This report presents the results of the study to extend the useful attenuation range of the Approximate Method outlined in the American National Standard, "Method for Calculation of the Absorption of Sound by the Atmosphere" (ANSI S1.26-1995), and provide a basis for replacing the current Society of Automotive Engineers Aerospace Recommended Practice 866A, "Standard Values of Atmospheric Absorption as a Function of Temperatureand Humidity" (SAE ARP866A). The report describes the implementation of the one-thirdoctave-band adaptations of the ISO/ANSI pure-tone equations, and the development and testingof the proposed SAE Method.