Theses and Dissertations at Montana State University (MSU)
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Item Assessing nonlinearity and memory extent in audio systems(Montana State University - Bozeman, College of Engineering, 2021) Hoerr, Ethan Randall; Chairperson, Graduate Committee: Robert C. MaherCreating digital models of existing audio devices is useful for increasing access to audio effects and for preserving audio history. In the work covered by this dissertation, we investigate the use of Volterra series modeling to assess the degree of nonlinearity of a system and time-delayed mutual information (TDMI) to estimate the length of the recovered impulse response. Using an arctangent function as an example system, comparing empirically generated Volterra series models containing anywhere from first- to fourth-order system kernels revealed that including the odd-ordered first and third kernels yielded the best-performing model. We propose that this benchmarking method can aid a system modeler by elucidating details about a system's nonlinear behavior. We also assess the utility of time-delayed mutual information (TDMI) as a method for revealing which samples of a recovered impulse response of a nonlinear system are significant. As a nonlinear metric of correlation between an input signal x[n] and output signal y[n], the TDMI method in MATLAB simulations accurately predicted the significant samples of delay lines, FIR moving average filters, and Schroeder all-pass filters. The TDMI method was less informative when applied to IIR low pass filters with and without an arctangent function appended to the output. Finally, we applied the TDMI approach to a real-world audio device, a distortion effects pedal designed for electric guitar players. In the presence of increasing nonlinear distortion, the calculated TDMI curve took the shape of a pronounced peak starting at T = 0 samples delay between x[n] and y[n], with T increasing as the distortion increased. A similar phenomenon was observed when lowering the pedal's low-pass filter cutoff frequency from 36.7 kHz to 620 Hz; in the 620 Hz test, the TDMI peak was significantly lower than the other test cases and featured a more gradual decay to the estimator bias noise floor. In summary, we demonstrated that Volterra series models are useful for assessing the degree of nonlinearity of a system and that time-delayed mutual information can inform which samples of a recovered impulse are significant. Both of these insights can aid in deciding how many Volterra series kernels and how much kernel memory to include when creating a black box system model.Item Observing variation of acoustical characteristics of several common firearms in a quasi anechoic environment at a high sampling rate(Montana State University - Bozeman, College of Engineering, 2016) Routh, Tushar Kanti; Chairperson, Graduate Committee: Robert C. MaherAudio recordings from a shooting incident may provide crucial information for a criminal investigation. A typical gunshot signal includes two high amplitude and short duration impulsive signature sounds, the muzzle blast, observed in all the gunshot waveforms, and the bullet s shock wave, which can only be detected if the bullet travels at supersonic speed. Acoustic gunshot analysis generally focuses on the study of muzzle blast signals, which last only a few milliseconds. Ideally, gunshot signals needed to be record at a very high sampling rate to reveal the muzzle blast details. Real life gunshot recordings are record with equipment not designed for these high-amplitude sounds. Moreover, the recordings contain the direct sound of the gun along with multiple overlapping signals due to sound reflections from the ground, nearby surfaces, and other obstacles. The resulting reverberant recording may be difficult to interpret. To study the details of these signals in a scientific manner, we have developed a quasi-anechoic procedure to capture gunshot signals at a very high sampling rate (500 kHz samples per second) using 12 microphones covering 180° in azimuth. The recordings are made in an open air environment with a raised shooting platform and microphone position, resulting in sufficient delay between the arrival of the direct sound at the microphones and the arrival of the first reflection (from the ground). The firearms used in this experiment include a Remington 870 shotgun, 308 Winchester rifle, AR15 rifle, and a 22LR rifle. Handguns tested include a Colt 1911A1, Glock 19 with 9mm ammunition, Glock 23, Sig 239, and a Ruger SP101 with both 357 Magnum and 38 Special ammunition. A number of successive shots were record for each of the firearm type. Based on analysis of the recorded data, we find that acoustic gunshot signals vary from one firearm to another in terms of peak sound pressure and Muzzle blast duration. For a given firearm, we observe significant differences in sound level and also Muzzle blast duration as a function of azimuth and find that there is measurable variation in signal details among successive shots from the same firearm.Item Virtual audio localization with simulated early reflections and generalized head-related transfer functions(Montana State University - Bozeman, College of Engineering, 2009) Reed, Darrin Kiyoshi; Chairperson, Graduate Committee: Robert C. MaherIn a natural sonic environment a listener is accustomed to hearing reflections and reverberation. It is conceived that early reflections could reduce front-back confusion in synthetic 3-D audio. This thesis describes experiments which seek to determine whether or not simulated reflections can reduce front-back confusions for audio presented with non-individualized head-related transfer functions (HRTFs) via headphones. To measure the contribution of the reflections, 13 human subjects participated in localization experiments which compared their localization ability with anechoic HRTF processing versus HRTF processing with a single early-reflection. The results were highly subject dependent; some showed improvement while others seemed to be inhibited by the reflections. Statistical analysis of the overall results concluded that a single reflection does not provide a significant difference in localization ability. Although this data rejects the hypothesis of this investigation, some suspicion regarding the contribution of lateral reflections in an auditory environment remains.