Rotorcraft flyover noise has long been a field of study for researchers. This is because for many people, the sounds produced by these vehicles are found to be extremely annoying. The focus of this thesis is to recreate the time-varying rotorcraft noise at the source for a single emission angle. Then, through interpolation between emission angles, produce a simulated flyover at the source that can then be propagated to a receiver. This will allow for the creation of a simulated flyover without the need of having to use a physical aircraft, or pre-existing data from some type of data collection means, such as a microphone array. The current methods are limited to a predefined length of data in order to synthesize signals. It has been documented that synthesizing flyover noise, from direct use of physical flyover recordings through an empirical approach, yields a high fidelity signal, as long as both unmodulated and modulated components are present. In order to extend these signals indefinitely, models for the amplitude and phase modulation must be developed. A band-limited random process will be explored for both the amplitude and phase modulations. An overlap-add technique, as well as a randomization technique and a modified phase modulation signal, defined as the "residual", will also be attempted in order to model the phase modulation. The results from this work have successfully found a means in which to produce a viable model of the amplitude modulation. Further investigation is still required in order to produce a model of the phase modulation which results in a high-fidelity model that can be extended indefinitely. / Master of Science / Helicopter noise has long been a field of study for researchers. This is because for many people, the sounds produced by these vehicles are found to be extremely annoying. The focus of this thesis is to recreate the sounds heard by an observer as a helicopter flies overhead. This will allow for the creation of a simulated flyover without the need of having to use a physical aircraft, or pre-existing data from some type of data collection means. The current methods used to produce helicopter flyovers are limited to a predefined length of data in order to create sounds an individual may hear on the ground. It has been documented that creating flyover noise, from direct use of physical flyover recordings, yields a high fidelity signal, as long as all components are present when recreating the new sound. In order to extend these signals indefinitely, models must be developed in order to model the key components heard as a helicopter passes over an observer. The results from this work have successfully found a means in which to produce a viable model for certain components of the original flyover. Further investigation is still required in order to produce a high-fidelity model that can be extended indefinitely with all the necessary components included. This research is part of a broader effort to study the effects flyovers have on the population in terms of annoyance and detection. The work done here will help to aid further models used to determine what individuals find annoying with regard to helicopters and the noises they produce.

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With the increasing demand for access to the crowded radio frequency spectrum, cognitive radios have been suggested as one solution. Cognitive radios would be frequency agile and able to sense their radio environment and opportunistically use empty spectrum. Spectrum sensing, monitoring a given band of spectrum to see if it is occupied, is an essential part of a cognitive radio. The preferred method of spectrum sensing is the energy detector, which does not require any a priori information about the signal to be detected and is computationally simple to implement. Man-made noise, impulsive in nature, has also become more prevalent with the widespread use of electronic devices. In this thesis, we took measurements of man-made impulsive noise in the broadcast digital television bands to measure its presence, power, and spatial correlation. The effects of impulsive noise on the detection performance of an energy detector were analyzed. Lastly, a wideband RF receiver was designed, built, and tested on the Virginia Tech campus, which would be well suited both to spectrum sensing and taking measurements of impulsive noise. / Master of Science

Measurements have been performed to understand the sound source mechanism in turbulent boundary layer flow over two and three-dimensional surface discontinuities whose height is smaller than the incoming boundary layer thickness. The work was performed in two different types of boundary layers: a wall-jet flow and a conventional high Reynolds boundary layer. In the wall-jet flow, measurements of far field sound from two-dimensional forward facing steps, gaps with rounded corners and swept forward facing steps with rounded corners were made. The sound from a forward facing step is shown to exhibit effects of non-compactness. Rounding the step corner results in consistent drop in sound levels but the directivity of the sound field remains unchanged. The sound from gaps is dominated by the forward step component and remains unaffected by rounding of the backward step portion. The sound from swept forward facing steps was found to approximately obey an acoustic sweep independence principle up to a sweep angle of 30 deg when the spanwise inhomogeneity in the flow is accounted for using a simple source distribution model. Sweep independence is also observed for steps with corner rounding radii up to 25% of the step height. The work performed in the high Reynolds number boundary layer included measurements on forward facing steps with rounded corners and a three-dimensional circular embossment with the same height as the forward step. The highest Reynolds number based on discontinuity height achieved in this work was approximately 93,000. The results show that rounding the forward step corner has the same qualitative effect on far field sound as in the wall-jet boundary layer. Quantitatively, for similar boundary layer edge velocity the sound is higher than in the wall-jet flow. The near field measurements show that the separation bubble downstream of the step shrinks as the step corner is rounded while the bubble upstream remains unaffected by it. The unsteady surface force in the lower half of the vertical face of the step was found to be independent of corner rounding. The force on the downstream surface shows similar character within the separation bubble for each rounding but decays faster with increasing downstream distance due to reduced bubble size. The unsteady force measurements were applied to the theory of Glegg et al. (2014) and the resultant of the unsteady forces on the vertical face and downstream surface placed at the top corner of the step is shown to qualitatively describe the far field sound. The acoustic sweep independence principle was applied to the far field sound from the circular embossment and it has been shown that the sound from the three-dimensional geometry can be predicted with reasonable accuracy using sound from a two-dimensional forward step with the same span. / Ph. D.

The supersonic jet plumes that exhaust from the engines of tactical aircraft produce intense noise signatures that expose the Navy personnel working on the deck of aircraft carriers to dangerously high levels of noise that often results in hearing damage. Reducing the noise radiated by these supersonic plumes is of interest to the Department of Defense and is the primary motivation of this research. Fundamentally, jet noise reduction is achieved by manipulating the nozzle boundary condition to produce changes in the turbulence development and decrease the acoustic efficiency of coherent structures. The research presented here focuses on a novel jet noise reduction technique involving a centered thermal non-uniformity that alters the base flow by introducing a temperature-driven centerline velocity deficit into a perfectly expanded Mach 1.5 jet. The results indicate $2 pm 0.5$ dB reductions in peak narrowband spectral sound pressure levels upstream of peak directivity directions for the non-uniform jet compared to a thermally uniform baseline, even for static thrust matched conditions. This reduction is hypothesized to be related to perturbations induced by the thermal non-uniformity that convect inside the irrotational core and reduce the correlation length scales of turbulence at locations far downstream. This hypothesis was evaluated by studying the coherent turbulence via its convective hydrodynamic footprint in the near-field. An indirect investigation of the near-field using a far-field-informed model of the wavenumber-frequency spectra indicate a reduction in the energy contained in the tail of the wavenumber spectra amplitude, suggesting a reduction in the size of large scale structures. A direct evaluation of the spatio-temporal behavior of the near-field was performed using temporally resolved schlieren images. Space-time correlations of the frequency-filtered near-field identified high frequency acoustic waves radiated by compactly coherent turbulent structures and low frequency Mach waves produced by large scale instabilities. In the thermally non-uniform case these features and their sources were found to be decorrelated at downstream regions. These results provide strong evidence that a centered thermal non-uniformity reduces the radiated noise compared to a uniform baseline by shortening the correlation length scales of coherent structures in regions far from the nozzle exhaust. / Doctor of Philosophy / A more complete understanding of the intense noise sources present in supersonic jet plumes is of value to both government and industry, and is a necessary step towards optimizing noise reduction techniques. Tactical aircraft that operate on the deck of aircraft carriers expose Navy personnel to dangerously high levels of noise that often results in permanent hearing damage. Supersonic jet noise reduction is also of relevance to the recent efforts to revitalize supersonic air transport over land. For supersonic air transport to become a reality, the noise produced by these future aircraft during takeoff and landing must meet the increasingly stringent community noise requirements. Fundamental jet noise research is needed to guide the design of future engine architectures for these aircraft to ensure their commercial success. The research presented herein examines a novel noise reduction technique that involves a centered thermal non-uniformity consisting of a heated jet plume with a spot of locally cooler, slower moving air concentrated along the centerline of a Mach 1.5 jet. This temperature driven velocity deficit is shown to reduce the radiated noise by up to 2.5 dB at peak frequencies and at angles just outside of the peak directivity direction. The cause of the noise reduction is hypothesized be related to a reduction in the size of the coherent structures that radiate a majority of the noise produced by turbulent jets. This hypothesis is evaluated by examining the 'footprint' of the coherent structures in the ambient field directly outside of the jet shear layer in an area called the near-field. An indirect investigation of the near-field using a far-field informed analytic model suggests a reduction in the size of large scale structures. A direct evaluation of the space time structure of the near-field was performed using temporally resolved schlieren images. Statistical processing of the density gradient provided by the schlieren images revealed acoustically intense structures known as Mach waves and high frequency acoustic waves. These features and their sources, large scale instabilities and compactly coherent turbulence, were found to be decorrelated by the introduction of the thermal non-uniformity. These results provide strong evidence that the centered thermal non-uniformity produces a noise benefit by reducing the size of the turbulent structures.

Weight is a critical parameter in the design of any system launched into space. Current launch costs are on the order of 10,000 dollars per pound of payload capacity. Reducing weight and thus increasing payload capacity is always in the forefront of the design process. One method of increasing the payload capacity of launch vehicles is to reduce the acoustic environment in the interior of the fairing. A major problem is that passive methods currently used for noise suppression do not exhibit significant energy dissipation at low frequencies. This motivates the use of active noise control. Using active noise control for frequencies below 200 to 300 Hz in addition to the passive control means has potential to provide broadband noise suppression and thus a smoother, cheaper ride for any payload. The problem with this technique is that active noise control commonly uses electromagnetic speakers as the control element. The weight of the speaker adds more cost to the application due to the approximate cost per pound to send a launch vehicle and payload to space. At 10,000 dollars per pound of payload capacity, the added cost spent on protecting the payload can potentially reduce the amount of payload capacity a customer receives due to monies spent on non-payload mass. Therefore, necessity dictates a light weight noise control solution. This work investigates the feasibility of a transducer with less mass than that of a conventional loudspeaker which dissipates energy at the acoustic resonances of an enclosed cavity. The test setup involves using the transducer to lower the sound pressure levels of acoustic resonances which are excited by an external source, thus simulating the launch phase of a launch vehicle. The transducer is used as an actuator to add damping through feedback control. The transducer is comprised of three thin flexures that are actuated by piezoceramic material attached to both sides. The flexures actuate a speaker cone that is attached to the end of the flexures. The transducer can act as a sensor or an actuator due to the nature of the piezoceramics. The sound absorbing transducer is modeled to couple to the first acoustic resonance of a six foot cylindrical cavity. The cavity acts as a simplified model of a launch vehicle payload fairing. Equations of motion are derived to model actuator motion and the acoustic impedance of the cavity. A state-space model of the system was derived for two cases: a collocated sensor/actuator pair exciting the tube and an external source exciting the tube with the transducer acting as an absorber. The transducer is designed to affect the first mode, however damping is noticed in the next acoustic resonance. Analysis of the theoretical model indicated up to 70 percent reduction of the open-loop RMS values or a reduction of 10 dB. Experimental results with the optimized transducer produced a 35 percent reduction of the open-loop RMS value or 3.73 dB. The first acoustic resonance coupled well with the first structural mode of the transducer providing optimal noise suppression for the first mode. Damping was also noted in the second acoustic mode. Neglecting the inertia of the tip mass introduced errors in the predictions of the transducer resonances at higher frequencies. This problem limited the ability to control the higher modes of the cavity. / Master of Science

Buildings located near airports may be subjected to significant noise levels due to aircraft flyovers. Aircraft noise is particularly annoying when compared to other traffic noises due to its intermittent nature. While noise control is typically performed at the source, sound insulation programs are in place to improve the acoustic performance of a residence affected by the flyovers. Noise Level Reduction (NLR) is a common metric used in the United States to determine whether a residence qualifies for such programs. Sound insulation programs are available to houses that have an indoor Day Night Average Sound Level (DNL) greater than 45 dBA. NLR is a single-number metric used to quantify the ability for a building or building element to reduce the transmission of external sound pressure levels generated by aircraft. In addition to determining whether a residence qualifies, NLR can be used to quantify the effectiveness of the modifications performed as a result of the sound insulation program. NLR measurements with a loudspeaker offer an alternative method to those performed with aircraft flyovers, offering flexibility to the consultants that perform these measurements in the field. The purpose of this research was to better understand and improve the loudspeaker test for measuring NLR, providing a resource to the aircraft noise industry. Testing was completed on a "test house" that was constructed on campus with construction methods typical of a mixed-humid climate. The angular dependency, repeatability, and reproducibility of NLR, among other factors, were evaluated with field measurements. Significant NLR variations were observed with changes in lateral and vertical angles of incidence.

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

In this dissertation the influence of vane sweep on rotor-stator interaction noise is investigated. In an analytical approach, the interaction of a convected gust, representing the rotor viscous wake, with a cascade of finite span swept airfoils, representing the stator, is analyzed. The analysis is based on the solution of the exact linearized equations of motion. High-frequency convected gusts for which noise generation is concentrated near the leading edge of the airfoils are considered. In a preliminary study, the problem of an isolated finite span swept airfoil interacting with a convected gust is analyzed. Using Fourier transform methods and the Wiener-Hopf technique, an approximate solution for this problem is developed. Closed form expressions for the acoustic farfield are obtained and used in a parametric study to assess the effect of airfoil sweep on noise generation. Results indicate that sweep can substantially reduce the farfield noise levels for a single airfoil. Utilizing the single airfoil model, an approximate solution to the problem of noise radiation from a cascade of finite span swept airfoils interacting with a convected gust is derived. Only upstream radiated noise is considered. Neglecting the weak coupling between the adjacent leading edges at high frequencies, the cascade solution is constructed as a superposition of acoustic farfields emanating from an infinite number of isolated airfoils. A parametric study of noise generated by gust-cascade interaction is then carried out to assess the effectiveness of vane sweep in reducing rotor-stator interaction noise. The results of the parametric study show that, over a fairly wide range of conditions, sweep is beneficial in reducing noise levels. One conclusion of particular importance is that rotor wake twist or circumferential lean substantially influences the effectiveness of vane sweep. The orientation of the vane sweep must be chosen to enhance the natural phase lag caused by wake lean, in which case rather small sweep angles substantially reduce the noise levels.

Master of Science / Department of Electrical and Computer Engineering / William B. Kuhn / A technique for measuring and characterizing transistor noise is presented. The primary goal of the measurements is to locate the 1/f noise corner for select transistors in Silicon-on-Sapphire processes. Additionally, the magnitude of the background channel noise of each transistor is measured. With this data, integrated circuit (IC) engineers will have a qualitative and quantitative resource for selecting transistors in designs with low noise requirements. During tests, transistor noise behavioral change is investigated over varying channel lengths, device type (N-type and P-type), threshold voltage, and bias voltage levels. Noise improvements for increased channel lengths from minimal, 1.0μm, and 4.0μm are measured. Transistors with medium and high threshold voltages are tested for comparison of their noise performance. The bias voltages are chosen to represent typical design values used in practice, with approximately 400 mV overdrive and a drain-to-source voltage range of 0.5 to 3.0V. The transistors subjected to tests are custom designed in Peregrine’s 0.5μm (FC) and 0.25μm (GC) Silicon-on-Sapphire (SOS) processes. In order to allow channel current noise to dominate over other circuit noise, the transistors have extraordinarily large aspect ratios (~2500 - 5000). The transistor noise produced is amplified and measured over a frequency range of 1kHz - 100MHz. This range allows the measurement of each device’s low and high frequency noise spectrum and resulting noise corner.