To address the question of the role of nonlinear effects in the propagation of noise radiated by high-power jet aircraft, extensive measurements were made of the F-22A Raptor during static engine run-ups. Data were acquired at low-, intermediate-, and high-thrust engine settings with microphones located 23-305 m from the aircraft along several angles. Comparisons between the results of a generalized-Burgers-equation-based nonlinear propagation model and the measurements yield favorable agreement, whereas application of a linear propagation model results in spectral predictions that are much too low at high frequencies. The results and analysis show that significant nonlinear propagation effects occur for even intermediate-thrust engine conditions and at angles well away from the peak radiation angle. This suggests that these effects are likely to be common in the propagation of noise radiated by high-power aircraft.

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As part of investigations into the design of next-generation launch vehicles, near and far-field data were collected during horizontal static firings of reusable solid rocket motors. In addition to spectral analysis at individual microphone locations, the spatial and temporal variation of overall and one-third octave band pressure levels at sideline and polar arc arrays is considered. Analysis of the probability density functions reveals positively skewed pressure waveforms, but extreme skewness in the first-order estimate of the time derivative because of the presence of significant acoustic shocks. However, plume impingement is the likely cause of reduced high-frequency levels and skewness at far-downstream positions.

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Far field propagation measurements of high-amplitude periodic signals generated by the U. S. Army Research Laboratory's Mobile Acoustic Source ͑MOAS͒ have been made. The MOAS is a large horn-coupled electropneumatic loudspeaker capable of producing sound at a few hundred hertz with a maximum overall sound pressure level of 155 dB re 20 Pa at 1 m. The possible influence of nonlinear effects have been investigated because the measurements exhibit greater sound pressure levels at high harmonics than are predicted by a linear propagation model. Between 100 and 375 m, nonlinearly predicted spectra obtained via a generalized Burgers equation-based model are consistently closer to measured spectra than are linear predictions, according to calculations of mean absolute error. These comparisons strengthen the assertion that nonlinearity is, in fact, the primary cause of disagreement between the measured and linearly predicted spectra at high frequencies.Comparisons between the nonlinear model and measurements, however, yield increased errors for greater propagation distances ͑ϳ1 km͒ and for measurements made later in the afternoon. For these cases, the nonlinear model calculations generally predict greater sound pressure levels at high frequencies than are actually present in the MOAS measurements. Despite the increased errors for these latter comparisons, the nonlinear model still typically performs better than the linear model. This provides additional confirmation of the presence of nonlinearity in the propagation, but may also point to the need to account for atmospheric variability in the numerical model to provide improved predictions.