Explosive Acoustics

Military jet aircraft are exposing both ground maintenance personnel and the community to high levels of noise. The US Department of Defense is funding research to develop advanced modeling tools for noise reduction techniques and community noise exposure. For these tools to achieve their full potential, innovative measurement and analysis methods are necessary to characterize the jet noise source region. To meet this need a portable near-field acoustic holography (NAH) system is under development to characterize full scale jet noise emissions. This paper will describe the basic design of the measurement array and data acquisition system for the NAH system, which will employ a patch measurement approach. With the patch measurement approach, multiple NAH reconstruction techniques can be used in tandem to provide improved confidence in the resulting reconstructions. 

The number of jet and rocket noise studies has increased in recent years as researchers have sought to better understand aeroacoustic source and radiation characteristics using predominantly linear reconstruction techniques. While jet and rocket noise is often finite-amplitude in nature, little is known about the existence of shock formation and coalescence close to the source. A numerical experiment determines that significant shock coalescence can occur when finiteamplitude noise is propagated over short distances at amplitudes similar to those expected of jet and rocket noise. Additionally, the errors associated with using linear reconstruction techniques are shown to be large when significant shock coalescence occurs. The results of this experiment point out the need for additional studies targeting shock coalescence and its possible role in near field jet and rocket noise propagation.

Recent experiments in active noise control (ANC) have used near-field error sensors whose locations are determined according to the minimization of sound power. Sensors should be placed in regions where the sound pressure reductions are the greatest during sound power minimization of the ANC system. Near-field pressure measurements of noise sources with and without ANC were made. With the error sensors in theoretically ideal locations, the measured near-field pressure map approximates the theoretical map created under the condition of minimized radiated power. Moving the error sensors to theoretically nonideal locations greatly reduces the attenuation of radiated sound power.

The number of jet and rocket noise studies has increased in recent years as researchers have sought to better understand aeroacoustic source and radiation characteristics. Although jet and rocket noise is finite-amplitude in nature, little is known about the existence of shock formation and coalescence close to the source. A numerical experiment is performed to propagate finite-amplitude noise and determine the extent of the nonlinearity over short distances with spherical spreading. The noise is filtered to have a haystack shape in the frequency domain, as is typical of such sources. The effect of the nonlinearity is compared in both the temporal and frequency domains as a function of distance. Additionally, the number of zero-crossings and overall sound pressure level is compared at several distances. The results indicate that the center frequency plays a particularly important role in the amount of coalescence and spectral redistribution that occurs. The general applicability of these results to actual near-field finite-amplitude jet and rocket noise experiments is also presented.

Near-field acoustical holography (NAH) techniques can be optimized if the method capitalizes on the geometry of the noise source under investigation. Helmholtz-equation least squares method (HELS) uses the solutions of Helmholtz equation in spherical coordinates as basis functions for the pressure field. HELS is an efficient NAH technique if the source and the measurement surfaces are spherical in nature. For nonspherical cases, such as radiation from a plate or bar, it takes a large number of functions to represent the field. In these cases, there is also a question about where to place the origin of the wave functions. In search of a HELS-type method that could be applied to nonspherical sources, a study into the features of conical coordinates has been conducted. Because Helmholtz equation is separable in conical coordinates, the solutions can be used, in a manner similar to HELS, as basis functions to represent the pressure field. For conical coordinates, the basis functions are spherical Hankel functions and Lame functions. Thus, for a conical source, a HELS-type formulation in conical coordinates could provide a natural choice for near-field acoustical holography.[Work supported by Blue Ridge Research and Consulting and Air Force Research Laboratory.]