Prediction of sound field from recoilless rifles in terms of source decomposition

In the present study, sound field from M40A1, a type of recoilless rifles, was measured using sound analyzers. Contrary to expectations based on the common characteristics of muzzle blast in previous studies, significantly different waveforms compared to those in other directions were observed in each acoustic pressure signal. In order to clarify how the surroundings affected the sound field, the temporal, spectral and directional characteristics were investigated using correlation functions and Fourier transforms. As a result, additional sources near the rear nozzle of the weapon were identified. Under the operating circumstances of recoilless weapons, blast and jet noise sources were found to be consistent with their acoustical characteristics. A hybrid model integrating the influences of blast and jet noise identified from source decomposition was derived according to the ISO standard 17201 and NASA empirical formulae. Since the predicted directivity pattern agrees moderately well with that measured in previous and present studies and the spectral distributions are in accordance with the measured ones, the hybrid model proposed in this study is considered to be appropriate for predicting the sound field from M40A1 recoilless rifle.