The design and analysis of finite length, multiple layered, induced strain actuators is investigated. A model of an arbitrary multiple layered actuator is utilized to predict the applied force and moment from the i^th layer onto a structure. The transverse equations of motion of a simply supported beam are derived using Timoshenko beam theory. This approach accounts for shear deformation and allows the actuator-applied moments to be directly incorporated into the equations of motion without further approximation. The model is cast in state space form and an assumed mode method is used to solve for the forced response of a nonuniform beam. Experiments are performed verifying the developed analytical model. The first experiment characterizes the dynamic properties of five different actuator/substructure configurations. Results indicate the system natural frequencies decreased and the structural damping increased with more attached actuators. Analytical predictions are shown to be in good agreement with the experimental results. / Master of Science

Laboratory-obtained, manufacturer-supplied hearing protector attenuation ratings typically overestimate the workers' protection level In the workplace. In addition, several work-related in-field factors often degrade protection performance of the hearing protection devices (HPDs), posing the threat of underprotection for industrial workers. This research investigated the effects of HPD wearing time, subject activity movement, and HPD fitting procedure on the frequency-specific attenuation and user-rated comfort achieved with a popular foam cushion earmuff, two types of earplugs (user-molded foam and pre-molded, triple-flanged polymer), and an earmuff over foam earplug combination. Both attenuation and comfort data were collected from 40 naive but audiometrically normal subjects. Using a psychophysical real-ear-attenuation-at-threshoId-measurement procedure, attenuation data were obtained before, during, and after the activity movement tasks, which induced typical worker movements, so that the influence of wearing time and activity movement could be determined. Bipolar comfort rating data were also collected before and after the activity movement tasks, The results of statistical analyses indicated that achieved attenuation and user comfort significantly decreased over a two-hour wearing period and that training to achieve better fitting markedly improved protection, although these changes were device- and frequency-specific. Loss in frequency-specific attenuation over the wearing period was up to 6.3 dB for all HPDs except the foam plug, and attenuation Improvement due to training ranged from 4 to 14 dB for all HPDs except the earmuff at 1000 Hz and below. Almost no difference In achieved attenuation or comfort was found between the two activity (head/torso and temporomandibular) movements, but the earmuff tended to slip during highly kinematic head/torso movement. In general, out of the four different HPD configurations used in the study, the foam plug was very resilient to either type of activity movement but did benefit more than the other devices from the training for proper tilting; it was also perceived as the most acceptable and stable HPD by the subjects. In summary, the research illuminated the strong influence of in-field factors on HPD effectiveness. / Master of Science

Low frequency noise is a common problem in aircraft and launch vehicles. New technologies must be investigated to reduce this noise while contributing minimal weight to the structure. This thesis investigates passive and active control methods to improve low frequency sound absorption and transmission loss using acoustic metamaterials. The acoustic metamaterials investigated consist of poro-elastic acoustic heterogeneous (HG) metamaterials and microperforated (MPP) acoustic metamaterials. HG metamaterials consist of poro-elastic material with a periodic arrangement of embedded masses acting as an array of mass-spring- damper systems. MPP acoustic metamaterials consist of periodic layers of micro-porous panels embedded in poro-elastic material. This thesis examines analytically, experimentally, and numerically the behavior of acoustic metamaterials compared to a baseline poro-elastic sample. The development of numerical techniques using finite element analysis will aid in understanding the physics behind their functionality and will influence their design. Design studies are performed to understand the effects of varying the density, size, shape, and placement of the embedded masses as well as the location and distribution of microperforated panels in poro- elastic material. An active HG metamaterial is investigated, consisting of an array of active masses embedded within poro-elastic material. Successful tonal and broadband noise control is achieved using a feedforward, filtered-x LMS control algorithm to minimize the downstream sound pressure level. Low-frequency absorption and transmission loss is successfully increased in the critical frequency range below 500 Hz. Acoustic metamaterials are compact compared to conventional materials and find applications in controlling low-frequency sound radiation in aircraft and launch vehicles. / Master of Science

Incubators in the Neonatal Intensive Care Unit (NICU) are known to produce high Sound Pressure Levels (SPL) that can have detrimental effects on infants. Currently measured SPL in NICU's using traditional incubators are above the recommended 45 dB[A] threshold value [1]. Due to operating equipment and environmental noise, the sound level that is perceived by the developing newborn can cause both short and long term hearing loss as well as psychological damage [1].This thesis presents a study on how passive noise control devices can be used to reduce SPL levels in incubator NICU environments. A combination of experimental testing coupled with Finite Element simulations were performed for a modern incubator. In the experimental portion, porous mattresses were analyzed to reduce SPL values. These same test scenarios were modeled using the FE software. Using this model, extensive studies were performed on an arrangement of porous mattress materials with simple foam shapes to determine sound absorbing characteristics of several designs. Data was collected and studied at a NICU at Children\'s Hospital in Norfolk, Va. Experimental work showed improvement in reducing SPL with multiple thicknesses for different sound absorbing mattresses. The experimental outcomes validated the FE simulation model by showing similar trends at the baby\'s ears. In simulation work, polyimide foam had the best low frequency performance while polyurethane had the greatest performance in middle and high frequencies. Designs that used full-width foam treatments across the incubator produced the overall greatest reduction in noise around the baby control volume by approximately 26%. / Master of Science

Traditional active noise control (ANC) systems rely on a physical sensor to measure the error signal at the desired location of attenuation. The error signal is then used to update an adaptive controller, which ultimately attenuates the measured response. However, it is not always practical to use traditional ANC in real-world applications. For example, as small unmanned aerial systems (UAS) become more commonly used, community noise exposure also increases, along with the desire to reduce UAS noise. Traditional ANC systems that rely on physical sensors at observer locations are impractical, since a UAS does not typically have real-time access to the response at an observer's ears, which is realistically in the far-field. Virtual error sensing (VES) can augment an ANC system using near-field measurements to estimate the response at a desired far-field location. In this way, the VES technique effectively shifts the zone of quiet from the location of the physical sensor(s) to a different "virtual" location. This thesis begins by outlining past work that used traditional ANC methods and virtual error sensing techniques. Numerical modeling results showing the predicted spatial change in SPL achieved using a virtual sensor will be presented. Experimental tests used ANC to attenuate the noise from a single UAS rotor at far-field locations using a near-field microphone and the remote microphone technique (RMT) to develop the VES. The results of the VES alone and with an ANC approach at several far-field virtual locations will be presented and discussed. / Master of Science / Small unmanned aerial systems (sUAS) are becoming increasingly common for private, military, and commercial use, and as such, community noise exposure is increasing. Reducing the noise produced by UAS could help improve community acceptance. Active noise control (ANC) might be used to attenuate noise produced by sUAS, however, traditional ANC systems would require a physical sensor in the far-field, which is not feasible. A virtual error sensor (VES) could eliminate the need for a far-field sensor. This thesis describes the proposed VES strategy, and presents numerical simulations and experimental results that highlight both the benefits and limitations of the approach. Results of the VES system with and without an ANC approach are discussed. Experimental testing focused on attenuating the tonal noise produced by one 2-bladed rotor with a tip radius of 4.7 inches. Pressure variations caused by the blade rotation were measured in the near and far-field using electret microphones and externally polarized condenser microphones, respectively. The ANC system used the filtered-x least mean squares algorithm in conjunction with the VES system to estimate the far-field response. A 2-inch diameter speaker served as the secondary source to provide the appropriate control input to the system. Experimental results show reductions between 6-13 dB at varying far-field locations and rotation rates.

Pneumatic percussion tools are extensively used in the construction industry. They are one of the noisiest machines in the construction industry generating noise levels above 110 dBA which are well beyond the permissible exposure limit (PEL) of 85 dBA. This work presents a comprehensive methodology for the acoustic characterization and noise source identification of these percussion tools. The methodology is applied to a representative pneumatic tool and the characterization results are described in detail. A mechanical analysis was performed on a chipping hammer finding mode shapes and natural frequencies of individual components. The mechanical analysis included modal hammer measurements and creating FE models. Fluid measurements were performed on the chipping hammer to find the velocity of the exhaust and pressure in the upper and lower chambers. The fluid tests found that the velocity of the exhaust is approximately Mach 1.0 or greater. Noise measurements were carried out on the chipping hammer to determine the spectral characteristics, overall sound power level, and spatial source strength maps of the tool. A spherical array of microphones was used to obtain an accurate estimate of the overall sound power levels and the directivity. The overall sound power radiation was found to be in the range of 110-115dBA. An advanced 63 microphone phased array was used to successfully locate and identify the major sources of noise from this tool via the use of beam-forming maps. This thesis also presents a preliminary noise control method employing commercial-off-the-shelf pneumatic silencers. The outcome of the tests is illustrated in detail in this thesis. / Master of Science

Hearing protection devices (HPDs), a widely used countermeasure against noise-induced hearing loss, are laboratory-tested for their attenuation (noise reduction) capabilities. Unfortunately, laboratory tests overestimate the in-workplace performance of the devices, potentially leading to inadequate protection for the user. Many factors affect in-field effectiveness, including the physical design and "wearability" of the protector. Wearability, a highly subjective aspect which encompasses such characteristics as user comfort, ease of use, and acceptability, directly affects performance by influencing the regularity of use and the manner in which a protector is worn. This research investigated the influence of the user's work-related activities over a prolonged wearing period, and of variations in headband compression force and cushion material (liquid- or foam-filled) on achieved noise attenuation and wearability (comfort and acceptability) of earmuff hearing protectors. REAT (real-ear attenuation at threshold) testing procedures were used to collect attenuation data on 24 subjects, both prior-to and following completion of a simulated work task. Bipolar rating scales were utilized to collect pre- and post-task wearabi1ity data. Statistical analyses demonstrated that the work-related movement and wearing time significantly reduced achieved attenuation and, for higher compression earmuffs, also degraded perceived comfort and acceptability. A high headband compression force was 1inked to increased attenuation and to poorer user comfort and acceptance. The data revealed no significant difference in achieved attenuation or wearability between cushion types. The results illustrate the powerful influence of physical activity on HPD effectiveness and the criticality of certain earmuff design parameters to both attenuation and wearability. Furthermore, the existing tradeoff in earmuff design between comfort and attenuation was clearly demonstrated. / Master of Science

Adaptive feedforward control of any physical system, acoustical, vibrational or other, requires what is termed as an uncontrollable coherent reference signal. That is, a signal which is highly representative (coherent) of the disturbance to be controlled which is not affected by the control actuator itself. Creating the <i>coherent</i> portion of this requirement for a certain class of problems is the motivation of this work. Most physical disturbances do not originate from a single source, but rather maintain contributions from a number of (possibly) correlated paths. For engineers who have access to only a single-input single-output (SISO) adaptive controller, the multi-source disturbance presents a difficult design issue. Simply adding the references in a linear combination can result in a signal which is not coherent at any frequency. Appropriately amplifying and suppressing coherent and incoherent signals prior to their linear combination can result in a signal which accurately represents the disturbance at all frequencies. This is precisely the task that the newly developed coherent output power (COP) filters perform. By calculating the coherent (or partial coherent) output power of each of the candidate references before control occurs, frequency domain filters are designed to remove incoherent portions of each signal. The advantages of performing the COP filtering procedure are very apparent when compared to the simple linear combination of signals. Coherence, and thus control performance, can be drastically improved. The COP filtering technique offers a means for system identification and computational savings not apparent in the conventional adaptive array, which solves the same multi-source problem. / Master of Science

This dissertation is concerned with the active control of local sound fields, as applied to an active headrest system. Using loudspeakers and microphones, an active headrest is capable of attenuating ambient noise and providing a comfortable acoustic environment for an occupant. A finite element (FE) model of an active headrest is built and analyzed such that the expected noise reduction levels could be quantified for various geometries as well as primary sound field conditions. Both plane wave and diffuse primary sound fields are considered and it is shown that the performance deteriorates for diffuse sound fields. It is then demonstrated that virtual sensing can greatly improve the spatial extent of the quiet zones as well as the attenuation levels. A prototype of the active headrest was constructed, with characteristics similar to those of the FE model, and tested in both anechoic and reverberant sound fields. Multichannel feedforward and feedback control architectures are implemented in real-time and it is shown that adaptive feedback systems are capable of attenuating band-limited disturbances. The spatial attenuation pattern surrounding the head is also measured by shifting the head to various positions and measuring the attenuation at the ears. Two virtual sensing techniques are compared in both feedback and feedforward architectures. The virtual microphone arrangement, which assumes that the primary sound field is equivalent at the physical and virtual locations, results in the best performance when used in a feedback system attenuating broadband disturbances. The remote microphone technique, which accounts for the transfer response between the physical and virtual locations, offers the best performance for tonal primary sound fields. In broadband sound fields, a causal relationship rarely exists between the physical and virtual microphones, resulting in poor performance. / PHD / Excessive noise and vibration levels in aircraft, rotorcraft, launch vehicles, and other aerospace vehicles may create harsh acoustic environments inside the vehicle. In some extreme cases, military applications being a prime example, hearing damage can occur due to the high noise levels associated with certain vehicles. Noise canceling headsets have been proven an effective solution to this problem, although in certain instances their use may not be safe or feasible. In this work, an active noise canceling headrest, or active headrest, is explored as an alternative solution to noise canceling headphones/headsets. An active headrest uses microphones and loudspeakers, typically located non-intrusively behind the head of the seat occupant, to reduce the ambient noise levels in the vicinity of the head and create a comfortable acoustic environment. A thorough investigation of the viability of such a system in a practical vehicle is assessed through the use of theoretical analysis, finite element modeling, and real-time performance experiments. Performance predictions generated using the finite element model were verified by performing real-time experiments, thus providing a level of confidence in additional predictions for alternative headrest geometries and configurations. Factors such as loudspeaker and microphone placement, head movements away from the nominal position, primary acoustic field characteristics, and choice of control strategy are all found to heavily influence the performance of an active headrest. Real-time experiments were performed in anechoic and reverberant sound fields and it is found that the noise canceling capability of the active headrest worsens in reverberant sound fields as compared to free field conditions.

In this thesis, the design and implementation of a Portable Feedback Active Noise Controller Headphone System, which is based on Texas Instruments TMS320VC5416PGE120 Fixed Point DSP, is described. Problems resulted from fixed-point implementation of LMS algorithm and delays existing in digital ANC implementation are determined. Effective solutions to overcome the aforementioned problems are proposed based on the literature survey. Design of the DSP based control card is explained and crucial points about analog-digital-mixed board design for noise sensitive applications are explained. Filtered input LMS algorithm, filtered input normalized LMS algorithm and filtered input sign-sign LMS algorithm are implemented as adaptation algorithms. The advantages and disadvantages of using modified LMS algorithms are indicated. The selection of the parameters of these algorithms is based on theoretical results and experiments. The real time performances of different adaptation algorithms are compared with each other as well as with a commercial analog ANC headphone under different types of artificial and natural noise signals. Moreover, practical conditions such as put on - put off case and dynamic range overflow case are handled with additional software implementations. It is shown that adaptive ANC systems improve the noise reduction significantly when the noise is within a narrow frequency range and this reduction can be applied to a wider frequency range. It is also shown that the problems of digitally implemented adaptive filters which are based on tracking capability, stability, dynamic range and portability can be fixed to challenge with the analog commercial ANC systems.