ONLINEARITY in the propagation of high-amplitude jet noise is a research topic that has been studied for decades. The combined results of early investigations such as those by Blackstock, 1 Morfey and Howell, 2 Crighton and Bashforth, 3 Crighton, 4 Lighthill, 5 and Gallagher and McLaughlin6 showed that nonlinearity should be a factor in the noise propagation for certain conditions. However, much was left undone regarding the relative significance of these effects. Recently, there has been a renewal of interest in nonlinear jet noise propagation that is largely attributable to next-generation military jet fighters coming online. Petitjean and McLaughlin7 and Petitjean et al. 8 have performed model-scale propagation experiments in anechoic facilities. From the full-scale jet perspective, Gee et al. 9, 10 have analyzed F/A-18E Super Hornet data for evidence of nonlinearity. Further indication of nonlinearity in full-scale jet noise propagation has come from work by Gee et al. 11 that involved field measurements of the noise radiated by the F-22 Raptor. Results from a nonlinear propagation model predicted significant waveform steepening and a spectral energy transfer to high frequencies that agreed closely with measured data.

Although the recent measurements have confirmed what has been suspected for decades regarding the nonlinear propagation of high-amplitude jet noise, both full-scale and model-scale experiments suffer from limitations. Measurements made to date have primarily shown evidence of nonlinear propagation by making a measurement at close range (but presumably in the geometric far field) and then assuming spherically spreading, free-field linear propagation out to a greater distance where this linear prediction is compared to measurement. For full-scale static measurements, this approach is potentially compromised by local meteorological effects (eg, turbulence and refraction) and by terrain effects. Model-scale measurements are typically carried out in an anechoic laboratory environment but are limited by bandwidth and/or propagation distance considerations. In a spectral sense, the first evidences of nonlinear propagation are seen at high frequencies due to the nonlinear steepening of the time waveform. If the measurement bandwidth (transducer or data acquisition system) is insufficient, these effects will not be readily seen in power spectral comparisons unless the propagation distance is large enough. However, since the size of an anechoic facility imposes a constraint on the allowable propagation distance, limited propagation distance for a given measurement bandwidth is a further limitation. In an effort to maximize the comparison range, measurements that are assumed to be in the far field may be actually located in the geometric near field. This practice would cause comparisons between spectra using a simple linear model that assumes far-field propagation erroneous.