The sound produced by military aircraft is dominated by noise generated by the turbulent mixing of the jetted exhaust with the ambient air. This jet noise has the potential to annoy the community and pose a hearing loss risk for military personnel. The goal of this dissertation is to characterize spatiospectral features in the field produced by full-scale military aircraft that are not traditionally seen at the laboratory scale and identify potential noise mechanisms for these features. Measurements of two military aircraft jet noise fields are found to be best described as a superposition of spatiospectral lobes, whose relative amplitudes dictate the overall directivity at each engine power. Near-field acoustical holography techniques are applied to one of the military aircraft measurements to characterize the behavior of the lobes as a function of engine power. The simulated jet noise of a highly heated laboratory-scale jet is then analyzed to compare with the military aircraft measurement and is found to only partially contain the spatiospectral lobe phenomenon. Application of near to far field coherence tracing and near-field acoustical holography to the simulations provides validation of the methods used on the military aircraft and illuminate potential source mechanisms that may explain the presence of the spatiospectral lobes.

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, to measured sound pressures in 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 reconstructions 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.

Near-field acoustical holography is a standard method for sound source visualization and localization. In this thesis, commonly used and newly published near-field acoustic holography methods are introduced and analysed. In addition, regularization methods are presented together with finding options of optimal regularization parameter. Based on theory, a LabVIEW library is built containing four implemented near-field acoustical holography algorithms and two regularization methods. To verify the correctness of implementations, a testing application has been made. This application allows testing of implemented algorithms with simulated or experimentally measured data. The correctness of implementation is verified, and algorithms are compared to each other with respect to accuracy and speed of calculation.

Near-field Acoustical Holography is a tool that is conventionally used to visualize sound fields through an inverse process in a three-dimensional space so that either sound field projections or sound source localization can be performed. The visualization is conducted by using sound pressure measurements taken in the near-field region close to the surface of the unknown sound source. Traditional Fourier-based Near-field Acoustical Holography requires a large number of measurement inputs to avoid spatial truncation effects. However, the use of a large number of measurements is usually not feasible since having a large number of microphones is costly, and usually the array is limited in size by the physical environment, thus limiting the practicality of this method. In the present work, because of the desire to reduce the number of microphones required to conduct acoustical holography, a method known as Statistically Optimized Near-field Acoustical Holography initially proposed by Steiner and Hald was analyzed. The main difference between the present work and the concept mentioned by Steiner and Hald is the cylindrical coordinate system employed here for the purpose of experimenting on a bladeless fan, which resembles a cylindrical structure and which could be assumed to be a cylindrical source. The algorithm was first verified <i>via</i> simulations and measurements, and was then applied to experimental data obtained <i>via</i> pressure measurements made with a cylindrical microphone array. Finally, suggestions for noise control strategies for the bladeless fan are described, based on the measurement results.<br>

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.

Recent advances in near-field acoustical holography (NAH) have expanded the theory to interior spaces where multiple sources and/or reflections are present. In 1990, Tamura presented the spatial Fourier transform separation method to measure the reflection coefficient at oblique angles using two measurement planes in the wave number domain. This paper adapts the spatial Fourier transform separation method for application in interior NAH. A practical exploration of important experimental parameters is performed, which include the relative amplitudes of primary and disturbing sources, the measurement plane separation distance, and an acceptable noise floor. This technique is successfully applied in a reverberant environment to reconstruct the velocity of a clamped vibrating plate. NAH methods based on the measurement of pressure and particle velocity have led to the ability to reduce the required measurement locations. Other recent advances in NAH have expanded the theory to interior spaces where multiple sources and/or reflections are present. This paper investigates the use of interpolation techniques to reduce the required measurement locations for interior NAH. Specifically, the benefits of a bi-cubic Hermite surface patch interpolation are discussed and compared to other interpolation routines. Although the required inputs for the Hermite interpolation can be measured using a variety of devices, a scanning six-microphone probe in a tetrahedral configuration is suggested. The six microphones are utilized to simultaneously sample pressure on two parallel planes and estimate the pressure gradients on both of these planes. The two interpolated measurement holograms are used to separate the incoming and outgoing waves using the spatial Fourier-transform method. Analytical simulations of simply supported plates are shown as well as experimental results in a reverberation room to characterize the reduction in measurement locations. Depending on the spatial frequency of the hologram, a measurement location reduction of 20–80% was observed.

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.