Moohyung Lee

Scan-based near-field acoustical holography and partial field decomposition in the presence of noise and source level variation

Practical holography measurements of composite sources are usually performed using a multireference cross-spectral approach, and the measured sound field must be decomposed into spatially coherent partial fields before holographic projection. The formulations by which the latter approach have been implemented have not taken explicit account of the effect of additive noise on the reference signals and so have strictly been limited to the case in which noise superimposed on the reference signals is negligible. Further, when the sound field is measured by scanning a subarray over a number of patches in sequence, the decomposed partial fields can suffer from corruption in the form of a spatially distributed error resulting from source level variation from scan-to-scan. In the present work, the effects of both noise included in the reference signals, and source level variation during a scan-based measurement, on partial field decomposition are described, and an integrated procedure for simultaneously suppressing the two effects is provided. Also, the relative performance of two partial field decomposition formulations is compared, and a strategy for obtaining the best results is described. The proposed procedure has been verified by using numerical simulations and has been applied to holographic measurements of a subsonic jet.

Source characterization of a subsonic jet by using near-field acoustical holography

In the present study, patch near-field acoustical holography was used in conjunction with a multireference, cross-spectral sound pressure measurement to visualize the sound field emitted by a subsonic jet and to predict its farfield radiation pattern. A strategy for microphone array design is described that accounts for the low spatial coherence of aeroacoustic sources and for microphone self-noise resulting from entrained flow near the jet. In the experiments, a 0.8-cm-diameter burner was used to produce a subsonic, turbulent jet with a Mach number of 0.26. Six fixed, linear arrays holding eight reference microphones apiece were disposed circumferentially around the jet, and a circular array holding sixteen, equally spaced field microphones was traversed along the jet axis to measure the sound field on a 30-cm-diameter cylindrical surface enclosing the jet. The results revealed that the jet could be modeled as a combination of eleven uncorrelated dipole-, quadrupole-, and octupole-like sources, and the contribution of each source type to the total radiated sound power could be identified. Both the total sound field reconstructed in a three-dimensional space and the farfield radiation directivity obtained by using the latter model were successfully validated by comparisons to directly measured results.

Patch near-field acoustical holography in cylindrical geometry

Spatial discrete Fourier transform-based near-field acoustical holography is known to suffer from windowing effects since the measurement aperture is necessarily finite. The latter effect can be mitigated by using patch holography, in which the measured field is extended beyond the measurement aperture based on successive smoothing operations. In this article, the application of a patch holography algorithm to cylindrical geometries is described. In planar geometry, initial zero-padding can be applied to the hologram pressure to an arbitrary degree in both in-plane directions since the pressure magnitude is expected to limit ultimately towards zero in both directions. In a cylindrical geometry, zero-padding can be implemented axially in the same way, but the number of zeros added in the circumferential direction is necessarily determined by the angular sample spacing owing to the periodic nature of the field in this direction. By using both numerical simulation and experimental results, it is shown that t...

Application of cylindrical near-field acoustical holography to the visualization of aeroacoustic sources.

The purpose of this study was to develop methods for visualizing the sound radiation from aeroacoustic sources in order to identify their source strength distribution, radiation patterns, and to quantify the performance of noise control solutions. Here, cylindrical Near-field Acoustical Holography was used for that purpose. In a practical holographic measurement of sources comprising either partially correlated or uncorrelated subsources, it is necessary to use a number of reference microphones so that the sound field on the hologram surface can be decomposed into mutually incoherent partial fields before holographic projection. In this article, procedures are described for determining the number of reference microphones required when visualizing partially correlated aeroacoustic sources; performing source nonstationarity compensation; and applying regularization. The procedures have been demonstrated by application to a ducted fan. Holographic tests were performed to visualize the sound radiation from that source in its original form. The system was then altered to investigate the effect of two modifications on the fans sound radiation pattern: first, leaks were created in the fan and duct assembly, and second, sound absorbing material was used to line the downstream duct section. Results in all three cases are shown at the blade passing frequency and for a broadband noise component. In the absence of leakage, both components were found to exhibit a dipole-like radiation pattern. Leakage was found to have a strong influence on the directivity of the blade passing tone. The increase of the flow resistance caused by adding the acoustical lining resulted in a nearly symmetric reduction of sound radiation.

A one-step patch near-field acoustical holography procedure.

When performing holography measurements over a limited area of a source, the hologram pressure typically remains finite at the edge of the measurement aperture. Patch near-field acoustical holography (NAH) has been developed specifically to mitigate the effects related to that windowing. In iterative patch NAH, the source distribution is reconstructed in two steps: first, the partially measured sound field is extended iteratively, and then the extended pressure is projected onto the source surface by using conventional NAH procedures. In the present work, a one-step procedure for performing that combined task is described. In this approach, the acoustical property to be reconstructed on a surface of interest is related to the partially measured pressure on the hologram surface in terms of sampling and bandlimiting matrices, and the reconstructed result is obtained by finding the regularized least squares solution of the latter relation; a procedure for determining the cutoff wave number of the bandlimiting matrix without a priori knowledge of the signal bandwidth is suggested. The proposed procedure was validated by using a synthetic sound field created by a point-driven, simply supported plated.

Estimation of the combustion-related noise transfer matrix of a multi-cylinder diesel engine

In the present paper, a procedure for estimating an engine-platform-dependent transfer matrix that relates in-cylinder pressures to radiated noise resulting from processes associated with the combustion process is described. A knowledge of that transfer matrix allows the combustion-related component of the noise radiated by a diesel engine to be estimated from a knowledge of cylinder pressure signals. The procedure makes use of multi-input/multi-output (MIMO) system modeling concepts in conjunction with cross-spectral measurements. To date, the empirical prediction of diesel engine combustion noise has usually been achieved by combining a cylinder pressure with a single, smooth structural attenuation function (e.g., the Lucas combustion noise meter) regardless of the specifications of the engine. In comparison, the procedure described in the present work provides the structural attenuation characteristics of a particular engine in the form of a transfer matrix, thus allowing accurate prediction by accounting fully for inter-cylinder correlation, cylinder-to-cylinder variation and the detailed characteristics of an engine structure. The procedure was applied to a six-cylinder diesel engine, and the various aspects of the new procedure are described.

Multi-Reference Methods for Nearfield Acoustical Holography

Holographic projection requires that sound fields be spatially coherent. That condition can be satisfied if data at all locations on the hologram surface are measured simultaneously. However, more commonly, a scanning procedure is used in which measurements are made on parts of the hologram surface in sequence. In the latter case, it is necessary to record signals from fixed‐location reference transducers so that the phase of the pressure in each scan segment can be determined. Further, if the sound field is radiated by statistically incoherent sub‐sources, the sound field must be decomposed into coherent partial fields before holographic projection is performed on the individual partial fields. For the partial field decomposition to be successful, the number of reference transducers must be greater than or equal to the number of incoherent sources generating the field. In this presentation, the state‐of‐the‐art of multi‐reference procedures applied to nearfield acoustical holography will be described. Re...