Selected Publications

BYU Authors: S. Hales Swift and Kent L. Gee, published in J. Acoust. Soc. Am.
A previous letter by Gee et al. [J. Acoust. Soc. Am. 121, EL1-EL7 (2007)] revealed likely shortcomings in using common, stationary (long-term) spectrum-based measures to quantify the perception of nonlinearly propagated noise. Here, the Glasberg and Moore [J. Audio Eng. Soc. 50, 331-342 (2002)] algorithm for time-varying loudness is investigated. Their short-term loudness, when applied to a shock-containing broadband signal and a phase-randomized signal with equivalent long-term spectrum, does not show a significant difference in loudness between the signals. Further analysis and discussion focus on the possible utility of the instantaneous loudness and the need for additional investigation in this area. (C) 2011 Acoustical Society of America. [DOI: 10.1121/1.3569710]
BYU Authors: Matthew D. Shaw and Kent L. Gee, published in Noise Control Eng. J.
An indoor test facility for 20-mm and 30-mm aircraft Gatling guns was recently constructed at Hill Air Force Base in Layton, UT. In designing the range, the primary concerns were that the 30-mm gun muzzle blast overpressures were large enough to a) cause significant spallation of the concrete walls during the anticipated 20-year building usage, and b) potentially pose an auditory risk for personnel working elsewhere in the test range building. This project consisted of three phases. First, levels, directivity, and geometric spreading of the 30-mm gun blast were characterized in outdoor measurements. Second, range impulse response estimates generated by a commercial room acoustics package were used to discover potential problem areas within the range and explore the effectiveness of treatments. Finally, data were collected on gun blasts in the completed range to confirm that the test facility meets all acoustical and occupational safety requirements. (C) 2010 Institute of Noise Control Engineering.
BYU Authors: Kent L. Gee and Micah R. Shepherd, published in J. Acoust. Soc. Am.
Bicoherence analysis has been used to characterize nonlinear effects in the propagation of noise from a model-scale, Mach-2.0, unheated jet. Nonlinear propagation effects are predominantly limited to regions near the peak directivity angle for this jet source and propagation range. The analysis also examines the practice of identifying nonlinear propagation by comparing spectra measured at two different distances and assuming far-field, linear propagation between them. This spectral comparison method can lead to erroneous conclusions regarding the role of nonlinearity when the observations are made in the geometric near field of an extended, directional radiator, such as a jet. (C) 2010 Acoustical Society of America
BYU Authors: Kent L. Gee, Julia A. Vernon, and Jeffrey H. Macedone, published in J. Chem. Educ.
Although hydrogen−oxygen balloon explosions are popular demonstrations, the acoustic impulse created poses a hearing damage risk if the peak level exceeds 140 dB at the listener’s ear. The results of acoustical measurements of hydrogen−oxygen balloons of varying volume and oxygen content are described. It is shown that hydrogen balloons may be used without auditory risk to typically situated participants. It is further shown that even small (0.1 mol total) hydrogen−oxygen balloons cannot be exploded without precaution, that is, ensuring that participants have hearing protection or are located sufficiently far away. In all cases, it is recommended that the presenter wear hearing protection.
BYU Authors: Zachary A. Collins, Kent L. Gee, Scott D. Sommerfeldt, and Jonathan D. Blotter, published in Proc. Meet. Acoust.
Near-field acoustical holography (NAH) is used to reconstruct three-dimensional acoustic fields from a two-dimensional planar measurement. During previous work at BYU, a method has been developed called energy based near-field acoustical holography which reduced the number of needed measurements by 75%. Other recent advances have expanded the theory to interior spaces where multiple sources and/or reflections are present. This paper presents a new method for reconstructing interior acoustic parameters using Fourier NAH and a single plane of energy density measurements. Energy density is measured using a six-microphone array. First, the probe measurements are used to create a Hermite surface pressure interpolation on two separate planes. These two planes are used to approximate the normal particle velocity as well as to separate the incoming and outgoing waves using the spatial Fourier-transform method. Once separated, traditional Fourier NAH is used to reconstruct the pressure and normal particle velocity at any point in space. Analytical and experimental results are shown and compared to exterior Fourier NAH approximations. Other drawbacks and benefits are discussed.
BYU Authors: Daniel A. Manwill, Jeff M. Fisher, Scott D. Sommerfeldt, Kent L. Gee, and Jonathan D. Blotter, published in Proc. Meet. Acoust.
Given the benefits of using acoustic energy density for active noise control in enclosures, it was hypothesized that active structural acoustic control (ASAC) might also benefit by incorporating an energy-based structural error quantity. Power flow, or structural intensity, and structural energy density were studied for use in an ASAC system. Power flow was found to be unsuitable for general-purpose use in this application. Its minimization properties are such that for a general case, it may be impossible to predict structural or acoustic response based on a minimization of power flow amplitude in a two-dimensional setting. Additionally, sensor placement is complicated by the large changes in power flow field orientation caused by a small mass loading for a lightweight structure. Structural energy density was found to be a suitable error metric, and provides a slight improvement over velocity-based ASAC in enclosed spaces. A genetic algorithm was used to study structural energy density sensor placement on a simply supported plate. At modal frequencies, optimum control was achieved by placing the sensor at antinodes. The placement of the control force was found to be less critical, but showed a slight tendency towards locations remote from the disturbance force with low velocity cross-derivative.

Theses, Captstones, and Dissertations

A large Balinese gamelan gong, the gong ageng wadon, is similar to other Indonesian gongs in that it is tonal. A previously undocumented phenomenon is the distinct acoustic beating this gong produces. In this study of the Brigham Young University's gong ageng wadon, acoustical and vibrometry measurements were performed. Scanning laser Doppler vibrometer results show a beat frequency of about 3 Hz is produced near 150 Hz and 160 Hz by closely spaced structural modes. A slightly slower beat frequency (around 2.5 Hz) is also produced at about 120 Hz. The difference in this case is that there is only one structural mode measured near this frequency. The acoustic radiation from this mode beats with a harmonic of the radiation from a lower structural mode, which harmonic is caused by nonlinear structural phenomena.
In order to facilitate the acquisition and accurate interpretation of intensity and energy density data in high-amplitude pressure fields, the expressions for intensity and energy density are examined to ascertain the impact of nonlinear processes on the standard expressions. Measurement techniques for estimating acoustic particle velocity are presented. The finite-difference method is developed in an alternate manner and presented along with bias and confidence estimates. Additionally, two new methods for estimating the local particle velocity are presented. These methods appears to eliminate the errors and bias associated with the finite-difference technique for certain cases.
The implementation of an automated positioning and measurement system in the fully anechoic chamber at Brigham Young University is described. The system is employed to study the frequency generation of a hypersonic speaker. The general positioning system design consists of suspended rails for planar motion which are moved using stepper motors controlled by a computer. Con- siderations for mounting, acoustically absorptive insulation, system error han- dling, and automated measurement techniques are addressed. Software design and necessary functionality is outlined in detail. This system is useful for high-resolution measurements in stationary sound fields that could not be car- ried out with a reasonable number of channels or in a reasonable amount of time. The system is used to measure hypersonic speaker frequency generation of using driving signals of 500 Hz, 3 kHz, 10 kHz and all of those frequencies combined. The speaker is also studied using a white noise input signal.
Recent experimental achievements in active noise control (ANC) for cooling fans have used near-field error sensors whose locations are determined according to a theoretical condition of minimized sound power. A theoretical point source model, based on the condition previously stated, reveals the location of near-field pressure nulls that may be used to optimize error sensor placement. The actual locations of these near-field pressure nulls for both an axial cooling fan and a monopole loudspeaker were measured over a two-dimensional grid with a linear array of microphones. The achieved global attenuation for each case is measured over a hemisphere located in the acoustic far field of the ANC system. The experimental results are compared to the theoretical pressure null locations in order to determine the efficacy of the point source model. The results closely matched the point source model with a loudspeaker as the primary source, and the sound power reduction was greatly reduced when error sensors were placed in non-ideal locations. A weakness of the current near-field modeling process is that a point monopole source is used to characterize the acoustic noise from an axial cooling fan, which may have multipole characteristics. A more complete characterization of fan noise may be obtained using a procedure based on the work of Martin and Roure [J. Sound Vib. 201 (5), 577–593 (1997)]. Pressure values are obtained over a hemisphere in the far field of a primary source and the contributions from point source distributions up to the second order, centered at the primary source, may be calculated using a multipole expansion. The source information is then used in the aforementioned theoretical near-field calculation of pressure. The error sensors are positioned using the complete fan characterization. The global far-field attenuation for the multipole expansion model of fan noise is compared to that of previous experiments. Results show that the multipole expansion model yields a more accurate representation the near field, but is not successful in achieving greater sound power reductions in the far field.
Near-field acoustical holography (NAH) has been used extensively for acoustical imaging of infinitesimal-amplitude (or small-amplitude) sources. However, recent interests are in the application of NAH to image finite-amplitude (or high-amplitude) sources such as jets and rockets. Since NAH is based on linear equations and finite-amplitude sources imply nonlinear effects, which cause shock formation and consequently an altered spectral shape, a feasibility study is carried out to determine the effect of nonlinear propagation on NAH. Jet and rocket sources typically have a distinct spectral shape resembling a ‘haystack’ and center frequencies varying from 30 to 300 Hz. To test the effect of nonlinear propagation on jet or rocket noise, several waveforms with varying spectral shapes and center frequencies were created and numerically propagated in one dimension using a nonlinear propagation algorithm. Bispectral methods were used to determine the amount and effect of nonlinearity, showing that higher center frequencies lead to more nonlinearities for a given amplitude. Also, higher-order statistical analysis of the time derivative of the waveforms was used to determine information about the relative amount of waveform steepening and shock coalescence occurring. NAH was then used to reconstruct the original waveform magnitude and the errors were determined. It was found that the ‘haystack’ spectral shape can be preserved by the nonlinear effects leading to low amplitude-reconstruction errors, whereas a narrow-band spectral shape will become altered and reconstruct very poorly. However, if nonlinear effects become strong due to higher center frequencies, longer propagation distances or higher amplitudes, even the ‘haystack’ shape will become altered enough to cause poor reconstruction. Two-dimensional propagation studies were also performed from two point sources, showing differences between linear and nonlinear propagation.
A balloon filled with a gas that has a different sound speed than that of air has been used by science teachers in the past as an ”acoustic lens.” One purpose of the lens is to show refraction of sound waves as an analogy to geometric optics [H. Kruglak and C. C. Kruse, Am. J. Phys. 8, 260-261 (1940)]. This paper discusses the physics involved with the balloon lens demonstration. In order to determine the validity of a gas-filled balloon as a classroom demonstration of an acoustic lens and to understand the corresponding phenomena, the problem is considered analytically, numerically, and experimentally. The results show that although a geometric analogy is a valid first order approximation, scattering theory is often required to fully understand the observed phenomena. Thus, this demonstration can be adapted to a wide range of students, from those learning basic principles of refraction to mathematically advanced students studying scattering.
The Rubens flame tube is a teaching demonstration that is over 100 years old that allows observers to visualize acoustic standing wave behavior [H. Rubens and O. Krigar-Menzel, Ann. Phys. (Leipzig) 17, 149 (1905)]. Flammable gas inside the tube flows through holes drilled along the top, and flames are then lit above. The tube is closed at one end and is driven with a loudspeaker at the other end. When the tube is driven at one of its resonance frequencies, the flames form a visual standing wave pattern as they vary in height according to the pressure amplitude in the tube. Although the basic performance of the tube has been explained [G. Ficken and C. Stephenson, Phys. Teach., 17, 306-310 (1979)], this paper discusses the previously unreported characteristic of the tube’s resonance frequency shifts. This study observed that this phenomenon involves an upward shift of the natural frequencies of the lower modes from what would ordinarily be expected in a closed-closed tube. Results from a numerical model suggest that the shift is primarily due to a Helmholtz resonance effect created by the drilled holes. The numerical model is explained and the numerical results are compared to experimental findings and discussed.
I have constructed a device that demonstrates the spectral decomposition of waves according to spatial location along a surface analogous to the function of the basilar membrane of the mammalian cochlea. The design is based on previous work done by Robert Keolian at Pennsylvania State University.1 I have modified Keolian’s design to incorporate variable mass as well as stiffness and an electronically driven shaker, which allows the device to be driven with complex waveforms. The model can be used to demonstrate beating, masking, and other psychoacoustical phenomena that occur on the basilar membrane.
Interest in predicting the community impact of flyover noise from high-performance aircraft has stimulated research into metrics that accurately represent human perception of the loudness of the noise thus created. This problem is complicated by both human factors involved in the perception of the noise waveforms, and physical ones, particularly the nonlinear process that underlies the transmission of high-amplitude noise through the atmosphere. An introduction will be given to the process of nonlinear propagation and how it is modeled to give our simulated nonlinear waveform. The processes underlying loudness perception in the hearing system and the methods used in this project to model them will be discussed. The reaction of the time-varying loudness metric, as described by Moore and Glasberg, to simulated linearly and nonlinearly propagated signals will be discussed and compared to its reaction to a “rephased” signal, in which the power spectrum of the nonlinearly propagated signal is maintained but the Fourier phase randomized to eliminate the shocks. Qualitative perceived reactions to the waveforms will be compared the quantitative reaction of the metric. These will be considered in the light of recent research into the phenomenon of “crackle” and the importance of the first derivative of the wave form in the qualitative experience of crackle.
In the active control of tonal noise from cooling fans, one factor that can limit the achievable attenuation is fluctuation of the blade passage frequency in time. Large fluctuations in a short time can hinder the algorithm from converging to the optimal solution. Some fans have steadier speeds than others, which can be due to unsteady driving mechanisms or the physical structure of the fan. Environmental effects such as back pressure and unsteady blade loading can also cause the fan speed to fluctuate. The shifting in the blade passage frequency will be measured using a zero-crossing technique to track the frequency of each cycle. Blade passage frequency fluctuations will be presented for various driving mechanisms and environmental conditions. Techniques to minimize frequency shifting will also be discussed.
Energy-based acoustic quantities provide vital information about acoustic fields and the characterization of acoustic sources. Recently, the phase and amplitude gradient estimator (PAGE) method has been developed to reduce error and extend bandwidth of energy-based quantity estimates. To inform uses and applications of the method, analytical and experimental characterizations of the method are presented. Analytical PAGE method bias errors are compared with those of traditional estimation for two- and three-microphone one-dimensional probes. For a monopole field when phase unwrapping is possible, zero bias error is achieved for active intensity using three-microphone PAGE and for specific acoustic impedance using two-microphone PAGE. A method for higher-order estimation in reactive fields is developed, and it is shown that a higher-order traditional method outperforms higher-order PAGE for reactive intensity in a standing wave field. Extending the applications of PAGE, the unwrapped phase gradient is used to develop a method for directional sensing with improved bandwidth and arbitrary array response.
Acoustical room characterization typically involves a lot of high end and expensive equipment for measuring, modeling, and finding solutions to problems. Recently, Room EQ Wizard (REW) was introduced to BYU as a less costly option. This paper will evaluate REW's effectiveness through the metrics of mapping sound coverage, reverberation time and clarity calculations, while providing the Missionary Training Center with acoustical consulting for their auditorium. REW proved to be efficient and effective in all metrics and helped narrow the problem down to an unforeseen interaction between the loudspeakers and podium microphone.
Sound generation and radiation properties are studied of full-scale tactical jet engine noise. This is motivated by the high sound exposure levels from jet noise, particularly for tactical engines. Acoustic source reconstruction methods are implemented computationally on existing jet noise data. A comparative study is performed using numerical simulations to understand the capabilities of more advanced beamforming methods to successfully estimate the source properties of a distributed, partially correlated source distribution. The properties and limitations of each beamforming method are described. Having validated the methods, beamforming with regularization—via the Hybrid Method—is implemented on linear array measurements near an installed tactical engine. A detailed analysis of the correlation and coherence properties associated with the phased array measurements guides the implementation of the beamforming. When the measurements are used as inputs to the beamforming, they produce partially correlated, distributed sources in a full-order model representation. A processing technique is also implemented that increases the usable bandwidth of the array measurements to almost an order of magnitude above the array design frequency. To more appropriately study the equivalent sources, a decomposition technique is designed and implemented to create a reduced-order wavepacket model of the jet noise. The wavepacket model is modular and scalable to allow for the efficient characterization of similar jet noise measurements. It is also appropriate for its physical significance, as wavepackets are attributed to the turbulent flow as well as the hydrodynamic and acoustic properties of the radiation. The reduced order model can estimate the levels and coherence properties of the acoustic radiation and represents a significant step towards a complete jet noise prediction model.
Acoustic intensity measurements traditionally use cross spectral processing methods with multi-microphone probes to estimate pressure and particle velocity. In 2015, Thomas showed that the phase and gradient estimator (PAGE) method increases probe bandwidth without modifying microphone spacing as compared to the traditional cross spectral method. In this study, acoustic intensity is estimated by both the PAGE method and the traditional method across two commercially built 3D intensity probes and three in-house built 2D intensity probes. Probe performance is compared in a broadband, white noise, anechoic sound field radiated from a loudspeaker. Probe orientation is considered by rotating each probe over a 360 degree horizontal plane at 2.5 degree increments. Results show increased frequency bandwidth using the PAGE method across all probe designs. 3D intensity level estimates suffered the least amount of error with the spherical probe. 2D intensity level and direction estimates suffered the least error with the 2D triangular probe with ½” microphones spaced at 2”. Measurement limitations concerning the 2D triangular probes with ¼” microphones are discussed
Energy quantities, which are calculated from pressure and particle velocity, yield a great deal of information about acoustic fields. Errors in pressure or particle velocity estimation lead to bias errors the estimation of energy quantities. The bias errors arise from different probe configurations and processing methods. Two processing methods are examined: the traditional method and the recently developed Phase and Amplitude Gradient Estimation (PAGE) method. These two methods are compared to investigate how each estimates pressure and particle velocity and the subsequent bias errors in a plane wave, standing wave, and spherical spreading wave field. Analytical expressions are derived for the energy quantity estimation using ideal one-dimensional probes. A simulation of the field from a baffled circular piston and measurements using ideal two-dimensional probes is computed. Compared to the traditional method, the PAGE method significantly extends the range of frequencies for which the results are accurate. It is found that a probe with a center microphone significantly reduces the estimation error and extends the usable range of frequencies. The PAGE method with unwrapping, perfectly matches the analytical results for plane waves, while the traditional method is only good at wavelengths that are large compared to the probe size. Furthermore, the PAGE method has a constant bias error in spherical wave fields due to the 1/r decrease in pressure. The traditional method has a frequency dependent bias error that is much worse at higher frequencies. Lastly, the PAGE method has the same or worse error for the standing wave. As an application of energy quantities, acoustic intensity is used to develop an equivalent source model for jet noise from an F-22 at military and afterburner engine conditions. An optimization is used to find the best-matching wavepacket model for measured intensity vectors. The results are compared to another intensity method of estimating the source region and source directivity, and the two methods have good agreement.
A recently developed phase and amplitude gradient estimator (PAGE) method for calculating acoustic intensity from multiple pressure measurements [Thomas et al., J. Acoust. Soc. Am. 134, 4058 (2013)] has been tested via anechoic laboratory measurements of the radiation from broadband sources. The measurements determine that the effective frequency bandwidth of valid acoustic intensity calculations can be substantially increased when using the PAGE method over the traditional cross-spectral approaches. Preliminary results are shown for two probe sizes and multiple broadband source configurations.
Mach stem formation in acoustic shocks is investigated using oxyacetylene balloon explosions conducted a short distance above pavement. As the shock wave propagates away from the blast site it transitions from a regular reflection to irregular reflection. The location of this transition point, as well as the path of the triple point, are experimentally resolved using microphone arrays and a high-speed camera. The measured transition point falls between that predicted from derivations based on weak shock waves and an empirical relationship derived from large-scale explosions. It also agrees well with predicted values based on von Neumann’s three shock theory.
An alternative pressure-sensor based method for estimating the acoustic intensity, the phase and amplitude gradient estimation (PAGE) method, is presented. This method is similar to the finite-difference p-p (FD) method, in which the intensity is estimated from pressure measurements made using an array of closely spaced microphones. The PAGE method uses the same hardware as the FD method, but does not suffer from the frequency-dependent bias inherent to the FD method. Detailed derivations of the new method and the traditional FD method are presented. Both methods are then compared using two acoustic fields: a plane wave and a three monopole system. The ability to unwrap the phase component of the PAGE method is discussed, which leads to accurate intensity estimates above previous frequency limits. The uncertainties associated with both methods of estimation are presented. It is shown that the PAGE method provides more accurate intensity estimates over a larger frequency bandwidth. The possibility of using a higher-order least-squares estimation with both methods is briefly demonstrated. A laboratory experiment designed to validate the PAGE method was conducted. The preliminary results from this experiment are presented and compared to analytical predictions. Finally, the application of the PAGE method to a static rocket test firing is presented. The PAGE method is shown to provide accurate intensity estimates at frequencies that are higher than possible with just the FD method.
In scan-based array measurements, stationary reference sensors are needed to temporally correlate the different measurement scans and produce coherent complex pressure fields that can be used to perform near-field acoustical holography (NAH). Because the number of references required increases with the number of subsources contributing to the sound field, an extended, partially correlated source (e.g., a turbulent jet) comprising many ill-defined sources can result in significantly increased measurement complexity and expense. Demonstrated here a is a new approach to creating spatiotemporally coherent pressures using self-referencing between overlapping measurement positions instead of separate reference channels. A laboratory experiment was designed and the data have been used to explore "stitching" together a complex pressure field. This experiment is described and the “stitching” method is detailed. To successfully execute the technique, unwrapping of intrascan phases is first accomplished with a two-dimensional phase unwrapping algorithm. Individual scan positions are then stitched together using median phase differences between multiple adjacent scans to create coherent planes of data. Amplitude-stitching is done by averaging across scans and preserving the integrated squared pressure across the overall aperture. The validity of this method is shown by showing that a consistent local coherence maintained through the stitching process. The technique is applied to jet noise, and the possibility of applying it to NAH is discussed. This technique provides direction for efficient experimental design for scan-based array measurements of extended sources.
Acoustic intensity measurements of the F-22A Raptor are analyzed as part of ongoing efforts to characterize the noise radiation from military jet aircraft. Data were recorded from a rig of microphones and an attached tetrahedral intensity probe at various locations to the sideline and aft of the aircraft. Numerical analysis of the intensity at one-third octave band center frequencies along various measurement planes and at a 23 m radius arc reveals the magnitude and directionality of the vector acoustic intensity. Differences in the trends for low-frequency and high-frequency data are discussed and, via a simple ray tracing method from maximum intensity regions, interpreted in terms of far-field behavior and source location. In particular, the extended source region contracts and moves upstream with increasing frequency, and vector directionalities point farther toward the sideline.
The radiation of finite-amplitude waves from the open end of a baffled, circular pipe is considered as a direct continuation of work begun by Kuhn, Blackstock, and Wright more than three decades ago [Kuhn et al., J. Acoust. Soc. Am. 63, S1, S84 (1978)]. Band-limited Gaussian noise, as well as 1 kHz, 1.5 kHz, and 2kHz sinusoidal pulses, with initial peak pressure amplitudes ranging from 0.5 – 1.2 kPa, have been propagated down a 6.1 m pipe, whose open end (5.1 cm inner diameter) has been placed off-center in a large rectangular baffle. As the steepened or shock-like waves exit the pipe, the measured waveforms are comprised of sharp impulses that are delta function-like in nature, particularly on axis. Although linear piston theory predicts similar waveform shapes, there is also evidence that nonlinear propagation of these impulses, which can exceed peak pressure amplitudes of 1.5 kPa near the pipe opening, is occurring.
Military aircraft generate high-amplitude noise which can cause injury to attending personnel. Efforts to mitigate the effects of this noise require a detailed understanding of the propagation of the noise, which was shown previously to be nonlinear. This thesis presents an analysis of highamplitude noise propagation, emphasizing measures used to quantify the importance of considering nonlinearity. Two measures of the importance of nonlinearity are compared. These measures are the wave steepening factor and a skewness estimate. The wave steepening factor is a measure of how much nonlinear waveform steepening has occurred in a waveform. The skewness estimate is the skewness of the first time-derivatives of the pressure amplitudes, and can be considered a measure of the shock content in a waveform. These two measures are analyzed analytically in terms of the Earnshaw, Fubini, Fay, and Khokhlov solutions to the Burgers equation. In addition, an analysis of how discrete sampling affects the estimation of these quantities is also presented. It is determined that the wave steepening factor is robust with respect to low sampling rates, but the skewness of the first time-derivatives of the pressure amplitudes is not robust, and requires very large sampling rates to be adequately estimated. Using numerical and experimental techniques, the two nonlinearity measures are applied to more complicated waveforms, such as Gaussian noise and noise with jet noise-like statistics. It is found that the evolution of the two nonlinearity measures discussed above for noise signals is distinctive in various ways. In particular, the skewness of the first time derivative of the pressure amplitudes suggest that noise waveforms experience nonlinear phenomena faster than initially sinusoidal signals, while the wave steepening factor suggests that they occur at approximately the same rate. The measures are then applied to full-scale military aircraft. By comparing these nonlinearity metrics with the results of the analytical, numerical, and experimental results found in this thesis, it is determined that nonlinearity is likely to be significant in the near field of a military aircraft at military and afterburner engine conditions.
The noise emissions of jets from full-scale engines installed on military aircraft pose a significant hearing loss risk to military personnel. Noise reduction technologies and the development of operational procedures that minimize noise exposure to personnel are enhanced by the accurate characterization of noise sources within a jet. Hence, more than six decades of research have gone into jet noise measurement and prediction. In the past decade, the noise-source visualization tool near-field acoustical holography (NAH) has been applied to jets. NAH fits a weighted set of expansion wave functions, typically planar, cylindrical, or spherical, t measured sound pressure sin the field. NAH measurements were made of a jet from an installed engine on a military aircraft. In the present study, the algorithm of statistically optimized NAH (SONAH) is modified to account for the presence of acoustic reflections from the concrete surface over which the jet was measured. The three dimensional field in the jet vicinity is reconstructed, and information about sources is inferred from reconstruction at the boundary of the turbulent jet flow. Then, a partial field decomposition (PFD) is performed, which represents the total field as the superposition of multiple, independent partial fields. This is the most direct attempt to equate partial fields with independent sources in a jet to date.
Microphone arrays are useful for measuring acoustic energy quantities (e.g. acoustic intensity) in the near-field of a full-scale solid rocket motor. Proper characterization of a rocket plume as a noise source will allow for more accurate predictions in engineering models that design for protection of structures, payloads and personnel near the rockets. Acoustic intensity and energy density quantities were measured in three rocket noise fields and have shown that the apparent source region of the rocket becomes smaller and moves upstream as frequency increases. Theoretical results accounting for some scattering and finite-difference errors arising in these types of energy-based measurements have been previously discussed by other authors. This thesis includes results from laboratory experiments which confirm some of this previous theoretical work as well as gives insight into the physical limitation of specific microphone array designs. Also, calibrations for both magnitude and directional response of the microphones are demonstrated. Of particular interest is the efficacy of phase calibration of array microphones for the low-frequency regime below 200 Hz.
For years, the effects of sound pressure level measurements upon human hearing and psychology have been studied, and only in recent years scientists have seen a link between these measurements and headphones connected to portable listening devices. The Brigham Young University Educational Acoustics Research Study (E.A.R.S) is an interactive demonstration devoted to acquiring samples of the student body that connect findings between the use of headphones with portable listening devices, such as mp3 players, and educating individuals in the process. This report will highlight the reasoning for conducting the study, describe the proper background in acoustics needed to understand the material, and analyze the results from the study, while concluding observations. The study will evaluate the overall sound pressure level due to a one second Leq, safe listening time according to non-occupational noise criterion, and demographics that define the individual, such as age, gender, listening preference, and mp3 player. Also, the study will ask the individual about their listening habits and if they will change based around the feedback received from the E.A.R.S. This study will ultimately benefit science with information to understand connections between listening habits and safe practices of listener protection.
Referee whistles output high-level short duration noise that has not been thoroughly studied. Damage risk criteria (DRC) exist to quantify the overall risk of sound exposure for continuous noise (OSHA, 1981; NIOSH, 1998) and other DRCs are intended for use with impulse noise (MIL-STD-1474D, 1991; Price, 2007). The noise from whistles is similar to impulse noise and the impulse DRC of equivalent A-weighted 8-hour energy (LeqA8), MIL-STD-1474D, Pfander, Smoorenburg, and the AHAAH model are used to analyze recordings of whistle-blows from a trained referee in a controlled environment. Recording locations were at the ear and one meter in front of the referee. Computational analysis shows that using some of the DRC, allowable exposures for referees during a sports match range from 18 to 118 exposures. Hearing protection is recommended for officials, especially for those who are exposed to other loud situations on the same day.
A statistical analysis of noise data from various-sized solid propellant rocket motors is presented. Time waveform data sampled at 204.8kHz using 6.35mm and 3.18mm microphones were collected near motors with nozzle exit diameters ranging from 0.13m to 1.22m. Non-Gaussian features of the data are explored by calculating estimates of the probability density functions of the data, its standard deviation, its skewness, and its kurtosis. This is carried out for both the pressure waveform and its first order time difference to reveal the formation of acoustic shocks within the noise. The analysis shows greater similarity between different rocket statistics for the pressure than for the time derivative.
The contributions of cavity and structural resonances to the sound radiation from a hammered dulcimer have been investigated. Results indicate that the sound quality of the lowest notes is affected by a relatively weak structural resonance in that frequency range. Additionally, near-field acoustical holography indicates significant sound radiation from front and back sound holes at frequencies of importance, contradicting the commonly held notion that the dulcimer’s sound holes serve no acoustical purpose.
Since the 1950s the jet aeroacoustics community has been involved in predicting and measuring the noise distribution in jets. In this work, cylindrical and planar Fourier near-field acoustical holography are used to investigate radiation from a full-scale, installed jet engine. Practical problems involving measurement aperture and the highly directional nature of the source are addressed. Insights from numerical simulations reveal usable reconstruction regions. A comparison of cylindrical and planar NAH for the respective measurement apertures shows cylindrical NAH outperforms planar NAH on reconstructions both towards and away from the source.
Exploding hydrogen-oxygen balloons are a popular demonstration in introductory chemistry and physical science courses. Initial research quantified potential hearing risk from exploding hydrogen-oxygen balloon demonstrations. Further time waveform and spectral analysis was conducted to characterize hydrogen-oxygen balloons as impulsive noise sources. Pure hydrogen balloons produce low levels (less than 140 dB) and inconsistent reactions. Balloons filled the stoichiometric ratio of hydrogen to oxygen produce very consistent results between trials. Hydrogen-oxygen balloons are also nearly omnidirectional, with little variation over angles. Levels for hydrogen-oxygen balloons increase as a function of balloon size and oxygen content. Comparison is provided with other impulsive noise sources, including various firearms, other explosive chemistry demonstrations, and firecrackers. Consideration is given to characteristics such as A-duration, rise time, and peak level. It is concluded that the hydrogen-oxygen balloons are less impulsive than other sources of similar amplitude, with longer A-durations and rise times. As such, the balloons have lower characteristic frequencies than other impulsive noise sources considered.
Exploding gas-filled balloons are common chemistry demonstrations. They provide an entertaining and educational means to experimentally verify nonlinear acoustical theory as described by the Earnshaw solution to the lossless Burgers equation and weak-shock theory. This paper describes the theory, the demonstration, and the results of a propagation experiment carried out to provide typical results. Data analysis shows that an acetylene-oxygen balloon produces an acoustic shock whose evolution agrees well with weak-shock theory. On the other hand, the pressure wave generated by a hydrogen-oxygen balloon also propagates nonlinearly, but does not approach N-wave-like, weak-shock formation over the propagation distance. High-speed video of these explosions provide discussion material on directionality of propagating acoustic shocks. Overall, the experiment shows that popular demonstrations of chemical reactions can be extended from chemistry classrooms to a pedagogical tool for the student of advanced physical acoustics.
A method of measuring angular dependence of acoustic transmission through supercritical plates in air is discussed. The coincidence effect occurs in a supercritical plate when the component of the acoustic wave number parallel to the plate matches the bending wave number in the plate. The transmission of sound is a maximum at the angle where this trace wave number matching occurs. The theory of the coincidence effect is well-defined for unbounded thin plates using plane-wave excitation. However, experimental results for finite plates are known to diverge from theory, especially near grazing angles. An experimental setup has been developed in order to observe the coincidence effect using continuous-wave excitation and phased-array methods. Experimental results through a 0.5 mm thick aluminum bar exhibit strong maxima at the predicted coincidence angles, showing that coincidence is observable using continuous waves. Also, transmission near grazing angles is seen to diverge from infinite plate theory. Further work is suggested to improve the measurement setup and explore the source of the divergence.
The acoustic field near large-scale solid rocket motors represents a harsh, high-amplitude noise environment rich with high-bandwidth acoustic shocks. Type-1 prepolarized microphones may be used in these environments with the benefit of reduced cost and measurement because they require only a constant-current supply available in many data acquisition systems. However, there are potential issues related to microphone response that should be considered. The main issue discussed here has to do with temporary failure of the constant-current supply due to an insufficiently fast response time in representing rapid voltage changes at shocks, which results in spurious, capacitive-like effects in the waveform data that are also manifest as a low-frequency roll-up in the spectrum noise floor. An experiment was conducted to identify under what circumstances these waveform effects arise. Data were measured from a solid rocket motor using several combinations of transducer, cable type, cable length and constant current supply. Results and mitigation methods found from the experiment are discussed. These include increasing the supply current, using low-impedance cables, and choosing the correct orientation for the transducers.
This study explores an alternative approach using a semi-empirical equivalent simple-source model to analyze and characterize the turbulence-induced aeroacoustic sources from military jet aircraft. Initially the model is a single point source located above a hard plane such that the locations of destructive interference from its mirror image matched the measured interference nulls. This provides useful information about the location of the dominant noise source region from the jet. The model then develops into a superposition of Rayleigh-distributed line arrays of uncorrelated and correlated monopoles. The model is tested on an extensive set of acoustic data taken on an F-22 Raptor. Although the model's line source characteristics are developed using data from only one measurement plane, it is able to accurately reproduce the radiation at other measurement planes. This equivalent line source matches the current prevailing theory that the sideline radiation is largely due to uncorrelated noise, whereas the correlated sources dominate downstream. In addition, the semi-empirical, simple-source model results corroborate the theory that the peak source location moves upstream with increasing frequency and lower engine conditions.
Scan-based near-field acoustical holography (NAH) is applied to partially correlated sources. Partial field decomposition via the virtual coherence method is used to implement the scan-based NAH. The background and theory of these methods are developed. Multiple stationary reference microphones are required for the partial field decomposition. Guidelines for reference microphone placement in the literature thus far have been limited. Improved guidelines for reference microphones are given after the results of two sets of experiments. The first set involves discrete, partially correlated sources, both physical and numerical. The second set of experiments is strictly numerical and involves continuous sources. Fewer microphones are required for partially correlated sources as compared to completely uncorrelated sources. Reference microphone number is found to be more critical to reducing holography reconstruction errors than is placement or location. For the continuous results, an appropriate figure of merit is created: reference microphones per coherence length. Based upon the definition of coherence length, two reference microphones per coherence length are required to minimize reconstruction error. Further practical reference microphone guidelines are given. These guidelines are to assist in preparing for a full-scale application of scanbased near-field acoustical holography to a military aircraft jet.
The United States Army Engineer District in Sacramento, California, selected HHI Corporation, a design-build construction and engineering firm of Farmington, Utah, to design and build an indoor test facility for the GAU-8 Avenger at Hill Air Force Base, Ogden, Utah. The blast pressures from this 30-mm Gatling gun, however, are large enough to cause spallation of the concrete walls over time. The facility is being designed and constructed to last for over 20 years, requiring several acoustical treatments. The pressures from the gun were measured outdoors, with maximum pressures exceeding 3000 Pa (163 dB) at a distance of 30 ft (9.1 m). A computer model of the room was designed using EASE, and impulse responses were generated at several positions. These impulse responses were convolved with an ideal blast wave pulse train to mimic the sound of the gun in the room. From these data and results collected from preliminary tests in the range, recommendations have been provided as to placement and types of necessary treatments. Final data confirm that the test facility meets all acoustical and occupational safety requirements.
A one-micron diameter platinum wire resistance thermometer measures temperature fluctuations generated by propagating noise produced by a horizontally-fired, static GEM-60 solid rocket motor. A relationship between small amplitude acoustic pressure and acoustic temperature is derived and the data are compared with those calculated from pressure data recorded by a nearby 3.18 mm condenser microphone. The validity of taking acoustic temperature measurements in a rocket field is discussed, particularly the role of turbulence induced temperature fluctuations.