Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 174-193). Also available via the Internet.

Thesis (M.S.)--Brigham Young University. Dept of Physics and Astronomy, 2006. / Includes bibliographical references (p. 57-59).

Thesis (M.S.)--Brigham Young University. Dept. of Physics and Astronomy, 2006. / Includes bibliographical references (p. 67-69).

No description available.

Includes bibliographical references (leaves 287-305) / 310 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1991

[Truncated abstract] The subject of fan noise generating mechanisms and its control has been studied intensively over the past few decades as a result of the ever-increasing demand for more powerful fans. A unique feature of fan noise is that it consists of high-level discrete frequency noise related to the blade passing frequency, and low-level broadband noise due mostly to turbulent airflow around the fan. Of the two types of fan noise, the discrete frequency noise is the more psychologically annoying component. Past research into fan noise has shown that the discrete frequency fan noise are dipole in nature and are caused predominantly by the fluctuating lift acting on the surfaces of the fan blades. Based on this, several theoretical models have been established to correlate these fluctuating lift forces to the far-field sound pressure. However, one general assumption in these models is that the fan blades are assumed rigid, and the consequence of such an assumption is that it is unclear if the far-field sound pressure is caused solely by the aerodynamic lift force, or whether the blade vibration also plays a substantial role in the generation of the far-field fan noise. One of the goal of this thesis was thus to experimentally quantify the contribution of blade vibration to far-field fan noise and it was found that blade vibration, whilst coherent with the far-field fan noise, did not contribute significantly. Aside of this, several experiments aimed at filling knowledge gaps in the understanding of fan noise characteristics were also be conducted, in particular, to understand the relationship between far-field sound pressure level to blade lengths as well as the number of blades on the fan. The experiments showed that for fans with many blades, the dependency of the far-field sound pressure on blade length is stronger than fans with less blades. Furthermore, dipole measurements showed that the dipole characteristics of fan noise does not occur only at the discrete frequencies, but also within a range of broadband frequencies, implying that the source for both discrete and broadband is the same. The second section of this thesis deals with the study of vortex shedding and its active control. When a circular cylinder (or any object) is placed in a flow within a specified Reynolds number range, flow separation and periodical wake motion is formed behind the cylinder, which is known as vortex shedding. It has been found in previous research that this wake motion is affected by acoustic field imposed on it via loudspeakers. This suggests that there is a strong acoustic-vortex relationship. However, little of this relationship is understood as conventional methods of studying vortex centre around the use of hot-wire anemometry, which effectively measures the velocity fluctuation in the flow. This thesis is the first in using a microphone to study the acoustic characteristic of the vortex wake, and experimental results shows that the two parallel shear layers of the wake carry the strongest pressure signals at the vortex shedding frequency, whilst the entrapped region between the layers carries the strongest pressure signals at the first harmonic.

No description available.

In some environments, there is a serious mismatch between the perceived (psychoacoustic) urgency of a warning and its situational urgency. In addition, many auditory warnings are not detectable within their environments. This research examined several prominent pulse parameters which affect the perceived urgency and detection time of auditory warning signals. These elements included pulse format (multitone sequential, multitone simultaneous, and rising sawtooth frequency-modulated pulse formats), pulse level (65 dBC and 79 dBC), and time between pulses (0 ms, 150 ms, and 300 ms). The environments of interest were those settings with steady-state broadband machinery noise. Conditions included a loading task which presented additional attentional demands upon the subject during the signal detection task. Free-modulus magnitude estimation quantified the relationship between auditory signal parameters and changes in perceived urgency. The method of paired comparisons was used to compare the perceived urgency of the auditory stimuli. Simple reaction time measured signal detectability. Signal effects were analyzed using a multivariate approach. Results indicated that there was a small but statistically significant relationship between perceived urgency and detection time. As perceived urgency increased, detection time decreased. Both perceived urgency and detection time were influenced by pulse level and format. The higher pulse level resulted in a greater perceived urgency of the signal and shorter detection time. Sequential signals were rated as less urgent than the other pulse formats, and subjects took longer to detect their occurrence. Under most conditions, there was no significant difference in the perceived urgency or detection time of simultaneous and frequency-modulated pulses. Time between pulses (inter-pulse interval) affected only perceived urgency, not detection time. The shorter the time between pulses, the greater the perceived urgency of the signal. / Ph. D.

An array of small, lightweight acoustic sources was investigated to determine how such an arrangement of sources would acoustically interact with a duct similar to that of a turbofan engine inlet. The sources were cylindrically curved aluminum panels excited in vibration by the application of a sinusoidally varying voltage to a piezoceramic actuator bonded to them. The finite element method was used as a design tool to size the panel based on desired vibration characteristics. A boundary element acoustic analysis was used to predict the acoustic output from various arrangements of sources. The central portion of the research was a series of experiments using an array of twelve sources arranged circumferentially in a duct. Measurements of the performance of each source revealed that the performance of the acoustic sources varied from source to source. This variation was assumed to have been caused by differences in the quality of the bond of each of the piezoceramic actuators to the panels. Directivity measurements were made in the far field. Measurements were also taken of the pressure field established in the duct cross-section. Modal decomposition was applied to the data. It was found that the dominant acoustic modes in the duct are those whose cut-on frequencies were near the frequency of excitation. / Master of Science

Through the use of multiple-input/single-output (MISO) modeling, the propagation paths of sound within a reciprocating refrigeration compressor have been investigated and ranked. By investigating the nature of sound propagation within reciprocating compressors, it is hoped that compressor manufacturers can effectively formulate strategies for compressor sound reduction. From experimental data of compressor far-field sound output, suspension spring forces, and internal pressure fluctuations, a MISO model has been developed. From this model, the importance of the suspension system to the compressor far-field sound spectrum has been identified. In the frequency range above 800 Hz, forces passing through the suspension system appear to be the dominant contributor to shell excitation and sound radiation. Based upon this finding, it is recommended that modified suspension systems be considered as an avenue for compressor sound reduction efforts in the future. / Master of Science