High-intensity rocket noise: nonlinear propagation, atmospheric absorption, and characterization.

Analyses of rocket noise data measured at far-field locations during the launch of a large rocket and a smaller rocket are presented. Weak shocks are present in all of the data sets. In order to characterize these shocks, those segments of the waveforms where the acoustic pressure is increasing are isolated and the rate of increase in pressure plotted versus magnitude of pressure rise. The plots follow a trend consistent with random noise at low values of pressure rise, then transition to the pressure-squared dependence expected for weak shocks at higher pressure rise values. Power spectral densities of the noise during the period of maximum overall sound-pressure levels display high- and low-frequency spectral slopes that are close to those predicted for shock-dominated noise. It is concluded that shocks must be included in propagation models if high frequency levels are to be estimated as a function of distance from the source. Initial shock thicknesses will have to be characterized experimentally and will require instrumentation with a bandwidth well in excess of 20 kHz. Reflection-free data are essential if meaningful assessments of the statistical properties of the noise are to be made.