By assuming air is composed of four gases, i.e., nitrogen, oxygen, water vapor, and carbon dioxide, and applying energy transfer rates for the binary collisions inherent in such a system, absorption of sound in the atmosphere has been predicted. The calculated curves based upon 24 energy transfer mechanisms are compared with experimental data over the humidity range of 0–100% relative humidity. Agreement between theory and experiment is good. By including classical absorption and rotational relaxation effects the total atmospheric absorption is also predicted. Calculations made for various concentrations of CO2 indicate that low-CO2 levels (less than 0.1% do not significantly affect absorption of audible sound at high humidities. At low humidities, however, CO2 is an important factor.