Numerical Propagation Of Aircraft En Route Noise PDF Download

Predictions of absorption and dispersion of en-route aircraft noise in the atmosphere are produced using the ANSI S1.26 [1] algorithm and an updated algorithm by Sutherland and Bass [2].Disagreement between predicted absorption coefficients are attributed to differences between the two models and reference atmospheric profiles, with the latter responsible for more variation than the former. In particular, the molar concentration of H2O is found to be a significant factor for absorption predictions in the atmosphere. Using the Sutherland and Bass algorithm, the relative contribution of separate physical absorption mechanisms, including classical thermoviscous effects, rotational relaxation, and vibrational relaxation losses, are compared as a function of frequency and altitude. It is found that vibrational relaxation is the dominant loss mechanism over the frequency range of 125 to 1000 Hz at altitudes of 0 to 10 km. Although vibrational relaxation gives way to classical losses above 10 km, it is shown that carbon dioxide-induced vibrational relaxation contributes up to 14% of the total losses at 15 km altitude. To evaluate the impact of absorption coefficients for a propagating wave, cumulative absorption over a vertical propagation path can be calculated by numerical integration. It is shown that the discretization step sizes should be no greater than 1 km to avoid under sampling the absorption curves from 0 to 18 km altitude. Finally, dispersion is analyzed by calculating the phase speed increment due to O2, N2, CO2, and O3 as a function of altitude. Dispersion due to O2 accounts for approximately 85% of the phase speed increment from 0 to 18 km. At 0 km, the N2-induced phase speed increment accounts for the majority of the remaining 15%, but decreases with increasing altitude. The percent contribution of CO2 is small at 0 km, and increases to 14% at 18 km. The CO2-induced dispersion, not typically included in sound propagation models, may effect sonic boom shock structure of supersonic aircraft at cruise altitudes.