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Partially absorbing parallel surfaces can be the dominant acoustical feature of many rooms. The sound fields in such rooms are not diffuse, which causes difficulty in predicting sound pressure levels. A method is developed for predicting sound pressure levels in these rooms. It rests on the prediction of sound pressure levels caused by a nondirectional source of known sound power radiating between absorbing parallel planes. The development proceeds from a geometrical acoustics viewpoint. Good correlation has been found between predicted and measured levels in existing rooms having simple geometries. National Science Foundation (U.S.) - Grant ENG74-00984A01

Geometric acoustics has been used to study the propagation of sound waves in a homogeneous moving medium with sheared flow bounded by the hard walls of a duct. Differential equations describing the ray trajectories and the spreading losses along each ray were developed and solved numerically for a range of centerline Mach numbers and shear boundary-layer thicknesses. Results were obtained which show the effects of upstream and downstream sound propagation on the ray paths. A method was also developed to allow the calculation of intensity loss profiles at specified downstream cross sections of the duct.

The sound absorption performance of an acoustic absorber consisting of a stretched circular membrane placed a short distance in front of a fiberglass blanket was both measured and predicted. Both theoretical and experimental analyses were restricted to plane acoustic waves. Theoretical predictions indicated that the membrane-blanket combination would have a sound power absorption coefficient nearly equal to the sound power absorption coefficient of the blanket alone if the incident acoustic plane wave drove the membrane at one of its resonance frequencies. Theoretical analysis also predicted that the sound power absorption coefficient would approach zero when the membrane was driven at an antiresonance frequency by the incident acoustic plane wave. Experimental agreement with theoretical predictions was good for several membrane-blanket combinations. The results show that membrane-blanket combinations can be effective acoustic absorbers in frequency ranges which do not include the antiresonance frequencies of the membrane. The equations developed may be used to predict the acoustic performance of any membrane-blanket combination.

The prediction of sound pressure levels in irregularly shaped factory spaces is approached through an analysis of acoustically coupled rooms. Previous work has indicated that room surface sound absorption characteristics create a theoretical interdependence between the sound pressure fields in each coupled volume. Whenever a barrier is interposed between a sound source and observer, the effects of diffraction must also be considered. A computer algorithm was developed to predict sound pressure levels in irregularly shaped rooms using a modified statistical acoustic theory. The program is based on a conversational technique and requires the user to enter information regarding room geometry, surface absorptions, receiver placements, and source sound power and placement information. Several output options are provided, among which is a plan view print-plot of sound pressure level contours in the analysis room. [Work supported by NSF.] National Science Foundation (U.S.)

Partially absorbing parallel surfaces can be dominant acoustical feature of many rooms. The sound fields in such rooms are not diffuse, which causes difficulty in predicting sound pressure levels. A method is developed for predicted sound pressure levels in these rooms. It rests on the prediction of sound pressure levels caused by a nondirectional source of known sound power radiating between absorbing parallel planes. The development proceeds from a geometrical acoustics viewpoint. Good correlation has been found between predicted and measured levels in existing rooms having simple geometries. [Work supported by NSF.] National Science Foundation (U.S.)

While photovoltaic research conducted to date has been primarily in the areas of utility peak shaving, residential applications, and stand alone applications, this research examines photovoltaic potential as a commercial customer initiated demand side management tool and develops a methodology which can be used to evaluate the value of a PV storage system. The primary focus is on the effect photovoltaic orientation and utility billing practices have on overall system value. Using a year's worth of 10 minute data collected at the Virginia Tech Solar Experimental Station and an academic building load, the effects of array orientation on clear sky photovoltaic production and the match between that production and the building load profile are discussed. The performance of five photovoltaic array configurations for both building energy and demand reduction were simulated. It was shown that arrays facing south-southwest and southwest have lower performance that a south facing array in spite of the presence of an afternoon building load peak. The impact of battery storage as a supplement to a PV system has been examined. The affect of battery efficiency, array orientation, and battery dispatch setpoint on ovarall system value has been investigated. Based on economic assumptions and billing schedules obtained from a medium sized utility, the present value of the various array performances were evaluated. It was found that O&M costs could significantly effect the value being assigned to array configurations with superior performance. Due to the variability of the available insolation, smaller arrays have a greater value than larger arrays. It was shown that the value of a PV storage system is greater than either of the two component systems taken alone. By adding battery storage to the PV system, the optimum system size was increased. Master of Science

Friction induced vibration of an epoxy coated shaft rotating in an elastomeric bushing is investigated. This study investigates the manner in which system dynamics and friction mechanisms are responsible for friction induced vibration and noise generation. A test method was developed to measure the friction torque and the system and acoustic response of the sliding system. Several materials including a fluorocarbon elastomer, a polydimethylsiloxane, and a natural rubber were tested. Three friction regimes were observed which were stick-slip oscillations, quasi-harmonic oscillations, and steady sliding. System stiffness and load were varied to observe changes in the critical velocities bounding each regime. System parameters were varied to determine sliding conditions leading to self-induced vibration, to establish how the character of vibration is affected, and to correlate friction torque with system and acoustic vibration for each elastomeric material. A two degree-of-freedom, lumped parameter model was developed to simulate the effect of system dynamics on the sliding behavior of the elastomeric bushing. The comparison of simulated and experimental response using analyses in the time and frequency domain indicate the predictive model provides an excellent representation of stick-slip behavior at various operating conditions. Master of Science

Five changes were suggested to improve the kidney dialysis machine, including a conductivity control feedback system, a positive pressure deaeration system, a balancing system for dialysate flow control, a diaphragm-type blood pump, and an ultrafiltration control system using blood pressure measurement. A kidney dialysis machine was constructed to test the changes for possible use in future dialysis machine designs. Tests were made on each of the five system improvements. All five ideas had some merit for a commercial dialysis machine. A description of the investigation and the various factors used in the development and testing of the five systems is included. Master of Science

Dynamic finite element modeling and analysis of a refrigeration compressor was investigated as part of a noise emission study. Natural frequencies and normal mode shapes were calculated for the major structural components of the compressor. The components were later combined to form a model of the compressor assembly which was subsequently solved for its dynamic properties. Model development included coordination with test data for verification and revision to improve model prediction accuracy. Considerable efforts were made to accurately represent the hermetic shell which presents several inherent modeling difficulties due to its geometry and other characteristics which result from a deep drawn manufacturing process. The importance of physical simplifications such as geometry representation, thickness variation, attachments, the welded seam, and residual stresses were established. In addition, theoretical limitations of the finite element method were addressed as a cause for analysis-test discrepancies. Housing models developed were found to agree within 12% of experimental natural frequencies up to 1100 Hz. Compatibility of analytical normal modes with resonant dwell experimental deflection shapes was considered. Analytical forced vibration response showed situations when the deflected shapes can be a superposition of modes rather than the pure mode shape. Analytical simulation of the test setup improved the agreement of analysis and test data. Additional components modeled include the internal compressor mechanism and its supports. Analysis showed that interactions with the internal components, particularly resonances within the suspension springs, are important for a valid representation of the compressor assembly. Resonances within the internal suspension components more than double or nearly triple the number of resonance frequencies in the compressor assembly. Master of Science

Equations were derived to calculate the reflection points of acoustic rays emanating from a source to a receiver. Reflection points for up to three reflections could be determined if the location of the source, receiver and all reflecting surfaces were known. Assuming a point source, the distances along the calculated ray paths were used along with the power level of the source and the absorptive characteristics of all the reflecting surfaces to determine the sound pressure levels at specified receiver locations. A computer program was developed to perform the necessary calculations for reflection points and sound pressure levels. The output was in the form of ray tracing plots which showed the unique reflection paths for up to three reflections and the sound pressure levels at each receiver location. Comparisons were made between predicted and calculated sound pressure levels in a rectangular parallelopiped shaped room and a long narrow hallway. The results of this investigation showed a promising potential in the area of sound pressure level predictions and the use of ray tracing plots to provide a means of reducing the sound pressure levels. A particularly interesting point of the program was its ability to handle very irregularly shaped rooms which include slanted surfaces. Master of Science

Fretting corrosion is a form of wear which occurs at the interface of two contacting solid materials as the result of small, relative vibratory motion. It is generally identified by the presence of a red oxide, Fe203. Previously, the majority of the investigators studied the fretting corrosion between two flat specimens or between a single ball and a flat plate held in contact by a normal force. There are a number of interrelated influencing factors involved in fretting corrosion including: the vibration frequency and amplitude, the environmental conditions, the characteristics of the material, and the type of lubrication. The present investigation was conducted in order to investigate the effects of frequency and amplitude of axial vibration and the consequent accelerations acting to produce fretting damage within an unlubricated instrument ball bearing. The effect of bearing axial play on the fretting damage was also examined. The reproducibility of the damage resulting from these variables was determined. Ph. D.

A dynamic pressure measuring system composed of long connecting tube, transducer, D-C amplifier and galvanometer oscillograph were used in a wind study conducted by the Department of Agricultural Engineering, Virginia Polytechnic Institute. The instrumentation was used to measure and record dynamic pressures due to wind velocity. Data recorded in that study indicated velocity head pressure oscillations on some surfaces of the test structure in excess of 100 Hz. This investigation was undertaken to see if the recorded oscillations could have been generated in the measuring system or if they were truly windpressure variations. Components of the pressure measuring system were modeled by transfer functions. From these transfer functions, a system operational transfer function was determined and used to define system frequency response. The frequency analysis indicated the system was severely limited in response by the oscillating air column constrained within the interconnecting tube. The usable frequency range of the system (+ 10% tolerable amplitude error allowable) was found to go from zero to 34.2 Hz. By eliminating the air column this frequency range could have been extended to 99.4 Hz. Phase shifts in these frequency ranges were found to be negligible. Transducers were mounted rigidly to exterior walls of the test structure. The measured fundamental natural frequencies of these walls (transducer mountings) were found to be well within the usable frequency range of the instrumentation; and, therefore, a source of vibration pickup. To improve the measuring system reliability, recommendations were made to eliminate the air column and stiffen the transducer mountings. Master of Science