INTRODUCTION There are a number of factors which influence the propagation of noise from an aircraft to a point on the ground. The purpose of this ARP@ however@ is to consider only the classical and molecular absorption of sound energy by the atmosphere. Spherical divergence@ scattering@ refraction@ and other effects should be treated separately. This ARP describes a method by which values can be obtained for the absorption of sound in air over a wide range of temperature and humidity conditions. Although it was developed primarily for use in evaluating aircraft flyover noise measurements@ the information should be applicable to other noise problems as well. The method presented is based on the theories of Kneser and Evans and Bazley@ the laboratory results of Harris and field data from various sources. Although the final information was used from these sources only@ the works of many other individuals and groups were used to arrive at a selection of the present method. The experimental results of Harris@ were obtained for a single temperature of 20 ??(68??. Essentially@ these data were used and curves based on Kneser's theory were modified to fit them. The modified curves then served as a basis for obtaining values over a wide range of temperatures@ humidifies and frequencies. Once these curves were established@ they were compared with field results to select a method for predicting absorption values for bands of noise by using the absorption value for a single frequency. Since AEP 866 was first published in 1964@ considerable data on the atmospheric absorption of sound have been collected. Most of these studies have indicated that@ on the average@ the atmospheric absorption coefficients determined by the method described herein are quite close to the measured values over a reasonably wide range of air temperatures and relative humidities. The purpose of this reissue of ARP 866 is to describe a mathematical procedure for determining atmospheric absorption coefficients that is suitable for use with machine computation techniques. Use of the mathematical representations of the various curves involved in the calculation routine should improve the precision and repeatability of determining atmospheric absorption coefficients. The calculation procedure is used to determine atmospheric absorption coefficients for 1/1- and 1/3 ?C octave bands of noise@ for air temperatures ranging from 1 to 100 ??and relative humidities from 1 to 100 percent. The calculated coefficients are presented in tabular and graphical forms. The tabulated coefficients provide a convenient method of reading all 1/1 ?C or 1/3 ?C octave band coefficients on one page for a variety of temperatures for a improved legibility and for easy comprehension of the dependence of the absorption coefficients on frequency@ temperature@ and humidity.