Effects of carbon dioxide on atmospheric absorption of noise for en-route aircraft and supersonic aircraft

The relative importance of carbon dioxide-induced absorption of noise is examined using the Sutherland and Bass 2004 atmospheric absorption model. Three cases are considered: first the vibrational relaxation loss due to CO₂ is compared with the vibrational losses due to N₂, O₂, and O₃. The second case compares the vibrational component of absorption to other mechanisms including rotational, classical, and diffusion losses. Finally, the relative contribution of CO₂-induced dispersion to total dispersion is evaluated. In all cases a frequency range of 125 to 1000 Hz and an atmosphere of 0 to 18 km are considered. Preliminary results indicate that carbon dioxide is not a major determinant of total absorption for altitudes typical of en-route subsonic aircraft in cruise, but may have a more pronounced effect at the higher altitudes involved for future supersonic aircraft. Additionally, CO₂ induced dispersion is found to be non-negligible at higher altitudes. The dominant absorption mechanisms rapidly shift over this range of altitudes, a phenomenon suggesting that the sonic boom shock structure could be influenced by the presence of CO₂ close to a cruising supersonic aircraft.