The major application of active noise control (ANC) has been focused on using a single reference signal; the work on multiple reference ANC is very scarce. Here, the behavior of multiple reference ANC is analyzed in both the frequency and time domain, and the coherence functions are provided to evaluate the effectiveness of multiple reference ANC. When there are multiple noise sources, multiple reference sensors are needed to generate complete reference signals. A simplified method combines those signals from multiple reference sensors into a single reference signal. Although this method could result in satisfactory noise control effects under special circumstances, the performance is generally compromised. A widely adopted method feeds each reference signal into a different control filter. This approach suffers from the problem of ill-conditioning when the reference signals are correlated. The problem of ill-conditioning results in slow convergence rate and high sensitivity to measurement error especially when the FXLMS algorithm is applied. To handle this particular problem, the decorrelated Filtered-X LMS (DFXLMS) algorithm is developed and studied in this thesis. Both simulations and experiments have been conducted to verify the DFXLMS algorithm and other issues associated with multiple reference ANC. The results presented herein are consistent with the theoretical analysis, and favorably indicate that the DFXLMS algorithm is effective in improving the convergence speed. To take the maximum advantage of the TMS320C30 DSP board used to implement the controller, several DSP programming issues are discussed, and assembly routines are given in the appendix. Furthermore, a graphical user interface (GUI) running under Windows' environment is introduced. The main purpose of the GUI is to facilitate parameters modification, real time data monitoring and DSP process control. / Master of Science

Previous work has shown that active noise control is a feasible solution to attenuate tonal noise radiated by small axial cooling fans, such as those found in desktop computers. One such control system reduced noise levels of a baffled 80-mm fan in the free field with four small loudspeakers surrounding the fan. Due to industry specified spatial constraints, a smaller fan and speaker configuration was desirable. The smaller configuration maintains similar control performance, further facilitating practical implementation of the control system. The smaller control system employs a smaller fan running at a higher speed. Different loudspeaker configurations for control exist and have been tested. A configuration consisting of four control sources spaced symmetrically around and coplanar to the fan exhibits global control of the tonal component of the fan noise. A configuration with three symmetrically spaced sources is shown to perform similarly, agreeing with theoretical prediction. An analysis of the control system in a non-ideal reflective environment is also discussed.

Feedback active noise control (ANC) has been applied as a means of attenuating broadband noise from a small axial cooling fan. Such fans are used to maintain thermal stability inside of computers, projectors, and other office equipment and home appliances. The type of low-level noise radiated from axial cooling fans has been classified as harmful to productivity and human well being. Previous research has successfully implemented feed-forward ANC, targeting specific narrow-band fan noise content related to the blade passage frequency (BPF) of the fan. The reference signal required for a feed-forward algorithm limits its ability to attenuate much of the noise content; however, it is also desirable to reduce broadband fan noise. Feedback control is a logical alternative in the absence of a valid reference signal. The fan used for this research was mounted in one of the six aluminum panels that constituted a mock computer case. The fan was surrounded by four miniature loudspeakers as control sources and four small electret microphones as error sensors. A feasibility study was conducted with a single channel of analog feedback control. However, for the majority of this research, the ANC algorithm was executed on a digital signal processor. Several electronic modules provided the necessary signal conditioning and conversion for the process. A method is proposed and validated for predicting the overall attenuation that can be obtained for a specific fan, based on its autocorrelation measurement. Studies were performed in order to determine the difference in performance between static and adaptive controllers. Comparisons are made between decentralized and centralized controllers, the results of which are presented in this thesis. Feedback ANC is demonstrated as a good alternative to feed-forward ANC for the reduction of BPF related tonal fan noise content. Some low-frequency broadband attenuation is achieved. The delay time associated with current DSP technology is shown to be too long to effectively attenuate flow noise (the main component of broadband fan noise). Adaptive control proved to be necessary for stability and performance in the feedback controller. Decentralized control is shown to outperform centralized control for this specific application.

No description available.

Methods for analyzing the structural-acoustic power efficiency of active structures are developed. For this work we define the power efficiency as the ratio of the sound power radiated by a structure to the maximum possible radiated sound power. An active structure is defined as one that has electromechanical actuators distributed over its surface for the purpose of structural-acoustic excitation. The power efficiency of planar, baffled structures with arbitrary boundary conditions is examined using a combination of methods based on numerical integration, variational principles, and finite element analysis. The fundamental result of this work is that computing the power efficiency of an active structure reduces to the solution of two eigenvalue problems. The maximum possible sound power radiated by a planar, baffled structure is shown to be equivalent to the largest eigenvalue of the acoustic power transfer matrix. The structural-acoustic power efficiency is the solution of a separate generalized eigenvalue problem whose parameters include the location of the electromechanical actuators and the type of electromechanical actuation. The advantage of this metric over other measures of radiation efficiency is that 0 and 1 bound the structural-acoustic power efficiency. Furthermore, solving for the power efficiency as a function of frequency yields a measure of the bandwidth of the structural-acoustic actuator. Power efficiency is analyzed for point force actuation and distributed moment actuation. Numerical simulations demonstrate that maximizing the power efficiency requires that the magnitude and phase of the structural modal velocity vector be matched to that of the eigenvector that corresponds to the maximum eigenvalue of the acoustic power transfer matrix. Matching the modal velocity to the maximizing eigenvector produces a vibration shape that maximizes the sound power radiation of the structure. Individual actuators are not able to achieve high efficiency over a broad frequency range for both types of electromechanical actuation. Multiple-actuator arrays are able to achieve higher average efficiency at the expense of increased number of actuators. An optimization problem is then posed to maximize the structural-acoustic power efficiency by varying the location and size of distributed moment actuators. We demonstrate that an average efficiency on the order of 0.85 is possible over a large bandwidth through optimal placement and sizing of a set of four distributed moment actuators. Experimental results on a baffled plate demonstrate that correct phasing of the actuators results in velocity distributions that correlate well with predicted results. / Ph. D.

A human factors experiment was conducted to assess whether a reduction in noise at the ear would cause an improvement in speech intelligibility, an improvement in cognitive performance, and/or a reduction in subjective mental workload. Modified Rhyme Test (MRT) stimuli were used to determine intelligibility and specific tests within the Complex Cognitive Assessment Battery (CCAB) were used to assess cognitive performance. The tests chosen from the CCAB were: Tower Puzzle, Logical Relations, and Numbers and Words. These tests were chosen because of the specific set of cognitive functions that they measure which corresponded to command and control tasks. Participants performed the MRT and CCAB tests simultaneously in a 114 dBA noise environment at two speech levels, 83 dB (linear) and 96 dB (linear), using two communication microphones, Gentex Model 1453 and a prototype communication microphone developed by Adaptive Technologies Inc. (ATI). The noise used in the experiment was from a recording made inside a US Army Bradley Fighting Vehicle. Subjective mental workload was assessed using the NASA-TLX and Modified Cooper-Harper (MCH) immediately after the experiment. Results indicated that the communication microphone developed by ATI reduced the noise level at the ear better than the current Gentex microphone. However, the Gentex microphone produced significantly higher speech intelligibility scores at the 96 dB speech level. Cognitive performance scores significantly improved with increasing speech level for both communication microphones, with the ATI microphone having the advantage at 83 dB and the Gentex at 96 dB. The results also indicated that the main effects of speech level and communication microphone did not have an effect on subjective mental workload. A correlation analysis revealed that there was a positive relationship between the two workload measurement tools, indicating that either scale may be used to assess mental workload. Therefore, it was concluded that the MCH could have been used instead of the NASA-TLX, since the overall workload score was of interest. / Ph. D.

Among the many occupational hazards to which Army rotary-wing aviators are exposed is intense noise generated from the aircraft. The potential for permanent hearing loss and difficulty communicating in helicopter noise is well known; an appropriate way to evaluate a hearing-impaired pilot's safety risk due to hearing loss is not as well known. Previous research has studied communication ability in helicopter cockpit noise under different headsets, but there are not conclusive data on the combined effects of degraded speech intelligibility due to noise and flight workload under the headset technology currently available to Army helicopter pilots. In particular, there is a scarcity of information on pilots with hearing loss. Currently, Army Aeromedical standards stipulate audiometric threshold criteria for rated helicopter pilots to ensure their safe flying. If the standard is not met, a flight waiver for hearing is generally granted if the pilot demonstrates good (at least 84%) binaural word recognition ability in a quiet environment. A research study was conducted to evaluate Army helicopter pilot performance with regard to flight workload, communication signal quality, headset configuration, and pilot hearing ability. Objectives of the study included the ability to refine current Army audiometric hearing waiver criteria, and to yield data on which to base flight and headset selection recommendations for pilots. In general, it was believed that flight performance and ratings of situation awareness (SA) would decrease as flight workload increased and communication signal quality decreased, and that assistive communication devices coupled with headsets would afford improved flight performance over their passive counterpart. It was also hypothesized that normal-hearing pilots would perform better than hearing-impaired pilots would. Twenty Army helicopter pilots (one group of 10 pilots without a hearing waiver and one group of 10 pilots with a hearing waiver) participated in this study. The pilots flew three flights in a Black Hawk flight simulator, each with a different headset configuration and with varying flight workload levels and varying air traffic control (ATC) communication signal quality. Objective flight performance parameters of heading, altitude, and airspeed deviation and ATC command readbacks were measured. Additionally, measurements were taken on subjective measures of workload, SA, and headset comfort/speech intelligibility. Experimental results partially supported the research hypotheses. Results indicated that flight performance and ratings of SA were negatively affected by increased flight workload and decreased communication signal quality for both groups of pilots. Results also showed that a passive headset/passive earplug combination use by the hearing-impaired group of pilots led to degradation of certain flight performance parameters and lower ratings of SA than the headsets equipped with assistive communication technology; however, the same headset effect was not seen with the group of normal-hearing pilots. This study yielded results that support a conclusion that factors other than hearing thresholds and word recognition ability in a quiet environment should be considered when evaluating Army helicopter pilots flight safety with regard to hearing sensitivity. Rather, the synergistic effects of flight workload and communication signal quality with individual hearing levels should be considered when making continued flight recommendations and headset choice recommendations. Results also support a recommendation requiring hearing-impaired pilots to use assistive communication technology and not be permitted to fly with passive headset devices. Further research should include a functional hearing assessment in which pilot hearing requirements are determined and individual hearing abilities are compared to the requirements. / Ph. D.

Techniques are available today to attenuate the output sound of the trumpet. All of these techniques involve using passive mutes. Due to the limitations in the sound one can obtain with passive mutes, another solution, using active noise control, is proposed to predictably attenuate the output sound of the trumpet. With the new system, it is theorized any desired output sound can be obtained. Within this thesis a model of the trumpet physics is derived and an investigation of the implementation of two analog feedback controllers and two digital LMS controllers is performed. The model of the trumpet mechanics is studied to understand the trumpet system before applying the control systems. Analysis is performed on the type and the location of the acoustic control actuator and the error sensor to be used. With the chosen actuator and sensor, the two types of controllers are designed and realized. The farfield spectrum of the trumpet's response to a single note is analyzed for each controller and the resulting attenuations compared. The model of the trumpet system is then used to demonstrate the coupling of the trumpet and the player and to show the effects of the controllers on the behavior of the player's embouchure. With the inclusion of the controllers in the trumpet system, the farfield spectrum was successfully attenuated at two harmonics of the tone passed through the trumpet. Testing was not performed with an actual trumpet player due to the high sound pressure levels (160 dB SPL) required from the control actuator. From a derived model of the control actuator, specifications for an acoustic driver capable of delivering the high sound pressure level were calculated. Design and fabrication of the proposed actuator will be completed during future work. / Master of Science

Thesis (PhD (Industrial Engineering))--University of Stellenbosch, 2007. / Off-road working vehicles are subjected to high levels of vibration input on the rough terrain and irregular roads they work. The human operators are therefore exposed to high levels of whole body vibration (WBV) and at risk of developing health problems. A number of international standards address the matter of whole body vibration, and the European Union issued a directive which limits the exposure of workers in the EU to WBV. Unfortunately, to date there is no law in South Africa requiring compliance with any of these EU standards nor guidelines. There are vehicles which are not fitted with suspension and/or suspension seats. The three wheeled logger used in forestry is a highly manoeuvrable and effective bulk handler, but without any form of suspension and no space under the operator’s seat to install a suspension seat. However, a suspension cushion can be retrofitted to existing vehicles largely alleviating the problem. To isolate low frequency vibration large suspension travel is required which makes an air suspension cushion attractive, as it can fully collapse. Additionally, a Helmholtz resonator if added to the cushion in the form of a pipe and tank, provides anti-resonance at a specific frequency. The resonator can be tuned by adjusting the pipe’s length and diameter as well as the volume of the tank. Larger diameter pipes have less friction and give better reduction of the transmissibility curve at the anti-resonance frequency. The SEAT value is a single number used to compare suspension seats for a specific input vibration. It is calculated from the weighted input acceleration power spectral density curve and the suspension seat transmissibility curve. The former is obtained from the vehicle and is vehicle, path and speed dependent. The latter is the only variable that can be improved by using a better suspension seat/cushion. The input power spectral density often contains significant energy at frequencies where the human operator is most sensitive. The cushion resonator could be tuned to position the anti-resonance in the transmissibility curve at these frequencies. The resultant output vibration would thus be lower than the input vibration at that frequency. In this dissertation an analytical model describes the state variables in the cushion, pipe and tank. A Simulink model predicts the transmissibility curve with a solid mass as well as with a two degree of freedom seated human model. Initially the prototype was tested with a solid mass to compare the transmissibility curve produced by the simulation with the experimental results. It was required to evaluate the contribution of the resonator without the complexity of the human impedance. Subsequent tests were carried out with human subjects. Test results showed high inter subject similarity at the anti-resonance frequencies. Design guidelines are formulated that can be used by the suspension cushion designer to specify the pipe diameter and length and the volume of the tank to determine the optimal transmissibility. Input psd from ISO7096 class EM3 vehicles is used as an example during the design process. A prototype air suspension cushion was designed to reduce output vibration on the three wheeled logger. Laboratory tests with human subjects showed a significant improvement at the problematic frequencies through the tuning of the resonator. Using a Helmholtz resonator with the air suspension cushion the overall SEAT value improved by 25% compared with a 100mm foam cushion. However, the current tank and pipe need to be reduced in size for practical implementation to the vehicle. Future work would include finding an alternative mass to replace the air in the pipe. This should reduce the size of the tank and the pipe required. Additionally the simultaneous effect of multiple resonators at different frequencies should be investigated. This is required for vehicles having an input psd with significant energy at more than one frequency band.

Use of mobile equipment has increased exponentially over the last decade. As use becomes more widespread so too does the demand for new functionalities. The limited memory and computational power of many mobile devices has proven to be a challenge resulting in many innovative solutions and a number of new standards. Despite this, there is often a requirement for additional enhancement to improve quality. The focus of this thesis work has been to perform enhancement within two different areas; audio or speech encoding and video encoding/decoding. The audio enhancement section of this thesis addresses the well known problem in the GSM system with an interfering signal generated by the switching nature of TDMA cellular telephony. Two different solutions are given to suppress such interference internally in the mobile handset. The first method involves the use of subtractive noise cancellation employing correlators, the second uses a structure of IIR noth filters. Both solutions use control algorithms based on the state of the communication between the mobile handset and the base station. The video section of this thesis presents two post-filters and one pre-filter. The two post-filters are designed to improve visual quality of highly compressed video streams from standard, block-based video codecs by combating both blocking and ringing artifacts. The second post-filter also performs sharpening. The pre-filter is designed to increase the coding efficiency of a standard block based video codec. By introducing a pre-processing algorithm before the encoder, the amount of camera disturbance and the complexity of the sequence can be decreased, thereby increasing coding efficiency.