Measurement and prediction of nonlinearity in outdoor propagation of periodic signals

Far field propagation measurements of high-amplitude periodic signals generated by the U. S. Army Research Laboratory's Mobile Acoustic Source (MOAS) 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 μPa 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 (∼1 km) 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.