OUTDOOR PROPAGATION OF FINITE-AMPLITUDE NOISE

Abstract.- Experiments on the propagation of finite-amplitude noise outdoors have been made. The source was an array of horns pointed upward to avoid ground effects. Spectra were recorded along the vertical propagation path at distances out to 80 m. In most of the experiments the source emitted noise approximately one octave in bandwidth. The frequency range was 2 to 10 kHz and the source le-vel range was 121 to 145 dB (overall) re 20 u Pa at 1 m. Measurements were compared with predictions based on linear theory. Major conclusions are as follows: 1. At the higher source levels nonlinear propagation distorsion caused a strong generation of high frequency noise over the entire propagation path. There was little generation of low frequency noise. 2. The generation occured in both the nearfield and the farfield of the source. Nearfield generation exceeded the high-frequency boost caused by diffraction. 3. At no measurement point was small-signal behavior established for the high-frequency noise. 4. Approprietely scaled, the measured spectra were found, to be well within the jet noise range. The-refore, nonlinear distortion of the sort we observed probably affects actual jet noise as well. 1. Introduction.- Experiment