and without the flanking path, was very good. A relatively new structural dynamics technique known as Statistical Energy Analysis (SEA) was used to predict the noise reduction between two adjacent cavities, the boundaries of which were structurally coupled. SEA is based on an analogy to conductive heat transfer. Therefore, it becomes more accurate at higher frequencies as the wave fields in the acoustic cavities become more diffuse (have more spatially uniform energy densities). The SEA model developed here, then applied over all but the lower end of the experimental panel response range. This lower limit corresponds to the breakdown of diffuse wave field conditions in the smaller of the two experimental cavities. The model allowed the effects of the variation of panel internal damping and bending stiffness upon noise reduction to be investigated and was successful in predicting the noise reduction between two aluminum walled model rooms (for two different partition thicknesses) to within 2 dB over most of the frequency range 400 to 20,000 Hz. The results of the above two experiments and the corresponding SEA values of noise reduction, showed that, except at low frequencies where panel response is predominantly mass controlled, the increasing of primary barrier (partition) surface density does little to increase the noise reduction between cavities bounded and coupled by relatively lightweight, resilient walls. Increased panel internal or joint damping, however, increased the noise reduction at all frequencies but especially those near panel coincidences (when flexural wave speed in a panel equals the acoustic wave speed). / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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The current work is devoted to the problem of sound wave scattering by elastic cylindrical objects in a plain acoustic waveguide. The Multi- Modal Integral Method (MMIM) is proposed, which is based on nonstandard representation of the Green's function. It combines advantages of integral equation and eigenfunction methods and provides a quickly converging and highly accurate solution, taking into account all the waveguide modes up to infinite order. As illustrations of application of this method, acoustic diffraction is calculated from a system of several parallel homogeneous cylinders and from an air-filled cylindrical elastic shell. Numerical solutions are found for various versions of the system of elastic cylinders in a fluid layer with perfectly soft and rigid boundaries. Phase - frequency and amplitude - frequency characteristics are found for modal coefficients of the scattered field. Sharp increase of their amplitudes is found near resonance frequencies of the waveguide and the scattering cylinders. Pictures of the source density on the surface of the cylinders show that the nature of their distribution strongly depends on the frequency and the mutual location of the cylinders in the waveguide. Field structure near the cylinders reveals that higher-order waveguide modes play a significant role in the scattering process. Spatial distribution of the acoustic power flow near the scattering object is calculated for several frequencies and two sets of elastic properties of the cylinder. It is shown that at the critical frequencies of the waveguide as well as at the internal resonances of the cylinder the acoustic energy flows in closed paths in some regions of the waveguide. Near the internal resonances of the cylinder the closed paths are located in the near vicinity of the scattering object and partially go through its interior. It is suggested that re-radiation of the energy stored in the vortices may contribute to the echo phenomenon. The integral reflection coefficient is calculated for a system waveguide/shell for different values of wall thickness and distance between the shell and the waveguide bottom. Maxima and minima in the reflection coefficient associated with cut-on frequencies of the waveguide modes and structural resonances of the shell are identified. The calculations show that the conventional definition of target strength in a shallow waveguide is inappropriate. Different kinds of resonances are identified in frequency and angular dependencies of the velocity amplitude of the shell surface. These resonances belong to the following groups: a) critical frequencies of the waveguide modes, b) structural resonances of the elastic shell, c) resonance oscillations of the gas-filled interior of the shell, d) resonance oscillations of the coupled fluid-shell. Application of the Multi-Modal Integral Method (MMIM) to Sound Wave Scattering in an Acoustic Waveguide. Temporal sequences of pictures showing the spatial structure of the total and scattered fields in the near and far field zones are obtained. It is shown that the incident field produces waves of acoustic pressure propagating along the boundary of the scattering object, which, in turn, generate the scattered acoustic field. In the process of propagation, the waves may interact with each other via the fluid or the scattering object. This leads to significant changes of the structure of the acoustic field and of the amount of acoustic energy reflected from the scatterer. It is also shown that, in most cases, standing waves exist between the scatterer and the waveguide boundaries. Accuracy of the Multi-Modal Integral Method is discussed. It is shown that the implementation of the method requires few computer resources for good accuracy of the solution. / Thesis (Ph.D.)--School of Mechanical Engineering, 1999.

Thesis (M.S.)--Naval Postgraduate School, 2002. / Thesis advisor(s): Thomas J. Hofler, Bruce Denardo. Includes bibliographical references (p. 41). Also available online.

Perceptual low bit-rate audio coding schemes cannot be evaluated with the traditional method of measuring the difference between input signal and output signal. A valid measure is the severity of differences between input and output that are audible to human listeners, measurement of which requires well-conducted subjective testing. Test sensitivity is improved through the use of "critical" audio program. Selection of such audio material has in the past been done by subjective listening. An objective method of selecting critical material could offer many advantages, including faster and simpler selection of new materials, the ability to evaluate a much larger number of potential materials, and the potential of much better critical materials as a larger portion of available music is explored. The use of perceptual models for selection of critical materials has been investigated. Prediction of criticality was found to be inadequate with the models used.

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Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1999. / Copies of author's previously published articles inserted. One computer disc (CD-ROM) in plastic jacket pasted onto back cover. Bibliography: leaves 319-339. Also available electronically.